I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
01
Feb
2021
Quantum process inference for a single qubit Maxwell’s demon
While quantum measurement theories are built around density matrices and observables, the laws of thermodynamics are based on processes such as are used in heat engines and refrigerators.
The study of quantum thermodynamics fuses these two distinct paradigms. In this article, we highlight the usage of quantum process matrices as a unified language for describing thermodynamic processes in the quantum regime. We experimentally demonstrate this in the context of a quantum Maxwell’s demon, where two major quantities are commonly investigated; the average work extraction ⟨W⟩ and the efficacy γ which measures how efficiently the feedback operation uses the obtained information. Using the tool of quantum process matrices, we develop the optimal feedback protocols for these two quantities and experimentally investigate them in a superconducting circuit QED setup.
31
Jan
2021
A superconductor free of quasiparticles for seconds
Superconducting devices, based on the Cooper pairing of electrons, are of outstanding importance in existing and emergent technologies, ranging from radiation detectors to quantum computers.
Their performance is limited by spurious broken Cooper pairs also known as quasiparticle excitations. In state-of-the-art devices, the time-averaged number of quasiparticles can be on the order of one. However, realizing a superconductor with no excitations remains an outstanding challenge. Here, we experimentally demonstrate a superconductor completely free of quasiparticles up to seconds. The quasiparticle number on a mesoscopic superconductor is monitored in real time by measuring the charge tunneling to a normal metal contact. Quiet excitation-free periods are interrupted by random-in-time events, where one or several Cooper pairs break, followed by a burst of charge tunneling within a millisecond. Our results vindicate the opportunity to operate devices without quasiparticles with potentially improved performance. In addition, our present experiment probes the origins of nonequilibrium quasiparticles in it; the decay of the Cooper pair breaking rate over several weeks following the initial cooldown rules out processes arising from cosmic or long-lived radioactive sources.
28
Jan
2021
Phase sensitive Landau-Zener-Stückelberg interference in superconducting quantum circuit
Superconducting circuit quantum electrodynamics (QED) architecture composed of superconducting qubit and resonator is a powerful platform for exploring quantum physics and quantum information
processing. By employing techniques developed for superconducting quantum computing, we experimentally investigate phase-sensitive Landau-Zener-Stückelberg (LZS) interference phenomena in a circuit QED. Our experiments cover a large range of LZS transition parameters, and demonstrate the LZS induced Rabi-like oscillation as well as phase-dependent steady-state population.
27
Jan
2021
Quasi-phasematching in a poled traveling-wave Josephson parametric amplifier with three-wave mixing
We develop the concept of quasi-phasematching (QPM) in the recently proposed traveling-wave Josephson parametric amplifier (TWJPA) with three wave mixing (3WM). The amplifier is based
on a ladder transmission line consisting of flux-biased radio-frequency SQUIDs possessing non-centrosymmetric nonlinearity of χ(2)-type. Due to design with periodically inverted groups of SQUIDs, giving reversal of sign of this nonlinearity, QPM in 3WM process, ωp=ωs+ωi, for pump (ωp), signal (ωs), and idler (ωi) frequency is achieved. Modeling shows that the TWJPA bandwidth is sufficiently large (ca. 0.4ωp) and flat, while propagating unwanted modes, including ω2p=2ωp, ω+=ωp+ωs, ω−=2ωp−ωs, etc., is suppressed with the help of engineered dispersion.
20
Jan
2021
Observation of thermalization and information scrambling in a superconducting quantum processor
Understanding various phenomena in non-equilibrium dynamics of closed quantum many-body systems, such as quantum thermalization, information scrambling, and nonergodic dynamics, is
a crucial for modern physics. Using a ladder-type superconducting quantum processor, we perform analog quantum simulations of both the XX ladder and one-dimensional (1D) XX model. By measuring the dynamics of local observables, entanglement entropy and tripartite mutual information, we signal quantum thermalization and information scrambling in the XX ladder. In contrast, we show that the XX chain, as free fermions on a 1D lattice, fails to thermalize, and local information does not scramble in the integrable channel. Our experiments reveal ergodicity and scrambling in the controllable qubit ladder, and opens the door to further investigations on the thermodynamics and chaos in quantum many-body systems.
