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
28
Aug
2023
Passive microwave circulation on a superconducting chip
Building large-scale superconducting quantum circuits will require miniaturisation and integration of supporting devices including microwave circulators, which are currently bulky,
stand-alone components. Here we report the realisation of a passive on-chip circulator which is made from a loop consisting of three tunnel-coupled superconducting islands, with DC-only control fields. We observe the effect of quasiparticle tunnelling, and we dynamically classify the system into different quasiparticle sectors. When tuned for circulation, the device exhibits strongly non-reciprocal 3-port scattering, with average on-resonance insertion loss of 2 dB, isolation of 14 dB, power reflectance of -11 dB, and a bandwidth of 200 MHz.
23
Aug
2023
Parity-protected superconducting qubit based on topological insulators
We propose a novel architecture that utilizes two 0-π qubits based on topological Josephson junctions to implement a parity-protected superconducting qubit. The topological Josephson
junctions provides protection against fabrication variations, which ensures the identical Josephson junctions required to implement the0-π qubit. By viewing the even and odd parity ground states of a 0-π qubit as spin-12 states, we construct the logic qubit states using the total parity odd subspace of two 0-π qubits. This parity-protected qubit exhibits robustness against charge noise, similar to a singlet-triplet qubit’s immunity to global magnetic field fluctuations. Meanwhile, the flux noise cannot directly couple two states with the same total parity and therefore is greatly suppressed. Benefiting from the simultaneous protection from both charge and flux noise, we demonstrate a dramatic enhancement of both T1 and T2 coherence times. Our work presents a new approach to engineer symmetry-protected superconducting qubits.
Digital-analog quantum computing of fermion-boson models in superconducting circuits
We propose a digital-analog quantum algorithm for simulating the Hubbard-Holstein model, describing strongly-correlated fermion-boson interactions, in a suitable architecture with superconducting
circuits. It comprises a linear chain of qubits connected by resonators, emulating electron-electron (e-e) and electron-phonon (e-p) interactions, as well as fermion tunneling. Our approach is adequate for a digital-analog quantum computing (DAQC) of fermion-boson models including those described by the Hubbard-Holstein model. We show the reduction in the circuit depth of the DAQC algorithm, a sequence of digital steps and analog blocks, outperforming the purely digital approach. We exemplify the quantum simulation of a half-filling two-site Hubbard-Holstein model. In such example we obtain fidelities larger than 0.98, showing that our proposal is suitable to study the dynamical behavior of solid-state systems. Our proposal opens the door to computing complex systems for chemistry, materials, and high-energy physics.
21
Aug
2023
Superconducting Quantum Circuits in the light of Dirac’s Constraint Analysis Framework
In this work we introduce a new framework – Dirac’s Hamiltonian formalism of constraint systems – to study different types of Superconducting Quantum Circuits (SQC)
in a {\it{unified}} and unambiguous way. The Lagrangian of a SQC reveals the constraints, that are classified in a Hamiltonian framework, such that redundant variables can be removed to isolate the canonical degrees of freedom for subsequent quantization of the Dirac Brackets via a generalized Correspondence Principle. This purely algebraic approach makes the application of concepts such as graph theory, null vector, loop charge,\ etc that are in vogue, (each for a specific type of circuit), completely redundant.
18
Aug
2023
Modular Superconducting Qubit Architecture with a Multi-chip Tunable Coupler
We use a floating tunable coupler to mediate interactions between qubits on separate chips to build a modular architecture. We demonstrate three different designs of multi-chip tunable
couplers using vacuum gap capacitors or superconducting indium bump bonds to connect the coupler to a microwave line on a common substrate and then connect to the qubit on the next chip. We show that the zero-coupling condition between qubits on separate chips can be achieved in each design and that the relaxation rates for the coupler and qubits are not noticeably affected by the extra circuit elements. Finally, we demonstrate two-qubit gate operations with fidelity at the same level as qubits with a tunable coupler on a single chip. Using one or more indium bonds does not degrade qubit coherence or impact the performance of two-qubit gates.
