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Fast coherent control of a charge qubit on solid neon with a spin-qubit-compatible resonator
Authors:
Jun Wang,
Yiran Tian,
Ivan Grytsenko,
Asher Jennings,
Beatriz Pérez González,
Xianjing Zhou,
Hirotaka Terai,
Dafei Jin,
Monica Benito,
Erika Kawakami
Abstract:
Electrons floating in vacuum provide a clean platform for quantum information processing owing to their isolation from material defects. In particular, electrons on solid neon have emerged as a promising qubit platform because of their potentially long coherence times. Here, toward spin-qubit realization, we couple a single electron on solid neon to a magnetic-field-compatible superconducting NbTi…
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Electrons floating in vacuum provide a clean platform for quantum information processing owing to their isolation from material defects. In particular, electrons on solid neon have emerged as a promising qubit platform because of their potentially long coherence times. Here, toward spin-qubit realization, we couple a single electron on solid neon to a magnetic-field-compatible superconducting NbTiN nanowire resonator. We realize a charge qubit and demonstrate microwave readout and coherent control, with Rabi frequencies up to 76 MHz, an order of magnitude larger than in previous studies. Under strong driving, we observe a qubit frequency shift from nonlinear interactions with the intense microwave field. Deterministic electron trapping at an intended position remains challenging due to solid neon surface roughness; we characterize the electron's position from its differential coupling to distinct electrodes. Although not trapped at an intended position, our estimates indicate that spin-qubit demonstrations remain feasible.
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Submitted 1 July, 2026; v1 submitted 29 May, 2026;
originally announced May 2026.
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High-$Q$ membrane resonators using ultra-high-stress crystalline TiN films
Authors:
Yuki Matsuyama,
Shotaro Shirai,
Ippei Nakamura,
Masao Tokunari,
Hirotaka Terai,
Yuji Hishida,
Ryo Sasaki,
Yusuke Tominaga,
Atsushi Noguchi
Abstract:
High-quality-factor ($Q$) mechanical resonators are essential components for precise sensing and control of mechanical motion at a quantum level. While amorphous materials such as SiN have been widely used in high-$Q$ mechanical resonators utilizing stress-induced dissipation dilution, crystalline materials have emerging potential to achieve higher quality factors by combining low intrinsic loss a…
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High-quality-factor ($Q$) mechanical resonators are essential components for precise sensing and control of mechanical motion at a quantum level. While amorphous materials such as SiN have been widely used in high-$Q$ mechanical resonators utilizing stress-induced dissipation dilution, crystalline materials have emerging potential to achieve higher quality factors by combining low intrinsic loss and high tensile stress. In this paper, we demonstrate high-Q membrane resonators using ultra-high-stress crystalline TiN. Our membrane resonator exhibits a tensile stress exceeding 2.3 GPa and a quality factor of $Q = 8.0 \times 10^6$ at 2.2 K. By estimating the dilution factor, we infer that our TiN resonator has a intrinsic quality factor comparable to that of SiN membrane resonators. With its ultra-high stress and crystalline properties, our TiN films can serve as a powerful tool for opto- and electromechanical systems, offering highly dissipation-diluted mechanical resonators.
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Submitted 2 September, 2025;
originally announced September 2025.
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NbTiN Nanowire Resonators for Spin-Photon Coupling on Solid Neon
Authors:
Y. Tian,
I. Grytsenko,
A. Jennings,
J. Wang,
H. Ikegami,
X. Zhou,
S. Tamate,
H. Terai,
H. Kutsuma,
D. Jin,
M. Benito,
E. Kawakami
Abstract:
Electrons floating on a solid neon exhibit long charge coherence times, making them attractive for hybrid quantum systems. When combined with high-quality, high-impedance superconducting resonators and a local magnetic field gradient, this platform enables strong charge--photon and spin--charge coupling-key ingredients for scalable spin qubit architectures. In this work, we demonstrate that NbTiN…
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Electrons floating on a solid neon exhibit long charge coherence times, making them attractive for hybrid quantum systems. When combined with high-quality, high-impedance superconducting resonators and a local magnetic field gradient, this platform enables strong charge--photon and spin--charge coupling-key ingredients for scalable spin qubit architectures. In this work, we demonstrate that NbTiN nanowire resonators maintain high quality factors around 10^5 after depositing solid neon onto the resonators and subsequently loading electrons onto the neon surface, validating their suitability for electrons-on-neon platforms. Building on these experimental results, we theoretically analyze micromagnet designs and coupling strategies that can enable spin-photon interactions in this platform. Our analysis outlines performance targets for next-generation devices, showing that, at the charge sweet spot, spin qubit gate fidelities exceeding 99.99% for single-qubit operations and 99.9% for two-qubit operations are achievable with natural neon.
