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WSi weak link element with a non-sinusoidal current-phase relation
Authors:
Sarah Garcia Jones,
Trevyn F. Q. Larson,
Sai Pavan Chitta,
Heli Vora,
Varun Verma,
Sae Woo Nam,
José Aumentado,
Jens Koch,
Raymond W. Simmonds,
András Gyenis
Abstract:
Nonlinearity is an essential ingredient for encoding quantum states with non-uniform energy spacing, implementing coherent quantum gates, reading out qubits, amplifying, and mixing electromagnetic signals. In this work, we demonstrate the nonlinear behavior of a constriction fabricated from an amorphous, high-kinetic inductance material, tungsten silicide, embedded in a three-dimensional RF-SQUID.…
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Nonlinearity is an essential ingredient for encoding quantum states with non-uniform energy spacing, implementing coherent quantum gates, reading out qubits, amplifying, and mixing electromagnetic signals. In this work, we demonstrate the nonlinear behavior of a constriction fabricated from an amorphous, high-kinetic inductance material, tungsten silicide, embedded in a three-dimensional RF-SQUID. We find that the results are consistent with the weak link behaving as a Josephson junction with a sawtooth-like current-phase relation or a quantum phase slip element. Finally, we measure relaxation times of the metastable, persistent-current states trapped in the local minima of the potential.
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Submitted 19 May, 2026;
originally announced May 2026.
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Reaching the intrinsic performance limits of superconducting nanowire single-photon detectors up to 0.1 mm wide
Authors:
Kristen M. Parzuchowski,
Eli Mueller,
Bakhrom G. Oripov,
Benedikt Hampel,
Ravin A. Chowdhury,
Sahil R. Patel,
Daniel Kuznesof,
Emma K. Batson,
Ryan Morgenstern,
Robert H. Hadfield,
Varun B. Verma,
Matthew D. Shaw,
Jason P. Allmaras,
Martin J. Stevens,
Alex Gurevich,
Adam N. McCaughan
Abstract:
Superconducting nanowire single-photon detectors (SNSPDs) combine high detection efficiency, low noise, and excellent timing resolution, making them a leading platform for photon-counting applications. However, despite decades of materials and fabrication research, detector performance has never been shown to match theoretical performance expectations. Here, we demonstrate for the first time in si…
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Superconducting nanowire single-photon detectors (SNSPDs) combine high detection efficiency, low noise, and excellent timing resolution, making them a leading platform for photon-counting applications. However, despite decades of materials and fabrication research, detector performance has never been shown to match theoretical performance expectations. Here, we demonstrate for the first time in situ tuning of a detector from its typical, suboptimal operation, to a regime limited only by material quality, allowing the device to reach its intrinsic performance limit. Our approach is based on current-biased superconducting "rails" placed on either side of the detector that redistribute current across its width to achieve its peak performance. This technique reduces the dark count rate by ten orders of magnitude. Further, we show operation at this intrinsic performance limit for devices up to 0.1 mm wide, and also demonstrate near-unity internal detection efficiency (IDE) at a wavelength of 4um for a 20um-wide detector--a factor of 20 wider than the current state of the art. This work enables future detectors to overcome the Pearl limit for device width, paving the way for arbitrarily large detectors.
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Submitted 23 July, 2026; v1 submitted 22 January, 2026;
originally announced January 2026.
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Tungsten Germanide Superconducting Nanowire Single-Photon Detectors with Saturated Internal Detection Efficiency at Wavelengths up to 29 μm
Authors:
Benedikt Hampel,
Daniel Kuznesof,
Andrew S. Mueller,
Sahil R. Patel,
Robert H. Hadfield,
Emma E. Wollman,
Matthew D. Shaw,
Dirk Schwarzer,
Alec M. Wodtke,
Khalid Hossain,
Allison V. Mis,
Alexana Roshko,
Richard P. Mirin,
Sae Woo Nam,
Martin J. Stevens,
Varun B. Verma
Abstract:
Superconducting nanowire single-photon detectors (SNSPDs) are among the most sensitive single-photon detectors available and have the potential to transform fields ranging from infrared astrophysics to molecular spectroscopy. However, extending their performance into the mid-infrared spectral region - crucial for applications such as exoplanet transit spectroscopy and vibrational fingerprinting of…
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Superconducting nanowire single-photon detectors (SNSPDs) are among the most sensitive single-photon detectors available and have the potential to transform fields ranging from infrared astrophysics to molecular spectroscopy. However, extending their performance into the mid-infrared spectral region - crucial for applications such as exoplanet transit spectroscopy and vibrational fingerprinting of molecules - has remained a major challenge, primarily due to material limitations and scalability constraints. Here, we report on the development of SNSPDs based on tungsten germanide, a novel material system that combines high mid-infrared sensitivity with compatibility for large-scale fabrication. Our detectors exhibit saturated internal detection efficiency at wavelengths up to 29 μm, while using 2.7x thicker films (8 nm vs 3 nm) and up to 4.5x wider nanowires (360 nm vs 80 nm) compared to mid-infrared-optimized SNSPDs fabricated from tungsten silicide. This advance will enable scalable, high-performance single-photon detection in a spectral region that was previously inaccessible, opening new frontiers in remote sensing, thermal imaging, environmental monitoring, molecular physics, and astronomy.
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Submitted 16 June, 2026; v1 submitted 25 November, 2025;
originally announced November 2025.
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Orbital magnetic moments in FeCr2S4 studied by x-ray magnetic circular dichroism
Authors:
V. K. Verma,
J. Patra,
V. R. Singh,
Y. Nonaka,
G. Shibata,
K. Ishigami,
A. Tanaka,
K. Ohgushi,
Y. Tokura,
T. Koide,
A. Fujimori
Abstract:
We have investigated the element specific magnetic characteristics of single-crystal FeCr2S4 using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). We have found that the Fe L2,3-edge XAS spectra do not exhibit clear multiplet structures, indicating strong hybridization between the Fe 3d and S 3p orbitals, leading to delocalized rather than localized electronic sta…
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We have investigated the element specific magnetic characteristics of single-crystal FeCr2S4 using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). We have found that the Fe L2,3-edge XAS spectra do not exhibit clear multiplet structures, indicating strong hybridization between the Fe 3d and S 3p orbitals, leading to delocalized rather than localized electronic states. The Fe 3d and Cr 3d spin moments are antiferromagnetically coupled, consistent with the Goodenough-Kanamori rule. The orbital magnetic moments of Fe and Cr are determined to be -0.23 and -0.017 μB/ion, respectively. The large orbital magnetic moment of Fe is due to the d6 configuration under the relatively weak tetrahedra crystal field at the Fe site, and the delocalized Fe electrons maintain the orbital degree of freedom in spite of their itinerant nature. To understand phenomena such as the gigantic Kerr rotation, it is essential to consider not only the orbital degrees of freedom but also the role of spin-orbit coupling, which induces a finite orbital magnetic moment through t2 and e level hybridization under the tetrahedral crystal field. This finite orbital moment serves as a direct indicator of spin-orbit interaction strength and links element-specific orbital magnetism to the large Kerr rotation. On the other hand, the octahedral crystal-field splitting of the Cr 3d level is large enough to result in the quenching of the orbital moment of the Cr ion in FeCr2S4.
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Submitted 15 September, 2025; v1 submitted 20 August, 2025;
originally announced August 2025.
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Localized quasiparticles in a fluxonium with quasi-two-dimensional amorphous kinetic inductors
Authors:
Trevyn F. Q. Larson,
Sarah Garcia Jones,
Tamás Kalmár,
Pablo Aramburu Sanchez,
Sai Pavan Chitta,
Varun Verma,
Kristen Genter,
Katarina Cicak,
Sae Woo Nam,
Gergő Fülöp,
Jens Koch,
Ray W. Simmonds,
András Gyenis
Abstract:
Disordered superconducting materials with high kinetic inductance are an important resource to generate nonlinearity in quantum circuits and create high-impedance environments. In thin films fabricated from these materials, the combination of disorder and the low effective dimensionality leads to increased order parameter fluctuations and enhanced kinetic inductance values. Among the challenges of…
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Disordered superconducting materials with high kinetic inductance are an important resource to generate nonlinearity in quantum circuits and create high-impedance environments. In thin films fabricated from these materials, the combination of disorder and the low effective dimensionality leads to increased order parameter fluctuations and enhanced kinetic inductance values. Among the challenges of harnessing these compounds in coherent devices are their proximity to the superconductor-insulator phase transition, the presence of broken Cooper pairs, and the two-level systems located in the disordered structure. In this work, we fabricate tungsten silicide wires from quasi-two-dimensional films with one spatial dimension smaller than the superconducting coherence length and embed them into microwave resonators and fluxonium qubits, where the kinetic inductance provides the inductive part of the circuits. We study the dependence of loss on the frequency, disorder, and geometry of the device, and find that the loss increases with the level of disorder and is dominated by the localized quasiparticles trapped in the spatial variations of the superconducting gap.
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Submitted 10 April, 2025;
originally announced April 2025.
