Showing 1–50 of 55 results for author: Kontos, T

Electrical control of the metal-insulator transition in a one dimensional device

Authors: J. Craquelin, L. Jarjat, B. Hue, A. Théry, C. Fruy, N. Struchkov, D. Stefani, M. M. Desjardins, A. Cottet, M. R. Delbecq, T. Kontos

Abstract: Controlling the low energy spectrum at the nanoscale with an external physical parameter has become an important resource for quantum devices. The emergence of an energy gap is one such key feature, linked to the mitigation of decoherence needed for quantum information processing. Indeed, the detrimental effects of high-energy uncontrolled excitations can only be cured at some specific tuning poin… ▽ More Controlling the low energy spectrum at the nanoscale with an external physical parameter has become an important resource for quantum devices. The emergence of an energy gap is one such key feature, linked to the mitigation of decoherence needed for quantum information processing. Indeed, the detrimental effects of high-energy uncontrolled excitations can only be cured at some specific tuning points in general. Achieving an energy gap is a natural way to extend decoherence countermeasures over a finite region of parameter space. This would be particularly useful in view of the recent efforts to build superconducting topological chains in a top-down approach. In this work, we demonstrate a large energy gap by spatially modulating the local potential of a suspended carbon nanotube, exploiting an analogy with condensed matter systems. This gap is homogeneous on the nanotube and tunable by about two orders of magnitude, bringing the electronic system from an insulating state to a near-metallic state at low temperatures. △ Less

Submitted 25 July, 2026; originally announced July 2026.

Comments: 22 pages including supplementary materials

Parametric Drive of a Double Quantum Dot in a Cavity

Authors: L. Jarjat, B. Hue, T. Philippe-Kagan, B. Neukelmance, J. Craquelin, A. Théry, C. Fruy, G. Abulizi, J. Becdelievre, M. M. Desjardins, T. Kontos, M. R. Delbecq

Abstract: We demonstrate the parametric modulation of a double quantum dot charge dipole coupled to a cavity, at the cavity frequency, achieving an amplified readout signal compared to conventional dispersive protocols. Our findings show that the observed cavity field displacement originates from dipole radiation within the cavity, rather than from a longitudinal coupling mechanism, yet exhibits the same si… ▽ More We demonstrate the parametric modulation of a double quantum dot charge dipole coupled to a cavity, at the cavity frequency, achieving an amplified readout signal compared to conventional dispersive protocols. Our findings show that the observed cavity field displacement originates from dipole radiation within the cavity, rather than from a longitudinal coupling mechanism, yet exhibits the same signatures while relying on a transverse coupling. By carefully tuning the phase and amplitude of the intra-cavity field, we achieve a $π$-phase shift between two dipole states, resulting in a substantial enhancement of the signal-to-noise ratio. In addition to its applications in quantum dot based qubits in cQED architectures, this protocol could serve as a new promising tool for probing exotic electronic states in mesoscopic circuits embedded in cavities. △ Less

Submitted 10 October, 2025; originally announced October 2025.

Comments: main text 6 pages, 4 figures; supplementary material 11 pages, 11 figures

Journal ref: Phys. Rev. Lett. 135, 153603 (2025)

Interferometric evidence of non-volatile anomalous phase shifts in exchange-spin-split Josephson supercurrent diodes

Authors: Kun-Rok Jeon, Jae-Keun Kim, Jiho Yoon, Jae-Chun Jeon, Hyeon Han, Audrey Cottet, Takis Kontos, Stuart S. P. Parkin

Abstract: The recent realization of zero-field, polarity-reversible supercurrent rectification in proximity-magnetized Rashba-type Pt Josephson junctions (JJs) enables the development of superconducting logic circuits and cryogenic memory applications. Here, we demonstrate a non-volatile anomalous phase shift φ_0 directly probed via superconducting quantum interferometry, providing phase-sensitive evidence… ▽ More The recent realization of zero-field, polarity-reversible supercurrent rectification in proximity-magnetized Rashba-type Pt Josephson junctions (JJs) enables the development of superconducting logic circuits and cryogenic memory applications. Here, we demonstrate a non-volatile anomalous phase shift φ_0 directly probed via superconducting quantum interferometry, providing phase-sensitive evidence of spontaneous time-reversal symmetry breaking in these Rashba-type systems. By replacing the Pt barrier with 5d or 4d element layers exhibiting different (para-)magnetic susceptibilities, spin-orbit coupling strengths, and electronic band structures, we elucidate the role of proximity effects in governing zero-field diode behavior. Ta (W) JJs exhibit zero-field diode efficiencies of ~17% (~5%) at 2 K, which are slightly (significantly) lower than those of Pt JJs. Notably, the diode polarity in Ta and W JJs is reversed relative to Pt JJs. Combined with the large zero-field diode efficiency (~15% at 2 K) observed in highly magnetic-susceptible Pd JJs, these results show that non-volatile φ_0 and, consequently, zero-field diode performance can be tuned through proximity engineering of interfacial magnetic ordering and Rashba spin-orbit interaction. △ Less

Submitted 23 June, 2026; v1 submitted 26 May, 2025; originally announced May 2025.

Comments: 21 pages, 5 figures

Microsecond-lived quantum states in a carbon-based circuit driven by cavity photons

Authors: B. Neukelmance, B. Hue, Q. Schaeverbeke, L. Jarjat, A. Théry, J. Craquelin, W. Legrand, T. Cubaynes, G. Abulizi, J. Becdelievre, M. El Abbassi, A. Larrouy, K. F. Ourak, D. Stefani, J. A. Sulpizio, A. Cottet, M. M. Desjardins, T. Kontos, M. R. Delbecq

Abstract: Semiconductor quantum dots are an attractive platform for the realisation of quantum processors. To achieve long-range coupling between them, quantum dots have been integrated into microwave cavities. However, it has been shown that their coherence is then reduced compared to their cavity-free implementations. Here, we manipulate the quantum states of a suspended carbon nanotube double quantum dot… ▽ More Semiconductor quantum dots are an attractive platform for the realisation of quantum processors. To achieve long-range coupling between them, quantum dots have been integrated into microwave cavities. However, it has been shown that their coherence is then reduced compared to their cavity-free implementations. Here, we manipulate the quantum states of a suspended carbon nanotube double quantum dot with ferromagnetic contacts embedded in a microwave cavity. By performing quantum manipulations via the cavity photons, we demonstrate coherence times of the order of $1.3μs$, two orders of magnitude larger than those measured so far in any carbon quantum circuit and one order of magnitude larger than silicon-based quantum dots in comparable environment. This holds promise for carbon as a host material for spin qubits in circuit quantum electrodynamics. △ Less

Submitted 11 July, 2025; v1 submitted 25 October, 2024; originally announced October 2024.

Comments: main text (7 pages, 4 figures); supplementary material (14 pages, 16 figures)

Journal ref: Nat Commun 16, 5636 (2025)

Observation of quantum oscillations in the extreme weak anharmonic limit

Authors: A. Théry, B. Neukelmance, B. Hue, W. Legrand, L. Jarjat, J. Craquelin, M. Villiers, A. Cottet, M. R. Delbecq, T. Kontos

Abstract: We investigate a granular aluminium quantum circuit with an anharmonicity of the order of its decoherence rate in a 3-dimensional microwave cavity. We perform single qubit-like manipulations such as Rabi oscillations and Ramsey fringes. Our findings, supported by quantitative numerical modeling, show that a very weakly anharmonic oscillator can also display quantum oscillations outside the qubit r… ▽ More We investigate a granular aluminium quantum circuit with an anharmonicity of the order of its decoherence rate in a 3-dimensional microwave cavity. We perform single qubit-like manipulations such as Rabi oscillations and Ramsey fringes. Our findings, supported by quantitative numerical modeling, show that a very weakly anharmonic oscillator can also display quantum oscillations outside the qubit regime. These oscillations are hard to disambiguate from qubit oscillations in time domain measurements for a single driving frequency. This sheds new light on recent findings for new material superconducting quantum bits. Our platform shows in addition large magnetic field resilience which could find applications for quantum enhanced dark matter search. △ Less

Submitted 8 January, 2024; originally announced January 2024.

Zero energy states clustering in an elemental nanowire coupled to a superconductor

Authors: L. C. Contamin, L. Jarjat, W. Legrand, A. Cottet, T. Kontos, M. R. Delbecq

Abstract: Nanoelectronic hybrid devices combining superconductors and a one-dimensional nanowire are promising platforms to realize topological superconductivity and its resulting exotic excitations. The bulk of experimental studies in this context are transport measurements where conductance peaks allow to perform a spectroscopy of the low lying electronic states and potentially to identify signatures of t… ▽ More Nanoelectronic hybrid devices combining superconductors and a one-dimensional nanowire are promising platforms to realize topological superconductivity and its resulting exotic excitations. The bulk of experimental studies in this context are transport measurements where conductance peaks allow to perform a spectroscopy of the low lying electronic states and potentially to identify signatures of the aforementioned excitations. The complexity of the experimental landscape calls for a benchmark in an elemental situation. The present work tackles such a task using an ultra-clean carbon nanotube circuit. Specifically, we show that the combination of magnetic field, weak disorder and superconductivity can lead to states clustering at low energy, as predicted by the random matrix theory predictions. Such a phenomenology is very general and should apply to most platforms trying to realize topological superconductivity in 1D systems, thus calling for alternative probes to reveal it. △ Less

Submitted 16 November, 2022; v1 submitted 7 September, 2022; originally announced September 2022.

