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Interferometric Signatures of Zero Modes in Fractional Quantum Hall-Superconductor Heterostructures
Authors:
Junyi Cao,
Ramanjit Sohal,
Angela Kou,
Eduardo Fradkin
Abstract:
Fractional quantum Hall-superconductor (FQH-SC) heterostructures are predicted to host defect-bound parafermion zero modes (PZMs). We propose two related configurations to probe their fusion structure. In a Josephson junction coupled to a single quantum point contact (QPC), quasiparticle tunneling switches the defect fusion channel, producing stochastic transitions between branches of the fraction…
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Fractional quantum Hall-superconductor (FQH-SC) heterostructures are predicted to host defect-bound parafermion zero modes (PZMs). We propose two related configurations to probe their fusion structure. In a Josephson junction coupled to a single quantum point contact (QPC), quasiparticle tunneling switches the defect fusion channel, producing stochastic transitions between branches of the fractional Josephson spectrum. Embedding the junction in a two-QPC Fabry-Pérot interferometer provides a complementary probe. Weak zero mode tunneling produces fusion-channel-dependent interference while strong tunneling makes the interferometer probe a superposition of fusion channels and strongly suppresses the signal: in the topological limit it vanishes exactly, revealing the defects' non-Abelian nature even when the parent FQH state is Abelian.
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Submitted 19 August, 2026;
originally announced August 2026.
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Structural control of two-level defect density revealed by high-throughput correlative measurements of Josephson junctions
Authors:
Oliver F. Wolff,
Harshvardhan Mantry,
Rahim Raja,
Wei-Hsiang Peng,
Kaushik Singirikonda,
Seungkyun Lee,
Shishir Sudhaman,
Rafael Goncalves,
Pinshane Y. Huang,
Angela Kou,
Wolfgang Pfaff
Abstract:
Materials defects in Josephson junctions (JJs), often referred to as two-level systems (TLS), couple to superconducting qubits and are a critical bottleneck for scalable quantum processors. Despite their importance, understanding the microscopic sources of TLS and how to mitigate them has remained a major challenge. Here, we demonstrate a high-throughput, correlated approach to trace the microstru…
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Materials defects in Josephson junctions (JJs), often referred to as two-level systems (TLS), couple to superconducting qubits and are a critical bottleneck for scalable quantum processors. Despite their importance, understanding the microscopic sources of TLS and how to mitigate them has remained a major challenge. Here, we demonstrate a high-throughput, correlated approach to trace the microstructural origins of strongly-coupled TLS in Josephson circuits. We assembled a massive dataset of TLS across 6,000 Al/AlOx/Al JJs and more than 600 atomic resolution transmission electron microscopy images. We statistically link fabrication, microstructure, and TLS occurrence, revealing a strong correlation between Al electrode thickness, Al grain size, and TLS density. Correspondingly, we find a two-thirds reduction in TLS prompted by a change in electrode fabrication parameters. These results demonstrate a robust, data-driven methodology to understand and control defects in quantum circuits and pave the way for significantly reducing TLS density.
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Submitted 11 February, 2026;
originally announced February 2026.
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A scanning resonator for probing quantum coherent devices
Authors:
Jared Gibson,
Zhanzhi Jiang,
Angela Kou
Abstract:
Superconducting resonators with high quality factors are extremely sensitive detectors of the complex impedance of materials and devices coupled to them. This capability has been used to measure losses in multiple different materials and, in the case of circuit quantum electrodynamics (circuit QED), has been used to measure the coherent evolution of multiple different types of qubits. Here, we rep…
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Superconducting resonators with high quality factors are extremely sensitive detectors of the complex impedance of materials and devices coupled to them. This capability has been used to measure losses in multiple different materials and, in the case of circuit quantum electrodynamics (circuit QED), has been used to measure the coherent evolution of multiple different types of qubits. Here, we report on the implementation of a scanning resonator for probing quantum coherent devices. Our scanning setup enables tunable coherent coupling to systems of interest without the need for fabricating on-chip superconducting resonators. We measure the internal quality factor of our resonator sensor in the single-photon regime to be > 10000 and demonstrate capacitive imaging using our sensor with zeptoFarad sensitivity and micron spatial resolution at milliKelvin temperatures. We then use our setup to characterize the energy spectrum and coherence times of multiple transmon qubits with no on-chip readout circuitry. Our work introduces a new tool for using circuit QED to measure existing and proposed qubit platforms.
