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
24
Dez
2022
DEC-QED: A flux-based 3D electrodynamic modeling approach to superconducting circuits and materials
Modeling the behavior of superconducting electronic circuits containing Josephson junctions is crucial for the design of superconducting information processors and devices. In this
paper, we introduce DEC-QED, a computational approach for modeling the electrodynamics of superconducting electronic circuits containing Josephson junctions in arbitrary three-dimensional electromagnetic environments. DEC-QED captures the non-linear response and induced currents of BCS superconductors and accurately captures phenomena such as the Meissner effect, flux quantization and Josephson effects. Using a finite-element construction based on Discrete Exterior Calculus (DEC), DEC-QED can accurately simulate transient and long-time dynamics in superconductors. The expression of the entire electrodynamic problem in terms of the gauge-invariant flux field and charges makes the resulting classical field theory suitable for second quantization.
23
Dez
2022
Quasiperiodic circuit quantum electrodynamics
Superconducting circuits are an extremely versatile platform to realize quantum information hardware, and, as was recently realized, to emulate topological materials, such as three-dimensional
Weyl semimetals or two-dimensional Chern insulators. We here show how a simple arrangement of capacitors and conventional superconductor-insulator-superconductor (SIS) junctions can realize a nonlinear capacitive element with a surprising property: it can be quasiperiodic with respect to the quantized Cooper-pair charge. Integrating this element into a larger circuit opens the door towards the engineering of an even broader class of systems. First, we use it to simulate a protected Dirac material defined in the transport degrees of freedom. The presence of the Dirac points leads to a suppression of the classical part of the finite-frequency current noise. Second, we exploit the quasiperiodicity to implement the Aubry-André model, and thereby emulate Anderson localization in charge space. Importantly, the realization by means of transport degrees of freedom allows for a straightforward generalization to arbitrary dimensions. Moreover, our setup implements a truly non-interacting version of the model, in which the macroscopic quantum mechanics of the circuit already incorporates microscopic interaction effects. We propose that measurements of the quantum fluctuations of the charge can be used to directly probe the localization effect. Finally, we present an outlook in which the nonlinear capacitance is employed in a quantum circuit emulating a magic-angle effect akin to twisted bilayer graphene.
Analogue black-white hole solitons in travelling wave parametric amplifiers with superconducting nonlinear asymmetric inductive elements
We propose an analogue black-white hole soliton realization using existing travelling wave parametric amplifier (TWPA) setups comprising superconducting nonlinear asymmetric inductive
elements (SNAILs). We show that the dynamics in terms of the phase difference coordinate across a SNAIL is described by the Korteweg-de Vries (KdV) or modified Korteweg-de Vries (mKdV) equations in the continuum approximation, depending on the external magnetic flux bias, which admit solitary wave solutions. We also validate the KdV-derived solitons by numerically solving the original, discrete element circuit equations. Soliton solutions to the (m)KdV equation spatially modulate the velocity for probe microwaves propagating in the TWPA, resulting in the effective realization of analogue black hole-white hole event horizon pairs.
A high on-off ratio beamsplitter interaction for gates on bosonically encoded qubits
Encoding a qubit in a high quality superconducting microwave cavity offers the opportunity to perform the first layer of error correction in a single device, but presents a challenge:
how can quantum oscillators be controlled while introducing a minimal number of additional error channels? We focus on the two-qubit portion of this control problem by using a 3-wave mixing coupling element to engineer a programmable beamsplitter interaction between two bosonic modes separated by more than an octave in frequency, without introducing major additional sources of decoherence. Combining this with single-oscillator control provided by a dispersively coupled transmon provides a framework for quantum control of multiple encoded qubits. The beamsplitter interaction gbs is fast relative to the timescale of oscillator decoherence, enabling over 103 beamsplitter operations per coherence time, and approaching the typical rate of the dispersive coupling χ used for individual oscillator control. Further, the programmable coupling is engineered without adding unwanted interactions between the oscillators, as evidenced by the high on-off ratio of the operations, which can exceed 105. We then introduce a new protocol to realize a hybrid controlled-SWAP operation in the regime gbs≈χ, in which a transmon provides the control bit for the SWAP of two bosonic modes. Finally, we use this gate in a SWAP test to project a pair of bosonic qubits into a Bell state with measurement-corrected fidelity of 95.5%±0.2%.
22
Dez
2022
Dual-rail encoding with superconducting cavities
The design of quantum hardware that reduces and mitigates errors is essential for practical quantum error correction (QEC) and useful quantum computations. To this end, we introduce
the circuit-QED dual-rail qubit in which our physical qubit is encoded in the single-photon subspace of two superconducting cavities. The dominant photon loss errors can be detected and converted into erasure errors, which are much easier to correct. In contrast to linear optics, a circuit-QED implementation of the dual-rail code offers completely new capabilities. Using a single transmon ancilla, we describe a universal gate set that includes state preparation, logical readout, and parametrizable single and two-qubit gates. Moreover, first-order hardware errors due to the cavity and transmon in all of these operations can be detected and converted to erasure errors, leaving background Pauli errors that are orders of magnitude smaller. Hence, the dual-rail cavity qubit delivers an optimal hierarchy of errors and rates, and is expected to be well below the relevant QEC thresholds with today’s devices.