Comments: main text (8 pages, 5 figures); supplementary material (7 pages, 6 figures)

Journal ref: Nat Commun 13, 6188 (2022)

Staggered spin-orbit interaction in a nanoscale device

Authors: L. C. Contamin, T. Cubaynes, W. Legrand, M. Marganska, M. M. Desjardins, M. Dartiailh, Z. Leghtas, A. Thiaville, S. Rohart, A. Cottet, M. R. Delbecq, T. Kontos

Abstract: The coupling of the spin and the motion of charge carriers stems directly from the atomic structure of a conductor. It has become an important ingredient for the emergence of topological matter, and, in particular, topological superconductivity which could host non-abelian excitations such as Majorana modes or parafermions. These modes are sought after mostly in semiconducting platforms which are… ▽ More The coupling of the spin and the motion of charge carriers stems directly from the atomic structure of a conductor. It has become an important ingredient for the emergence of topological matter, and, in particular, topological superconductivity which could host non-abelian excitations such as Majorana modes or parafermions. These modes are sought after mostly in semiconducting platforms which are made of heavy atoms and therefore exhibit naturally a large spin-orbit interaction. Creating domain walls in the spin orbit interaction at the nanoscale may turn out to be a crucial resource for engineering topological excitations suitable for universal topological quantum computing. For example, it has been proposed for exploring exotic electronic states or for creating hinge states. Realizing this in natural platforms remains a challenge. In this work, we show how this can be alternatively implemented by using a synthetic spin orbit interaction induced by two lithographically patterned magnetically textured gates. By using a double quantum dot in a light material -- a carbon nanotube -- embedded in a microwave cavity, we trigger hopping between two adjacent orbitals with the microwave photons and directly compare the wave functions separated by the domain wall via the light-matter coupling. We show that we can achieve an engineered staggered spin-orbit interaction with a change of strength larger than the hopping energy between the two sites. △ Less

Submitted 25 April, 2021; originally announced April 2021.

Comments: comments/questions are highly welcome

Journal ref: Phys. Rev. B 107, 085152 (2023)

Hybrid light-matter networks of Majorana zero modes

Authors: L. C. Contamin, M. R. Delbecq, B. Dou çot, A. Cottet, T. Kontos

Abstract: Topological excitations, such as Majorana zero modes, are a promising route for encoding quantum information. Topologically protected gates of Majorana qubits, based on their braiding, will require some form of network. Here, we propose to build such a network by entangling Majorana matter with light in a microwave cavity QED setup. Our scheme exploits a light-induced interaction which is universa… ▽ More Topological excitations, such as Majorana zero modes, are a promising route for encoding quantum information. Topologically protected gates of Majorana qubits, based on their braiding, will require some form of network. Here, we propose to build such a network by entangling Majorana matter with light in a microwave cavity QED setup. Our scheme exploits a light-induced interaction which is universal to all the Majorana nanoscale circuit platforms. This effect stems from a parametric drive of the light-matter coupling in a one-dimensional chain of physical Majorana modes. Our setup enables all the basic operations needed in a Majorana quantum computing platform such as fusing, braiding, the crucial T-gate, the read-out and, importantly, the stabilization or correction of the physical Majorana modes. △ Less

Submitted 30 March, 2021; originally announced March 2021.

Comments: comments are highly welcome

Magnifying quantum phase fluctuations with Cooper-pair pairing

Authors: W. C. Smith, M. Villiers, A. Marquet, J. Palomo, M. R. Delbecq, T. Kontos, P. Campagne-Ibarcq, B. Douçot, Z. Leghtas

Abstract: Remarkably, complex assemblies of superconducting wires, electrodes, and Josephson junctions are compactly described by a handful of collective phase degrees of freedom that behave like quantum particles in a potential. The inductive wires contribute a parabolic confinement, while the tunnel junctions add a cosinusoidal corrugation. Usually, the ground state wavefunction is localized within a sing… ▽ More Remarkably, complex assemblies of superconducting wires, electrodes, and Josephson junctions are compactly described by a handful of collective phase degrees of freedom that behave like quantum particles in a potential. The inductive wires contribute a parabolic confinement, while the tunnel junctions add a cosinusoidal corrugation. Usually, the ground state wavefunction is localized within a single potential well -- that is, quantum phase fluctuations are small -- although entering the regime of delocalization holds promise for metrology and qubit protection. A direct route is to loosen the inductive confinement and let the ground state phase spread over multiple Josephson periods, but this requires a circuit impedance vastly exceeding the resistance quantum and constitutes an ongoing experimental challenge. Here we take a complementary approach and fabricate a generalized Josephson element that can be tuned in situ between one- and two-Cooper-pair tunneling, doubling the frequency of the corrugation and thereby magnifying the number of wells probed by the ground state. We measure a tenfold suppression of flux sensitivity of the first transition energy, implying a twofold increase in the vacuum phase fluctuations. △ Less

Submitted 5 April, 2022; v1 submitted 29 October, 2020; originally announced October 2020.

Comments: 11 pages, 8 figures

Journal ref: Phys. Rev. X 12, 021002 (2022)

Nanoassembly technique of carbon nanotubes for hybrid circuit-QED

Authors: T. Cubaynes, L. C. Contamin, M. C. Dartiailh, M. M. Desjardins, A. Cottet, M. R. Delbecq, T. Kontos

Abstract: A complex quantum dot circuit based on a clean and suspended carbon nanotube embedded in a circuit quantum electrodynamique (cQED) architecture is a very attractive platform to investigate a large spectrum of physics phenomena ranging from qubit physics to nanomechanics. We demonstrate a carbon nanotube transfer process allowing us to integrate clean carbon nanotubes into complex quantum dot circu… ▽ More A complex quantum dot circuit based on a clean and suspended carbon nanotube embedded in a circuit quantum electrodynamique (cQED) architecture is a very attractive platform to investigate a large spectrum of physics phenomena ranging from qubit physics to nanomechanics. We demonstrate a carbon nanotube transfer process allowing us to integrate clean carbon nanotubes into complex quantum dot circuits inside a cQED platform. This technique is compatible with various contacting materials such as superconductors or ferromagnets. This makes it suitable for hybrid quantum devices. Our results are based on 8 different devices demonstrating the robustness of this technique. △ Less

Submitted 22 July, 2020; originally announced July 2020.

Comments: 5 pages, 4 figures, 1 table

Ultrasensitive Photoresponse of Graphene Quantum Dot in the Coulomb Blockade Regime to THz Radiation

Authors: E. Riccardi, S. Massabeau, F. Valmorra, S. Messelot, M. Rosticher, J. Tignon, K. Watanabe, T. Taniguchi, M. Delbecq, S. Dhillon, R. Ferreira, S. Balibar, T. Kontos, J. Mangeney

Abstract: Graphene quantum dots (GQDs) have recently attracted considerable attention, with appealing properties for terahertz (THz) technology. This includes the demonstration of large thermal bolometric effects in GQDs when illuminated by THz radiation. However, the interaction of THz photons with GQDs in the Coulomb blockade regime - single electron transport regime - remains unexplored. Here, we demonst… ▽ More Graphene quantum dots (GQDs) have recently attracted considerable attention, with appealing properties for terahertz (THz) technology. This includes the demonstration of large thermal bolometric effects in GQDs when illuminated by THz radiation. However, the interaction of THz photons with GQDs in the Coulomb blockade regime - single electron transport regime - remains unexplored. Here, we demonstrate the ultrasensitive photoresponse to THz radiation (from <0.1 to 10 THz) of a hBN-encapsulated GQD in the Coulomb blockade regime at low temperature (170 mK). We show that THz radiation of $\sim$10 pW provides a photocurrent response in the nanoampere range, resulting from a renormalization of the chemical potential of the GQD of $\sim$0.15 meV. We attribute this photoresponse to an interfacial photogating effect. Furthermore, our analysis reveals the absence of thermal effects, opening new directions in the study of coherent quantum effects at THz frequencies in GQDs. △ Less

Submitted 4 June, 2020; originally announced June 2020.