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Submitted 17 October, 2025; v1 submitted 30 May, 2025;
originally announced May 2025.
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Superconducting flux qubit with ferromagnetic Josephson $π$-junction operating at zero magnetic field
Authors:
Sunmi Kim,
Leonid V. Abdurakhimov,
Duong Pham,
Wei Qiu,
Hirotaka Terai,
Sahel Ashhab,
Shiro Saito,
Taro Yamashita,
Kouichi Semba
Abstract:
Conventional superconducting flux qubits require the application of a precisely tuned magnetic field to set the operation point at half a flux quantum through the qubit loop, which complicates the on-chip integration of this type of device. It has been proposed that by inducing a $π$-phase shift in the superconducting order parameter using a precisely controlled nanoscale-thickness superconductor/…
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Conventional superconducting flux qubits require the application of a precisely tuned magnetic field to set the operation point at half a flux quantum through the qubit loop, which complicates the on-chip integration of this type of device. It has been proposed that by inducing a $π$-phase shift in the superconducting order parameter using a precisely controlled nanoscale-thickness superconductor/ferromagnet/superconductor Josephson junction, commonly referred to as $π$-junction, it is possible to realize a flux qubit operating at zero magnetic flux. Here, we report the realization of a zero-flux-biased flux qubit based on three NbN/AlN/NbN Josephson junctions and a NbN/PdNi/NbN ferromagnetic $π$-junction. The qubit lifetime is in the microsecond range, which we argue is limited by quasiparticle excitations in the metallic ferromagnet layer. Our results pave the way for developing quantum coherent devices, including qubits and sensors, that utilize the interplay between ferromagnetism and superconductivity.
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Submitted 16 October, 2024; v1 submitted 25 January, 2024;
originally announced January 2024.
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Efficient low-energy single-electron detection using a large-area superconducting microstrip
Authors:
Masato Shigefuji,
Alto Osada,
Masahiro Yabuno,
Shigehito Miki,
Hirotaka Terai,
Atsushi Noguchi
Abstract:
Superconducting strip single-photon detectors (SSPDs) are excellent tools not only for single-photon detection but also for single-particle detection owing to their high detection efficiency, low dark counts, and low time jitter. Although the detection of various particles, including electrons with keV-scale energy, has been reported so far, there have been no studies for detecting low-energy elec…
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Superconducting strip single-photon detectors (SSPDs) are excellent tools not only for single-photon detection but also for single-particle detection owing to their high detection efficiency, low dark counts, and low time jitter. Although the detection of various particles, including electrons with keV-scale energy, has been reported so far, there have been no studies for detecting low-energy electrons. It has yet to be clarified how low-energy electrons interact with electrons and/or phonons in a superconductor during electron detection. Here we report the detection property of a superconducting micro-strip single-electron detector (SSED) for electrons with energy below 200 eV. The detection efficiency is estimated as at least 37 % when electrons impinging on the stripline possess an energy of 200 eV. We also show that the minimum detectable energy of electrons is about 10 eV with our SSED, much lower than those of ions, which implies that the electron-electron interaction plays a significant role. SSEDs might open a wide range of applications, from condensed matter physics to quantum information science, because of their compatibility with the cryogenic environment.
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Submitted 26 January, 2023;
originally announced January 2023.