21
Dez
2022
Realization of a quantum neural network using repeat-until-success circuits in a superconducting quantum processor
Artificial neural networks are becoming an integral part of digital solutions to complex problems. However, employing neural networks on quantum processors faces challenges related
to the implementation of non-linear functions using quantum circuits. In this paper, we use repeat-until-success circuits enabled by real-time control-flow feedback to realize quantum neurons with non-linear activation functions. These neurons constitute elementary building blocks that can be arranged in a variety of layouts to carry out deep learning tasks quantum coherently. As an example, we construct a minimal feedforward quantum neural network capable of learning all 2-to-1-bit Boolean functions by optimization of network activation parameters within the supervised-learning paradigm. This model is shown to perform non-linear classification and effectively learns from multiple copies of a single training state consisting of the maximal superposition of all inputs.
Error-detectable bosonic entangling gates with a noisy ancilla
Bosonic quantum error correction has proven to be a successful approach for extending the coherence of quantum memories, but to execute deep quantum circuits, high-fidelity gates between
encoded qubits are needed. To that end, we present a family of error-detectable two-qubit gates for a variety of bosonic encodings. From a new geometric framework based on a „Bloch sphere“ of bosonic operators, we construct ZZL(θ) and eSWAP(θ) gates for the binomial, 4-legged cat, dual-rail and several other bosonic codes. The gate Hamiltonian is simple to engineer, requiring only a programmable beamsplitter between two bosonic qubits and an ancilla dispersively coupled to one qubit. This Hamiltonian can be realized in circuit QED hardware with ancilla transmons and microwave cavities. The proposed theoretical framework was developed for circuit QED but is generalizable to any platform that can effectively generate this Hamiltonian. Crucially, one can also detect first-order errors in the ancilla and the bosonic qubits during the gates. We show that this allows one to reach error-detected gate fidelities at the 10−4 level with today’s hardware, limited only by second-order hardware errors.
16
Dez
2022
Wideband Josephson Parametric Isolator
The cryogenic hardware needed to build a superconducting qubit based quantum computer requires a variety of microwave components including microwave couplers, filters, amplifiers, and
circulators/isolators. Traditionally, these are implemented via discrete components inserted in to the signal path. As qubit counts climb over the 100+ mark, the integration of these peripheral components, in an effort to reduce overall footprint, thermal load, and added noise in the overall system, is a key challenge to scaling. Ferrite-based microwave isolators are one of the physically largest devices that continue to remain as discrete components. They are generally employed in the readout chain to protect qubits and resonators from broadband noise and unwanted signals emanating from downstream components such as amplifiers. Here we present an alternative two-port isolating integrated circuit derived from the DC Superconducting Quantum Interference Device (DC-SQUID). The non-reciprocal transmission is achieved using the three-wave microwave mixing properties of a flux-modulated DC-SQUID. We show experimentally that, when multiple DC-SQUIDs are embedded in a multi-pole admittance inverting filter structure, RF flux pumping of the DC-SQUIDs can provide directional microwave power flow. For a three-pole filter device, we experimentally demonstrate a directionality greater than 15 dB over a 600 MHz bandwidth.
14
Dez
2022
Photon-Pressure with a Negative Mass Microwave Mode
Harmonic oscillators belong to the most fundamental concepts in physics and are central to many current research fields such as circuit QED, cavity optomechanics and photon-pressure
systems. Here, we engineer an effective negative mass harmonic oscillator mode in a superconducting microwave LC circuit and couple it via photon-pressure to a second low-frequency circuit. We demonstrate that the effective negative mass leads to an inversion of dynamical backaction and to sideband-cooling of the low-frequency circuit by a blue-detuned pump field, naturally explained by the inverted energy ladder of the negative mass oscillator.
Fast parametric two-qubit gate for highly detuned fixed-frequency superconducting qubits using a double-transmon coupler
High-performance two-qubit gates have been reported with superconducting qubits coupled via a single-transmon coupler (STC). Most of them are implemented for qubits with a small detuning
since reducing residual ZZ coupling for highly detuned qubits by an STC is challenging. In terms of the frequency crowding and crosstalk, however, highly detuned qubits are desirable. Here, we numerically demonstrate a high-performance parametric gate for highly detuned fixed-frequency qubits using a recently proposed tunable coupler called a double-transmon coupler (DTC). Applying an ac flux pulse, we can perform a maximally entangling universal gate (iSWAP‾‾‾‾‾‾‾√) with an average fidelity over 99.99% and a short gate time of about 24 ns. This speed is comparable to resonance-based gates for slightly detuned tunable qubits. Moreover, using a dc flux pulse alternatively, we can achieve another kind of entangling gate called a CZ gate with an average fidelity over 99.99% and a gate time of about 18 ns. Given the frexibility and feasible settings, we can expect that the DTC will contribute to realizing a high-performance quantum computer in the near future.