Journal ref: Nano lett. 2020

Highly coherent spin states in carbon nanotubes coupled to cavity photons

Authors: T. Cubaynes, M. R. Delbecq, M. C. Dartiailh, R. Assouly, M. M. Desjardins, L. C. Contamin, L. E. Bruhat, Z. Leghtas, F. Mallet, A. Cottet, T. Kontos

Abstract: Spins confined in quantum dots are considered as a promising platform for quantum information processing. While many advanced quantum operations have been demonstrated, experimental as well as theoretical efforts are now focusing on the development of scalable spin quantum bit architectures. One particularly promising method relies on the coupling of spin quantum bits to microwave cavity photons.… ▽ More Spins confined in quantum dots are considered as a promising platform for quantum information processing. While many advanced quantum operations have been demonstrated, experimental as well as theoretical efforts are now focusing on the development of scalable spin quantum bit architectures. One particularly promising method relies on the coupling of spin quantum bits to microwave cavity photons. This would enable the coupling of distant spins via the exchange of virtual photons for two qubit gate applications, which still remains to be demonstrated with spin qubits. Here, we use a circuit QED spin-photon interface to drive a single electronic spin in a carbon nanotube based double quantum dot using cavity photons. The microwave spectroscopy allows us to identify an electrically controlled spin transition with a decoherence rate which can be tuned to be as low as $250kHz$. We show that this value is consistent with the expected hyperfine coupling in carbon nanotubes. These coherence properties, which can be attributed to the use of pristine carbon nanotubes stapled inside the cavity, should enable coherent spin-spin interaction via cavity photons and compare favourably to the ones recently demonstrated in Si-based circuit QED experiments. △ Less

Submitted 12 March, 2019; originally announced March 2019.

Journal ref: npj Quantum Information 5, 47 (2019)

Synthetic spin orbit interaction for Majorana devices

Authors: M. M. Desjardins, L. C. Contamin, M. R. Delbecq, M. C Dartiailh, L. E. Bruhat, T. Cubaynes, J. J. Viennot, F. Mallet, S. Rohart, A. Thiaville, A. Cottet, T. Kontos

Abstract: The interplay of superconductivity with a non-trivial spin texture holds promises for the engineering of non-abelian Majorana quasi-particles. A wide class of systems expected to exhibit exotic correlations are based on nanoscale conductors with strong spin-orbit interaction, subject to a strong external magnetic field. The strength of the spin-orbit coupling is a crucial parameter for the topolog… ▽ More The interplay of superconductivity with a non-trivial spin texture holds promises for the engineering of non-abelian Majorana quasi-particles. A wide class of systems expected to exhibit exotic correlations are based on nanoscale conductors with strong spin-orbit interaction, subject to a strong external magnetic field. The strength of the spin-orbit coupling is a crucial parameter for the topological protection of Majorana modes as it forbids other trivial excitations at low energy. The spin-orbit interaction is in principle intrinsic to a material. As a consequence, experimental efforts have been recently focused on semiconducting nano-conductors or spin-active atomic chains contacted to a superconductor. Alternatively, we show how both a spin-orbit and a Zeeman effect can be autonomously induced by using a magnetic texture coupled to any low dimensional conductor, here a carbon nanotube. Transport spectroscopy through superconducting contacts reveals oscillations of Andreev like states under a change of the magnetic texture. These oscillations are well accounted for by a scattering theory and are absent in a control device with no magnetic texture. A large synthetic spin-orbit energy of about 1.1 meV, larger than the intrinsic spin orbit energy in many other platforms, is directly derived from the number of oscillations. Furthermore, a robust zero energy state, the hallmark of devices hosting localized Majorana modes, emerges at zero magnetic field. Our findings synthetize all the features for the emergence of Majorana modes at zero magnetic field in a controlled, local and autonomous fashion. It could be used for advanced experiments, including microwave spectroscopy and braiding operations, which are at the heart of new schemes of topological quantum computation. △ Less

Submitted 20 February, 2019; originally announced February 2019.

Journal ref: Nature Materials 18, 1060 (2019)

Scaling laws of the Kondo problem at finite frequency

Authors: L. E Bruhat, J. J. Viennot, M. C. Dartiailh, M. M. Desjardins, A. Cottet, T. Kontos

Abstract: Driving a quantum system at finite frequency allows one to explore its dynamics. This has become a well mastered resource for controlling the quantum state of two level systems in the context of quantum information processing. However, this can also be of fundamental interest, especially with many-body systems which display an intricate finite frequency behavior. In condensed matter, the Kondo eff… ▽ More Driving a quantum system at finite frequency allows one to explore its dynamics. This has become a well mastered resource for controlling the quantum state of two level systems in the context of quantum information processing. However, this can also be of fundamental interest, especially with many-body systems which display an intricate finite frequency behavior. In condensed matter, the Kondo effect epitomizes strong electronic correlations, but the study of its dynamics and the related scaling laws has remained elusive so far. Here, we fill this gap by studying a carbon nanotube based Kondo quantum dot driven by a microwave signal. Our findings not only confirm long-standing theoretical predictions, but also allow us to establish a simple ansatz for the scaling laws on the Kondo problem at finite frequency. More generally, our technique opens a new path for understanding the dynamics of complex quantum dot circuits in the context of quantum simulation of strongly correlated electron fluids. △ Less

Submitted 23 July, 2018; originally announced July 2018.

Observation of the frozen charge of a Kondo resonance

Authors: M. M. Desjardins, J. J. Viennot, M. C Dartiailh, L. E. Bruhat, M. R. Delbecq, M. Lee, M. -S. Choi, A. Cottet, T. Kontos

Abstract: The ability to control electronic states at the nanoscale has contributed to our modern understanding of condensed matter. In particular, quantum dot circuits represent model systems for the study of strong electronic correlations, epitomized by the Kondo effect. Here, we show that circuit Quantum Electrodynamics architectures can be used to study the internal degrees of freedom of such a many-bod… ▽ More The ability to control electronic states at the nanoscale has contributed to our modern understanding of condensed matter. In particular, quantum dot circuits represent model systems for the study of strong electronic correlations, epitomized by the Kondo effect. Here, we show that circuit Quantum Electrodynamics architectures can be used to study the internal degrees of freedom of such a many-body phenomenon. We couple a quantum dot to a high finesse microwave cavity to measure with an unprecedented sensitivity the dot electronic compressibility i.e. the ability of the dot to accommodate charges. Because it corresponds solely to the charge response of the electronic system, this quantity is not equivalent to the conductance which involves in general other degrees of freedom such as spin. By performing dual conductance/compressibility measurements in the Kondo regime, we uncover directly the charge dynamics of this peculiar mechanism of electron transfer. Strikingly, the Kondo resonance, visible in transport measurements, is transparent to microwave photons trapped in the high finesse cavity. This reveals that, in such a many body resonance, finite conduction is achieved from a charge frozen by Coulomb interaction. This previously elusive freezing of charge dynamics is in stark contrast with the physics of a free electron gas. Our setup highlights the power of circuit quantum electrodynamics architectures to study condensed matter problems. The tools of cavity quantum electrodynamics could be used in other types of mesoscopic circuits with many-body correlations and bring a promising platform to perform quantum simulation of fermion-boson problems. △ Less

Submitted 23 July, 2018; originally announced July 2018.

Comments: minor differences with published version

Journal ref: Nature 545, 71 (2017)

Cavity QED with hybrid nanocircuits: from atomic-like physics to condensed matter phenomena

Authors: Audrey Cottet, Matthieu C. Dartiailh, Matthieu M. Desjardins, Tino Cubaynes, Lauriane C. Contamin, Matthieu Delbecq, Jérémie J. Viennot, Laure E. Bruhat, Benoit Douçot, Takis Kontos

Abstract: Circuit QED techniques have been instrumental to manipulate and probe with exquisite sensitivity the quantum state of superconducting quantum bits coupled to microwave cavities. Recently, it has become possible to fabricate new devices where the superconducting quantum bits are replaced by hybrid mesoscopic circuits combining nanoconductors and metallic reservoirs. This mesoscopic QED provides a n… ▽ More Circuit QED techniques have been instrumental to manipulate and probe with exquisite sensitivity the quantum state of superconducting quantum bits coupled to microwave cavities. Recently, it has become possible to fabricate new devices where the superconducting quantum bits are replaced by hybrid mesoscopic circuits combining nanoconductors and metallic reservoirs. This mesoscopic QED provides a new experimental playground to study the light-matter interaction in electronic circuits. Here, we present the experimental state of the art of Mesoscopic QED and its theoretical description. A first class of experiments focuses on the artificial atom limit, where some quasiparticles are trapped in nanocircuit bound states. In this limit, the Circuit QED techniques can be used to manipulate and probe electronic degrees of freedom such as confined charges, spins, or Andreev pairs. A second class of experiments consists in using cavity photons to reveal the dynamics of electron tunneling between a nanoconductor and fermionic reservoirs. For instance, the Kondo effect, the charge relaxation caused by grounded metallic contacts, and the photo-emission caused by voltage-biased reservoirs have been studied. The tunnel coupling between nanoconductors and fermionic reservoirs also enable one to obtain split Cooper pairs, or Majorana bound states. Cavity photons represent a qualitatively new tool to study these exotic condensed matter states. △ Less

Submitted 1 September, 2017; originally announced September 2017.