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Enhanced coherence of all-nitride superconducting qubits epitaxially grown on silicon substrate
Authors:
Sunmi Kim,
Hirotaka Terai,
Taro Yamashita,
Wei Qiu,
Tomoko Fuse,
Fumiki Yoshihara,
Sahel Ashhab,
Kunihiro Inomata,
Kouichi Semba
Abstract:
Improving the coherence of superconducting qubits is a fundamental step towards the realization of fault-tolerant quantum computation. However, coherence times of quantum circuits made from conventional aluminium-based Josephson junctions are limited by the presence of microscopic two-level systems in the amorphous aluminum oxide tunnel barriers. Here, we have developed superconducting qubits base…
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Improving the coherence of superconducting qubits is a fundamental step towards the realization of fault-tolerant quantum computation. However, coherence times of quantum circuits made from conventional aluminium-based Josephson junctions are limited by the presence of microscopic two-level systems in the amorphous aluminum oxide tunnel barriers. Here, we have developed superconducting qubits based on NbN/AlN/NbN epitaxial Josephson junctions on silicon substrates which promise to overcome the drawbacks of qubits based on Al/AlO$_{x}$/Al junctions. The all-nitride qubits have great advantages such as chemical stability against oxidation, resulting in fewer two-level fluctuators, feasibility for epitaxial tunnel barriers that reduce energy relaxation and dephasing, and a larger superconducting gap of $\sim$5.2 meV for NbN, compared to $\sim$0.3 meV for aluminium, which suppresses the excitation of quasiparticles. By replacing conventional MgO by a silicon substrate with a TiN buffer layer for epitaxial growth of nitride junctions, we demonstrate a qubit energy relaxation time $T$$_{1}$=16.3 $μ$s and a spin-echo dephasing time $T$$_{2}$=21.5 $μ$s. These significant improvements in quantum coherence are explained by the reduced dielectric loss compared to previously reported NbN-based qubits with MgO substrates ($T$$_{1}$$\approx$$T$$_{2}$$\approx$0.5 $μ$s). These results are an important step towards constructing a new platform for superconducting quantum hardware.
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Submitted 11 October, 2021; v1 submitted 13 March, 2021;
originally announced March 2021.
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Scalable readout interface for superconducting nanowire single-photon detectors using AQFP and RSFQ logic families
Authors:
Naoki Takeuchi,
Fumihiro China,
Shigehito Miki,
Shigeyuki Miyajima,
Masahiro Yabuno,
Nobuyuki Yoshikawa,
Hirotaka Terai
Abstract:
We propose a scalable readout interface for superconducting nanowire single-photon detector (SSPD) arrays, which we call the AQFP/RSFQ interface. This interface is composed of adiabatic quantum-flux-parametron (AQFP) and rapid single-flux-quantum (RSFQ) logic families. The AQFP part reads out the spatial information of an SSPD array via a single cable, and the RSFQ part reads out the temporal info…
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We propose a scalable readout interface for superconducting nanowire single-photon detector (SSPD) arrays, which we call the AQFP/RSFQ interface. This interface is composed of adiabatic quantum-flux-parametron (AQFP) and rapid single-flux-quantum (RSFQ) logic families. The AQFP part reads out the spatial information of an SSPD array via a single cable, and the RSFQ part reads out the temporal information via a single cable. The hybrid interface has high temporal resolution owing to low timing jitter in the operation of the RSFQ part. In addition, the hybrid interface achieves high circuit scalability because of low supply current in the operation of the AQFP part. Therefore, the hybrid interface is suitable for handling many-pixel SSPD arrays. We demonstrate a four-pixel SSPD array using the hybrid interface as proof of concept. The measurement results show that the hybrid interface can read out all of the pixels with a low error rate and low timing jitter.
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Submitted 8 May, 2020; v1 submitted 3 April, 2020;
originally announced April 2020.
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Fabrication of Deep-Sub-Micrometer NbN/AlN/NbN Epitaxial Junctions on a Si Substrate
Authors:
Wei Qiu,
Hirotaka Terai
Abstract:
We have developed a novel fabrication process for ultra-small, full-epitaxial NbN Josephson junctions on a silicon (Si) substrate. A full-epitaxial NbN/AlN/NbN tri-layer was grown on a Si (100) wafer with a (200)-oriented TiN buffer layer. It was patterned into Josephson junctions by an electron beam lithography (EBL) for junction definition followed by a reactive ion etch (RIE). A chemical mechan…
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We have developed a novel fabrication process for ultra-small, full-epitaxial NbN Josephson junctions on a silicon (Si) substrate. A full-epitaxial NbN/AlN/NbN tri-layer was grown on a Si (100) wafer with a (200)-oriented TiN buffer layer. It was patterned into Josephson junctions by an electron beam lithography (EBL) for junction definition followed by a reactive ion etch (RIE). A chemical mechanical polishing (CMP) process and an additional RIE by using CHF$_3$ gas formed reliable electrical contacts between the junction counter electrodes and the wiring layer. All fabricated junctions, with a junction size down to 0.27 $μ$m in diameter, showed excellent current-voltage characteristics with a clear gap structure and a small sub-gap leakage current. The dielectric layer of SiO$_2$ that served as an insulator between base and counter electrodes was removed in a wet etching process using a buffered HF solution. We have confirmed that the quality of the junctions was maintained after the removal of the SiO$_2$ dielectric layer.
