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
19
Apr
2021
Practical quantum error correction with the XZZX code and Kerr-cat qubits
The development of robust architectures capable of large-scale fault-tolerant quantum computation should consider both their quantum error-correcting codes, and the underlying physical
qubits upon which they are built, in tandem. Following this design principle we demonstrate remarkable error correction performance by concatenating the XZZX surface code with Kerr-cat qubits. We contrast several variants of fault-tolerant systems undergoing different circuit noise models that reflect the physics of Kerr-cat qubits. Our simulations show that our system is scalable below a threshold gate infidelity of pCX∼6.5% within a physically reasonable parameter regime, where pCX is the infidelity of the noisiest gate of our system; the controlled-not gate. This threshold can be reached in a superconducting circuit architecture with a Kerr-nonlinearity of 10MHz, a ∼6.25 photon cat qubit, single-photon lifetime of ≳64μs, and thermal photon population ≲8%. Such parameters are routinely achieved in superconducting circuits.
Canonical Quantization of Superconducting Circuits
In the quest to produce quantum technology, superconducting networks, working at temperatures just above absolute zero, have arisen as one of the most promising physical implementations.
The precise analysis and synthesis of such circuits have required merging the fields of physics, engineering, and mathematics.
In this dissertation, we develop mathematically consistent and precise Hamiltonian models to describe ideal superconducting networks made of an arbitrary number of lumped elements, such as capacitors, inductors, Josephson and phase-slip junctions, gyrators, etc., and distributed ones like transmission lines. We give formal proofs for the decoupling at high and low frequencies of lumped degrees of freedom from infinite-dimensional systems in different coupling configurations in models based on the effective Kirchhoff’s laws. We extend the standard theory to quantize circuits that include ideal nonreciprocal elements all the way to their Hamiltonian descriptions in a systematic way. Finally, we pave the way on how to quantize general frequency-dependent gyrators and circulators coupled to both transmission lines and other lumped-element networks.
We have explicitly shown, that these models, albeit ideal, are finite and present no divergence issues. We explain and dispel misunderstandings from the previous literature. Furthermore, we have demonstrated the usefulness of a redundant basis for performing separation of variables of the transmission line (1D) fields in the presence of point-like (lumped-element) couplings by time-reversal symmetry-breaking terms, i.e. nonreciprocal elements.
Microwave Optomechanically Induced Transparency and Absorption
High-quality microwave amplifiers and notch-filters can be made from microwave optomechanical systems in which a mechanical resonator is coupled to a microwave cavity by radiation pressure.
These amplifiers and filters rely on optomechanically induced transparency (OMIT) and absorption (OMIA), respectively. Such devices can amplify microwave signals with large, controllable gain, high dynamic range and very low noise. Furthermore, extremely narrowband filters can be constructed with this technique. We briefly review previous measurements of microwave OMIT and OMIA before reporting our own measurements of these phenomena, which cover a larger parameter space than has been explored in previous works. We find excellent agreement between our measurements and the predictions of input/output theory, thereby guiding further development of microwave devices based on nanomechanics.
16
Apr
2021
Near-Ideal Quantum Efficiency with a Floquet Mode Traveling Wave Parametric Amplfier
Broadband quantum-limited amplifiers would advance applications in quantum information processing, metrology, and astronomy. However, conventional traveling-wave parametric amplifiers
(TWPAs) support broadband amplification at the cost of increased added noise. In this work, we develop and apply a multi-mode, quantum input-output theory to quantitatively identify the sidebands as a primary noise mechanism in all conventional TWPAs. We then propose an adiabatic Floquet mode scheme that effectively eliminates the sideband-induced noise and subsequently overcomes the trade-off between quantum efficiency (QE) and bandwidth. We then show that a Floquet mode Josephson traveling-wave parametric amplifier implementation can simultaneously achieve >20dB gain and a QE of η/ηideal>99.9% of the quantum limit over more than an octave of bandwidth. Crucially, Floquet mode TWPAs also strongly suppress the nonlinear forward-backward wave coupling and are therefore genuinely directional. Floquet mode TWPAs can thus be directly integrated on-chip without isolators, making near-perfect measurement efficiency possible. The proposed Floquet scheme is also widely applicable to other platforms such as kinetic inductance traveling-wave amplifiers and optical parametric amplifiers.
Dynamical sweet spot engineering via two-tone flux modulation of superconducting qubits
Current superconducting quantum processors require strategies for coping with material defects and imperfect parameter targeting in order to scale up while maintaining high performance.