Comments: 34 pages, 18 figures, 1 table, minor differences with the published version to appear in Journal of Physics: Condensed Matter as a topical review

Journal ref: J. Phys.: Condens. Matter 29 433002 (2017)

Direct cavity detection of Majorana pairs

Authors: Matthieu Dartiailh, Takis Kontos, Benoit Douçot, Audrey Cottet

Abstract: No experiment could directly test the particle/antiparticle duality of Majorana fermions, so far. However, this property represents a necessary ingredient towards the realization of topological quantum computing schemes. Here, we show how to complete this task by using microwave techniques. The direct coupling between a pair of overlapping Majorana bound states and the electric field from a microw… ▽ More No experiment could directly test the particle/antiparticle duality of Majorana fermions, so far. However, this property represents a necessary ingredient towards the realization of topological quantum computing schemes. Here, we show how to complete this task by using microwave techniques. The direct coupling between a pair of overlapping Majorana bound states and the electric field from a microwave cavity is extremely difficult to detect due to the self-adjoint character of Majorana fermions which forbids direct energy exchanges with the cavity. We show theoretically how this problem can be circumvented by using photo-assisted tunneling to fermionic reservoirs. The absence of direct microwave transition inside the Majorana pair in spite of the light-Majorana coupling would represent a smoking gun for the Majorana self-adjoint character. △ Less

Submitted 6 February, 2017; originally announced February 2017.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Lett. 118, 126803 (2017)

Strong coupling between an electron in a quantum dot circuit and a photon in a cavity

Authors: L. E Bruhat, T. Cubaynes, J. J. Viennot, M. C. Dartiailh, M. M. Desjardins, A. Cottet, T. Kontos

Abstract: Circuit quantum electrodynamics allows one to probe, manipulate and couple superconducting quantum bits using cavity photons at an exquisite level. One of its cornerstones is the possibility to achieve the strong coupling which allows one to hybridize coherently light and matter. Its transposition to quantum dot circuits could offer the opportunity to use new degrees of freedom such as individual… ▽ More Circuit quantum electrodynamics allows one to probe, manipulate and couple superconducting quantum bits using cavity photons at an exquisite level. One of its cornerstones is the possibility to achieve the strong coupling which allows one to hybridize coherently light and matter. Its transposition to quantum dot circuits could offer the opportunity to use new degrees of freedom such as individual charge or spin. However, the strong coupling of quantum dot circuits to cavity photons remains to be observed. Here, we demonstrate a hybrid superconductor-quantum dot circuit which realizes the strong coupling of an individual electronic excitation to microwave photons. We observe a vacuum Rabi splitting 2g~10 MHz which exceeds by a factor of 3 the linewidth of the hybridized light-matter states. Our findings open the path to ultra-long distance entanglement of quantum dot based qubits. They could be adapted to many other circuit designs, shedding new light on the roadmap for scalability of quantum dot setups. △ Less

Submitted 15 December, 2016; originally announced December 2016.

Journal ref: Phys. Rev. B 98, 155313 (2018)

Towards hybrid circuit quantum electrodynamics with quantum dots

Authors: J. J. Viennot, M. R. Delbecq, L. E. Bruhat, M. C. Dartiailh, M. Desjardins, M. Baillergeau, A. Cottet, T. Kontos

Abstract: Cavity quantum electrodynamics allows one to study the interaction between light and matter at the most elementary level. The methods developed in this field have taught us how to probe and manipulate individual quantum systems like atoms and superconducting quantum bits with an exquisite accuracy. There is now a strong effort to extend further these methods to other quantum systems, and in partic… ▽ More Cavity quantum electrodynamics allows one to study the interaction between light and matter at the most elementary level. The methods developed in this field have taught us how to probe and manipulate individual quantum systems like atoms and superconducting quantum bits with an exquisite accuracy. There is now a strong effort to extend further these methods to other quantum systems, and in particular hybrid quantum dot circuits. This could turn out to be instrumental for a noninvasive study of quantum dot circuits and a realization of scalable spin quantum bit architectures. It could also provide an interesting platform for quantum simulation of simple fermion-boson condensed matter systems. In this short review, we discuss the experimental state of the art for hybrid circuit quantum electrodynamics with quantum dots, and we present a simple theoretical modeling of experiments. △ Less

Submitted 7 October, 2016; originally announced October 2016.

Comments: Minor differences with published version

Journal ref: Comptes Rendus Physique 17 (7), 705-717 (2016)

Cavity photons as a probe for charge relaxation resistance and photon emission in a quantum dot coupled to normal and superconducting continua

Authors: L. E. Bruhat, J. J. Viennot, M. C. Dartiailh, M. M. Desjardins, T. Kontos, A. Cottet

Abstract: Microwave cavities have been widely used to investigate the behavior of closed few-level systems. Here, we show that they also represent a powerful probe for the dynamics of charge transfer between a discrete electronic level and fermionic continua. We have combined experiment and theory for a carbon nanotube quantum dot coupled to normal metal and superconducting contacts. In equilibrium conditio… ▽ More Microwave cavities have been widely used to investigate the behavior of closed few-level systems. Here, we show that they also represent a powerful probe for the dynamics of charge transfer between a discrete electronic level and fermionic continua. We have combined experiment and theory for a carbon nanotube quantum dot coupled to normal metal and superconducting contacts. In equilibrium conditions, where our device behaves as an effective quantum dot-normal metal junction, we approach a universal photon dissipation regime governed by a quantum charge relaxation effect. We observe how photon dissipation is modified when the dot admittance turns from capacitive to inductive. When the fermionic reservoirs are voltage biased, the dot can even cause photon emission due to inelastic tunneling to/from a Bardeen-Cooper-Schrieffer peak in the density of states of the superconducting contact. We can model these numerous effects quantitatively in terms of the charge susceptibility of the quantum dot circuit. This validates an approach that could be used to study a wide class of mesoscopic QED devices. △ Less

Submitted 16 May, 2016; originally announced May 2016.

Comments: 15 pages, 8 figures, minor differences with published version

Journal ref: Phys. Rev. X 6, 021014 (2016)

Coherent coupling of a single spin to microwave cavity photons

Authors: J. J. Viennot, M. C. Dartiailh, A. Cottet, T. Kontos

Abstract: Electron spins and photons are complementary quantum-mechanical objects that can be used to carry, manipulate and transform quantum information. To combine these resources, it is desirable to achieve the coherent coupling of a single spin to photons stored in a superconducting resonator. Using a circuit design based on a nanoscale spin-valve, we coherently hybridize the individual spin and charge… ▽ More Electron spins and photons are complementary quantum-mechanical objects that can be used to carry, manipulate and transform quantum information. To combine these resources, it is desirable to achieve the coherent coupling of a single spin to photons stored in a superconducting resonator. Using a circuit design based on a nanoscale spin-valve, we coherently hybridize the individual spin and charge states of a double quantum dot while preserving spin coherence. This scheme allows us to achieve spin-photon coupling up to the MHz range at the single spin level. The cooperativity is found to reach 2.3, and the spin coherence time is about 60ns. We thereby demonstrate a mesoscopic device suitable for non-destructive spin read-out and distant spin coupling. △ Less

Submitted 13 September, 2015; originally announced September 2015.

Comments: minor differences with published version

Journal ref: Science 349, 408 (2015)

Electron-photon coupling in Mesoscopic Quantum Electrodynamics

Authors: Audrey Cottet, Takis Kontos, Benoit Douçot

Abstract: Understanding the interaction between cavity photons and electronic nanocircuits is crucial for the development of Mesoscopic Quantum Electrodynamics (QED). One has to combine ingredients from atomic Cavity QED, like orbital degrees of freedom, with tunneling physics and strong cavity field inhomogeneities, specific to superconducting circuit QED. It is therefore necessary to introduce a formalism… ▽ More Understanding the interaction between cavity photons and electronic nanocircuits is crucial for the development of Mesoscopic Quantum Electrodynamics (QED). One has to combine ingredients from atomic Cavity QED, like orbital degrees of freedom, with tunneling physics and strong cavity field inhomogeneities, specific to superconducting circuit QED. It is therefore necessary to introduce a formalism which bridges between these two domains. We develop a general method based on a photonic pseudo-potential to describe the electric coupling between electrons in a nanocircuit and cavity photons. In this picture, photons can induce simultaneously orbital energy shifts, tunneling, and local orbital transitions. We study in details the elementary example of a single quantum dot with a single normal metal reservoir, coupled to a cavity. Photon-induced tunneling terms lead to a non-universal relation between the cavity frequency pull and the damping pull. Our formalism can also be applied to multi quantum dot circuits, molecular circuits, quantum point contacts, metallic tunnel junctions, and superconducting nanostructures enclosing Andreev bound states or Majorana bound states, for instance. △ Less

Submitted 9 May, 2015; v1 submitted 5 January, 2015; originally announced January 2015.