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Synthesis and Stability Kinetics of Nanoporous TaC Derived from Ta Precursors
Authors:
Catherine Ott,
Vaibhav Verma,
Adam Peters,
Ian McCue
Abstract:
Ultra-high temperature ceramics (UHTCs) are promising materials for use in next-generation aerospace structures but have primarily been used as monolithic materials or coatings due to processing limitations. Here, new functionality (e.g., ablation resistance) is introduced to these materials by developing a porous form factor that can be later infiltrated with a secondary phase. This UHTC scaffold…
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Ultra-high temperature ceramics (UHTCs) are promising materials for use in next-generation aerospace structures but have primarily been used as monolithic materials or coatings due to processing limitations. Here, new functionality (e.g., ablation resistance) is introduced to these materials by developing a porous form factor that can be later infiltrated with a secondary phase. This UHTC scaffold is synthesized via gas-phase carburization of nanoporous tantalum to the ultra-high-temperature ceramic, TaC. The kinetics of Ta conversion in a carburizing environment was examined over a range of temperatures to determine rate-limiting behavior and activation energy for the process. A 1-D moving interface model was constructed to predict carburization depth and compare data from the present work to that in the literature. It was found that the activation energy for carburization increases as conversion proceeds, suggesting a transition from grain boundary to bulk diffusion. Additionally, to simulate potential use cases of this nanoporous ceramic, the compositional and morphological stability was evaluated in a high temperature environment. Finally, the utility of this thermally stable porous UHTC was demonstrated through synthesis of a nanostructured composite of TaC and oxidation-resistant material, SiO2.
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Submitted 24 December, 2024;
originally announced December 2024.
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Harnessing the Duality of Magnetism and Conductivity: A Review of Oxide based Dilute Magnetic Semiconductors
Authors:
Pankaj Bhardwaj,
Jarnail Singh,
Vikram Verma,
Ravi Kumar
Abstract:
Over the last two decades, the new branch of spintronics, i.e., semiconductor spintronics, has gained more attention because it integrates the characteristics of conventional semiconductors, such as optical bandgap and charge carriers, helpful for processing and computing pieces of information combined with magnets for data storage applications in a single device. Likewise, substituting transition…
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Over the last two decades, the new branch of spintronics, i.e., semiconductor spintronics, has gained more attention because it integrates the characteristics of conventional semiconductors, such as optical bandgap and charge carriers, helpful for processing and computing pieces of information combined with magnets for data storage applications in a single device. Likewise, substituting transition metal (TM) ions to induce magnetic qualities into semiconductors or oxides creates dilute magnetic semiconductors (DMSs) or oxides (DMOs) with high electronic, photonic, and magnetic functionality. This review article discusses the historical outline of magnetic semiconductors with their origin and mechanism. It also includes a concise overview of various DMO systems based on their conductivity (p-type and n-type) to elucidate the synthesis, origin, and control mechanisms and further evoke the prepared spintronics devices. The occurrence of RTFM with transparency and conductivity can be helpful in spintronics device fabrications, which was assumed to be governed by the formation of intrinsic defects, charge carriers, morphology, and the induced exchange interactions between ions. The DMOs-based spintronics devices, such as magneto-optical devices, transparent ferromagnets, and spin-based solar cells, exploit both semiconducting and magnetic properties, which have also been discussed in this review article with outlook and perspectives.
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Submitted 6 May, 2024; v1 submitted 28 February, 2024;
originally announced February 2024.
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A 64-pixel mid-infrared single-photon imager based on superconducting nanowire detectors
Authors:
Benedikt Hampel,
Richard P. Mirin,
Sae Woo Nam,
Varun B. Verma
Abstract:
A large-format mid-infrared single-photon imager with very low dark count rates would enable a broad range of applications in fields like astronomy and chemistry. Superconducting nanowire single-photon detectors (SNSPDs) are a mature photon-counting technology as demonstrated by their figures of merit. However, scaling SNSPDs to large array sizes for mid-infrared applications requires sophisticate…
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A large-format mid-infrared single-photon imager with very low dark count rates would enable a broad range of applications in fields like astronomy and chemistry. Superconducting nanowire single-photon detectors (SNSPDs) are a mature photon-counting technology as demonstrated by their figures of merit. However, scaling SNSPDs to large array sizes for mid-infrared applications requires sophisticated readout architectures in addition to superconducting materials development. In this work, an SNSPD array design that combines a thermally coupled row-column multiplexing architecture with a thermally coupled time-of-flight transmission line was developed for mid-infrared applications. The design requires only six cables and can be scaled to larger array sizes. The demonstration of a 64-pixel array shows promising results for wavelengths between $\mathrm{3.4\,μm}$ and $\mathrm{10\,μm}$, which will enable the use of this single-photon detector technology for a broad range of new applications.
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Submitted 28 September, 2023;
originally announced September 2023.
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Trap-Integrated Superconducting Nanowire Single-Photon Detectors with Improved RF Tolerance for Trapped-Ion Qubit State Readout
Authors:
Benedikt Hampel,
Daniel H. Slichter,
Dietrich Leibfried,
Richard P. Mirin,
Sae Woo Nam,
Varun B. Verma
Abstract:
State readout of trapped-ion qubits with trap-integrated detectors can address important challenges for scalable quantum computing, but the strong rf electric fields used for trapping can impact detector performance. Here, we report on NbTiN superconducting nanowire single-photon detectors (SNSPDs) employing grounded aluminum mirrors as electrical shielding that are integrated into linear surface-…
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State readout of trapped-ion qubits with trap-integrated detectors can address important challenges for scalable quantum computing, but the strong rf electric fields used for trapping can impact detector performance. Here, we report on NbTiN superconducting nanowire single-photon detectors (SNSPDs) employing grounded aluminum mirrors as electrical shielding that are integrated into linear surface-electrode rf ion traps. The shielded SNSPDs can be successfully operated at applied rf trapping potentials of up to $\mathrm{54\,V_{peak}}$ at $\mathrm{70\,MHz}$ and temperatures of up to $\mathrm{6\,K}$, with a maximum system detection efficiency of $\mathrm{68\,\%}$. This performance should be sufficient to enable parallel high-fidelity state readout of a wide range of trapped ion species in typical cryogenic apparatus.
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Submitted 2 February, 2023;
originally announced February 2023.
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Cr doping-induced ferromagnetism in the spin-glass Cd1-xMnxTe studied by x-ray magnetic circular dichroism
Authors:
V. K. Verma,
S. Sakamoto,
K. Ishikawa,
V. R. Singh,
K. Ishigami,
G. Shibata,
T. Kadono,
T. Koide,
S. Kuroda,
A. Fujimori
Abstract:
The prototypical diluted magnetic semiconductor Cd1-xMnxTe is a spin glass (x<0.6) or an antiferromagnet (x>0.6), but becomes ferromagnetic upon doping with a small amount of Cr atoms substituting for Mn. In order to investigate the origin of the ferromagnetism in Cd1-x-yMnxCryTe, we have studied its element specific magnetic properties by x-ray absorption spectroscopy (XAS) and x-ray magnetic cir…
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The prototypical diluted magnetic semiconductor Cd1-xMnxTe is a spin glass (x<0.6) or an antiferromagnet (x>0.6), but becomes ferromagnetic upon doping with a small amount of Cr atoms substituting for Mn. In order to investigate the origin of the ferromagnetism in Cd1-x-yMnxCryTe, we have studied its element specific magnetic properties by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the Cr and Mn L2,3 edges. Thin films were grown by molecular beam epitaxy with a fixed Mn content of x = 0.2 and varying Cr content in the range of y = 0 - 0.04. Measured XAS and XMCD spectra indicate that both Cr and Mn atoms are divalent and that the ferromagnetic or superparamagnetic components of Cr and Mn are aligned in the same directions. The magnetization of Mn increases with increasing Cr content. These results can be explained if ferromagnetic interaction exists between neighboring Mn and Cr ions although interaction between Mn atoms is largely antiferromagnetic. We conclude that each ferromagnetic or superparamagnetic cluster consists of ferromagnetically coupled several Cr and a much larger number of Mn ions.
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Submitted 5 April, 2022;
originally announced April 2022.
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Size dependent nature of the magnetic-field driven superconductor-to-insulator quantum-phase transitions
Authors:
Xiaofu Zhang,
Adriana E. Lita,
Huanlong Liu,
Varun B. Verma,
Qiang Zhou,
Sae Woo Nam,
Andreas Schilling
Abstract:
The nature of the magnetic-field driven superconductor-to-insulator quantum-phase transition in two-dimensional systems at zero temperature has been under debate since the 1980s, and became even more controversial after the observation of a quantum-Griffiths singularity. Whether it is induced by quantum fluctuations of the superconducting phase and the localization of Cooper pairs, or is directly…
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The nature of the magnetic-field driven superconductor-to-insulator quantum-phase transition in two-dimensional systems at zero temperature has been under debate since the 1980s, and became even more controversial after the observation of a quantum-Griffiths singularity. Whether it is induced by quantum fluctuations of the superconducting phase and the localization of Cooper pairs, or is directly driven by depairing of these pairs, remains an open question. We herein experimentally demonstrate that in weakly-pinning systems and in the limit of infinitely wide films, a sequential superconductor-to-Bose insulator-to-Fermi insulator quantum-phase transition takes place. By limiting their size smaller than the effective penetration depth, however, the vortex interaction alters, and the superconducting state re-enters the Bose-insulating state. As a consequence, one observes a direct superconductor-to-Fermi insulator in the zero-temperature limit. In narrow films, the associated critical-exponent products diverge along the corresponding phase boundaries with increasing magnetic field, which is a hallmark of the quantum-Griffiths singularity.