15
Aug
2023
High-frequency suppression of inductive coupling between flux qubit and transmission line resonator
We perform theoretical calculations to investigate the naturally occurring high-frequency cutoff in a circuit comprising a flux qubit coupled inductively to a transmission line resonator
(TLR). Our results agree with those of past studies that considered somewhat similar circuit designs. In particular, a decoupling occurs between the qubit and the high-frequency modes. As a result, the coupling strength between the qubit and resonator modes increases with mode frequency ω as ω‾‾√ at low frequencies and decreases as 1/ω‾‾√ at high frequencies. We derive expressions for the multimode-resonator-induced Lamb shift in the qubit’s characteristic frequency. Because of the natural decoupling between the qubit and high-frequency modes, the Lamb-shift-renormalized qubit frequency remains finite.
04
Aug
2023
Radiatively-cooled quantum microwave amplifiers
Superconducting microwave amplifiers are essential for sensitive signal readout in superconducting quantum processors. Typically based on Josephson Junctions, these amplifiers require
operation at milli-Kelvin temperatures to achieve quantum-limited performance. Here we demonstrate a quantum microwave amplifier that employs radiative cooling to operate at elevated temperatures. This kinetic-inductance-based parametric amplifier, patterned from a single layer of high-Tc NbN thin film\cmt{in the form of a nanobridge}, maintains a high gain and meanwhile enables low added noise of 1.3 quanta when operated at 1.5 Kelvin. Remarkably, this represents only a 0.2 quanta increase compared to the performance at a base temperature of 0.1 Kelvin. By uplifting the parametric amplifiers from the mixing chamber without compromising readout efficiency, this work represents an important step for realizing scalable microwave quantum technologies.
Optimizing quantum gates towards the scale of logical qubits
A foundational assumption of quantum error correction theory is that quantum gates can be scaled to large processors without exceeding the error-threshold for fault tolerance. Two major
challenges that could become fundamental roadblocks are manufacturing high performance quantum hardware and engineering a control system that can reach its performance limits. The control challenge of scaling quantum gates from small to large processors without degrading performance often maps to non-convex, high-constraint, and time-dependent control optimization over an exponentially expanding configuration space. Here we report on a control optimization strategy that can scalably overcome the complexity of such problems. We demonstrate it by choreographing the frequency trajectories of 68 frequency-tunable superconducting qubits to execute single- and two-qubit gates while mitigating computational errors. When combined with a comprehensive model of physical errors across our processor, the strategy suppresses physical error rates by ∼3.7× compared with the case of no optimization. Furthermore, it is projected to achieve a similar performance advantage on a distance-23 surface code logical qubit with 1057 physical qubits. Our control optimization strategy solves a generic scaling challenge in a way that can be adapted to other quantum algorithms, operations, and computing architectures.
03
Aug
2023
Model-based Optimization of Superconducting Qubit Readout
Measurement is an essential component of quantum algorithms, and for superconducting qubits it is often the most error prone. Here, we demonstrate model-based readout optimization achieving
low measurement errors while avoiding detrimental side-effects. For simultaneous and mid-circuit measurements across 17 qubits, we observe 1.5% error per qubit with a 500ns end-to-end duration and minimal excess reset error from residual resonator photons. We also suppress measurement-induced state transitions achieving a leakage rate limited by natural heating. This technique can scale to hundreds of qubits and be used to enhance the performance of error-correcting codes and near-term applications.
Dissipative Dynamics of Graph-State Stabilizers with Superconducting Qubits
We study the noisy evolution of multipartite entangled states, focusing on superconducting-qubit devices accessible via the cloud. We experimentally characterize the single-qubit coherent
and incoherent error parameters together with the effective two-qubit interactions, whose combined action dominates the decoherence of quantum memory states. We find that a valid modeling of the dynamics of superconducting qubits requires one to properly account for coherent frequency shifts, caused by stochastic charge-parity fluctuations. We present a numerical approach that is scalable to tens of qubits, allowing us to simulate efficiently the dissipative dynamics of some large multiqubit states. Comparing our simulations to measurements of stabilizers dynamics of graph states realized experimentally with up to 12 qubits on a ring, we find that a very good agreement is achievable. Our approach allows us to probe nonlocal state characteristics that are inaccessible in the experiment. We show evidence for a significant improvement of the many-body state fidelity using dynamical decoupling sequences, mitigating the effect of charge-parity oscillations and two-qubit crosstalk.