Comments: 17 pages, 5 figures

Journal ref: Phys. Rev. B 91, 205417 (2015)

Stamping single wall nanotubes for circuit quantum electrodynamics

Authors: J. J. Viennot, J. Palomo, T. Kontos

Abstract: We report on a dry transfer technique for single wall carbon nanotube devices which allows to embed them in high finesse microwave cavity. We demonstrate the ground state charge readout and a quality factor of about 3000 down to the single photon regime. This technique allows to make devices such as double quantum dots which could be instrumental for achieving the strong spin photon coupling. It c… ▽ More We report on a dry transfer technique for single wall carbon nanotube devices which allows to embed them in high finesse microwave cavity. We demonstrate the ground state charge readout and a quality factor of about 3000 down to the single photon regime. This technique allows to make devices such as double quantum dots which could be instrumental for achieving the strong spin photon coupling. It can easily be extended to generic carbon nanotube based microwave devices. △ Less

Submitted 1 April, 2014; originally announced April 2014.

Comments: Version similar to the one accepted

Journal ref: Appl. Phys. Lett. 104 , 113108 (2014)

Out of equilibrium charge dynamics in a hybrid cQED architecture

Authors: J. J. Viennot, M. R. Delbecq, M. C. Dartiailh, A. Cottet, T. Kontos

Abstract: The recent development of hybrid cQED allows one to study how cavity photons interact with a system driven out of equilibrium by fermionic reservoirs. We study here one of the simplest combination : a double quantum dot coupled to a single mode of the electromagnetic field. We are able to couple resonantly the charge levels of a carbon nanotube based double dot to cavity photons. We perform a micr… ▽ More The recent development of hybrid cQED allows one to study how cavity photons interact with a system driven out of equilibrium by fermionic reservoirs. We study here one of the simplest combination : a double quantum dot coupled to a single mode of the electromagnetic field. We are able to couple resonantly the charge levels of a carbon nanotube based double dot to cavity photons. We perform a microwave read out of the charge states of this system which allows us to unveil features of the out of equilibrium charge dynamics, otherwise invisible in the DC current. We extract relaxation rate, dephasing rate and photon number of the hybrid system using a theory based on a master equation technique. These findings open the path for manipulating other degrees of freedom e.g. the spin and/or the valley in nanotube based double dots using microwave light. △ Less

Submitted 20 December, 2013; v1 submitted 16 October, 2013; originally announced October 2013.

Comments: added new data on two tone microwave spectroscopy of the double dot

Journal ref: Phys. Rev. B 89, 165404 (2014)

Squeezing light with Majorana fermions

Authors: Audrey Cottet, Takis Kontos, Benoit Douçot

Abstract: Coupling a semiconducting nanowire to a microwave cavity provides a powerfull means to assess the presence or absence of isolated Majorana fermions in the nanowire. These exotic bound states can cause a significant cavity frequency shift but also a strong cavity nonlinearity leading for instance to light squeezing. The dependence of these effects on the nanowire gate voltages gives direct signatur… ▽ More Coupling a semiconducting nanowire to a microwave cavity provides a powerfull means to assess the presence or absence of isolated Majorana fermions in the nanowire. These exotic bound states can cause a significant cavity frequency shift but also a strong cavity nonlinearity leading for instance to light squeezing. The dependence of these effects on the nanowire gate voltages gives direct signatures of the unique properties of Majorana fermions, such as their self-adjoint character and their exponential confinement. △ Less

Submitted 12 November, 2013; v1 submitted 16 July, 2013; originally announced July 2013.

Comments: long version: 11 pages, 5 figures

Journal ref: Phys. Rev. B 88, 195415 (2013)

Photon mediated interaction between distant quantum dot circuits

Authors: M. R. Delbecq, L. E. Bruhat, J. J. Viennot, S. Datta, A. Cottet, T. Kontos

Abstract: Engineering the interaction between light and matter is an important goal in the emerging field of quantum opto-electronics. Thanks to the use of cavity quantum electrodynamics architectures, one can envision a fully hybrid multiplexing of quantum conductors. Here, we use such an architecture to couple two quantum dot circuits . Our quantum dots are separated by 200 times their own size, with no d… ▽ More Engineering the interaction between light and matter is an important goal in the emerging field of quantum opto-electronics. Thanks to the use of cavity quantum electrodynamics architectures, one can envision a fully hybrid multiplexing of quantum conductors. Here, we use such an architecture to couple two quantum dot circuits . Our quantum dots are separated by 200 times their own size, with no direct tunnel and electrostatic couplings between them. We demonstrate their interaction, mediated by the cavity photons. This could be used to scale up quantum bit architectures based on quantum dot circuits or simulate on-chip phonon-mediated interactions between strongly correlated electrons. △ Less

Submitted 11 March, 2013; originally announced March 2013.

Journal ref: Nature Communications 4, 1400 (2013)

Spectroscopy of non-local superconducting correlations in a double quantum dot

Authors: L. G. Herrmann, P. Burset, W. J. Herrera, F. Portier, P. Roche, C. Strunk, A. Levy Yeyati, T. Kontos

Abstract: We investigate non-linear transport in a double quantum dot connected to two normal electrodes and a central superconducting finger. By this means, we perform a transport spectroscopy of such a system which implements a Cooper pair splitter. The non-linear conductance exhibits strong subgap features which can be associated with the coherence of the injected Cooper pairs. Our findings are well acco… ▽ More We investigate non-linear transport in a double quantum dot connected to two normal electrodes and a central superconducting finger. By this means, we perform a transport spectroscopy of such a system which implements a Cooper pair splitter. The non-linear conductance exhibits strong subgap features which can be associated with the coherence of the injected Cooper pairs. Our findings are well accounted for by the recently developed microscopic theory of Cooper pairs splitters made in SWNTs. △ Less

Submitted 9 May, 2012; originally announced May 2012.

Comments: 4 pages 4 figures

Subradiant split Cooper pairs

Authors: Audrey Cottet, Takis Kontos, Alfredo Levy Yeyati

Abstract: We suggest a way to characterize the coherence of the split Cooper pairs emitted by a double-quantum-dot based Cooper pair splitter (CPS), by studying the radiative response of such a CPS inside a microwave cavity. The coherence of the split pairs manifests in a strongly nonmonotonic variation of the emitted radiation as a function of the parameters controlling the coupling of the CPS to the cavit… ▽ More We suggest a way to characterize the coherence of the split Cooper pairs emitted by a double-quantum-dot based Cooper pair splitter (CPS), by studying the radiative response of such a CPS inside a microwave cavity. The coherence of the split pairs manifests in a strongly nonmonotonic variation of the emitted radiation as a function of the parameters controlling the coupling of the CPS to the cavity. The idea to probe the coherence of the electronic states using the tools of Cavity Quantum Electrodynamics could be generalized to many other nanoscale circuits. △ Less

Submitted 4 April, 2012; v1 submitted 30 November, 2011; originally announced November 2011.

Comments: Main text + Supplemental material file (15 pages, 5 figures), to appear in Physical Review Letters

Journal ref: Phys. Rev. Lett. 108, 166803 (2012)

Coupling a quantum dot, fermionic leads and a microwave cavity on-chip

Authors: M. R. Delbecq, V. Schmitt, F. D. Parmentier, N. Roch, J. J. Viennot, G. Fève, B. Huard, C. Mora, A. Cottet, T. Kontos

Abstract: We demonstrate a hybrid architecture consisting of a quantum dot circuit coupled to a single mode of the electromagnetic field. We use single wall carbon nanotube based circuits inserted in superconducting microwave cavities. By probing the nanotube-dot using a dispersive read-out in the Coulomb blockade and the Kondo regime, we determine an electron-photon coupling strength which should enable ci… ▽ More We demonstrate a hybrid architecture consisting of a quantum dot circuit coupled to a single mode of the electromagnetic field. We use single wall carbon nanotube based circuits inserted in superconducting microwave cavities. By probing the nanotube-dot using a dispersive read-out in the Coulomb blockade and the Kondo regime, we determine an electron-photon coupling strength which should enable circuit QED experiments with more complex quantum dot circuits. △ Less

Submitted 22 August, 2011; originally announced August 2011.

Comments: 4 pages, 4 figures

Journal ref: Phys. Rev. Lett. 107, 256804 (2011)

Mesoscopic admittance of a double quantum dot

Authors: Audrey Cottet, Christophe Mora, Takis Kontos

Abstract: We calculate the mesoscopic admittance $G(ω)$ of a double quantum dot (DQD),which can be measured directly using microwave techniques. This quantity reveals spectroscopic information on the DQD and is also directly sensitive to a Pauli spin blockade effect. We then discuss the problem of a DQD coupled to a high quality photonic resonator. When the photon correlation functions can be developed alon… ▽ More We calculate the mesoscopic admittance $G(ω)$ of a double quantum dot (DQD),which can be measured directly using microwave techniques. This quantity reveals spectroscopic information on the DQD and is also directly sensitive to a Pauli spin blockade effect. We then discuss the problem of a DQD coupled to a high quality photonic resonator. When the photon correlation functions can be developed along a random-phase-approximation-like scheme, the response of the resonator gives an access to $G(ω)$. △ Less

Submitted 28 March, 2011; v1 submitted 1 November, 2010; originally announced November 2010.