19
Jan
2021
Tunable Coupling Architecture for Fixed-frequency Transmons
Implementation of high-fidelity two-qubit operations is a key ingredient for scalable quantum error correction. In superconducting qubit architectures tunable buses have been explored
as a means to higher fidelity gates. However, these buses introduce new pathways for leakage. Here we present a modified tunable bus architecture appropriate for fixed-frequency qubits in which the adiabaticity restrictions on gate speed are reduced. We characterize this coupler on a range of two-qubit devices achieving a maximum gate fidelity of 99.85%. We further show the calibration is stable over one day.
17
Jan
2021
Coherent manipulation of an Andreev spin qubit
Two promising architectures for solid-state quantum information processing are electron spins in semiconductor quantum dots and the collective electromagnetic modes of superconducting
circuits. In some aspects, these two platforms are dual to one another: superconducting qubits are more easily coupled but are relatively large among quantum devices (∼mm), while electrostatically-confined electron spins are spatially compact (∼μm) but more complex to link. Here we combine beneficial aspects of both platforms in the Andreev spin qubit: the spin degree of freedom of an electronic quasiparticle trapped in the supercurrent-carrying Andreev levels of a Josephson semiconductor nanowire. We demonstrate coherent spin manipulation by combining single-shot circuit-QED readout and spin-flipping Raman transitions, finding a spin-flip time TS=17 μs and a spin coherence time T2E=52 ns. These results herald a new spin qubit with supercurrent-based circuit-QED integration and further our understanding and control of Andreev levels — the parent states of Majorana zero modes — in semiconductor-superconductor heterostructures.
14
Jan
2021
A reversed Kerr traveling wave parametric amplifier
Traveling wave parametric amplification in a nonlinear medium provides broadband quantum-noise limited gain and is a remarkable resource for the detection of electromagnetic radiation.
This nonlinearity is at the same time the key to the amplification phenomenon but also the cause of a fundamental limitation: poor phase matching between the signal and the pump. Here we solve this issue with a new phase matching mechanism based on the sign reversal of the Kerr nonlinearity. We present a novel traveling wave parametric amplifier composed of a chain of superconducting nonlinear asymmetric inductive elements (SNAILs) which allows this sign reversal when biased with the proper magnetic flux. Compared to previous state of the art phase matching approaches, this reversed Kerr phase matching mechanism avoids the presence of gaps in transmission, reduces gain ripples, and allows in situ tunability of the amplification band over an unprecedented wide range. Besides such notable advancements in the amplification performance, with direct applications to superconducting quantum computing, the in-situ tunability of the nonlinearity in traveling wave structures, with no counterpart in optics to the best of our knowledge, opens exciting experimental possibilities in the general framework of microwave quantum optics and single-photon detection.
Quantum computing with superconducting circuits in the picosecond regime
We discuss the realization of a universal set of ultrafast single- and two-qubit operations with superconducting quantum circuits and investigate the most relevant physical and technical
limitations that arise when pushing for faster and faster gates. With the help of numerical optimization techniques, we establish a fundamental bound on the minimal gate time, which is determined independently of the qubit design solely by its nonlinearity. In addition, important practical restrictions arise from the finite qubit transition frequency and the limited bandwidth of the control pulses. We show that for highly anharmonic flux qubits and commercially available control electronics, elementary single- and two-qubit operations can be implemented in about 100 picoseconds with residual gate errors below 10−4. Under the same conditions, we simulate the complete execution of a compressed version of Shor’s algorithm for factoring the number 15 in about one nanosecond. These results demonstrate that compared to state-of-the-art implementations with transmon qubits, a hundredfold increase in the speed of gate operations with superconducting circuits is still feasible.
13
Jan
2021
Magnetic-Field-Compatible Superconducting Transmon Qubit
We present a hybrid semiconductor-based superconducting qubit device which remains coherent at magnetic fields up to 1 T. The qubit transition frequency exhibits periodic oscillations
with magnetic field, consistent with interference effects due to the magnetic flux threading the cross section of the proximitized semiconductor nanowire junction. As induced superconductivity revives, additional coherent modes emerge at high magnetic fields, which we attribute to the interaction of the qubit and low-energy Andreev states.