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Submitted 27 June, 2025;
originally announced June 2025.
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Fluxonium as a control qubit for bosonic quantum information
Authors:
Ke Nie,
J. Nofear Bradford,
Supriya Mandal,
Aayam Bista,
Wolfgang Pfaff,
Angela Kou
Abstract:
Bosonic codes in superconducting resonators are a hardware-efficient avenue for quantum error correction and benefit from favorable error hierarchies provided by long-lived cavities compared to typical superconducting qubits. The required coupling to an ancillary control qubit, however, can negate these benefits by inducing highly detrimental effects such as excess decoherence and undesired nonlin…
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Bosonic codes in superconducting resonators are a hardware-efficient avenue for quantum error correction and benefit from favorable error hierarchies provided by long-lived cavities compared to typical superconducting qubits. The required coupling to an ancillary control qubit, however, can negate these benefits by inducing highly detrimental effects such as excess decoherence and undesired nonlinearities. An important question is thus whether a cavity-qubit coupling can be realized that offers readout and control capabilities without spoiling the cavity. Here, motivated by its long lifetime and design flexibility of its Hamiltonian, we experimentally investigate the fluxonium as a control qubit for superconducting cavities. We couple a fluxonium qubit to a superconducting resonator in the strong-dispersive regime and use it to measure the coherence and inherited nonlinearities of the resonator. We then demonstrate universal control by preparing and characterizing resonator Fock states and their superpositions, with fidelities limited by resonator decay in our planar prototype device. Finally, we use the predictability of the resonator's inherited nonlinearities to show numerically that the fluxonium can reach cavity-coupling regimes that eliminate undesirable cavity nonlinearities. These results demonstrate the potential of the fluxonium as a high-performance bosonic control qubit for superconducting cavities.
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Submitted 10 June, 2025; v1 submitted 29 May, 2025;
originally announced May 2025.
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Operation of a high-frequency, phase-slip qubit
Authors:
Cheeranjeev Purmessur,
Kaicheung Chow,
Bernard van Heck,
Angela Kou
Abstract:
Aluminum-based Josephson junctions are currently the main sources of nonlinearity for control and manipulation of superconducting qubits. A constriction-based junction provides an alternative source of nonlinearity that promises new types of protected qubits and the possibility of high-temperature and high-frequency operation through the use of superconductors with larger energy gaps. Junctions ma…
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Aluminum-based Josephson junctions are currently the main sources of nonlinearity for control and manipulation of superconducting qubits. A constriction-based junction provides an alternative source of nonlinearity that promises new types of protected qubits and the possibility of high-temperature and high-frequency operation through the use of superconductors with larger energy gaps. Junctions made from such superconductors have been challenging to incorporate into superconducting qubits because of difficulty controlling junction parameters and have had extremely low lifetimes, which limited their utility. Here we demonstrate that junctions made using titanium nitride (TiN) are a promising and controllable qubit platform. We use TiN junctions to build superconducting qubits based on quantum phase slips through the junction. We operate the qubit at zero flux where the qubit frequency (~17 GHz) is mainly determined by the inductance of the qubit. We perform readout and coherent control of the superconducting qubit, and measure qubit lifetimes >60 $μ$s. Finally, we demonstrate operation of the qubit at temperatures exceeding 300 mK. Our results add the TiN-based junction as a tool for superconducting quantum information processing and opens avenues for new classes of superconducting qubits.
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Submitted 9 July, 2026; v1 submitted 10 February, 2025;
originally announced February 2025.