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Submitted 13 May, 2021;
originally announced May 2021.
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State Readout of a Trapped Ion Qubit Using a Trap-Integrated Superconducting Photon Detector
Authors:
S. L. Todaro,
V. B. Verma,
K. C. McCormick,
D. T. C. Allcock,
R. P. Mirin,
D. J. Wineland,
S. W. Nam,
A. C. Wilson,
D. Leibfried,
D. H. Slichter
Abstract:
We report high-fidelity state readout of a trapped ion qubit using a trap-integrated photon detector. We determine the hyperfine qubit state of a single $^9$Be$^+$ ion held in a surface-electrode rf ion trap by counting state-dependent ion fluorescence photons with a superconducting nanowire single-photon detector (SNSPD) fabricated into the trap structure. The average readout fidelity is 0.9991(1…
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We report high-fidelity state readout of a trapped ion qubit using a trap-integrated photon detector. We determine the hyperfine qubit state of a single $^9$Be$^+$ ion held in a surface-electrode rf ion trap by counting state-dependent ion fluorescence photons with a superconducting nanowire single-photon detector (SNSPD) fabricated into the trap structure. The average readout fidelity is 0.9991(1), with a mean readout duration of 46 $μ$s, and is limited by the polarization impurity of the readout laser beam and by off-resonant optical pumping. Because there are no intervening optical elements between the ion and the detector, we can use the ion fluorescence as a self-calibrated photon source to determine the detector quantum efficiency and its dependence on photon incidence angle and polarization.
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Submitted 31 July, 2020;
originally announced August 2020.
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Methodical engineering of defects in Mn$_x$Zn$_{1-x}$ O($x$ = 0.03, and 0.05) nanostructures by electron beam for nonlinear optical applications: A new insight
Authors:
Albin Antony,
P. Poornesh,
I. V. Kityk,
K. Ozga,
J. Jedryka,
Reji Philip,
Ganesh Sanjeev,
Vikash Chandra Petwal,
Vijay Pal Verma,
Jishnu Dwivedi
Abstract:
A series of MnxZn1-xO (x=0.03, 0.05) nanostructures have been grown via the solution based chemical spray pyrolysis technique. Electron beam induced modifications on structural, linear and nonlinear optical and surface morphological properties have been studied and elaborated. GXRD (glancing angle X-ray diffraction) patterns show sharp diffraction peaks matching with the hexagonal wurtzite structu…
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A series of MnxZn1-xO (x=0.03, 0.05) nanostructures have been grown via the solution based chemical spray pyrolysis technique. Electron beam induced modifications on structural, linear and nonlinear optical and surface morphological properties have been studied and elaborated. GXRD (glancing angle X-ray diffraction) patterns show sharp diffraction peaks matching with the hexagonal wurtzite structure of ZnO thin films. The upsurge in ebeam dosage resulted in the shifting of XRD peaks (101) and (002) towards lower angle side, and increase in FWHM value. Gaussian deconvolution on PL spectra reveals the quenching of defect centers, implying the role of electron beam irradiation regulating luminescence and defect centers in the nanostructures. Irradiation induced spatial confinement and phonon localization effects have been observed in the films via micro Raman studies. The later are evident from spectral peak shifts and broadening. Detailed investigations on the effect of electron beam irradiation on third order nonlinear optical properties under continuous and pulsed mode of laser operation regimes are deliberated. Third order absorptive nonlinearity of the nanostructures evaluated using the open aperture Z-scan technique in both continuous and pulsed laser regimes shows strong nonlinear absorption coefficient \b{eta} eff of the order 10-4 cm/W confirming their suitability for passive optical limiting applications under intense radiation environments. Laser induced third harmonic generation (LITHG) experiment results supports the significant variation in nonlinearities upon electron beam irradiation, and the effect can be utilized for frequency conversion mechanisms in high power laser sources and UV light emitters.
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Submitted 8 January, 2020;
originally announced January 2020.
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Strong suppression of the resistivity near the transition to superconductivity in narrow micro-bridges in external magnetic fields
Authors:
Xiaofu Zhang,
Adriana E. Lita,
Konstantin Smirnov,
HuanLong Liu,
Dong Zhu,
Varun B. Verma,
Sae Woo Nam,
Andreas Schilling
Abstract:
We have investigated a series of superconducting bridges based on homogeneous amorphous WSi and MoSi films, with bridge widths w ranging from 2 um to 1000 um and film thicknesses d ~ 4-6 nm and 100 nm. Upon decreasing the bridge widths below the respective Pearl lengths, we observe in all cases distinct changes in the characteristics of the resistive transitions to superconductivity. For each of t…
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We have investigated a series of superconducting bridges based on homogeneous amorphous WSi and MoSi films, with bridge widths w ranging from 2 um to 1000 um and film thicknesses d ~ 4-6 nm and 100 nm. Upon decreasing the bridge widths below the respective Pearl lengths, we observe in all cases distinct changes in the characteristics of the resistive transitions to superconductivity. For each of the films, the resistivity curves R(B,T) separate at a well-defined and field-dependent temperature T*(B) with decreasing the temperature, resulting in a dramatic suppression of the resistivity and a sharpening of the transitions with decreasing bridge width w. The associated excess conductivity in all the bridges scales as 1/w, which may suggest the presence of a highly conducting region that is dominating the electric transport in narrow bridges. We argue that this effect can only be observed in materials with sufficiently weak vortex pinning.
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Submitted 21 February, 2020; v1 submitted 6 September, 2019;
originally announced September 2019.
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Determining the depairing current in superconducting nanowire single-photon detectors
Authors:
S. Frasca,
B. Korzh,
M. Colangelo,
D. Zhu,
A. E. Lita,
J. P. Allmaras,
E. E. Wollman,
V. B. Verma,
A. E. Dane,
E. Ramirez,
A. D. Beyer,
S. W. Nam,
A. G. Kozorezov,
M. D. Shaw,
K. K. Berggren
Abstract:
We estimate the depairing current of superconducting nanowire single photon detectors (SNSPDs) by studying the dependence of the nanowires kinetic inductance on their bias current. The kinetic inductance is determined by measuring the resonance frequency of resonator style nanowire coplanar waveguides both in transmission and reflection configurations. Bias current dependent shifts in the measured…
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We estimate the depairing current of superconducting nanowire single photon detectors (SNSPDs) by studying the dependence of the nanowires kinetic inductance on their bias current. The kinetic inductance is determined by measuring the resonance frequency of resonator style nanowire coplanar waveguides both in transmission and reflection configurations. Bias current dependent shifts in the measured resonant frequency correspond to the change in the kinetic inductance, which can be compared with theoretical predictions. We demonstrate that the fast relaxation model described in the literature accurately matches our experimental data and provides a valuable tool for direct determination of the depairing current. Accurate and direct measurement of the depairing current is critical for nanowire quality analysis, as well as modeling efforts aimed at understanding the detection mechanism in SNSPDs.
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Submitted 18 April, 2019;
originally announced April 2019.
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An ultrahigh-impedance superconducting thermal switch for interfacing superconductors to semiconductors and optoelectronics
Authors:
A. N. McCaughan,
V. B. Verma,
S. Buckley,
J. P. Allmaras,
A. G. Kozorezov,
A. N. Tait,
S. W. Nam,
J. M. Shainline
Abstract:
A number of current approaches to quantum and neuromorphic computing use superconductors as the basis of their platform or as a measurement component, and will need to operate at cryogenic temperatures. Semiconductor systems are typically proposed as a top-level control in these architectures, with low-temperature passive components and intermediary superconducting electronics acting as the direct…
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A number of current approaches to quantum and neuromorphic computing use superconductors as the basis of their platform or as a measurement component, and will need to operate at cryogenic temperatures. Semiconductor systems are typically proposed as a top-level control in these architectures, with low-temperature passive components and intermediary superconducting electronics acting as the direct interface to the lowest-temperature stages. The architectures, therefore, require a low-power superconductor-semiconductor interface, which is not currently available. Here we report a superconducting switch that is capable of translating low-voltage superconducting inputs directly into semiconductor-compatible (above 1,000 mV) outputs at kelvin-scale temperatures (1 K or 4 K). To illustrate the capabilities in interfacing superconductors and semiconductors, we use it to drive a light-emitting diode (LED) in a photonic integrated circuit, generating photons at 1 K from a low-voltage input and detecting them with an on-chip superconducting single-photon detector. We also characterize our device's timing response (less than 300 ps turn-on, 15 ns turn-off), output impedance (greater than 1 MΩ), and energy requirements (0.18 fJ/um^2, 3.24 mV/nW).
