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
11
Feb
2021
Removing leakage-induced correlated errors in superconducting quantum error correction
Quantum computing can become scalable through error correction, but logical error rates only decrease with system size when physical errors are sufficiently uncorrelated. During computation,
unused high energy levels of the qubits can become excited, creating leakage states that are long-lived and mobile. Particularly for superconducting transmon qubits, this leakage opens a path to errors that are correlated in space and time. Here, we report a reset protocol that returns a qubit to the ground state from all relevant higher level states. We test its performance with the bit-flip stabilizer code, a simplified version of the surface code for quantum error correction. We investigate the accumulation and dynamics of leakage during error correction. Using this protocol, we find lower rates of logical errors and an improved scaling and stability of error suppression with increasing qubit number. This demonstration provides a key step on the path towards scalable quantum computing.
10
Feb
2021
Observing a Topological Transition in Weak-Measurement-Induced Geometric Phases
Measurement plays a quintessential role in the control of quantum systems. Beyond initialization and readout which pertain to projective measurements, weak measurements in particular,
through their back-action on the system, may enable various levels of coherent control. The latter ranges from observing quantum trajectories to state dragging and steering. Furthermore, just like the adiabatic evolution of quantum states that is known to induce the Berry phase, sequential weak measurements may lead to path-dependent geometric phases. Here we measure the geometric phases induced by sequences of weak measurements and demonstrate a topological transition in the geometric phase controlled by measurement strength. This connection between weak measurement induced quantum dynamics and topological transitions reveals subtle topological features in measurement-based manipulation of quantum systems. Our protocol could be implemented for classes of operations (e.g. braiding) which are topological in nature. Furthermore, our results open new horizons for measurement-enabled quantum control of many-body topological states.
Error mitigation via stabilizer measurement emulation
Dynamical decoupling (DD) is a widely-used quantum control technique that takes advantage of temporal symmetries in order to partially suppress quantum errors without the need resource-intensive
error detection and correction protocols. This and other open-loop error mitigation techniques are critical for quantum information processing in the era of Noisy Intermediate-Scale Quantum technology. However, despite its utility, dynamical decoupling does not address errors which occur at unstructured times during a circuit, including certain commonly-encountered noise mechanisms such as cross-talk and imperfectly calibrated control pulses. Here, we introduce and demonstrate an alternative technique – `quantum measurement emulation‘ (QME) – that effectively emulates the measurement of stabilizer operators via stochastic gate application, leading to a first-order insensitivity to coherent errors. The QME protocol enables error suppression based on the stabilizer code formalism without the need for costly measurements and feedback, and it is particularly well-suited to discrete coherent errors that are challenging for DD to address.
Large-scale GHZ states through topologically protected zero-energy mode in a superconducting qutrit-resonator chain
We propose a superconducting qutrit-resonator chain model, and analytically work out forms of its topological edge states. The existence of the zero-energy mode enables to generate
a state transfer between two ends of the chain, accompanied with state flips of all intermediate qutrits, based on which N-body Greenberger-Horne-Zeilinger (GHZ) states can be generated with great robustness against disorders of coupling strengths. Three schemes of generating large-scale GHZ states are designed, each of which possesses the robustness against loss of qutrits or of resonators, meeting a certain performance requirement of different experimental devices. With experimentally feasible qutrit-resonator coupling strengths and available coherence times of qutrits and resonators, it has a potential to generate large-scale GHZ states among dozens of qutrits with a high fidelity. Further, we show the experimental consideration of generating GHZ states based on the circuit QED system, and discuss the prospect of realizing fast GHZ states.
04
Feb
2021
Dissipative stabilization of squeezing beyond \SI{3}{dB} in a microwave mode
While a propagating state of light can be generated with arbitrary squeezing by pumping a parametric resonator, the intra-resonator state is limited to 3 dB of squeezing. Here, we implement
a reservoir engineering method to surpass this limit using superconducting circuits. Two-tone pumping of a three-wave-mixing element implements an effective coupling to a squeezed bath which stabilizes a squeezed state inside the resonator. Using an ancillary superconducting qubit as a probe allows us to perform a direct Wigner tomography of the intra-resonator state. The raw measurement provides a lower bound on the squeezing at about 6.7±0.2 dB below the zero-point level. Further, we show how to correct for resonator evolution during the Wigner tomography and obtain a squeezing as high as 8.2±0.8 dB. Moreover, this level of squeezing is achieved with a purity of −0.4±0.4 dB.
02
Feb
2021
Exploiting dynamic quantum circuits in a quantum algorithm with superconducting qubits
The execution of quantum circuits on real systems has largely been limited to those which are simply time-ordered sequences of unitary operations followed by a projective measurement.
As hardware platforms for quantum computing continue to mature in size and capability, it is imperative to enable quantum circuits beyond their conventional construction. Here we break into the realm of dynamic quantum circuits on a superconducting-based quantum system. Dynamic quantum circuits involve not only the evolution of the quantum state throughout the computation, but also periodic measurements of a subset of qubits mid-circuit and concurrent processing of the resulting classical information within timescales shorter than the execution times of the circuits. Using noisy quantum hardware, we explore one of the most fundamental quantum algorithms, quantum phase estimation, in its adaptive version, which exploits dynamic circuits, and compare the results to a non-adaptive implementation of the same algorithm. We demonstrate that the version of real-time quantum computing with dynamic circuits can offer a substantial and tangible advantage when noise and latency are sufficiently low in the system, opening the door to a new realm of available algorithms on real quantum systems.
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.