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Readout-induced leakage of the fluxonium qubit
Authors:
Aayam Bista,
Matthew Thibodeau,
Ke Nie,
Kaicheung Chow,
Bryan K. Clark,
Angela Kou
Abstract:
Dispersive readout is widely used to perform high-fidelity measurement of superconducting qubits. Much work has been focused on the qubit readout fidelity, which depends on the achievable signal-to-noise ratio and the qubit relaxation time. As groups have pushed to increase readout fidelity by increasing readout photon number, dispersive readout has been shown to strongly affect the post-measureme…
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Dispersive readout is widely used to perform high-fidelity measurement of superconducting qubits. Much work has been focused on the qubit readout fidelity, which depends on the achievable signal-to-noise ratio and the qubit relaxation time. As groups have pushed to increase readout fidelity by increasing readout photon number, dispersive readout has been shown to strongly affect the post-measurement qubit state. Such effects hinder the effectiveness of quantum error correction, which requires measurements that both have high readout fidelity and are quantum non-demolition (QND). Here, we experimentally investigate non-QND effects in the fluxonium. We map out the state evolution of fluxonium qubits in the presence of resonator photons and observe that these photons induce transitions in the fluxonium both within and outside the qubit subspace. We numerically model our system and find that transitions to higher-excited states and coupling to an external spurious mode are necessary to explain observed non-QND effects.
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Submitted 16 December, 2025; v1 submitted 29 January, 2025;
originally announced January 2025.
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Parametrically-controlled microwave-photonic interface for the fluxonium
Authors:
Ke Nie,
Aayam Bista,
Kaicheung Chow,
Wolfgang Pfaff,
Angela Kou
Abstract:
Converting quantum information from stationary qubits to traveling photons enables both fast qubit initialization and efficient generation of flying qubits for redistribution of quantum information. This conversion can be performed using cavity-sideband transitions. In the fluxonium, however, direct cavity-sideband transitions are forbidden due to parity symmetry. Here we circumvent this parity se…
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Converting quantum information from stationary qubits to traveling photons enables both fast qubit initialization and efficient generation of flying qubits for redistribution of quantum information. This conversion can be performed using cavity-sideband transitions. In the fluxonium, however, direct cavity-sideband transitions are forbidden due to parity symmetry. Here we circumvent this parity selection rule by using a three-wave mixing element to couple the fluxonium to a resonator. We experimentally demonstrate a scheme for interfacing the fluxonium with traveling photons through microwave-induced parametric conversion. We perform fast reset on the fluxonium qubit, initializing it with >95% ground-state population. We then implement controlled release and temporal shaping of a flying photon, useful for quantum state transfer and remote entanglement. The simplicity and flexibility of our demonstrated scheme enables fluxonium-based remote entanglement architectures.
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Submitted 13 December, 2024; v1 submitted 17 April, 2024;
originally announced April 2024.
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The Floquet Fluxonium Molecule: Driving Down Dephasing in Coupled Superconducting Qubits
Authors:
Matthew Thibodeau,
Angela Kou,
Bryan K. Clark
Abstract:
High-coherence qubits, which can store and manipulate quantum states for long times with low error rates, are necessary building blocks for quantum computers. Here we propose a driven superconducting erasure qubit, the Floquet fluxonium molecule, which minimizes bit-flip rates through disjoint support of its qubit states and suppresses phase flips by a novel second-order insensitivity to flux-nois…
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High-coherence qubits, which can store and manipulate quantum states for long times with low error rates, are necessary building blocks for quantum computers. Here we propose a driven superconducting erasure qubit, the Floquet fluxonium molecule, which minimizes bit-flip rates through disjoint support of its qubit states and suppresses phase flips by a novel second-order insensitivity to flux-noise dephasing. We estimate the bit-flip, phase-flip, and erasure rates through numerical simulations, with predicted coherence times of approximately 50 ms in the computational subspace and erasure lifetimes of about 500 $μ$s. We also present a protocol for performing high-fidelity single-qubit rotation gates via additional flux modulation, on timescales of roughly 500 ns, and propose a scheme for erasure detection and logical readout. Our results demonstrate the utility of drives for building new qubits that can outperform their static counterparts.
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Submitted 7 November, 2024; v1 submitted 16 January, 2024;
originally announced January 2024.