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Submitted 30 September, 2019; v1 submitted 25 March, 2019;
originally announced March 2019.
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Detecting Dark Matter with Superconducting Nanowires
Authors:
Yonit Hochberg,
Ilya Charaev,
Sae-Woo Nam,
Varun Verma,
Marco Colangelo,
Karl K. Berggren
Abstract:
We propose the use of superconducting nanowires as both target and sensor for direct detection of sub-GeV dark matter. With excellent sensitivity to small energy deposits on electrons, and demonstrated low dark counts, such devices could be used to probe electron recoils from dark matter scattering and absorption processes. We demonstrate the feasibility of this idea using measurements of an exist…
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We propose the use of superconducting nanowires as both target and sensor for direct detection of sub-GeV dark matter. With excellent sensitivity to small energy deposits on electrons, and demonstrated low dark counts, such devices could be used to probe electron recoils from dark matter scattering and absorption processes. We demonstrate the feasibility of this idea using measurements of an existing fabricated tungsten-silicide nanowire prototype with 0.8 eV energy threshold and 4.3 nanograms with 10 thousand seconds of exposure, which showed no dark counts. The results from this device already place meaningful bounds on dark matter-electron interactions, including the strongest terrestrial bounds on sub-eV dark photon absorption to date. Future expected fabrication on larger scales and with lower thresholds should enable probing new territory in the direct detection landscape, establishing the complementarity of this approach to other existing proposals.
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Submitted 12 March, 2019;
originally announced March 2019.
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Superconducting fluctuations and characteristic time scales in amorphous WSi
Authors:
Xiaofu Zhang,
Adriana E. Lita,
Mariia Sidorova,
Varun B. Verma,
Qiang Wang,
Sae Woo Nam,
Alexej Semenov,
Andreas Schilling
Abstract:
We study magnitudes and temperature dependences of the electron-electron and electron-phonon interaction times which play the dominant role in the formation and relaxation of photon induced hotspot in two dimensional amorphous WSi films. The time constants are obtained through magnetoconductance measurements in perpendicular magnetic field in the superconducting fluctuation regime and through time…
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We study magnitudes and temperature dependences of the electron-electron and electron-phonon interaction times which play the dominant role in the formation and relaxation of photon induced hotspot in two dimensional amorphous WSi films. The time constants are obtained through magnetoconductance measurements in perpendicular magnetic field in the superconducting fluctuation regime and through time-resolved photoresponse to optical pulses. The excess magnetoconductivity is interpreted in terms of the weak-localization effect and superconducting fluctuations. Aslamazov-Larkin, and Maki-Thompson superconducting fluctuation alone fail to reproduce the magnetic field dependence in the relatively high magnetic field range when the temperature is rather close to Tc because the suppression of the electronic density of states due to the formation of short lifetime Cooper pairs needs to be considered. The time scale τ_i of inelastic scattering is ascribed to a combination of electron-electron (τ_(e-e)) and electron-phonon (τ_(e-ph)) interaction times, and a characteristic electron-fluctuation time (τ_(e-fl)), which makes it possible to extract their magnitudes and temperature dependences from the measured τ_i. The ratio of phonon-electron (τ_(ph-e)) and electron-phonon interaction times is obtained via measurements of the optical photoresponse of WSi microbridges. Relatively large τ_(e-ph)/τ_(ph-e) and τ_(e-ph)/τ_(e-e) ratios ensure that in WSi the photon energy is more efficiently confined in the electron subsystem than in other materials commonly used in the technology of superconducting nanowire single-photon detectors (SNSPDs). We discuss the impact of interaction times on the hotspot dynamics and compare relevant metrics of SNSPDs from different materials.
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Submitted 13 December, 2017;
originally announced December 2017.
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Direct observation of nanofabrication influence on the optical properties of single self-assembled InAs/GaAs quantum dots
Authors:
Jin Liu,
Kumarasiri Konthasinghe,
Marcelo Davanco,
John Lawall,
Vikas Anant,
Varun Verma,
Richard Mirin,
Sae Woo Nam,
Jin Dong Song,
Ben Ma,
Ze Sheng Chen,
Hai Qiao Ni,
Zhi Chuan Niu,
Kartik Srinivasan
Abstract:
Single self-assembled InAs/GaAs quantum dots are a promising solid-state quantum technology, with which vacuum Rabi splitting, single-photon-level nonlinearities, and bright, pure, and indistinguishable single-photon generation having been demonstrated. For such achievements, nanofabrication is used to create structures in which the quantum dot preferentially interacts with strongly-confined optic…
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Single self-assembled InAs/GaAs quantum dots are a promising solid-state quantum technology, with which vacuum Rabi splitting, single-photon-level nonlinearities, and bright, pure, and indistinguishable single-photon generation having been demonstrated. For such achievements, nanofabrication is used to create structures in which the quantum dot preferentially interacts with strongly-confined optical modes. An open question is the extent to which such nanofabrication may also have an adverse influence, through the creation of traps and surface states that could induce blinking, spectral diffusion, and dephasing. Here, we use photoluminescence imaging to locate the positions of single InAs/GaAs quantum dots with respect to alignment marks with < 5 nm uncertainty, allowing us to measure their behavior before and after fabrication. We track the quantum dot emission linewidth and photon statistics as a function of distance from an etched surface, and find that the linewidth is significantly broadened (up to several GHz) for etched surfaces within a couple hundred nanometers of the quantum dot. However, we do not observe appreciable reduction of the quantum dot radiative efficiency due to blinking. We also show that atomic layer deposition can stabilize spectral diffusion of the quantum dot emission, and partially recover its linewidth.
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Submitted 26 October, 2017;
originally announced October 2017.
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Towards integrated superconducting detectors on lithium niobate waveguides
Authors:
Jan Philipp Höpker,
Moritz Bartnick,
Evan Meyer-Scott,
Frederik Thiele,
Stephan Krapick,
Nicola Montaut,
Matteo Santandrea,
Harald Herrmann,
Sebastian Lengeling,
Raimund Ricken,
Viktor Quiring,
Torsten Meier,
Adriana Lita,
Varun Verma,
Thomas Gerrits,
Sae Woo Nam,
Christine Silberhorn,
Tim J. Bartley
Abstract:
Superconducting detectors are now well-established tools for low-light optics, and in particular quantum optics, boasting high-efficiency, fast response and low noise. Similarly, lithium niobate is an important platform for integrated optics given its high second-order nonlinearity, used for high-speed electro-optic modulation and polarization conversion, as well as frequency conversion and source…
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Superconducting detectors are now well-established tools for low-light optics, and in particular quantum optics, boasting high-efficiency, fast response and low noise. Similarly, lithium niobate is an important platform for integrated optics given its high second-order nonlinearity, used for high-speed electro-optic modulation and polarization conversion, as well as frequency conversion and sources of quantum light. Combining these technologies addresses the requirements for a single platform capable of generating, manipulating and measuring quantum light in many degrees of freedom, in a compact and potentially scalable manner. We will report on progress integrating tungsten transition-edge sensors (TESs) and amorphous tungsten silicide superconducting nanowire single-photon detectors (SNSPDs) on titanium in-diffused lithium niobate waveguides. The travelling-wave design couples the evanescent field from the waveguides into the superconducting absorber. We will report on simulations and measurements of the absorption, which we can characterize at room temperature prior to cooling down the devices. Independently, we show how the detectors respond to flood illumination, normally incident on the devices, demonstrating their functionality.
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Submitted 18 August, 2017;
originally announced August 2017.
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UV superconducting nanowire single-photon detectors with high efficiency, low noise, and 4 K operating temperature
Authors:
Emma E. Wollman,
Varun B. Verma,
Andrew D. Beyer,
Ryan M. Briggs,
Francesco Marsili,
Jason P. Allmaras,
Adriana E. Lita,
Richard P. Mirin,
Sae Woo Nam,
Matthew D. Shaw
Abstract:
For photon-counting applications at ultraviolet wavelengths, there are currently no detectors that combine high efficiency (> 50%), sub-nanosecond timing resolution, and sub-Hz dark count rates. Superconducting nanowire single-photon detectors (SNSPDs) have seen success over the past decade for photon-counting applications in the near-infrared, but little work has been done to optimize SNSPDs for…
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For photon-counting applications at ultraviolet wavelengths, there are currently no detectors that combine high efficiency (> 50%), sub-nanosecond timing resolution, and sub-Hz dark count rates. Superconducting nanowire single-photon detectors (SNSPDs) have seen success over the past decade for photon-counting applications in the near-infrared, but little work has been done to optimize SNSPDs for wavelengths below 400 nm. Here, we describe the design, fabrication, and characterization of UV SNSPDs operating at wavelengths between 250 and 370 nm. The detectors have active areas up to 56 $μ$m in diameter, 70 - 80% efficiency, timing resolution down to 60 ps FWHM, blindness to visible and infrared photons, and dark count rates of ~ 0.25 counts/hr for a 56 $μ$m diameter pixel. By using the amorphous superconductor MoSi, these UV SNSPDs are also able to operate at temperatures up to 4.2 K. These performance metrics make UV SNSPDs ideal for applications in trapped-ion quantum information processing, lidar studies of the upper atmosphere, UV fluorescent-lifetime imaging microscopy, and photon-starved UV astronomy.
