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
23
Apr
2019
Realization of a Carbon-Nanotube-Based Superconducting Qubit
Hybrid circuit quantum electrodynamics (QED) involves the study of coherent quantum physics in solid state systems via their interactions with superconducting microwave circuits. Here
we present an implementation of a hybrid superconducting qubit that employs a carbon nanotube as a Josephson junction. We realize the junction by contacting a carbon nanotube with a superconducting Pd/Al bi-layer, and implement voltage tunability of the qubit frequency using a local electrostatic gate. We demonstrate strong dispersive coupling to a coplanar waveguide resonator via observation of a resonator frequency shift dependent on applied gate voltage. We extract qubit parameters from spectroscopy using dispersive readout and find qubit relaxation and coherence times in the range of 10−200 ns.
An experimental platform for hybridization of atomic and superconducting quantum systems
Hybrid quantum systems have the potential of mitigating current challenges in developing a scalable quantum computer. Of particular interest is the hybridization between atomic and
superconducting qubits. We demonstrate a novel experimental setup for transferring and trapping ultracold atoms inside a millikelvin cryogenic environment, where interactions between atomic and superconducting qubits can be established, paving the way for hybrid quantum systems. 87Rb atoms are prepared in a conventional magneto-optical trap and transported via a magnetic conveyor belt into a UHV compatible dilution refrigerator with optical access. We store 5×108 atoms with a lifetime of 794 seconds in the vicinity of the millikelvin stage.
20
Apr
2019
Repetition cat-qubits: fault-tolerant quantum computation with highly reduced overhead
Is it possible to reduce the complexity of quantum error correction close to that of a classical one? We present a repetition code based on the so-called cat-qubits as a viable approach
towards a massive reduction in the hardware requirements for universal and fault-tolerant quantum computation. The cat-qubits that are stabilized by a two-photon driven dissipative process, exhibit a tunable noise bias where the effective bit-flip errors are exponentially suppressed with the average number of photons. We propose a realization of a set of gates on the cat-qubits that preserve such a noise bias. Combining these base qubit operations, we build, at the level of the repetition cat-qubit, a universal set of fully protected logical gates. This set includes single qubit preparations and measurements, NOT, controlled-NOT and controlled-controlled-NOT (Toffoli) gates. Remarkably, this construction does not come with significant overhead as is the case with more conventional schemes requiring magic states preparation, distillation and injection. Finally, all required operations on the cat-qubits could be performed with simple modifications of existing experimental setups.
19
Apr
2019
Introduction to Experimental Quantum Measurement with Superconducting Qubits
Quantum technology has been rapidly growing due to its potential revolutionary applications. In particular, superconducting qubits provide a strong light-matter interaction as required
for quantum computation and in principle can be scaled up to a high level of complexity. However, obtaining the full benefit of quantum mechanics in superconducting circuits requires a deep understanding of quantum physics in such systems in all aspects. One of the most crucial aspects is the concept of measurement and the dynamics of the quantum systems under the measurement process. This document is intended to be a pedagogical introduction to the concept of quantum measurement from an experimental perspective. We study the dynamics of a single superconducting qubit under continuous monitoring. We demonstrate that weak measurement is a versatile tool to investigate fundamental questions in quantum dynamics and quantum thermodynamics for open quantum systems.
16
Apr
2019
Loss and Saturation in Superconducting Travelling-Wave Parametric Amplifiers
We have developed a coupled-mode analysis framework for superconducting travelling-wave parametric amplifiers using the full Telegrapher’s equations to incorporate loss-related
behaviour. Our model provides an explanation of previous experimental observations regarding loss in amplifiers, advantages of concatenating amplifiers to achieve high gains, and signal gain saturation. This work can be used to guide the design of amplifiers in terms of the choice of material systems, transmission line geometry, operating conditions, and pump strength.
13
Apr
2019
A Quantum Engineer’s Guide to Superconducting Qubits
The aim of this review is to provide quantum engineers with an introductory guide to the central concepts and challenges in the rapidly accelerating field of superconducting quantum
circuits. Over the past twenty years, the field has matured from a predominantly basic research endeavor to one that increasingly explores the engineering of larger-scale superconducting quantum systems. Here, we review several foundational elements — qubit design, noise properties, qubit control, and readout techniques — developed during this period, bridging fundamental concepts in circuit quantum electrodynamics (cQED) and contemporary, state-of-the-art applications in gate-model quantum computation.
12
Apr
2019
Photon-assisted charge-parity jumps in a superconducting qubit
We evaluate the rates of energy and phase relaxation of a superconducting qubit caused by stray photons with energy exceeding the threshold for breaking a Cooper pair. All channels
of relaxation within this mechanism are associated with the change in the charge parity of the qubit, enabling the separation of the photon-assisted processes from other contributions to the relaxation rates. Among the signatures of the new mechanism is the same order of rates of the transitions in which a qubit looses or gains energy.
09
Apr
2019
Sequential measurement of a superconducting qubit
We present a superconducting device that realizes the sequential measurement of a transmon qubit. The unitary evolution between system and probe is indeed separated in time and space
from the measurement of the probe itself. The device disables common limitations of dispersive readout such as Purcell effect or transients in the cavity mode by tuning the coupling to the measurement channel on demand. The probe is initially stored in a memory mode and coupled to the qubit until a microwave pump releases it into an output line in a characteristic time as low as 10 ns, which is 400 times shorter than the memory lifetime. The Wigner function of the memory allows us to characterize the non-Gaussian nature of the probe and its dynamics. A direct measurement of the released probe field quadratures demonstrates a readout fidelity of 97.5 % in a total measurement time of 220 ns.
03
Apr
2019
A Doubly non-Linear Superconducting Qubit
We describe a superconducting circuit consisting of a Josephson junction in parallel with a quantum phase slip wire, which implements a Hamiltonian that is periodic in both charge and
flux. This Hamiltonian is exactly diagonalisable in a double-Bloch band, and the eigenstates are shown to be code states of the Gottesman-Kitaev-Preskill quantum error correcting code. The eigenspectrum has several critical points, where the linear sensitivity to external charge and flux noise vanishes. The states at these critical points thus hold promise as qubit states that are insensitive to external noise sources.
Characterizing Low-Quality-Factor Dissipative Superconducting Resonators
Characterizing superconducting microwave resonators with highly dissipative elements is a technical challenge, but a requirement for implementing and understanding the operation of
hybrid quantum devices involving dissipative elements, e.g. for thermal engineering and detection. We present experiments on λ/4 superconducting niobium coplanar waveguide (CPW) resonators, shunted at the antinode by a dissipative copper microstrip via aluminium leads, yielding a quality factor unresolvable from the typical microwave environment. By measuring the transmission both above and below this transition, we are able to isolate the resonance. We then experimentally verify this method with copper microstrips of increasing thicknesses, from 50 nm to 150 nm, and measure quality factors in the range of 10∼67 in a consistent way.