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Signatures of Parafermion Zero Modes in Fractional Quantum Hall-Superconductor Heterostructures
Authors:
Junyi Cao,
Angela Kou,
Eduardo Fradkin
Abstract:
Parafermion zero modes can arise in hybrid structures composed of $ν=1/m$ fractional quantum Hall edges proximitized with an s-wave superconductor. Here we consider parafermion and Cooper pair tunneling, and backscattering in a junction formed in such hybrid structures. We find that the $4πm$ periodicity due to parafermion-only tunneling reduces, in the presence of backscattering, to $4π$-periodic…
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Parafermion zero modes can arise in hybrid structures composed of $ν=1/m$ fractional quantum Hall edges proximitized with an s-wave superconductor. Here we consider parafermion and Cooper pair tunneling, and backscattering in a junction formed in such hybrid structures. We find that the $4πm$ periodicity due to parafermion-only tunneling reduces, in the presence of backscattering, to $4π$-periodic at zero temperature and $2π$-periodic at finite temperature unless the fermion parity is fixed. Nevertheless, a clear signature of parafermion tunneling remains in the shape of the current-phase relation.
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Submitted 27 March, 2024; v1 submitted 25 September, 2023;
originally announced September 2023.
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Direct manipulation of a superconducting spin qubit strongly coupled to a transmon qubit
Authors:
Marta Pita-Vidal,
Arno Bargerbos,
Rok Žitko,
Lukas J. Splitthoff,
Lukas Grünhaupt,
Jaap J. Wesdorp,
Yu Liu,
Leo P. Kouwenhoven,
Ramón Aguado,
Bernard van Heck,
Angela Kou,
Christian Kraglund Andersen
Abstract:
Spin qubits in semiconductors are currently one of the most promising architectures for quantum computing. However, they face challenges in realizing multi-qubit interactions over extended distances. Superconducting spin qubits provide a promising alternative by encoding a qubit in the spin degree of freedom of an Andreev level. Such an Andreev spin qubit could leverage the advantages of circuit q…
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Spin qubits in semiconductors are currently one of the most promising architectures for quantum computing. However, they face challenges in realizing multi-qubit interactions over extended distances. Superconducting spin qubits provide a promising alternative by encoding a qubit in the spin degree of freedom of an Andreev level. Such an Andreev spin qubit could leverage the advantages of circuit quantum electrodynamic, enabled by an intrinsic spin-supercurrent coupling. The first realization of an Andreev spin qubit encoded the qubit in the excited states of a semiconducting weak-link, leading to frequent decay out of the computational subspace. Additionally, rapid qubit manipulation was hindered by the need for indirect Raman transitions. Here, we exploit a different qubit subspace, using the spin-split doublet ground state of an electrostatically-defined quantum dot Josephson junction with large charging energy. Additionally, we use a magnetic field to enable direct spin manipulation over a frequency range of 10 GHz. Using an all-electric microwave drive we achieve Rabi frequencies exceeding 200 MHz. We furthermore embed the Andreev spin qubit in a superconducting transmon qubit, demonstrating strong coherent qubit-qubit coupling. These results are a crucial step towards a hybrid architecture that combines the beneficial aspects of both superconducting and semiconductor qubits.
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Submitted 28 August, 2022; v1 submitted 22 August, 2022;
originally announced August 2022.
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Spectroscopy of spin-split Andreev levels in a quantum dot with superconducting leads
Authors:
Arno Bargerbos,
Marta Pita-Vidal,
Rok Žitko,
Lukas J. Splitthoff,
Lukas Grünhaupt,
Jaap J. Wesdorp,
Yu Liu,
Leo P. Kouwenhoven,
Ramón Aguado,
Christian Kraglund Andersen,
Angela Kou,
Bernard van Heck
Abstract:
We use a hybrid superconductor-semiconductor transmon device to perform spectroscopy of a quantum dot Josephson junction tuned to be in a spin-1/2 ground state with an unpaired quasiparticle. Due to spin-orbit coupling, we resolve two flux-sensitive branches in the transmon spectrum, depending on the spin of the quasi-particle. A finite magnetic field shifts the two branches in energy, favoring on…
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We use a hybrid superconductor-semiconductor transmon device to perform spectroscopy of a quantum dot Josephson junction tuned to be in a spin-1/2 ground state with an unpaired quasiparticle. Due to spin-orbit coupling, we resolve two flux-sensitive branches in the transmon spectrum, depending on the spin of the quasi-particle. A finite magnetic field shifts the two branches in energy, favoring one spin state and resulting in the anomalous Josephson effect. We demonstrate the excitation of the direct spin-flip transition using all-electrical control. Manipulation and control of the spin-flip transition enable the future implementation of charging energy protected Andreev spin qubits.
