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
20
Feb
2025
Randomized benchmarking of a high-fidelity remote CNOT gate over a meter-scale microwave interconnect
In the modular superconducting quantum processor architecture, high-fidelity, meter-scale microwave interconnect between processor modules is a key technology for extending system size
beyond constraints imposed by device manufacturing equipment, yield, and signal delivery. While there have been many demonstrations of remote state transfer between modules, these relied on tomographic experiments for benchmarking, but this technique does not reliably separate State Preparation And Measurement (SPAM) error from error per state transfer. Recent developments based on randomized benchmarking provide a compatible theory for separating these two errors. In this work, we present a module-to-module interconnect based on Tunable-Coupling Qubits (TCQs) and benchmark, in a SPAM error tolerant manner, a remote state transfer fidelity of 0.988 across a 60cm long coplanar waveguide (CPW). The state transfer is implemented via superadiabatic transitionless driving method, which suppresses intermediate excitation in internal modes of CPW. We also introduce the frame tracking technique to correct unintended qubit phase rotations before and after the state transfers, which enables the SPAM-error-tolerant benchmarking of the state transfers. We further propose and construct a remote CNOT gate between modules, composed of local CZ gates in each module and remote state transfers, and report a high gate fidelity of 0.933 using randomized benchmarking method. The remote CNOT construction and benchmarking we present is a more complete metric that fully characterizes the module to module link operation going forward as it more closely represents interconnect operation in a circuit.
Experimental demonstrations of Josephson threshold detectors for broadband microwave photons detection
Current-biased Josephson junctions (CBJJs) have been demonstrated as sensitive Josephson threshold detectors (JTDs) in the infrared range. In this letter, we show this kind of detector
could also be used to detect broadband microwave photons. Based on the numerical simulations of the noise-driving phase dynamics of an underdamped Josephson junction, driven by the low-frequency triangular wave current, we argue that the microwave photons flowing across the JJ can be detected by probing the voltage switched signals of the JJ. Experimentally, we designed and fabricated the relevant Al/AlOx/Al Josephson device and measured its response to microwave photons at 50~mK temperature. Experimental results indicate that the weak microwave signals could be threatened as the additional noises modify the phase dynamics of the CBJJ, which could thus be detected by the generated JTD. The detection sensitivity is characterized by using the Kumar-Caroll index to differentiate the junction switched duration distributions, with and without microwave signal input. Although the demonstrated detection sensitivity is just −92~dBm (corresponding to approximately 30~photon/ns) for the microwave photons at ∼5GHz (which is manifestly deviated from the plasma frequency of the fabricated JJ), we argued, based on the relevant numerical simulations, that the generated JTD could be used to achieve the sensitive detection of the microwave photons at the plasma frequency of the JJ.
Passive leakage removal unit based on a disordered transmon array
Leakage out from the qubit subspace compromises standard quantum error correction protocols and is a challenge for practical quantum computing. We propose a passive leakage removal
unit based on an array of coupled disordered transmons and last-site reset by feedback-measurement or dissipation. The transmons have parametric disorder both in frequency and anharmonicity such that the qubit subspace is protected by localization through energy level mismatch while the energy levels for leakage excitations are in resonance for maximized leakage mobility. Leakage excitations propagate through the idle transmons until reaching the last site with feedback-measurement or dissipation removing them. For removing leakage excitations, we find two optimal measurement rates, which are comprehensively understood through two distinct timescales between the propagation and disintegration of leakage excitations. Based only on an array of standard transmon devices, our approach is readily compatible with existing superconducting quantum processor designs under realistic conditions.
Time-adaptive single-shot crosstalk detector on superconducting quantum computer
Quantum crosstalk which stems from unwanted interference of quantum operations with nearby qubits is a major source of noise or errors in a quantum processor. In the context of shared
quantum computing, it is challenging to mitigate the crosstalk effect between quantum computations being simultaneously run by multiple users since the exact spatio-temporal gate distributions are not apparent due to privacy concerns. It is therefore important to develop techniques for accurate detection and mitigation of crosstalk to enable high-fidelity quantum computing. Assuming prior knowledge of crosstalk parameters, we propose a time-adaptive detection method leveraging spectator qubits and multiple quantum coherence to amplify crosstalk-induced perturbations. We demonstrate its utility in detecting random sparsely distributed crosstalk within a time window. Our work evaluates its performance in two scenarios: simulation using an artificial noise model with gate-induced crosstalk and always-on idlings channels; and the simulation using noise sampled from an IBM quantum computer parametrised by the reduced HSA error model. The presented results show our method’s efficacy hinges on the dominance of single-qubit coherent noise across channels, and the impact of angle mismatching is suppressed as spectator qubits increase. From simulation using real-device noise parameters, our strategy outperforms the previous constant-frequency detection method of Harper et. al. [arXiv: 2402.02753 (2024)] in the detection success rate, achieving an average detection success probability of 0.852±0.022 (equally scaled noise channels) and 0.933±0.024 (asymmetrically scaled noise channels) from 1 to 7 crosstalk counts.