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Submitted 2 April, 2020;
originally announced April 2020.
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Nonreciprocal Terahertz Second Harmonic Generation in Superconducting NbN under Supercurrent Injection
Authors:
Sachiko Nakamura,
Kota Katsumi,
Hirotaka Terai,
Ryo Shimano
Abstract:
Giant second-harmonic generation (SHG) in the terahertz (THz) frequency range is observed in a thin film of an s-wave superconductor NbN, where the time-reversal ($\mathcal{T}$-) and space-inversion ($\mathcal{P}$-) symmetries are simultaneously broken by supercurrent injection. We demonstrate that the phase of the second-harmonic (SH) signal flips when the direction of supercurrent is inverted, i…
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Giant second-harmonic generation (SHG) in the terahertz (THz) frequency range is observed in a thin film of an s-wave superconductor NbN, where the time-reversal ($\mathcal{T}$-) and space-inversion ($\mathcal{P}$-) symmetries are simultaneously broken by supercurrent injection. We demonstrate that the phase of the second-harmonic (SH) signal flips when the direction of supercurrent is inverted, i.e., the signal is ascribed to the nonreciprocal response that occurs under broken $\mathcal{P}$- and $\mathcal{T}$-symmetries. The temperature dependence of the SH signal exhibits a sharp resonance, which is accounted for by the vortex motion driven by the THz electric field in an anharmonic pinning potential. The maximum conversion ratio $η_{\mathrm{SHG}}$ reaches $\approx10^{-2}$ in a thin film NbN with the thickness of 25 nm after the field cooling with a very small magnetic field of $\approx1$ Oe, for a relatively weak incident THz electric field of 2.8 kV/cm at 0.48 THz.
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Submitted 24 August, 2020; v1 submitted 1 March, 2020;
originally announced March 2020.
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Infrared activation of the Higgs mode by supercurrent injection in superconducting NbN
Authors:
Sachiko Nakamura,
Yudai Iida,
Yuta Murotani,
Ryusuke Matsunaga,
Hirotaka Terai,
Ryo Shimano
Abstract:
Higgs mode in superconductors, i.e. the collective amplitude mode of the order parameter does not associate with charge nor spin fluctuations, therefore it does not couple to the electromagnetic field in the linear response regime. On the contrary to this common understanding, here, we demonstrate that, if the dc supercurrent is introduced into the superconductor, the Higgs mode becomes infrared a…
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Higgs mode in superconductors, i.e. the collective amplitude mode of the order parameter does not associate with charge nor spin fluctuations, therefore it does not couple to the electromagnetic field in the linear response regime. On the contrary to this common understanding, here, we demonstrate that, if the dc supercurrent is introduced into the superconductor, the Higgs mode becomes infrared active and is directly observed in the linear optical conductivity measurement. We observed a sharp resonant peak at $ω=2Δ$ in the optical conductivity spectrum of a thin-film NbN in the presence of dc supercurrent, showing a reasonable agreement with the recent theoretical prediction. The method as proven by this work opens a new pathway to study the Higgs mode in a wide variety of superconductors.
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Submitted 16 June, 2019; v1 submitted 26 September, 2018;
originally announced September 2018.