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Submitted 12 September, 2022; v1 submitted 19 August, 2022;
originally announced August 2022.
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Singlet-doublet transitions of a quantum dot Josephson junction detected in a transmon circuit
Authors:
Arno Bargerbos,
Marta Pita-Vidal,
Rok Žitko,
Jesús Ávila,
Lukas J. Splitthoff,
Lukas Grünhaupt,
Jaap J. Wesdorp,
Christian K. Andersen,
Yu Liu,
Leo P. Kouwenhoven,
Ramón Aguado,
Angela Kou,
Bernard van Heck
Abstract:
We realize a hybrid superconductor-semiconductor transmon device in which the Josephson effect is controlled by a gate-defined quantum dot in an InAs/Al nanowire. Microwave spectroscopy of the transmon's transition spectrum allows us to probe the ground state parity of the quantum dot as a function of gate voltages, external magnetic flux, and magnetic field applied parallel to the nanowire. The m…
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We realize a hybrid superconductor-semiconductor transmon device in which the Josephson effect is controlled by a gate-defined quantum dot in an InAs/Al nanowire. Microwave spectroscopy of the transmon's transition spectrum allows us to probe the ground state parity of the quantum dot as a function of gate voltages, external magnetic flux, and magnetic field applied parallel to the nanowire. The measured parity phase diagram is in agreement with that predicted by a single-impurity Anderson model with superconducting leads. Through continuous time monitoring of the circuit we furthermore resolve the quasiparticle dynamics of the quantum dot Josephson junction across the phase boundaries. Our results can facilitate the realization of semiconductor-based $0-π$ qubits and Andreev qubits.
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Submitted 25 February, 2022;
originally announced February 2022.
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Observation of vanishing charge dispersion of a nearly-open superconducting island
Authors:
Arno Bargerbos,
Willemijn Uilhoorn,
Chung-Kai Yang,
Peter Krogstrup,
Leo P. Kouwenhoven,
Gijs de Lange,
Bernard van Heck,
Angela Kou
Abstract:
Isolation from the environment determines the extent to which charge is confined on an island, which manifests as Coulomb oscillations such as charge dispersion. We investigate the charge dispersion of a nanowire transmon hosting a quantum dot in the junction. We observe rapid suppression of the charge dispersion with increasing junction transparency, consistent with the predicted scaling law whic…
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Isolation from the environment determines the extent to which charge is confined on an island, which manifests as Coulomb oscillations such as charge dispersion. We investigate the charge dispersion of a nanowire transmon hosting a quantum dot in the junction. We observe rapid suppression of the charge dispersion with increasing junction transparency, consistent with the predicted scaling law which incorporates two branches of the Josephson potential. We find improved qubit coherence times at the point of highest suppression, suggesting novel approaches for building charge-insensitive qubits.
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Submitted 28 February, 2020; v1 submitted 22 November, 2019;
originally announced November 2019.
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A gate-tunable, field-compatible fluxonium
Authors:
Marta Pita-Vidal,
Arno Bargerbos,
Chung-Kai Yang,
David J. van Woerkom,
Wolfgang Pfaff,
Nadia Haider,
Peter Krogstrup,
Leo P. Kouwenhoven,
Gijs de Lange,
Angela Kou
Abstract:
Circuit quantum electrodynamics, where photons are coherently coupled to artificial atoms built with superconducting circuits, has enabled the investigation and control of macroscopic quantum-mechanical phenomena in superconductors. Recently, hybrid circuits incorporating semiconducting nanowires and other electrostatically-gateable elements have provided new insights into mesoscopic superconducti…
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Circuit quantum electrodynamics, where photons are coherently coupled to artificial atoms built with superconducting circuits, has enabled the investigation and control of macroscopic quantum-mechanical phenomena in superconductors. Recently, hybrid circuits incorporating semiconducting nanowires and other electrostatically-gateable elements have provided new insights into mesoscopic superconductivity. Extending the capabilities of hybrid flux-based circuits to work in magnetic fields would be especially useful both as a probe of spin-polarized Andreev bound states and as a possible platform for topological qubits. The fluxonium is particularly suitable as a readout circuit for topological qubits due to its unique persistent-current based eigenstates. In this Letter, we present a magnetic-field compatible hybrid fluxonium with an electrostatically-tuned semiconducting nanowire as its non-linear element. We operate the fluxonium in magnetic fields up to 1T and use it to observe the $\varphi_0$-Josephson effect. This combination of gate-tunability and field-compatibility opens avenues for the exploration and control of spin-polarized phenomena using superconducting circuits and enables the use of the fluxonium as a readout device for topological qubits.