Journal ref: Physical Review B 83, 121311(R) (2011)

Noisy Kondo impurities

Authors: T. Delattre, C. Feuillet-Palma, L. G. Herrmann, P. Morfin, J. -M. Berroir, G. Fève, B. Plaçais, D. C. Glattli, M. -S. Choi, C. Mora, T. Kontos

Abstract: The anti-ferromagnetic coupling of a magnetic impurity carrying a spin with the conduction electrons spins of a host metal is the basic mechanism responsible for the increase of the resistance of an alloy such as Cu${}_{0.998}$Fe${}_{0.002}$ at low temperature, as originally suggested by Kondo . This coupling has emerged as a very generic property of localized electronic states coupled to a contin… ▽ More The anti-ferromagnetic coupling of a magnetic impurity carrying a spin with the conduction electrons spins of a host metal is the basic mechanism responsible for the increase of the resistance of an alloy such as Cu${}_{0.998}$Fe${}_{0.002}$ at low temperature, as originally suggested by Kondo . This coupling has emerged as a very generic property of localized electronic states coupled to a continuum . The possibility to design artificial controllable magnetic impurities in nanoscopic conductors has opened a path to study this many body phenomenon in unusual situations as compared to the initial one and, in particular, in out of equilibrium situations. So far, measurements have focused on the average current. Here, we report on \textit{current fluctuations} (noise) measurements in artificial Kondo impurities made in carbon nanotube devices. We find a striking enhancement of the current noise within the Kondo resonance, in contradiction with simple non-interacting theories. Our findings provide a test bench for one of the most important many-body theories of condensed matter in out of equilibrium situations and shed light on the noise properties of highly conductive molecular devices. △ Less

Submitted 22 October, 2010; originally announced October 2010.

Comments: minor differences with published version

Report number: Nature Physics 5, 208 (2009)

Journal ref: Nature Physics 5, 208 (2009)

Transport scattering time probed through rf admittance of a graphene capacitor

Authors: E. Pallecchi, A. C. Betz, J. Chaste, G. Fève, B. Huard, T. Kontos, J. -M. Berroir, B. Plaçais

Abstract: We have investigated electron dynamics in top gated graphene by measuring the gate admittance of a diffusive graphene capacitor in a broad frequency range as a function of carrier density. The density of states, conductivity and diffusion constant are deduced from the low frequency gate capacitance, its charging time and their ratio. The admittance evolves from an RC-like to a skin-effect response… ▽ More We have investigated electron dynamics in top gated graphene by measuring the gate admittance of a diffusive graphene capacitor in a broad frequency range as a function of carrier density. The density of states, conductivity and diffusion constant are deduced from the low frequency gate capacitance, its charging time and their ratio. The admittance evolves from an RC-like to a skin-effect response at GHz frequency with a crossover given by the Thouless energy. The scattering time is found to be independent of energy in the 0 - 200 meV investigated range at room temperature. This is consistent with a random mass model for Dirac Fermions. △ Less

Submitted 23 March, 2011; v1 submitted 19 May, 2010; originally announced May 2010.

Comments: 6 pages, 6 figures

Journal ref: Phys. Rev. B 83, 125408 (2011)

A spin quantum bit with ferromagnetic contacts for circuit QED

Authors: Audrey Cottet, Takis Kontos

Abstract: We theoretically propose a scheme for a spin quantum bit based on a double quantum dot contacted to ferromagnetic elements. Interface exchange effects enable an all electric manipulation of the spin and a switchable strong coupling to a superconducting coplanar waveguide cavity. Our setup does not rely on any specific band structure and can in principle be realized with many different types of nan… ▽ More We theoretically propose a scheme for a spin quantum bit based on a double quantum dot contacted to ferromagnetic elements. Interface exchange effects enable an all electric manipulation of the spin and a switchable strong coupling to a superconducting coplanar waveguide cavity. Our setup does not rely on any specific band structure and can in principle be realized with many different types of nanoconductors. This allows to envision on-chip single spin manipulation and read-out using cavity QED techniques. △ Less

Submitted 16 October, 2010; v1 submitted 11 May, 2010; originally announced May 2010.

Journal ref: Phys.Rev.Lett.105:160502,2010

Current correlations of an on-demand electron source as an evidence of single particle emission

Authors: A. Mahé, F. D. Parmentier, E. Bocquillon, J. -M. Berroir, D. C. Glattli, T. Kontos, B. Plaçais, G. Fève, A. Cavanna, Y. Jin

Abstract: In analogy with quantum optics, short time correlations of the current fluctuations are used to characterize an on-demand electron source consisting of a quantum dot connected to a conductor via a tunable tunnel barrier. We observe a new fundamental noise for electrons associated with the quantum fluctuations of the electron emission time, which we call quantum jitter. In optimum operating conditi… ▽ More In analogy with quantum optics, short time correlations of the current fluctuations are used to characterize an on-demand electron source consisting of a quantum dot connected to a conductor via a tunable tunnel barrier. We observe a new fundamental noise for electrons associated with the quantum fluctuations of the electron emission time, which we call quantum jitter. In optimum operating conditions of the source, the noise reduces to the quantum jitter limit, which demonstrates single particle emission. Combined with the coherent manipulations of single electrons in a quantum conductor, this electron quantum optics experiment opens the way to explore new problems including quantum statistics and interactions at the single electron level. △ Less

Submitted 22 December, 2010; v1 submitted 12 April, 2010; originally announced April 2010.

Journal ref: Physical Review B 82, 201309(R) (2010)

Conserved spin and orbital phase along carbon nanotubes connected with multiple ferromagnetic contacts

Authors: C. Feuillet-Palma, T. Delattre, P. Morfin, J. -M. Berroir, G. Fève, D. C. Glattli, B. Placais, A. Cottet, T. Kontos

Abstract: We report on spin dependent transport measurements in carbon nanotubes based multi-terminal circuits. We observe a gate-controlled spin signal in non-local voltages and an anomalous conductance spin signal, which reveal that both the spin and the orbital phase can be conserved along carbon nanotubes with multiple ferromagnetic contacts. This paves the way for spintronics devices exploiting both th… ▽ More We report on spin dependent transport measurements in carbon nanotubes based multi-terminal circuits. We observe a gate-controlled spin signal in non-local voltages and an anomalous conductance spin signal, which reveal that both the spin and the orbital phase can be conserved along carbon nanotubes with multiple ferromagnetic contacts. This paves the way for spintronics devices exploiting both these quantum mechanical degrees of freedom on the same footing. △ Less

Submitted 28 March, 2010; originally announced March 2010.

Comments: 8 pages - minor differences with published version

Journal ref: Phys. Rev. B 81, 115414 (2010)

Thermal shot noise in top-gated single carbon nanotube field effect transistors

Authors: J. Chaste, E. Pallecchi, P. Morfin, G. Feve, T. Kontos, J. -M. Berroir, P. Hakonen, B. Placais

Abstract: The high-frequency transconductance and current noise of top-gated single carbon nanotube transistors have been measured and used to investigate hot electron effects in one-dimensional transistors. Results are in good agreement with a theory of 1-dimensional nano-transistor. In particular the prediction of a large transconductance correction to the Johnson-Nyquist thermal noise formula is confirme… ▽ More The high-frequency transconductance and current noise of top-gated single carbon nanotube transistors have been measured and used to investigate hot electron effects in one-dimensional transistors. Results are in good agreement with a theory of 1-dimensional nano-transistor. In particular the prediction of a large transconductance correction to the Johnson-Nyquist thermal noise formula is confirmed experimentally. Experiment shows that nanotube transistors can be used as fast charge detectors for quantum coherent electronics with a resolution of $13\mathrm{μe/\sqrt{Hz}}$ in the 0.2-$0.8 \mathrm{GHz}$ band. △ Less

Submitted 12 May, 2010; v1 submitted 3 February, 2010; originally announced February 2010.

Comments: 3 pages, 4 figures

Journal ref: Applied Physics Letters 96, 192103 (2010)

Carbon Nanotubes as Cooper Pair Beam Splitters

Authors: L. G. Herrmann, F. Portier, P. Roche, A. Levy Yeyati, T. Kontos, C. Strunk

Abstract: We report on conductance measurements in carbon nanotube based double quantum dots connected to two normal electrodes and a central superconducting finger. By operating our devices as beam splitters, we provide evidence for Crossed Andreev Reflections \textit{tunable in situ}. This opens an avenue to more sophisticated quantum optics-like experiments with spin entangled electrons. We report on conductance measurements in carbon nanotube based double quantum dots connected to two normal electrodes and a central superconducting finger. By operating our devices as beam splitters, we provide evidence for Crossed Andreev Reflections \textit{tunable in situ}. This opens an avenue to more sophisticated quantum optics-like experiments with spin entangled electrons. △ Less

Submitted 17 September, 2009; originally announced September 2009.