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Submitted 11 August, 2017;
originally announced August 2017.
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Entanglement and Quantum phase transition in topological insulators
Authors:
Anvesh Raja Kovela,
Anant Vijay Verma,
Prasanta K. Panigrahi,
Bhavesh Chauhan
Abstract:
Presence of entangled states is explicitly shown in Topological insulator (TI) $Bi_2Te_3$. The surface and bulk state are found to have the different structures of entanglement. The surface states live as maximally entangled states in the four-dimensional subspace of total Hilbert space (spin, orbital, space). However, bulk states are entangled in the whole Hilbert space. Bulk states are found to…
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Presence of entangled states is explicitly shown in Topological insulator (TI) $Bi_2Te_3$. The surface and bulk state are found to have the different structures of entanglement. The surface states live as maximally entangled states in the four-dimensional subspace of total Hilbert space (spin, orbital, space). However, bulk states are entangled in the whole Hilbert space. Bulk states are found to be entangled maximally by controlled injection of electrons with momentum only along the z-direction. Scheme to detect entanglement in a 2-D model using measurement, confirming natural implementation of universal Hadamard with Controlled-NOT gates is explicated.
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Submitted 3 April, 2017;
originally announced April 2017.
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Fano fluctuations in superconducting nanowire single-photon detectors
Authors:
A. G. Kozorezov,
C. Lambert,
F. Marsili,
M. J. Stevens,
V. B. Verma,
J. P. Allmaras,
M. D. Shaw,
R. P. Mirin,
Sae Woo Nam
Abstract:
Because of their universal nature, Fano fluctuations are expected to influence the response of superconducting nanowire single-photon detectors (SNSPDs). We predict that photon counting rate ($PCR$) as a function of bias current ($I_B$) in SNSPDs is described by an integral over a transverse coordinate-dependent complementary error function. The latter describes smearing of local responses due to…
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Because of their universal nature, Fano fluctuations are expected to influence the response of superconducting nanowire single-photon detectors (SNSPDs). We predict that photon counting rate ($PCR$) as a function of bias current ($I_B$) in SNSPDs is described by an integral over a transverse coordinate-dependent complementary error function. The latter describes smearing of local responses due to Fano fluctuations of the amount of energy deposited into electronic system. The finite width, $σ$, of the $PCR$ vs $I_B$ arises from fluctuations in the energy partition between quasiparticles and phonons during the energy down-conversion cascade. In narrow-nanowire SNSPDs the local responses are uniform, and the effect of Fano-fluctuations on $σ$ is dominant. In wide-nanowire SNSPDs with strong coordinate dependence of local responses due to vortex-antivortex unbinding and vortex entry from edges, Fano-fluctuations smear singularities imprinted by vorticity on the transition part of $PCR$ curve. We demonstrate good agreement between theory and experiments for a series of bath temperatures and photon energies in narrow-wire WSi SNSPDs. The time-resolved hotspot relaxation curves predicted by Fano fluctuations match the Lorentzian shapes observed in experiments over the whole range of bias currents investigated except for their tails.
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Submitted 9 February, 2017;
originally announced February 2017.
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UV-sensitive superconducting nanowire single photon detectors for integration in an ion trap
Authors:
D. H. Slichter,
V. B. Verma,
D. Leibfried,
R. P. Mirin,
S. W. Nam,
D. J. Wineland
Abstract:
We demonstrate superconducting nanowire single photon detectors with 76 +/- 4 % system detection efficiency at a wavelength of 315 nm and an operating temperature of 3.2 K, with a background count rate below 1 count per second at saturated detection efficiency. We propose integrating these detectors into planar surface electrode radio-frequency Paul traps for use in trapped ion quantum information…
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We demonstrate superconducting nanowire single photon detectors with 76 +/- 4 % system detection efficiency at a wavelength of 315 nm and an operating temperature of 3.2 K, with a background count rate below 1 count per second at saturated detection efficiency. We propose integrating these detectors into planar surface electrode radio-frequency Paul traps for use in trapped ion quantum information processing. We operate detectors integrated into test ion trap structures at 3.8 K both with and without typical radio-frequency trapping electric fields. The trapping fields reduce system detection efficiency by 9 %, but do not increase background count rates.
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Submitted 10 April, 2017; v1 submitted 29 November, 2016;
originally announced November 2016.
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Efficient fiber-coupled single-photon source based on quantum dots in a photonic-crystal waveguide
Authors:
Raphaël S. Daveau,
Krishna C. Balram,
Tommaso Pregnolato,
Jin Liu,
Eun H. Lee,
Jin D. Song,
Varun Verma,
Richard Mirin,
Sae Woo Nam,
Leonardo Midolo,
Søren Stobbe,
Kartik Srinivasan,
Peter Lodahl
Abstract:
Many photonic quantum information processing applications would benefit from a high brightness, fiber-coupled source of triggered single photons. Here, we present a fiber-coupled photonic-crystal waveguide single-photon source relying on evanescent coupling of the light field from a tapered out-coupler to an optical fiber. A two-step approach is taken where the performance of the tapered out-coupl…
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Many photonic quantum information processing applications would benefit from a high brightness, fiber-coupled source of triggered single photons. Here, we present a fiber-coupled photonic-crystal waveguide single-photon source relying on evanescent coupling of the light field from a tapered out-coupler to an optical fiber. A two-step approach is taken where the performance of the tapered out-coupler is recorded first on an independent device containing an on-chip reflector. Reflection measurements establish that the chip-to-fiber coupling efficiency exceeds 80 %. The detailed characterization of a high-efficiency photonic-crystal waveguide extended with a tapered out-coupling section is then performed. The corresponding overall single-photon source efficiency is 10.9 % $\pm$ 2.3 %, which quantifies the success probability to prepare an exciton in the quantum dot, couple it out as a photon in the waveguide, and subsequently transfer it to the fiber. The applied out-coupling method is robust, stable over time, and broadband over several tens of nanometers, which makes it a highly promising pathway to increase the efficiency and reliability of planar chip-based single-photon sources.
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Submitted 27 October, 2016;
originally announced October 2016.
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Experimental investigation of the detection mechanism in WSi nanowire superconducting single photon detectors
Authors:
Rosalinda Gaudio,
Jelmer J. Renema,
Zili Zhou,
Varun B. Verma,
Adriana E. Lita,
Jeffrey Shainline,
Martin J. Stevens,
Richard P. Mirin,
Sae Woo Nam,
Martin P. van Exter,
Michiel J. A. de Dood,
Andrea Fiore
Abstract:
We use quantum detector tomography to investigate the detection mechanism in WSi nanowire superconducting single photon detectors (SSPDs). To this purpose, we fabricated a 250nm wide and 250nm long WSi nanowire and measured its response to impinging photons with wavelengths ranging from $λ$ = 900 nm to $λ$ = 1650 nm. Tomographic measurements show that the detector response depends on the total exc…
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We use quantum detector tomography to investigate the detection mechanism in WSi nanowire superconducting single photon detectors (SSPDs). To this purpose, we fabricated a 250nm wide and 250nm long WSi nanowire and measured its response to impinging photons with wavelengths ranging from $λ$ = 900 nm to $λ$ = 1650 nm. Tomographic measurements show that the detector response depends on the total excitation energy only. Moreover, for energies Et > 0.8eV the current energy relation is linear, similar to what was observed in NbN nanowires, whereas the current-energy relation deviates from linear behaviour for total energies below 0.8eV.
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Submitted 7 June, 2016; v1 submitted 24 February, 2016;
originally announced February 2016.
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Hotspot Relaxation Dynamics in a Current Carrying Superconductor
Authors:
F. Marsili,
M. J. Stevens,
A. Kozorezov,
V. B. Verma,
Colin Lambert,
J. A. Stern,
R. Horansky,
S. Dyer,
S. Duff,
D. P. Pappas,
A. Lita,
M. D. Shaw,
R. P. Mirin,
S. W. Nam
Abstract:
We experimentally studied the dynamics of optically excited hotspots in current carrying WSi superconducting nanowires as a function of bias current, bath temperature and excitation wavelength. We discovered that: (1) the hotspot relaxation is a factor of ~ 4 slower in WSi than in NbN; (2) the hotspot relaxation time depends on bias current, and (3) the current dependence of the hotspot relaxation…
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We experimentally studied the dynamics of optically excited hotspots in current carrying WSi superconducting nanowires as a function of bias current, bath temperature and excitation wavelength. We discovered that: (1) the hotspot relaxation is a factor of ~ 4 slower in WSi than in NbN; (2) the hotspot relaxation time depends on bias current, and (3) the current dependence of the hotspot relaxation time changes with temperature and wavelength. We explained all of these effects with a model based on quasi particle recombination.