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Submitted 1 October, 2020; v1 submitted 17 October, 2019;
originally announced October 2019.
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Superconducting circuit protected by two-Cooper-pair tunneling
Authors:
W. C. Smith,
A. Kou,
X. Xiao,
U. Vool,
M. H. Devoret
Abstract:
We present a protected superconducting qubit based on an effective circuit element that only allows pairs of Cooper pairs to tunnel. These dynamics give rise to a nearly degenerate ground state manifold indexed by the parity of tunneled Cooper pairs. We show that, when the circuit element is shunted by a large capacitance, this manifold can be used as a logical qubit that we expect to be insensiti…
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We present a protected superconducting qubit based on an effective circuit element that only allows pairs of Cooper pairs to tunnel. These dynamics give rise to a nearly degenerate ground state manifold indexed by the parity of tunneled Cooper pairs. We show that, when the circuit element is shunted by a large capacitance, this manifold can be used as a logical qubit that we expect to be insensitive to multiple relaxation and dephasing mechanisms.
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Submitted 21 January, 2020; v1 submitted 3 May, 2019;
originally announced May 2019.
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Gated conditional displacement readout of superconducting qubits
Authors:
S. Touzard,
A. Kou,
N. E. Frattini,
V. V. Sivak,
S. Puri,
A. Grimm,
L. Frunzio,
S. Shankar,
M. H. Devoret
Abstract:
We have realized a new interaction between superconducting qubits and a readout cavity that results in the displacement of a coherent state in the cavity, conditioned on the state of the qubit. This conditional state, when it reaches the cavity-following, phase-sensitive amplifier, matches its measured observable, namely the in-phase quadrature. In a setup where several qubits are coupled to the s…
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We have realized a new interaction between superconducting qubits and a readout cavity that results in the displacement of a coherent state in the cavity, conditioned on the state of the qubit. This conditional state, when it reaches the cavity-following, phase-sensitive amplifier, matches its measured observable, namely the in-phase quadrature. In a setup where several qubits are coupled to the same readout resonator, we show it is possible to measure the state of a target qubit with minimal dephasing of the other qubits. Our results suggest novel directions for faster readout of superconducting qubits and implementations of bosonic quantum error-correcting codes.
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Submitted 18 September, 2018;
originally announced September 2018.
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Driving forbidden transitions in the fluxonium artificial atom
Authors:
U. Vool,
A. Kou,
W. C. Smith,
N. E. Frattini,
K. Serniak,
P. Reinhold,
I. M. Pop,
S. Shankar,
L. Frunzio,
S. M. Girvin,
M. H. Devoret
Abstract:
Atomic systems display a rich variety of quantum dynamics due to the different possible symmetries obeyed by the atoms. These symmetries result in selection rules that have been essential for the quantum control of atomic systems. Superconducting artificial atoms are mainly governed by parity symmetry. Its corresponding selection rule limits the types of quantum systems that can be built using ele…
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Atomic systems display a rich variety of quantum dynamics due to the different possible symmetries obeyed by the atoms. These symmetries result in selection rules that have been essential for the quantum control of atomic systems. Superconducting artificial atoms are mainly governed by parity symmetry. Its corresponding selection rule limits the types of quantum systems that can be built using electromagnetic circuits at their optimal coherence operation points ("sweet spots"). Here, we use third-order nonlinear coupling between the artificial atom and its readout resonator to drive transitions forbidden by the parity selection rule for linear coupling to microwave radiation. A Lambda-type system emerges from these newly accessible transitions, implemented here in the fluxonium artificial atom coupled to its "antenna" resonator. We demonstrate coherent manipulation of the fluxonium artificial atom at its sweet spot by stimulated Raman transitions. This type of transition enables the creation of new quantum operations, such as the control and readout of physically protected artificial atoms.