Comments: submitted

Journal ref: Phys. Rev. Lett. 104, 026801 (2010)

Multi-terminal spin-dependent transport in ballistic carbon nanotubes

Authors: Audrey Cottet, Chéryl Feuillet-Palma, Takis Kontos

Abstract: We study theoretically nonlocal spin transport in a ballistic carbon nanotube contacted to two ferromagnetic leads and two normal-metal leads. When the magnetizations of the two ferromagnets are changed from a parallel to an antiparallel configuration, the circuit shows a hysteretic behavior which is specific to the few-channel regime. In the coherent limit, the amplitude of the magnetic signals… ▽ More We study theoretically nonlocal spin transport in a ballistic carbon nanotube contacted to two ferromagnetic leads and two normal-metal leads. When the magnetizations of the two ferromagnets are changed from a parallel to an antiparallel configuration, the circuit shows a hysteretic behavior which is specific to the few-channel regime. In the coherent limit, the amplitude of the magnetic signals is strongly enhanced due to resonance effects occurring inside the nanotube. Our calculations pave the way for experiments on low-dimensional nonlocal spin transport, which should give results remarkably different from the experiments realized so far in the multichannel diffusive incoherent regime. △ Less

Submitted 23 March, 2009; originally announced March 2009.

Comments: 9 pages, 8 figures

Journal ref: Phys. Rev. B 79, 125422 (2009)

Subnanosecond single electron source in the time-domain

Authors: Adrien Mahé, François Parmentier, Gwendal Fève, Jean-Marc Berroir, Takis Kontos, A. Cavanna, Bernard Etienne, Yong Jin, Christian Glattli, Bernard Plaçais

Abstract: We describe here the realization of a single electron source similar to single photon sources in optics. On-demand single electron injection is obtained using a quantum dot connected to the conductor via a tunnel barrier of variable transmission (quantum point contact). Electron emission is triggered by a sudden change of the dot potential which brings a single energy level above the Fermi energ… ▽ More We describe here the realization of a single electron source similar to single photon sources in optics. On-demand single electron injection is obtained using a quantum dot connected to the conductor via a tunnel barrier of variable transmission (quantum point contact). Electron emission is triggered by a sudden change of the dot potential which brings a single energy level above the Fermi energy in the conductor. A single charge is emitted on an average time ranging from 100 ps to 10 ns ultimately determined by the barrier transparency and the dot charging energy. The average single electron emission process is recorded with a 0.5 ns time resolution using a real-time fast acquisition card. Single electron signals are compared tosimulation based on scattering theory approach adapted for finite excitation energies. △ Less

Submitted 16 September, 2008; originally announced September 2008.

Single Carbon Nanotube Transistor at GHz Frequency

Authors: Julien Chaste, Lorenz Lechner, Pascal Morfin, Gwendal Feve, Takis Kontos, Jean-Marc Berroir, Christian Glattli, Henri Happy, Perti Hakonen, Bernard Placais

Abstract: We report on microwave operation of top-gated single carbon nanotube transistors. From transmission measurements in the 0.1-1.6 GHz range, we deduce device transconductance gm and gate-nanotube capacitance Cg of micro- and nanometric devices. A large and frequency-independent gm of about 20 microSiemens is observed on short devices, which meets the best dc results. The capacitance per unit gate… ▽ More We report on microwave operation of top-gated single carbon nanotube transistors. From transmission measurements in the 0.1-1.6 GHz range, we deduce device transconductance gm and gate-nanotube capacitance Cg of micro- and nanometric devices. A large and frequency-independent gm of about 20 microSiemens is observed on short devices, which meets the best dc results. The capacitance per unit gate length of 60 aF/micrometer is typical of top gates on a conventional oxide with a dielectric constant equal to 10. This value is a factor of 3-5 below the nanotube quantum capacitance which, according to recent simulations, favors high transit frequencies . For our smallest devices, we find a large transit frequency equal to 50 GHz with no evidence of saturation in length dependence. △ Less

Submitted 4 June, 2008; v1 submitted 2 June, 2008; originally announced June 2008.

Journal ref: Nano Letters 8, 2 (2008) 525

An On-Demand Coherent Single Electron Source

Authors: Gwendal Fève, Adrien Mahé, Jean-Marc Berroir, Takis Kontos, Bernard Plaçais, Christian Glattli, A. Cavanna, Bernard Etienne, Yong Jin

Abstract: We report on the electron analog of the single photon gun. On demand single electron injection in a quantum conductor was obtained using a quantum dot connected to the conductor via a tunnel barrier. Electron emission is triggered by application of a potential step which compensates the dot charging energy. Depending on the barrier transparency the quantum emission time ranges from 0.1 to 10 nan… ▽ More We report on the electron analog of the single photon gun. On demand single electron injection in a quantum conductor was obtained using a quantum dot connected to the conductor via a tunnel barrier. Electron emission is triggered by application of a potential step which compensates the dot charging energy. Depending on the barrier transparency the quantum emission time ranges from 0.1 to 10 nanoseconds. The single electron source should prove useful for the implementation of quantum bits in ballistic conductors. Additionally periodic sequences of single electron emission and absorption generate a quantized AC-current. △ Less

Submitted 30 May, 2007; originally announced May 2007.

Journal ref: Science 316 (2007) 1169

Nanospintronics with carbon nanotubes

Authors: Audrey Cottet, Takis Kontos, Sangeeta Sahoo, Hon Tin Man, Mahn-Soo Choi, Wolfgang Belzig, Christoph Bruder, Alberto Morpurgo, Christian Schoenenberger

Abstract: One of the actual challenges of spintronics is the realization of a spin-transistor allowing to control spin transport through an electrostatic gate. In this review, we report on different experiments which demonstrate a gate control of spin transport in a carbon nanotube connected to ferromagnetic leads. We also discuss some theoretical approaches which can be used to analyze spin transport in… ▽ More One of the actual challenges of spintronics is the realization of a spin-transistor allowing to control spin transport through an electrostatic gate. In this review, we report on different experiments which demonstrate a gate control of spin transport in a carbon nanotube connected to ferromagnetic leads. We also discuss some theoretical approaches which can be used to analyze spin transport in these systems. We emphasize the roles of the gate-tunable quasi-bound states inside the nanotube and the coherent spin-dependent scattering at the interfaces between the nanotube and its ferromagnetic contacts. △ Less

Submitted 18 March, 2007; originally announced March 2007.

Comments: 35 pages, 15 figures, some figures in gif

Journal ref: Semicond. Sci. Technol. 21, S78 (2006)

Shot noise in carbon nanotube based Fabry-Perot interferometers

Authors: L. G. Herrmann, T. Delattre, P. Morfin, J. -M. Berroir, B. Plaçais, D. C. Glattli, T. Kontos

Abstract: We report on shot noise measurements in carbon nanotube based Fabry-Perot electronic interferometers. As a consequence of quantum interferences, the noise power spectral density oscillates as a function of the voltage applied to the gate electrode. The quantum shot noise theory accounts for the data quantitatively. It allows to confirm the existence of two nearly degenerate orbitals. At resonanc… ▽ More We report on shot noise measurements in carbon nanotube based Fabry-Perot electronic interferometers. As a consequence of quantum interferences, the noise power spectral density oscillates as a function of the voltage applied to the gate electrode. The quantum shot noise theory accounts for the data quantitatively. It allows to confirm the existence of two nearly degenerate orbitals. At resonance, the transmission of the nanotube approaches unity, and the nanotube becomes noiseless, as observed in quantum point contacts. In this weak backscattering regime, the dependence of the noise on the backscattering current is found weaker than expected, pointing either to electron-electron interactions or to weak decoherence. △ Less

Submitted 5 March, 2007; originally announced March 2007.

Journal ref: Phys. Rev. Lett. 99, 156804 (2007)

The relaxation time of a chiral quantum R-L circuit

Authors: Julien Gabelli, Gwendal Fève, Takis Kontos, Jean-Marc Berroir, Bernard Plaçais, Christian Glattli, Bernard Etienne, Yong Jin, Markus Büttiker

Abstract: We report on the GHz complex admittance of a chiral one dimensional ballistic conductor formed by edge states in the quantum Hall regime. The circuit consists of a wide Hall bar (the inductor L) in series with a tunable resistor (R) formed by a quantum point contact. Electron interactions between edges are screened by a pair of side gates. Conductance steps are observed on both real and imaginar… ▽ More We report on the GHz complex admittance of a chiral one dimensional ballistic conductor formed by edge states in the quantum Hall regime. The circuit consists of a wide Hall bar (the inductor L) in series with a tunable resistor (R) formed by a quantum point contact. Electron interactions between edges are screened by a pair of side gates. Conductance steps are observed on both real and imaginary parts of the admittance. Remarkably, the phase of the admittance is transmission-independent. This shows that the relaxation time of a chiral R-L circuit is resistance independent. A current and charge conserving scattering theory is presented that accounts for this observation with a relaxation time given by the electronic transit time in the c cuit. △ Less

Submitted 14 May, 2007; v1 submitted 26 January, 2007; originally announced January 2007.

