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
17
Mrz
2024
Dynamics and Resonance Fluorescence from a Superconducting Artificial Atom Doubly Driven by Quantized and Classical Fields
We report an experimental demonstration of resonance fluorescence in a two-level superconducting artificial atom under two driving fields coupled to a detuned cavity. One of the fields
is classical and the other is varied from quantum (vacuum fluctuations) to classical one by controlling the photon number inside the cavity. The device consists of a transmon qubit strongly coupled to a one-dimensional transmission line and a coplanar waveguide resonator. We observe a sideband anti-crossing and asymmetry in the emission spectra of the system through a one-dimensional transmission line, which is fundamentally different from the weak coupling case. By changing the photon number inside the cavity, the emission spectrum of our doubly driven system approaches to the case when the atom is driven by two classical bichromatic fields. We also measure the dynamical evolution of the system through the transmission line and study the properties of the first-order correlation function, Rabi oscillations and energy relaxation in the system. The study of resonance fluorescence from an atom driven by two fields promotes understanding decoherence in superconducting quantum circuits and may find applications in superconducting quantum computing and quantum networks.
14
Mrz
2024
Engineering nonequilibrium steady states through Floquet Liouvillians
We experimentally study the transient dynamics of a dissipative superconducting qubit under periodic drive towards its nonequilibrium steady states. The corresponding stroboscopic evolution,
given by the qubit states at times equal to integer multiples of the drive period, is determined by a (generically non-Hermitian) Floquet Liouvillian. The drive period controls both the transients across its non-Hermitian degeneracies and the resulting nonequilibrium steady states. These steady states can exhibit higher purity compared to those achieved with a constant drive. We further study the dependence of the steady states on the direction of parameter variation and relate these findings to the recent studies of dynamically encircling exceptional points. Our work provides a new approach to control non-Hermiticity in dissipative quantum systems and presents a new paradigm in quantum state preparation and stabilization.
13
Mrz
2024
Gauge invariant quantization for circuits including Josephson junctions
Recently, a new theory of superconductivity has been put forward that attributes the origin of superconductivity to the appearance of a non-trivial Berry connection from many-electron
wave functions.
This theory reproduces the major results of the BCS theory with conserving the particle number, and predicts the single-electron supercurrent tunneling across the Josephson junction with keeping the correct Josephson relation. We re-examine the quantization of superconducting qubit circuits by taking into account the above development, and show that the dynamical variables used in the standard theory, the flux nodes relating to the voltage, should be replaced by those relating to the electromagnetic vector potential. The fact that the Josephson junction tunneling allows the single-electron supercurrent tunneling is the reason for the existence of excited single electrons in superconducting qubits with Josephson junctions. We predict that it will be avoided by weakening the coupling between two superconductors in the Josephson junction.
Coherent competition and control between three-wave mixing and four-wave mixing in superconducting circuits
Exploring intermixing and interplay between different frequency-mixing processes has always been one of the interesting subjects at the interface of nonlinear optics with quantum optics.
Here we investigate coherent competition and control between three-wave mixing (TWM) and four-wave mixing (FWM) in a cyclic three-level superconducting quantum system. In the weak control-field regime, strong competition leads to an alternating oscillation between TWM and FWM signals and this oscillation is a signature of strong energy exchange between these two nonlinear processes. In particular, such oscillation is absent from conventional multi-wave mixing in atomic systems. Surprisingly, synchronous TWM and FWM processes are demonstrated in the strong control-field regime and, at the same time, their efficiencies can be as high as 40% and 45%, respectively. Our study shows that these competitive behaviors between TWM and FWM can be manipulated by tuning the control-field intensity.
12
Mrz
2024
Anomalous magnetic flux via junction twist-angle in a triplet-superconducting transmon qubit
Superconducting transmon qubits with strong anharmonicity and insensitivity to offset charge are highly desirable for low-error implementation. In this work we propose a c-axis junction,
comprising triplet superconductors, and set at a relative twist angle. Invoking spin-orbit coupling and spin polarization, which are known to occur in the material platform of choice, we examine the resulting transmon Hamiltonian. This junction allows for direct control of the single and double Cooper pair tunneling strength, and most remarkably, an anomalous magnetic flux — i.e. a phase offset equivalent to magnetic flux, yet in zero magnetic field. Having control over these three parameters — single and double pair tunneling and anomalous flux — allows for optimal design of the transmon qubit. Interestingly, in this architecture, the anomalous flux is determined by the twist angle of the junction, thereby offering a novel zero-field functionality. Our key results rely on symmetry arguments, for concreteness we demonstrate the implementation of our concept using a model of moiré graphene-based c-axis junctions.