To that end, in-situ control of qubit frequencies with magnetic flux can be used to avoid spurious resonances. However, increased dephasing due to 1/f flux noise limits performance at all of these operating points except for noise-protected sweet spots, which are sparse under DC flux bias and monochromatic flux modulation. Here we experimentally demonstrate that two-tone flux modulation can be used to create a continuum of dynamical sweet spots, greatly expanding the range of qubit frequencies achievable while first-order insensitive to slow flux noise. To illustrate some advantages of this flexibility, we use bichromatic flux control to reduce the error rates and gate times of parametric entangling operations between transmons. Independent of gate scheme, the ability to use flux control to freely select qubit frequencies while maintaining qubit coherence represents an important step forward in the robustness and scalability of near-term superconducting qubit devices.
15
Apr
2021
True Differential Superconducting On-Chip Output Amplifier
The true-differential superconductor on-chip amplifier has complementary outputs that float with respect to chip ground. This improves signal integrity and compatibility with the receiving
semiconductor stage. Both source-terminated and non-source-terminated designs producing 4mV demonstrated rejection of a large common mode interference in the package. Measured margins are ±8.5% on the output bias, and ±28% on AC clock amplitude. Waveforms and eye diagrams are taken at 2.9-10Gb/s. Direct measurement of bit-error rates are better than the resolution limit of 1e-12 at 2.9Gb/s, and better than 1e-9 at 10Gb/s.
14
Apr
2021
Circuit quantum electrodynamics: A new look toward developing full-wave numerical models
Devices built using circuit quantum electrodynamics architectures are one of the most popular approaches currently being pursued to develop quantum information processing hardware.
Although significant progress has been made over the previous two decades, there remain many technical issues limiting the performance of fabricated systems. Addressing these issues is made difficult by the absence of rigorous numerical modeling approaches. This work begins to address this issue by providing a new mathematical description of one of the most commonly used circuit quantum electrodynamics systems, a transmon qubit coupled to microwave transmission lines. Expressed in terms of three-dimensional vector fields, our new model is better suited to developing numerical solvers than the circuit element descriptions commonly used in the literature. We present details on the quantization of our new model, and derive quantum equations of motion for the coupled field-transmon system. These results can be used in developing full-wave numerical solvers in the future. To make this work more accessible to the engineering community, we assume only a limited amount of training in quantum physics and provide many background details throughout derivations.
Measurement and Data-Assisted Simulation of Bit Error Rate in RQL Circuits
A circuit-simulation-based method is used to determine the thermally-induced bit error rate of superconducting logic circuits. Simulations are used to evaluate the multidimensional
Gaussian integral across noise current sources attached to the active devices. The method is data-assisted and has predictive power. Measurement determines the value of a single parameter, effective noise bandwidth, for each error mechanism. The errors in the distributed networks of comparator-free RQL logic nucleate across multiple Josephson junctions, so the effective critical current is about three times that of the individual devices. The effective noise bandwidth is only 6-23% of the junction plasma frequency at a modest clock rate of 3.4GHz, which is 1% of the plasma frequency. This analysis shows the ways measured bit error rate comes out so much lower than simplistic estimates based on isolated devices.
Quantum beats of a magnetic fluxon in a two-cell SQUID
We report a detailed theoretical study of a coherent macroscopic quantum-mechanical phenomenon – quantum beats of a single magnetic fluxon trapped in a two-cell SQUID of high
kinetic inductance. We calculate numerically and analytically the low-lying energy levels of the fluxon, and explore their dependence on externally applied magnetic fields. The quantum dynamics of the fluxon shows quantum beats originating from its coherent quantum tunneling between the SQUID cells. We analyze the experimental setup based on a three-cell SQUID, allowing for time-resolved measurements of quantum beats of the fluxon.
13
Apr
2021
Emergence of nonlinear friction from quantum fluctuations
Nonlinear damping, a force of friction that depends on the amplitude of motion, plays an important role in many electrical, mechanical and even biological oscillators. In novel technologies
such as carbon nanotubes, graphene membranes or superconducting resonators, the origin of nonlinear damping is sometimes unclear. This presents a problem, as the damping rate is a key figure of merit in the application of these systems to extremely precise sensors or quantum computers. Through measurements of a superconducting circuit, we show that nonlinear damping can emerge as a direct consequence of quantum fluctuations and the conservative nonlinearity of a Josephson junction. The phenomenon can be understood and visualized through the flow of quasi-probability in phase space, and accurately describes our experimental observations. Crucially, the effect is not restricted to superconducting circuits: we expect that quantum fluctuations or other sources of noise give rise to nonlinear damping in other systems with a similar conservative nonlinearity, such as nano-mechanical oscillators or even macroscopic systems.