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Submitted 31 May, 2018; v1 submitted 20 November, 2017;
originally announced November 2017.
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Simultaneous monitoring of fluxonium qubits in a waveguide
Authors:
A. Kou,
W. C. Smith,
U. Vool,
I. M. Pop,
K. M. Sliwa,
M. Hatridge,
L. Frunzio,
M. H. Devoret
Abstract:
Most quantum-error correcting codes assume that the decoherence of each physical qubit is independent of the decoherence of any other physical qubit. We can test the validity of this assumption in an experimental setup where a microwave feedline couples to multiple qubits by examining correlations between the qubits. Here, we investigate the correlations between fluxonium qubits located in a singl…
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Most quantum-error correcting codes assume that the decoherence of each physical qubit is independent of the decoherence of any other physical qubit. We can test the validity of this assumption in an experimental setup where a microwave feedline couples to multiple qubits by examining correlations between the qubits. Here, we investigate the correlations between fluxonium qubits located in a single waveguide. Despite being in a wide-bandwidth electromagnetic environment, the qubits have measured relaxation times in excess of 100 us. We use cascaded Josephson parametric amplifiers to measure the quantum jumps of two fluxonium qubits simultaneously. No correlations are observed between the relaxation times of the two fluxonium qubits, which indicates that the sources of relaxation are local to each qubit. Our architecture can easily be scaled to monitor larger numbers of qubits.
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Submitted 17 September, 2017; v1 submitted 16 May, 2017;
originally announced May 2017.
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A fluxonium-based artificial molecule with a tunable magnetic moment
Authors:
A. Kou,
W. C. Smith,
U. Vool,
R. T. Brierley,
H. Meier,
L. Frunzio,
S. M. Girvin,
L. I. Glazman,
M. H. Devoret
Abstract:
Engineered quantum systems allow us to observe phenomena that are not easily accessible naturally. The LEGO-like nature of superconducting circuits makes them particularly suited for building and coupling artificial atoms. Here, we introduce an artificial molecule, composed of two strongly coupled fluxonium atoms, which possesses a tunable magnetic moment. Using an applied external flux, one can t…
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Engineered quantum systems allow us to observe phenomena that are not easily accessible naturally. The LEGO-like nature of superconducting circuits makes them particularly suited for building and coupling artificial atoms. Here, we introduce an artificial molecule, composed of two strongly coupled fluxonium atoms, which possesses a tunable magnetic moment. Using an applied external flux, one can tune the molecule between two regimes: one in which the ground-excited state manifold has a magnetic dipole moment and one in which the ground-excited state manifold has only a magnetic quadrupole moment. By varying the applied external flux, we find the coherence of the molecule to be limited by local flux noise. The ability to engineer and control artificial molecules paves the way for building more complex circuits for protected qubits and quantum simulation.
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Submitted 4 December, 2016; v1 submitted 4 October, 2016;
originally announced October 2016.
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Quantization of inductively-shunted superconducting circuits
Authors:
W. C. Smith,
A. Kou,
U. Vool,
I. M. Pop,
L. Frunzio,
R. J. Schoelkopf,
M. H. Devoret
Abstract:
We present a method for calculating the energy levels of superconducting circuits that contain highly anharmonic, inductively-shunted modes with arbitrarily strong coupling. Our method starts by calculating the normal modes of the linearized circuit and proceeds with numerical diagonalization in this basis. As an example, we analyze the Hamiltonian of a fluxonium qubit inductively coupled to a rea…
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We present a method for calculating the energy levels of superconducting circuits that contain highly anharmonic, inductively-shunted modes with arbitrarily strong coupling. Our method starts by calculating the normal modes of the linearized circuit and proceeds with numerical diagonalization in this basis. As an example, we analyze the Hamiltonian of a fluxonium qubit inductively coupled to a readout resonator. While elementary, this simple example is nontrivial because it cannot be efficiently treated by the method known as "black-box quantization," numerical diagonalization in the bare harmonic oscillator basis, or perturbation theory. Calculated spectra are compared to measured spectroscopy data, demonstrating excellent quantitative agreement between theory and experiment.