Journal ref: Physical Review Letters 98 (2007) 166806

Controlling spin in an electronic interferometer with spin-active interfaces

Authors: A. Cottet, T. Kontos, W. Belzig, C. Schonenberger, C. Bruder

Abstract: We consider electronic current transport through a ballistic one-dimensional quantum wire connected to two ferromagnetic leads. We study the effects of the spin-dependence of interfacial phase shifts (SDIPS) acquired by electrons upon scattering at the boundaries of the wire. The SDIPS produces a spin splitting of the wire resonant energies which is tunable with the gate voltage and the angle be… ▽ More We consider electronic current transport through a ballistic one-dimensional quantum wire connected to two ferromagnetic leads. We study the effects of the spin-dependence of interfacial phase shifts (SDIPS) acquired by electrons upon scattering at the boundaries of the wire. The SDIPS produces a spin splitting of the wire resonant energies which is tunable with the gate voltage and the angle between the ferromagnetic polarizations. This property could be used for manipulating spins. In particular, it leads to a giant magnetoresistance effect with a sign tunable with the gate voltage and the magnetic field applied to the wire. △ Less

Submitted 29 March, 2006; v1 submitted 8 December, 2005; originally announced December 2005.

Comments: 5 pages, 3 figures. to be published in Europhysics Letters

Journal ref: Europhys. Lett. 74, 320 (2006)

Electric Field Control of Spin Transport

Authors: Sangeeta Sahoo, Takis Kontos, Jürg Furer, Christian Hoffmann, Matthias Gräber, Audrey Cottet, Christian Schönenberger

Abstract: Spintronics is an approach to electronics in which the spin of the electrons is exploited to control the electric resistance R of devices. One basic building block is the spin-valve, which is formed if two ferromagnetic electrodes are separated by a thin tunneling barrier. In such devices, R depends on the orientation of the magnetisation of the electrodes. It is usually larger in the antiparall… ▽ More Spintronics is an approach to electronics in which the spin of the electrons is exploited to control the electric resistance R of devices. One basic building block is the spin-valve, which is formed if two ferromagnetic electrodes are separated by a thin tunneling barrier. In such devices, R depends on the orientation of the magnetisation of the electrodes. It is usually larger in the antiparallel than in the parallel configuration. The relative difference of R, the so-called magneto-resistance (MR), is then positive. Common devices, such as the giant magneto-resistance sensor used in reading heads of hard disks, are based on this phenomenon. The MR may become anomalous (negative), if the transmission probability of electrons through the device is spin or energy dependent. This offers a route to the realisation of gate-tunable MR devices, because transmission probabilities can readily be tuned in many devices with an electrical gate signal. Such devices have, however, been elusive so far. We report here on a pronounced gate-field controlled MR in devices made from carbon nanotubes with ferromagnetic contacts. Both the amplitude and the sign of the MR are tunable with the gate voltage in a predictable manner. We emphasise that this spin-field effect is not restricted to carbon nanotubes but constitutes a generic effect which can in principle be exploited in all resonant tunneling devices. △ Less

Submitted 3 November, 2005; originally announced November 2005.

Comments: 22 pages, 5 figures

Journal ref: Nature Physics Vol 1, pages 99-102 (2005)

Ferromagnetic 0-pi Junctions as Classical Spins

Authors: M. L. Della Rocca, M. Aprili, T. Kontos, A. Gomez, P. Spatkis

Abstract: The ground state of highly damped PdNi based 0-pi ferromagnetic Josephson junctions shows a spontaneous half quantum vortex, sustained by a supercurrent of undetermined sign. This supercurrent flows in the electrode of a Josephson junction used as a detector and produces a phi(0)/4 shift in its magnetic diffraction pattern. We have measured the statistics of the positive or negative sign shift o… ▽ More The ground state of highly damped PdNi based 0-pi ferromagnetic Josephson junctions shows a spontaneous half quantum vortex, sustained by a supercurrent of undetermined sign. This supercurrent flows in the electrode of a Josephson junction used as a detector and produces a phi(0)/4 shift in its magnetic diffraction pattern. We have measured the statistics of the positive or negative sign shift occurring at the superconducting transition of such a junction. The randomness of the shift sign, the reproducibility of its magnitude and the possibility of achieving exact flux compensation upon field cooling: all these features show that 0-pi junctions behave as classical spins, just as magnetic nanoparticles with uniaxial anisotropy. △ Less

Submitted 19 January, 2005; originally announced January 2005.

Comments: 4 pages, 4 figures

Electrical Spin Injection in Multi-Wall carbon NanoTubes with transparent ferromagnetic contacts

Authors: S. Sahoo, T. Kontos, C. Schönenberger, C. Sürgers

Abstract: We report on electrical spin injection measurements on MWNTs . We use a ferromagnetic alloy Pd$_{1-x}$Ni$_{x}$ with x $\approx$ 0.7 which allows to obtain devices with resistances as low as 5.6 $kΩ$ at 300 $K$. The yield of device resistances below 100 $kΩ$, at 300 $K$, is around 50%. We measure at 2 $K$ a hysteretic magneto-resistance due to the magnetization reversal of the ferromagnetic leads… ▽ More We report on electrical spin injection measurements on MWNTs . We use a ferromagnetic alloy Pd$_{1-x}$Ni$_{x}$ with x $\approx$ 0.7 which allows to obtain devices with resistances as low as 5.6 $kΩ$ at 300 $K$. The yield of device resistances below 100 $kΩ$, at 300 $K$, is around 50%. We measure at 2 $K$ a hysteretic magneto-resistance due to the magnetization reversal of the ferromagnetic leads. The relative difference between the resistance in the antiparallel (AP) orientation and the parallel (P) orientation is about 2%. △ Less

Submitted 16 December, 2004; v1 submitted 24 November, 2004; originally announced November 2004.

Comments: submitted to APL version without figures version with figures available on http://www.unibas.ch/phys-meso/

Shot-noise and conductance measurements of transparent superconductor / two-dimensional electron gas junctions

Authors: B. -R. Choi, A. E. Hansen, T. Kontos, C. Hoffmann, S. Oberholzer, W. Belzig, C. Schoenenberger, T. Akazaki, H. Takayanagi

Abstract: We have measured the conductance and shot-noise of superconductor-normal metal (S-N) junctions between a Niobium (Nb) film and a 2-dimensional electron gas (2DEG), formed in an InAs-based semiconductor heterostructure. Adjacent to the junction, the 2DEG is shaped into a submicrometer beam-splitter. The current shot-noise measured through one arm of the beam-splitter is found to be enhanced due t… ▽ More We have measured the conductance and shot-noise of superconductor-normal metal (S-N) junctions between a Niobium (Nb) film and a 2-dimensional electron gas (2DEG), formed in an InAs-based semiconductor heterostructure. Adjacent to the junction, the 2DEG is shaped into a submicrometer beam-splitter. The current shot-noise measured through one arm of the beam-splitter is found to be enhanced due to Andreev reflection. Both noise and conductance measurements indicate that the Nb-2DEG interface is of high quality with a transparency approaching approx. 60-70 %. The present device can be seen as a quasi-ballistic S-N beam-splitter junction. △ Less

Submitted 25 October, 2004; originally announced October 2004.

Comments: full paper including figures can be retrieved from: http://www.unibas.ch/phys-meso/Research/Papers/2004/Shot-Noise-Yjunction.pdf

On the Kondo effect in carbon nanotubes at half halfing

Authors: B. Babic, T. Kontos, C. Schoenenberger

Abstract: In a single state of a quantum dot the Kondo effect arises due to the spin-degeneracy, which is present if the dot is occupied with one electron (N = 1). The eigenstates of a carbon nanotube quantum dot possess an additional orbital degeneracy leading to a four-fold shell pattern. This additional degeneracy increases the possibility for the Kondo effect to appear. We revisit the Kondo problem in… ▽ More In a single state of a quantum dot the Kondo effect arises due to the spin-degeneracy, which is present if the dot is occupied with one electron (N = 1). The eigenstates of a carbon nanotube quantum dot possess an additional orbital degeneracy leading to a four-fold shell pattern. This additional degeneracy increases the possibility for the Kondo effect to appear. We revisit the Kondo problem in metallic carbon nanotubes by linear and non-linear transport measurement in this regime, in which the four-fold pattern is present. We have analyzed the ground state of CNTs, which were grown by chemical vapor deposition, at filling N = 1, N = 2, and N = 3. Of particular interest is the half-filled shell, i.e. N = 2. In this case, the ground state is either a paired electron state or a state for which the singlet and triplet states are effectively degenerate, allowing in the latter case for the appearance of the Kondo effect. We deduce numbers for the effective missmatch d of the levels from perfect degeneracy and the exchange energy J. While d ~ 0.1 - 0.2 (in units of level spacing) is in agreement with previous work, the exchange term is found to be surprisingly small: J < 0.02. In addition we report on the observation of gaps, which in one case is seen at N = 3 and in another is present over an extended sequence of levels. △ Less

Submitted 8 July, 2004; originally announced July 2004.

Comments: full paper including figures at: http://www.unibas.ch/phys-meso/Research/Papers/2004/Kondo-4shell-SWNT.pdf