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Polarization-resolved terahertz third-harmonic generation in a superconductor NbN: dominance of Higgs mode beyond the BCS approximation
Authors:
Ryusuke Matsunaga,
Naoto Tsuji,
Kazumasa Makise,
Hirotaka Terai,
Hideo Aoki,
Ryo Shimano
Abstract:
Recent advances in time-domain terahertz (THz) spectroscopy have unveiled that resonantly-enhanced strong THz third-harmonic generation (THG) mediated by the collective Higgs amplitude mode occurs in s-wave superconductors, where charge-density fluctuations (CDF) have also been shown to contribute to the nonlinear third-order susceptibility. It has been theoretically proposed that the nonlinear re…
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Recent advances in time-domain terahertz (THz) spectroscopy have unveiled that resonantly-enhanced strong THz third-harmonic generation (THG) mediated by the collective Higgs amplitude mode occurs in s-wave superconductors, where charge-density fluctuations (CDF) have also been shown to contribute to the nonlinear third-order susceptibility. It has been theoretically proposed that the nonlinear responses of Higgs and CDF exhibit essentially different polarization dependences. Here we experimentally discriminate the two contributions by polarization-resolved intense THz transmission spectroscopy for a single-crystal NbN film. The result shows that the resonant THG in the transmitted light always appears in the polarization parallel to that of the incident light with no appreciable crystal axis dependence. When we compare this with the theoretical calculation here with the BCS approximation and the dynamical mean-field theory for a model of NbN constructed from first principles, the experimental result strongly indicates that the Higgs mode rather than the CDF dominates the THG resonance in NbN. A possible mechanism for this is discussed such as the retardation effect in the phonon-mediated pairing interaction beyond BCS.
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Submitted 8 March, 2017; v1 submitted 8 March, 2017;
originally announced March 2017.
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Stable, high-performance operation of a fiber-coupled superconducting nanowire avalanche photon detector
Authors:
Shigehito Miki,
Masahiro Yabuno,
Taro Yamashita,
Hirotaka Terai
Abstract:
We present a stable and high-performance fiber-coupled NbTiN superconducting nanowire avalanche photon detector (SNAP). We demonstrate afterpulse-free operation in serially connected two SNAPs (SC-2SNAP), even in the absence of a choke inductor, achieving a 7.7 times faster response speed than standard SSPDs. The SC-2SNAP device showed a system detection efficiency (SDE) of 81.0% with wide bias cu…
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We present a stable and high-performance fiber-coupled NbTiN superconducting nanowire avalanche photon detector (SNAP). We demonstrate afterpulse-free operation in serially connected two SNAPs (SC-2SNAP), even in the absence of a choke inductor, achieving a 7.7 times faster response speed than standard SSPDs. The SC-2SNAP device showed a system detection efficiency (SDE) of 81.0% with wide bias current margin, a dark count rate of 6.8 counts/s, and full width at half maximum timing jitter of 68 ps, operating at 2.3 K.
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Submitted 25 January, 2017;
originally announced January 2017.
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Low-Filling-Factor Superconducting Single Photon Detector with High System Detection Efficiency
Authors:
Taro Yamashita,
Shigehito Miki,
Hirotaka Terai,
Zhen Wang
Abstract:
We designed, fabricated, and measured superconducting nanowire single-photon detectors (SSPDs) with low filling factor which achieve high system detection efficiency (SDE) and counting rate simultaneously. Numerical simulation reveals that high optical absorptance is possible in SSPDs even for low filing factor by tuning the device design. The SDEs of fabricated 18-50% filling factor SSPDs were me…
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We designed, fabricated, and measured superconducting nanowire single-photon detectors (SSPDs) with low filling factor which achieve high system detection efficiency (SDE) and counting rate simultaneously. Numerical simulation reveals that high optical absorptance is possible in SSPDs even for low filing factor by tuning the device design. The SDEs of fabricated 18-50% filling factor SSPDs were measured systematically, and all SSPDs showed high SDEs of 61-80% and the lowest 18% filling factor SSPD achieved a high SDE of 69%.
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Submitted 15 November, 2013; v1 submitted 13 May, 2013;
originally announced May 2013.
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Higgs Amplitude Mode in BCS Superconductors Nb$_{1-x}$Ti$_{x}$N induced by Terahertz Pulse Excitation
Authors:
Ryusuke Matsunaga,
Yuki I. Hamada,
Kazumasa Makise,
Yoshinori Uzawa,
Hirotaka Terai,
Zhen Wang,
Ryo Shimano
Abstract:
Ultrafast responses of BCS superconductor Nb1-xTixN films in a nonadiabatic excitation regime were investigated by using terahertz (THz) pump-THz probe spectroscopy. After an instantaneous excitation with the monocycle THz pump pulse, a transient oscillation emerges in the electromagnetic response in the BCS gap energy region. The oscillation frequency coincides with the asymptotic value of the BC…
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Ultrafast responses of BCS superconductor Nb1-xTixN films in a nonadiabatic excitation regime were investigated by using terahertz (THz) pump-THz probe spectroscopy. After an instantaneous excitation with the monocycle THz pump pulse, a transient oscillation emerges in the electromagnetic response in the BCS gap energy region. The oscillation frequency coincides with the asymptotic value of the BCS gap energy, indicating the appearance of the theoretically-anticipated collective amplitude mode of the order parameter, namely the Higgs amplitude mode. Our result opens a new pathway to the ultrafast manipulation of the superconducting order parameter by optical means.