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Submitted 9 June, 2015;
originally announced June 2015.
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Thickness-dependent magnetic properties and strain-induced orbital magnetic moment in SrRuO3 thin films
Authors:
K. Ishigami,
K. Yoshimatsu,
D. Toyota,
M. Takizawa,
T. Yoshida,
G. Shibata,
T. Harano,
Y. Takahashi,
T. Kadono,
V. K. Verma,
V. R. Singh,
Y. Takeda,
T. Okane,
Y. Saitoh,
H. Yamagami,
T. Koide,
M. Oshima,
H. Kumigashira,
A. Fujimori
Abstract:
Thin films of the ferromagnetic metal SrRuO3 (SRO) show a varying easy magnetization axis depending on the epitaxial strain and undergo a metal-to-insulator transition with decreasing film thickness. We have investigated the magnetic properties of SRO thin films with varying thicknesses fabricated on SrTiO3(001) substrates by soft x-ray magnetic circular dichroism (XMCD) at the Ru M2,3 edge. Resul…
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Thin films of the ferromagnetic metal SrRuO3 (SRO) show a varying easy magnetization axis depending on the epitaxial strain and undergo a metal-to-insulator transition with decreasing film thickness. We have investigated the magnetic properties of SRO thin films with varying thicknesses fabricated on SrTiO3(001) substrates by soft x-ray magnetic circular dichroism (XMCD) at the Ru M2,3 edge. Results have shown that, with decreasing film thickness, the film changes from ferromagnetic to non-magnetic around 3monolayer thickness, consistent with previous magnetization and magneto-optical Kerr effect measurements. The orbital magnetic moment perpendicular to the film was found to be ~ 0.1μB/Ru atom, and remained nearly unchanged with decreasing film thickness while the spin magnetic moment decreases. Mechanism for the formation of the orbital magnetic moment is discussed based on the electronic structure of the compressively strained SRO film.
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Submitted 4 July, 2015; v1 submitted 21 May, 2015;
originally announced May 2015.
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Electronic and magnetic properties of off-stoichiometric Co$_\mathrm{2}$Mn$_β$Si/MgO interfaces studied by x-ray magnetic circular dichroism
Authors:
V. R. Singh,
V. K. Verma,
K. Ishigami,
G. Shibata,
A. Fujimori,
T. Koide,
Y. Miura,
M. Shirai,
T. Ishikawa,
G. f. Li,
M. Yamamoto
Abstract:
We have studied the electronic and magnetic states of Co and Mn atoms at the interface of the Co$_\mathrm{2}$Mn$_β$Si (CMS)/MgO ($β$=0.69, 0.99, 1.15 and 1.29) magnetic tunnel junction (MTJ) by means of x-ray magnetic circular dichroism. In particular, the Mn composition ($β$) dependences of the Mn and Co magnetic moments were investigated. The experimental spin magnetic moments of Mn,…
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We have studied the electronic and magnetic states of Co and Mn atoms at the interface of the Co$_\mathrm{2}$Mn$_β$Si (CMS)/MgO ($β$=0.69, 0.99, 1.15 and 1.29) magnetic tunnel junction (MTJ) by means of x-ray magnetic circular dichroism. In particular, the Mn composition ($β$) dependences of the Mn and Co magnetic moments were investigated. The experimental spin magnetic moments of Mn, $m_\mathrm{spin}$(Mn), derived from XMCD weakly decreased with increasing Mn composition $β$ in going from Mn-deficient to Mn-rich CMS films. This behavior was explained by first-principles calculations based on the antisite-based site-specific formula unit (SSFU) composition model, which assumes the formation of only antisite defect, not vacancies, to accommodate off-stoichiometry. Furthermore, the experimental spin magnetic moments of Co, $m_\mathrm{spin}$(Co), also weakly decreased with increasing Mn composition. This behavior was consistently explained by the antisite-based SSFU model, in particular, by the decrease in the concentration of Co$_\mathrm{Mn}$ antisites detrimental to the half-metallicity of CMS with increasing $β$. This finding is consistent with the higher TMR ratios which have been observed for CMS/MgO/CMS MTJs with Mn-rich CMS electrodes.
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Submitted 9 April, 2015;
originally announced April 2015.
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Quantum storage of entangled telecom-wavelength photons in an erbium-doped optical fibre
Authors:
Erhan Saglamyurek,
Jeongwan Jin,
Varun B. Verma,
Matthew D. Shaw,
Francesco Marsili,
Sae Woo Nam,
Daniel Oblak,
Wolfgang Tittel
Abstract:
The realization of a future quantum Internet requires processing and storing quantum information at local nodes, and interconnecting distant nodes using free-space and fibre-optic links. Quantum memories for light are key elements of such quantum networks. However, to date, neither an atomic quantum memory for non-classical states of light operating at a wavelength compatible with standard telecom…
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The realization of a future quantum Internet requires processing and storing quantum information at local nodes, and interconnecting distant nodes using free-space and fibre-optic links. Quantum memories for light are key elements of such quantum networks. However, to date, neither an atomic quantum memory for non-classical states of light operating at a wavelength compatible with standard telecom fibre infrastructure, nor a fibre-based implementation of a quantum memory has been reported. Here we demonstrate the storage and faithful recall of the state of a 1532 nm wavelength photon, entangled with a 795 nm photon, in an ensemble of cryogenically cooled erbium ions doped into a 20 meter-long silicate fibre using a photon-echo quantum memory protocol. Despite its currently limited efficiency and storage time, our broadband light-matter interface brings fibre-based quantum networks one step closer to reality. Furthermore, it facilitates novel tests of light-matter interaction and collective atomic effects in unconventional materials.
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Submitted 13 January, 2015; v1 submitted 2 September, 2014;
originally announced September 2014.
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High-efficiency WSi superconducting nanowire single-photon detectors operating at 2.5 K
Authors:
V. B. Verma,
B. Korzh,
F. Bussières,
R. D. Horansky,
A. E. Lita,
F. Marsili,
M. D. Shaw,
H. Zbinden,
R. P. Mirin,
S. W. Nam
Abstract:
We investigate the operation of WSi superconducting nanowire single-photon detectors (SNSPDs) at 2.5 K, a temperature which is ~ 70 % of the superconducting transition temperature (TC) of 3.4 K. We demonstrate saturation of the system detection efficiency at 78 +- 2 % with a jitter of 191 ps. We find that the jitter at 2.5 K is limited by the noise of the readout, and can be improved through the u…
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We investigate the operation of WSi superconducting nanowire single-photon detectors (SNSPDs) at 2.5 K, a temperature which is ~ 70 % of the superconducting transition temperature (TC) of 3.4 K. We demonstrate saturation of the system detection efficiency at 78 +- 2 % with a jitter of 191 ps. We find that the jitter at 2.5 K is limited by the noise of the readout, and can be improved through the use of cryogenic amplifiers. Operation of SNSPDs with high efficiency at temperatures very close to TC appears to be a unique property of amorphous WSi.
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Submitted 6 June, 2014;
originally announced June 2014.
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Superconducting nanowire single photon detectors fabricated from an amorphous Mo0.75Ge0.25 thin film
Authors:
V. B. Verma,
A. E. Lita,
M. R. Vissers,
F. Marsili,
D. P. Pappas,
R. P. Mirin,
S. W. Nam
Abstract:
We present the characteristics of superconducting nanowire single photon detectors (SNSPDs) fabricated from amorphous Mo0.75Ge0.25 thin-films. Fabricated devices show a saturation of the internal detection efficiency at temperatures below 1 K, with system dark count rates below 500 counts per second. Operation in a Gifford-McMahon (GM) cryocooler at 2.5 K is possible with system detection efficien…
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We present the characteristics of superconducting nanowire single photon detectors (SNSPDs) fabricated from amorphous Mo0.75Ge0.25 thin-films. Fabricated devices show a saturation of the internal detection efficiency at temperatures below 1 K, with system dark count rates below 500 counts per second. Operation in a Gifford-McMahon (GM) cryocooler at 2.5 K is possible with system detection efficiencies (SDE) exceeding 20% for SNSPDs which have not been optimized for high detection efficiency.
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Submitted 18 February, 2014;
originally announced February 2014.