Designing high-fidelity two-qubit gates between fluxonium qubits
We take a bottom-up, first-principles approach to design a two-qubit gate between fluxonium qubits for minimal error, speed, and control simplicity. Our proposed architecture consists
of two fluxoniums coupled via a linear resonator. Using a linear coupler introduces the possibility of material optimization for suppressing its loss, enables efficient driving of state-selective transitions through its large charge zero point fluctuation, reduces sensitivity to junction aging, and partially mitigates coherent coupling to two-level systems. Crucially, a resonator-as-coupler approach also suggests a clear path to increased connectivity between fluxonium qubits, by reducing capacitive loading when the coupler has a high impedance. After performing analytic and numeric analyses of the circuit Hamiltonian and gate dynamics, we tune circuit parameters to destructively interfere sources of coherent error, revealing an efficient, fourth-order scaling of coherent error with gate duration. For component properties from the literature, we predict an open-system average CZ gate infidelity of 1.86×10−4 in 70ns.
10
Mrz
2024
Higher-order exceptional surface in a pseudo-Hermitian superconducting circuit
In the last few years, much attention has been paid to exceptional surfaces (ESs) owing to various important physical phenomena and potential applications. However, high-order ESs in
pseudo-Hermitian systems have not been reported until now. Here, we study the high-order ES in a pseudo-Hermitian superconducting (SC) circuit system. In our proposal, the SC circuit system is composed of three circularly coupled SC cavities, where the gain and loss are balanced. According to the eigenvalue properties of the pseudo-Hermitian Hamiltonian, we derive the general pseudo-Hermitian conditions for the ternary SC system. In the special pseudo-Hermitian case with parity-time symmetry, all third-order exceptional points (EP3s) of the SC system form a third-order exceptional line in the parameter space. Under the general pseudo-Hermitian conditions, more EP3s are found, and all EP3s are located on a surface, i.e., a third-order exceptional surface is constructed. Moreover, we also investigate the eigenvalues of the pseudo-Hermitian SC circuit around EP3s. Our work opens up a door for exploring high-order ESs and related applications in pseudo-Hermitian systems.
07
Mrz
2024
Bloch oscillations in a transmon embedded in a resonant electromagnetic environment
Recently developed Josephson junction array transmission lines implement strong-coupling circuit electrodynamics compatible with a range of superconducting quantum devices. They provide
both the high impedance which allows for strong quantum fluctuations, and photon modes with which to probe a quantum device, such as a small Josephson junction. In this high-impedance environment, current through the junction is accompanied by charge Bloch oscillations analogous to those in crystalline systems. However, the interplay between Bloch oscillations and environmental photon resonances remains largely unexplored. Here we describe the Bloch oscillations in a transmon-type qubit attached to high-impedance transmission lines with discrete photon spectra. Transmons are characterized by well-separated charge bands, favoring Bloch oscillations over Landau-Zener tunneling. We find resonances in the voltage–current relation and the spectrum of photons emitted by the Bloch oscillations. The transmon also scatters photons inelastically; we find the cross-section for a novel anti-Stokes-like process whereby photons gain a Bloch oscillation quantum. Our results outline how Bloch oscillations leave fingerprints for experiments across the DC, MHz, and GHz ranges.
06
Mrz
2024
Mechanically Designing Protected Superconducting Qubits
Significant progress is required in the engineering of large, interacting quantum systems in order to realize the promises of gate-model quantum computing. Designing such systems is
challenging, as the dynamics of continuous variable quantum systems are generally unintuitive, and brute-force numerical solutions are difficult to impossible in more than a few dimensions. In this work, I draw analogies between modern superconducting qubits and mechanical mass-spring systems in attempt to gain a simple intuition for what makes each design special. In particular, I analyze superconducting qubits that are inherently protected from noise, and connect this protection to features of the corresponding mechanical system. The hope is that intuition gained from analyzing these systems mechanically will allow for intuitive design of useful superconducting circuits in the future.
A multiplexed control architecture for superconducting qubits with row-column addressing
In state-of-the-art superconducting quantum processors, each qubit is controlled by at least one control line that delivers control pulses generated at room temperature to qubits at
millikelvin temperatures. This strategy has been successfully applied to control hundreds of qubits but is unlikely to be scalable to control thousands of qubits, let alone millions or even billions of qubits needed in fault-tolerance quantum computing. The reason for this is due to the wiring challenge, the number of accommodated control lines is limited by factors, such as the cooling power and physical space of the cryogenic system, the control footprint area at the qubit chip level, and so on. Here, we introduce a multiplexed control architecture for superconducting qubits with two types of shared control lines, row and column lines, providing an efficient approach for parallel controlling N qubits with O(N‾‾√) control lines. With the combination of the two-type shared lines, unique pairs of control pulses are delivered to qubits on each row-column intersection, enabling parallel qubit addressing. Of particular concern here is that, unlike traditional gate schemes, both single- and two-qubit gates are implemented with pairs of control pulses. Considering the inherent parallelism and the control limitations, the integration of the architecture into quantum computing systems should be tailored as much as possible to the specific properties of the quantum circuits to be executed. As such, the architecture could be scalable for executing structured quantum circuits, such as quantum error correction circuits.