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Submitted 20 October, 2016; v1 submitted 4 February, 2016;
originally announced February 2016.
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Signatures of quantum phase transitions in the dynamic response of fluxonium qubit chains
Authors:
Hendrik Meier,
R. T. Brierley,
Angela Kou,
S. M. Girvin,
Leonid I. Glazman
Abstract:
We evaluate the microwave admittance of a one-dimensional chain of fluxonium qubits coupled by shared inductors. Despite its simplicity, this system exhibits a rich phase diagram. A critical applied magnetic flux separates a homogeneous ground state from a phase with a ground state exhibiting inhomogeneous persistent currents. Depending on the parameters of the array, the phase transition may be a…
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We evaluate the microwave admittance of a one-dimensional chain of fluxonium qubits coupled by shared inductors. Despite its simplicity, this system exhibits a rich phase diagram. A critical applied magnetic flux separates a homogeneous ground state from a phase with a ground state exhibiting inhomogeneous persistent currents. Depending on the parameters of the array, the phase transition may be a conventional continuous one, or of a commensurate-incommensurate nature. Furthermore, quantum fluctuations affect the transition and possibly lead to the presence of gapless "floating phases". The signatures of the soft modes accompanying the transitions appear as a characteristic frequency dependence of the dissipative part of admittance.
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Submitted 24 August, 2015; v1 submitted 22 July, 2015;
originally announced July 2015.
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Confining the state of light to a quantum manifold by engineered two-photon loss
Authors:
Zaki Leghtas,
Steven Touzard,
Ioan M. Pop,
Angela Kou,
Brian Vlastakis,
Andrei Petrenko,
Katrina M. Sliwa,
Anirudh Narla,
Shyam Shankar,
Michael J. Hatridge,
Matthew Reagor,
Luigi Frunzio,
Robert J. Schoelkopf,
Mazyar Mirrahimi,
Michel H. Devoret
Abstract:
Physical systems usually exhibit quantum behavior, such as superpositions and entanglement, only when they are sufficiently decoupled from a lossy environment. Paradoxically, a specially engineered interaction with the environment can become a resource for the generation and protection of quantum states. This notion can be generalized to the confinement of a system into a manifold of quantum state…
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Physical systems usually exhibit quantum behavior, such as superpositions and entanglement, only when they are sufficiently decoupled from a lossy environment. Paradoxically, a specially engineered interaction with the environment can become a resource for the generation and protection of quantum states. This notion can be generalized to the confinement of a system into a manifold of quantum states, consisting of all coherent superpositions of multiple stable steady states. We have experimentally confined the state of a harmonic oscillator to the quantum manifold spanned by two coherent states of opposite phases. In particular, we have observed a Schrodinger cat state spontaneously squeeze out of vacuum, before decaying into a classical mixture. This was accomplished by designing a superconducting microwave resonator whose coupling to a cold bath is dominated by photon pair exchange. This experiment opens new avenues in the fields of nonlinear quantum optics and quantum information, where systems with multi-dimensional steady state manifolds can be used as error corrected logical qubits.
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Submitted 15 December, 2014;
originally announced December 2014.
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Coulomb Oscillations in Antidots in the Integer and Fractional Quantum Hall Regimes
Authors:
A. Kou,
C. M. Marcus,
L. N. Pfeiffer,
K. W. West
Abstract:
We report measurements of resistance oscillations in micron-scale antidots in both the integer and fractional quantum Hall regimes. In the integer regime, we conclude that oscillations are of the Coulomb type from the scaling of magnetic field period with the number of edges bound to the antidot. Based on both gate-voltage and field periods, we find at filling factor ν = 2 a tunneling charge of e…
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We report measurements of resistance oscillations in micron-scale antidots in both the integer and fractional quantum Hall regimes. In the integer regime, we conclude that oscillations are of the Coulomb type from the scaling of magnetic field period with the number of edges bound to the antidot. Based on both gate-voltage and field periods, we find at filling factor ν = 2 a tunneling charge of e and two charged edges. Generalizing this picture to the fractional regime, we find (again, based on field and gate-voltage periods) at ν = 2/3 a tunneling charge of (2/3)e and a single charged edge.
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Submitted 7 January, 2012;
originally announced January 2012.