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Thickness-dependent ferromagnetic metal to paramagnetic insulator transition in La$_{0.6}$Sr$_{0.4}$MnO$_3$ thin films studied by x-ray magnetic circular dichroism
Authors:
Goro Shibata,
Kohei Yoshimatsu,
Enju Sakai,
Vijay Raj Singh,
Virendra Kumar Verma,
Keisuke Ishigami,
Takayuki Harano,
Toshiharu Kadono,
Yukiharu Takeda,
Tetsuo Okane,
Yuji Saitoh,
Hiroshi Yamagami,
Akihito Sawa,
Hiroshi Kumigashira,
Masaharu Oshima,
Tsuneharu Koide,
Atsushi Fujimori
Abstract:
Metallic transition-metal oxides undergo a metal-to-insulator transition (MIT) as the film thickness decreases across a ritical thickness of several monolayers (MLs), but its driving mechanism remains controversial. We have studied the thickness-dependent MIT of the ferromagnetic metal La$_{0.6}$Sr$_{0.4}$MnO$_3$ by x-ray absorption spectroscopy and x-ray magnetic circular dichroism. As the film t…
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Metallic transition-metal oxides undergo a metal-to-insulator transition (MIT) as the film thickness decreases across a ritical thickness of several monolayers (MLs), but its driving mechanism remains controversial. We have studied the thickness-dependent MIT of the ferromagnetic metal La$_{0.6}$Sr$_{0.4}$MnO$_3$ by x-ray absorption spectroscopy and x-ray magnetic circular dichroism. As the film thickness was decreased across the critical thickness of the MIT (6-8 ML), a gradual decrease of the ferromagnetic signals and a concomitant increase of paramagnetic signals were observed, while the Mn valence abruptly decreased towards Mn$^{3+}$. These observations suggest that the ferromagnetic phase gradually and most likely inhomogeneously turns into the paramagnetic phase and both phases abruptly become insulating at the critical thickness.
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Submitted 24 June, 2014; v1 submitted 3 November, 2013;
originally announced November 2013.
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Orbital magnetic moment and coercivity of SiO$_{2}$-coated FePt nanoparticles studied by x-ray magnetic circular dichroism
Authors:
Y. Takahashi,
T. Kadono,
V. R. Singh,
V. K. Verma,
K. Ishigami,
G. Shibata,
T. Harano,
A. Fujimori,
Y. Takeda,
T. Okane,
Y. Saitoh,
H. Yamagami,
S. Yamamoto,
M. Takano
Abstract:
We have investigated the spin and orbital magnetic moments of Fe in FePt nanoparticles in the $L$1$_{0}$-ordered phase coated with SiO$_{2}$ by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Fe $L_{\rm 2,3}$ absorption edges. Using XMCD sum rules, we evaluated the ratio of the orbital magnetic moment ($M_{\rm orb}$) to the spin magnetic moment…
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We have investigated the spin and orbital magnetic moments of Fe in FePt nanoparticles in the $L$1$_{0}$-ordered phase coated with SiO$_{2}$ by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Fe $L_{\rm 2,3}$ absorption edges. Using XMCD sum rules, we evaluated the ratio of the orbital magnetic moment ($M_{\rm orb}$) to the spin magnetic moment ($M_{\rm spin}$) of Fe to be $M_{\rm orb}/M_{\rm spin}$ = 0.08. This $M_{\rm orb}/M_{\rm spin}$ value is comparable to the value (0.09) obtained for FePt nanoparticles prepared by gas phase condensation, and is larger than the values ($\sim$0.05) obtained for FePt thin films, indicating a high degree of $L$1$_{0}$ order. The hysteretic behavior of the FePt component of the magnetization was measured by XMCD. The magnetic coercivity ($H_{\rm c}$) was found to be as large as 1.8 T at room temperature, $\sim$3 times larger than the thin film value and $\sim$50 times larger than that of the gas phase condensed nanoparticles. The hysteresis curve is well explained by the Stoner-Wohlfarth model for non-interacting single-domain nanoparticles with the $H_{\rm c}$ distributed from 1 T to 5 T.
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Submitted 30 October, 2013;
originally announced October 2013.
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Phase diagram of Ca$_{1-x}$Ce$_x$MnO$_3$ thin films studied by X-ray magnetic circular dichroism
Authors:
T. Harano,
G. Shibata,
K. Yoshimatsu,
K. Ishigami,
V. K. Verma,
Y. Takahashi,
T. Kadono,
T. Yoshida,
A. Fujimori,
T. Koide,
F. -H. Chang,
H. -J. Lin,
D. -J. Huang,
C. -T. Chen,
P. -H. Xiang,
H. Yamada,
A. Sawa
Abstract:
In the perovskite-type Ca$_{1-x}$Ce$_{x}$MnO$_{3}$ (CCMO), one can control the transport and magnetic properties through varying Ce content. In the case of thin films, the properties can also be controlled by epitaxial strain from the substrate through changing it such as YAlO$_{3}$ (YAO), NdAlO$_{3}$ (NAO), and LaSrAlO$_{4}$ (LSAO). However, one cannot measure the magnetization of thin films on N…
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In the perovskite-type Ca$_{1-x}$Ce$_{x}$MnO$_{3}$ (CCMO), one can control the transport and magnetic properties through varying Ce content. In the case of thin films, the properties can also be controlled by epitaxial strain from the substrate through changing it such as YAlO$_{3}$ (YAO), NdAlO$_{3}$ (NAO), and LaSrAlO$_{4}$ (LSAO). However, one cannot measure the magnetization of thin films on NAO substrates by conventional magnetization measurements because of the strong paramagnetic signals from the Nd$^{3+}$ ions. In order to eliminate the influence of Nd$^{3+}$ and to identify magnetic phases of the CCMO thin films, we have performed element-selective X-ray magnetic circular dichroism (XMCD) measurements of the Mn 2{\it p} core level. By studying the anisotropy of the XMCD intensity, we could unambiguously determine the magnetic phase diagram of the CCMO thin films.
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Submitted 23 October, 2013;
originally announced October 2013.
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Role of doped Ru in coercivity-enhanced La$_{0.6}$Sr$_{0.4}MnO$_3$ thin film studied by x-ray magnetic circular dichroism
Authors:
T. Harano,
G. Shibata,
K. Ishigami,
Y. Takashashi,
V. K. Verma,
V. R. Singh,
T. Kadono,
A. Fujimori,
Y. Takeda,
T. Okane,
Y. Saitoh,
H. Yamagami,
T. Koide,
H. Yamada,
A. Sawa,
M. Kawasaki,
Y. Tokura,
A. Tanaka
Abstract:
The coercivity of La$_{1-x}$Sr$_x$MnO$_3$ thin films can be enhanced by Ru substitution for Mn. In order to elucidate its mechanism, we performed soft x-ray absorption and magnetic circular dichroism measurements at the Ru M$_{2,3}$ and Mn L$_{2,3}$ edges. We found that the spin direction of Ru and Mn are opposite and that Ru has a finite orbital magnetic moment. Cluster-model analysis indicated t…
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The coercivity of La$_{1-x}$Sr$_x$MnO$_3$ thin films can be enhanced by Ru substitution for Mn. In order to elucidate its mechanism, we performed soft x-ray absorption and magnetic circular dichroism measurements at the Ru M$_{2,3}$ and Mn L$_{2,3}$ edges. We found that the spin direction of Ru and Mn are opposite and that Ru has a finite orbital magnetic moment. Cluster-model analysis indicated that the finite orbital magnetic moment as well as the reduced spin moment of Ru result from local lattice distortion caused by epitaxial strain from the SrTiO$_3$ substrate in the presence of spin-orbit interaction.
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Submitted 10 September, 2013;
originally announced September 2013.
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Enhanced ferromagnetic moment in Co-doped BiFeO3 thin films studied by soft X-ray circular dichroism
Authors:
V. R. Singh,
V. K. Verma,
K. Ishigami,
G. Shibata,
Y. Yamazaki,
A. Fujimori,
Y. Takeda,
T. Okane,
Y. Saitoh,
H. Yamagami,
Y. Nakamura,
M. Azuma,
Y. Shimakawa
Abstract:
BiFeO$_3$ (BFO) shows both ferroelectricity and magnetic ordering at room temperature but its ferromagnetic component, which is due to spin canting, is negligible. Substitution of transition-metal atoms such as Co for Fe is known to enhance the ferromagnetic component in BFO. In order to reveal the origin of such magnetization enhancement, we performed soft x-ray absorption spectroscopy (XAS) and…
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BiFeO$_3$ (BFO) shows both ferroelectricity and magnetic ordering at room temperature but its ferromagnetic component, which is due to spin canting, is negligible. Substitution of transition-metal atoms such as Co for Fe is known to enhance the ferromagnetic component in BFO. In order to reveal the origin of such magnetization enhancement, we performed soft x-ray absorption spectroscopy (XAS) and soft x-ray magnetic circular dichroism (XMCD) studies of BiFe$_{1-x}$Co$_x$O$_3$ ({\it x} = 0 to 0.30) (BFCO) thin films grown on LaAlO$_3$(001) substrates. The XAS results indicated that the Fe and Co ions are in the Fe$^{3+}$ and Co$^{3+}$ states. The XMCD results showed that the Fe ions show ferromagnetism while the Co ions are antiferromagnetic at room temperature. The XAS and XMCD measurements also revealed that part of the Fe$^{3+}$ ions are tetrahedrally co-ordinated by oxygen ions but that the XMCD signals of the octahedrally coordinated Fe$^{3+}$ ions increase with Co content. The results suggest that an impurity phase such as the ferrimagnetic $γ$-Fe$_2$O$_3$ which exists at low Co concentration decreases with increasing Co concentration and that the ferromagnetic component of the Fe$^{3+}$ ion in the octrahedral crystal fields increases with Co concentration, probably reflecting the increased canting of the Fe$^{3+}$ ions.