12
Feb
2025
Theory of an autonomous quantum heat engine based on superconducting electric circuits with non-Markovian heat baths
We propose and theoretically analyze a realistic superconducting electric circuit that can be used to realize an autonomous quantum heat engine in circuit quantum electrodynamics. Using
a quasiclassical, non-Markovian theoretical model, we demonstrate that coherent microwave photon generation can emerge solely from heat flow through the circuit and its nonlinear internal dynamics. The predicted generation rate is sufficiently high for experimental observation in circuit quantum electrodynamics, making this work a significant step toward the first experimental realization of an autonomous quantum heat engine in superconducting circuits.
Multiplexed qubit readout quality metric beyond assignment fidelity
The accurate measurement of quantum two-level objects (qubits) is crucial for developing quantum computing hardware. Over the last decade, the measure of choice for benchmarking readout
routines for superconducting qubits has been assignment fidelity. However, this method only focuses on the preparation of computational basis states and therefore does not provide a complete characterization of the readout. Here, we expand the focus to the use of detector tomography to fully characterize multi-qubit readout of superconducting transmon qubits. The impact of different readout parameters on the rate of information extraction is studied using quantum state reconstruction infidelity as a proxy. The results are then compared with assignment fidelities, showing good agreement for separable two-qubit states. We therefore propose the rate of infidelity convergence as an alternative and more comprehensive benchmark for single- and multi-qubit readout optimization. We find the most effective allocation of a fixed shot budget between detector tomography and state reconstruction in single- and two-qubit experiments. To address the growing interest in three-qubit gates, we perform three-qubit quantum state tomography that goes beyond conventional readout error mitigation methods and find a factor of 30 reduction in quantum infidelity. Our results demonstrate that neither quantum nor classical qubit readout correlations are induced even by very high levels of readout noise. Consequently, correlation coefficients can serve as a valuable tool in qubit readout optimization.
11
Feb
2025
Kinetic inductance coupling for circuit QED with spins
In contrast to the commonly used qubit resonator transverse coupling via the σxy-degree of freedom, longitudinal coupling through σz presents a tantalizing alternative: it does not
hybridize the modes, eliminating Purcell decay, and it enables quantum-non-demolishing qubit readout independent of the qubit-resonator frequency detuning. Here, we demonstrate longitudinal coupling between a {Cr7Ni} molecular spin qubit ensemble and the kinetic inductance of a granular aluminum superconducting microwave resonator. The inherent frequency-independence of this coupling allows for the utilization of a 7.8 GHz readout resonator to measure the full {Cr7Ni} magnetization curve spanning 0-600 mT, corresponding to a spin frequency range of fspin=0-15 GHz. For 2 GHz detuning from the readout resonator, we measure a 1/e spin relaxation time τ=0.38 s, limited by phonon decay to the substrate. Based on these results, we propose a path towards longitudinal coupling of single spins to a superconducting fluxonium qubit.
A proposal for charge basis tomography of superconducting qubits
We introduce a general protocol for obtaining the charge basis density matrix of a superconducting quantum circuit. Inspired by cavity state tomography, our protocol combines Josephson-energy
pulse sequences and projective charge-basis readout to access the off-diagonal elements of the density matrix, a scheme we thus dub charge basis tomography. We simulate the reconstruction of the ground state of a target transmon using the Aharonov-Casher effect in a probe qubit to realise projective readout and show the Hilbert-Schmidt distance can detect deviations from the correct model Hamiltonian. Unlocking this ability to validate models using the ground state sets the stage for using transmons to detect interacting and topological phases, particularly in materials where time-domain and spectroscopic probes can be limited by intrinsic noise.
Tomographic Signatures of Interacting Majorana and Andreev States in Superconductor-Semiconductor Transmon Qubits
Semiconductor-based Josephson junctions embedded within a Cooper-pair-box can host complex many-body states, such as interacting Andreev states and potentially other quasi-particles
of topological origin. Here, we study the insights that could be revealed from a tomographic reconstruction of the Cooper-pair charge distribution of the junction prepared in its ground state. We posit that interacting and topological states can be identified from distinct signatures within the probability distribution of the charge states. Furthermore, the comprehensive dataset provides direct access to information theory metrics elucidating the entanglement between the charge sector of the superconductor and the microscopic degrees of freedom in the junction. We demonstrate how these metrics serve to further classify differences between the types of excitations in the junction.
Enhancing dissipative cat qubit protection by squeezing
Dissipative cat-qubits are a promising architecture for quantum processors due to their built-in quantum error correction. By leveraging two-photon stabilization, they achieve an exponentially
suppressed bit-flip error rate as the distance in phase-space between their basis states increases, incurring only a linear increase in phase-flip rate. This property substantially reduces the number of qubits required for fault-tolerant quantum computation. Here, we implement a squeezing deformation of the cat qubit basis states, further extending the bit-flip time while minimally affecting the phase-flip rate. We demonstrate a steep reduction in the bit-flip error rate with increasing mean photon number, characterized by a scaling exponent γ=4.3, rising by a factor of 74 per added photon. Specifically, we measure bit-flip times of 22 seconds for a phase-flip time of 1.3 μs in a squeezed cat qubit with an average photon number n¯=4.1, a 160-fold improvement in bit-flip time compared to a standard cat. Moreover, we demonstrate a two-fold reduction in Z-gate infidelity, with an estimated phase-flip probability of ϵX=0.085 and a bit-flip probability of ϵZ=2.65⋅10−9 which confirms the gate bias-preserving property. This simple yet effective technique enhances cat qubit performances without requiring design modification, moving multi-cat architectures closer to fault-tolerant quantum computation.