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Submitted 8 July, 2013; v1 submitted 2 May, 2013;
originally announced May 2013.
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High performance fiber-coupled NbTiN superconducting nanowire single photon detectors with Gifford-McMahon cryocooler
Authors:
Shigehito Miki,
Taro Yamashita,
Hirotaka Terai,
Zhen Wang
Abstract:
We present high performance fiber-coupled niobium titanium nitride superconducting nanowire single photon detectors fabricated on thermally oxidized silicon substrates. The best device showed a system detection efficiency (DE) of 74%, dark count rate of 100 c/s, and full width at half maximum timing jitter of 68 ps under a bias current of 18.0 uA with a practical Gifford-McMahon cryocooler system.…
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We present high performance fiber-coupled niobium titanium nitride superconducting nanowire single photon detectors fabricated on thermally oxidized silicon substrates. The best device showed a system detection efficiency (DE) of 74%, dark count rate of 100 c/s, and full width at half maximum timing jitter of 68 ps under a bias current of 18.0 uA with a practical Gifford-McMahon cryocooler system. We also introduced six detectors into the cryocooler and confirmed that the system DE of all detectors was higher than 67% at the dark count rate of 100 c/s.
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Submitted 11 April, 2013; v1 submitted 26 March, 2013;
originally announced March 2013.
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Crosstalk-free operation of multi-element SSPD array integrated with SFQ circuit in a 0.1 Watt GM cryocooler
Authors:
Taro Yamashita,
Shigehito Miki,
Hirotaka Terai,
Kazumasa Makise,
Zhen Wang
Abstract:
We demonstrate the successful operation of a multi-element superconducting nanowire single-photon detector (SSPD) array integrated with a single-flux-quantum (SFQ) readout circuit in a compact 0.1 W Gifford-McMahon cryocooler. A time-resolved readout technique, where output signals from each element enter the SFQ readout circuit with finite time intervals, revealed crosstalk-free operation of the…
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We demonstrate the successful operation of a multi-element superconducting nanowire single-photon detector (SSPD) array integrated with a single-flux-quantum (SFQ) readout circuit in a compact 0.1 W Gifford-McMahon cryocooler. A time-resolved readout technique, where output signals from each element enter the SFQ readout circuit with finite time intervals, revealed crosstalk-free operation of the four-element SSPD array connected with the SFQ readout circuit. The timing jitter and the system detection efficiency were measured to be 50 ps and 11.4%, respectively, which were comparable to the performance of practical single-pixel SSPD systems.
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Submitted 17 July, 2012;
originally announced July 2012.
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Origin of intrinsic dark count in superconducting nanowire single-photon detectors
Authors:
T. Yamashita,
S. Miki,
K. Makise,
W. Qiu,
H. Terai,
M. Fujiwara,
M. Sasaki,
Z. Wang
Abstract:
The origin of the decoherence in superconducting nanowire single-photon detectors, the so-called dark count, was investigated. We measured the direct-current characteristics and bias-current dependencies of the dark count rate in a wide range of temperatures from 0.5 K to 4 K, and analyzed the results by theoretical models of thermal fluctuations of vortices. Our results indicate that the current-…
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The origin of the decoherence in superconducting nanowire single-photon detectors, the so-called dark count, was investigated. We measured the direct-current characteristics and bias-current dependencies of the dark count rate in a wide range of temperatures from 0.5 K to 4 K, and analyzed the results by theoretical models of thermal fluctuations of vortices. Our results indicate that the current-assisted unbinding of vortex-antivortex pairs is the dominant origin of the dark count.
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Submitted 26 October, 2011; v1 submitted 15 March, 2011;
originally announced March 2011.