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Submitted 25 August, 2013;
originally announced August 2013.
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Observation of magnetically hard grain boundaries in double-perovskite Sr$_{2}$FeMoO$_{6}$
Authors:
Y. Takahashi,
V. K. Verma,
G. Shibata,
T. Harano,
K. Ishigami,
K. Yoshimatsu,
T. Kadono,
A. Fujimori,
A. Tanaka,
F. -H. Chang,
H. -J. Lin,
D. J. Huang,
C. T. Chen,
B. Pal,
D. D. Sarma
Abstract:
Unusual low temperature magneto-resistance (MR) of ferromagnetic Sr$_{2}$FeMoO$_{6}$ polycrystals has been attributed to magnetically hard grain boundaries which act as spin valves. We detected the different magnetic hysteresis curves for the grains and the grain boundaries of polycrystalline Sr$_{2}$FeMoO$_{6}$ by utilizing the different probing depths of the different detection modes of x-ray ab…
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Unusual low temperature magneto-resistance (MR) of ferromagnetic Sr$_{2}$FeMoO$_{6}$ polycrystals has been attributed to magnetically hard grain boundaries which act as spin valves. We detected the different magnetic hysteresis curves for the grains and the grain boundaries of polycrystalline Sr$_{2}$FeMoO$_{6}$ by utilizing the different probing depths of the different detection modes of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), namely, the total electron yield (TEY) mode (probing depth $\sim$5 nm) and the total fluorescence yield (TFY) mode (probing depth $\sim$100 nm). At 20 K, the magnetic coercivity detected in the TEY mode ($H_{\rm c,TEY}$) was several times larger than that in the TFY mode ($H_{\rm c,TFY}$), indicating harder ferromagnetism of the grain boundaries than that of the grains. At room temperature, the grain boundary magnetism became soft and $H_{\rm c,TEY}$ and $H_{\rm c,TFY}$ were nearly the same. From line-shape analysis of the XAS and XMCD spectra, we found that in the grain boundary region the ferromagnetic component is dominated by Fe$^{2+}$ or well-screened signals while the non-magnetic component is dominated by Fe$^{3+}$ or poorly-screened signals.
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Submitted 20 May, 2013;
originally announced May 2013.
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Detecting Single Infrared Photons with 93% System Efficiency
Authors:
F. Marsili,
V. B. Verma,
J. A. Stern,
S. Harrington,
A. E. Lita,
T. Gerrits,
I. Vayshenker,
B. Baek,
M. D. Shaw,
R. P. Mirin,
S. W. Nam
Abstract:
Single-photon detectors (SPDs) at near infrared wavelengths with high system detection efficiency (> 90%), low dark count rate (< 1 counts per second, cps), low timing jitter (< 100 ps), and short reset time (< 100 ns) would enable landmark experiments in a variety of fields. Although some of the existing approaches to single-photon detection fulfill one or two of the above specifications, to date…
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Single-photon detectors (SPDs) at near infrared wavelengths with high system detection efficiency (> 90%), low dark count rate (< 1 counts per second, cps), low timing jitter (< 100 ps), and short reset time (< 100 ns) would enable landmark experiments in a variety of fields. Although some of the existing approaches to single-photon detection fulfill one or two of the above specifications, to date no detector has met all of the specifications simultaneously. Here we report on a fiber-coupled single-photon-detection system employing superconducting nanowire single photon detectors (SNSPDs) that closely approaches the ideal performance of SPDs. Our detector system has a system detection efficiency (SDE), including optical coupling losses, greater than 90% in the wavelength range λ= 1520-1610 nm; device dark count rate (measured with the device shielded from room-temperature blackbody radiation) of ~ 0.01 cps; timing jitter of ~ 150 ps FWHM; and reset time of 40 ns.
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Submitted 25 September, 2012;
originally announced September 2012.
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X-ray absorption spectroscopy and X-ray magnetic circular dichroism studies of transition-metal-co-doped ZnO nano-particles
Authors:
T. Kataoka,
Y. Yamazaki,
V. R. Singh,
Y. Sakamoto,
K. Ishigami,
V. K. Verma,
A. Fujimori,
F. -H. Chang,
H. -J. Lin,
D. J. Huang,
C. T. Chen,
D. Asakura,
T. Koide,
A. Tanaka,
D. Karmakar,
S. K. Mandal,
T. K. Nath,
I. Dagupta
Abstract:
We report on x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) studies of the paramagnetic (Mn,Co)-co-doped ZnO and ferromagnetic (Fe,Co)-co-doped ZnO nano-particles. Both the surface-sensitive total-electron-yield mode and the bulk-sensitive total-fluorescence-yield mode have been employed to extract the valence and spin states of the surface and inner core regions…
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We report on x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) studies of the paramagnetic (Mn,Co)-co-doped ZnO and ferromagnetic (Fe,Co)-co-doped ZnO nano-particles. Both the surface-sensitive total-electron-yield mode and the bulk-sensitive total-fluorescence-yield mode have been employed to extract the valence and spin states of the surface and inner core regions of the nano-particles. XAS spectra reveal that significant part of the doped Mn and Co atoms are found in the trivalent and tetravalent state in particular in the surface region while majority of Fe atoms are found in the trivalent state both in the inner core region and surface region. The XMCD spectra show that the Fe$^{3+}$ ions in the surface region give rise to the ferromagnetism while both the Co and Mn ions in the surface region show only paramagnetic behaviors. The transition-metal atoms in the inner core region do not show magnetic signals, meaning that they are antiferromagnetically coupled. The present result combined with the previous results on transition-metal-doped ZnO nano-particles and nano-wires suggest that doped holes, probably due to Zn vacancy formation at the surfaces of the nano-particles and nano-wires, rather than doped electrons are involved in the occurrence of ferromagnetism in these systems.
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Submitted 18 August, 2012; v1 submitted 15 August, 2012;
originally announced August 2012.
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Ferromagnetism of cobalt-doped anatase TiO$_2$ studied by bulk- and surface-sensitive soft x-ray magnetic circular dichroism
Authors:
V. R. Singh,
K. Ishigami,
V. K. Verma,
G. Shibata,
Y. Yamazaki,
T. Kataoka,
A. Fujimori,
F. -H. Chang,
D. -J. Huang,
H. -J. Lin,
C. T. Chen,
Y. Yamada,
T. Fukumura,
M. Kawasaki
Abstract:
We have studied magnetism in anatase Ti$_{1-x}$Co$_x$O$_{2-δ}$ ({\it x} = 0.05) thin films with various electron carrier densities, by soft x-ray magnetic circular dichroism (XMCD) measurements at the Co $L_{2,3}$ absorption edges. For electrically conducting samples, the magnetic moment estimated by XMCD was $<$ 0.3 $μ_B$/Co using the surface-sensitive total electron yield (TEY) mode, while it wa…
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We have studied magnetism in anatase Ti$_{1-x}$Co$_x$O$_{2-δ}$ ({\it x} = 0.05) thin films with various electron carrier densities, by soft x-ray magnetic circular dichroism (XMCD) measurements at the Co $L_{2,3}$ absorption edges. For electrically conducting samples, the magnetic moment estimated by XMCD was $<$ 0.3 $μ_B$/Co using the surface-sensitive total electron yield (TEY) mode, while it was 0.3-2.4 $μ_B$/Co using the bulk-sensitive total fluorescence yield (TFY) mode. The latter value is in the same range as the saturation magnetization 0.6-2.1 $μ_B$/Co deduced by SQUID measurement. The magnetization and the XMCD intensity increased with carrier density, consistent with the carrier-induced origin of the ferromagnetism.
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Submitted 1 June, 2012;
originally announced June 2012.
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Phase-field simulations of viscous fingering in shear-thinning fluids
Authors:
Sebastien Nguyen,
Roger Folch,
Vijay K. Verma,
Hervé Henry,
Mathis Plapp
Abstract:
A phase-field model for the Hele-Shaw flow of non-Newtonian fluids is developed. It extends a previous model for Newtonian fluids to a wide range of shear-dependent fluids. The model is applied to perform simulations of viscous fingering in shear- thinning fluids, and it is found to be capable of describing the complete crossover from the Newtonian regime at low shear rate to the strongly shear-…
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A phase-field model for the Hele-Shaw flow of non-Newtonian fluids is developed. It extends a previous model for Newtonian fluids to a wide range of shear-dependent fluids. The model is applied to perform simulations of viscous fingering in shear- thinning fluids, and it is found to be capable of describing the complete crossover from the Newtonian regime at low shear rate to the strongly shear-thinning regime at high shear rate. The width selection of a single steady-state finger is studied in detail for a 2-plateaux shear-thinning law (Carreau law) in both its weakly and strongly shear-thinning limits, and the results are related to previous analyses. In the strongly shear-thinning regime a rescaling is found for power-law (Ostwald-de-Waehle) fluids that allows for a direct comparison between simulations and experiments without any adjustable parameters, and good agreement is obtained.
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Submitted 1 December, 2009;
originally announced December 2009.