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
07
Apr
2015
Violating the Bell-Leggett-Garg inequality with weak measurement of an entangled state
Since the inception of quantum mechanics, its validity as a complete description of reality has been challenged due to predictions that defy classical intuition. For many years it was
unclear whether predictions like entanglement and projective measurement represented real phenomena or artifacts of an incomplete model. Bell inequalities (BI) provided the first quantitative test to distinguish between quantum entanglement and a yet undiscovered classical hidden variable theory. The Leggett-Garg inequality (LGI) provides a similar test for projective measurement, and more recently has been adapted to include variable strength measurements to study the process of measurement itself. Here we probe the intersection of both entanglement and measurement through the lens of the hybrid Bell-Leggett-Garg inequality (BLGI). By correlating data from ancilla-based weak measurements and direct projective measurements, we for the first time quantify the effect of measurement strength on entanglement collapse. Violation of the BLGI, which we achieve only at the weakest measurement strengths, offers compelling evidence of the completeness of quantum mechanics while avoiding several loopholes common to previous experimental tests. This uniquely quantum result significantly constrains the nature of any possible classical theory of reality. Additionally, we demonstrate that with sufficient scale and fidelity, a universal quantum processor can be used to study richer fundamental physics.
Dispersive Response of a Disordered Superconducting Quantum Metamaterial
We consider a disordered quantum metamaterial formed by an array of superconducting flux qubits coupled to microwave photons in a cavity. We map the system on the Tavis-Cummings model
accounting for the disorder in frequencies of the qubits. The complex transmittance is calculated with the parameters taken from state-of-the-art experiments. We demonstrate that photon phase shift measurements allow to distinguish individual resonances in the metamaterial with up to 100 qubits, in spite of the decoherence spectral width being remarkably larger than the effective coupling constant. Our simulations are in agreement with the results of the recently reported experiment.
03
Apr
2015
Non-Hermitian Hamiltonian approach to the microwave transmission through one- dimensional qubit chain
We investigate the propagation of microwave photons in a one-dimensional waveguide interacting with a number of artificial atoms (qubits). Within the formalism of projection operators
and non-Hermitian Hamiltonian approach we develop a one-photon approximation scheme for the calculation of the transmission and reflection factors of the microwave signal in a waveguide which contains an arbitrary number \emph{N} of non-interacting qubits. It is shown that for identical qubits in the long-wave limit a coherent superradiance state is formed with the width being equal to the sum of the widths of spontaneous transitions of \emph{N} individual qubits.
01
Apr
2015
Autonomous quantum thermal machine for generating steady-state entanglement
We discuss a simple quantum thermal machine for the generation of steady-state entanglement between two interacting qubits. The machine is autonomous in the sense that it uses only
incoherent interactions with thermal baths, but no source of coherence or external control. By weakly coupling the qubits to thermal baths at different temperatures, inducing a heat current through the system, steady-state entanglement is generated far from thermal equilibrium. Finally, we discuss two possible implementations, using superconducting flux qubits or a semiconductor double quantum dot. Experimental prospects for steady-state entanglement are promising in both systems.
Quantum technologies with hybrid systems
An extensively pursued current direction of research in physics aims at the development of practical technologies that exploit the effects of quantum mechanics. As part of this ongoing
effort, devices for information processing, secure communication and high-precision sensing are being implemented with diverse systems, ranging from photons, atoms and spins to mesoscopic superconducting and nanomechanical structures. Their physical properties make some of these systems better suited than others for specific tasks; thus, photons are well suited for transmitting quantum information, weakly interacting spins can serve as long-lived quantum memories, and superconducting elements can rapidly process information encoded in their quantum states. A central goal of the envisaged quantum technologies is to develop devices that can simultaneously perform several of these tasks, namely, reliably store, process, and transmit quantum information. Hybrid quantum systems composed of different physical components with complementary functionalities may provide precisely such multi-tasking capabilities. This article reviews some of the driving theoretical ideas and first experimental realizations of hybrid quantum systems and the opportunities and the challenges they present and offers a glance at the near- and long-term perspectives of this fascinating and rapidly expanding field.
31
Mä
2015
Long-time coherent quantum behaviour of the D-Wave machine
Extensive experiments have demonstrated quantum behaviour in the long-time operation of the D-Wave quantum computer. The decoherence time of a single flux qubit is reported to be on
the order of nanoseconds, which is much shorter than the time required to carry out a computation on the timescale of seconds. In our contribution we investigate a model of four qubits with one qubit coupled to a phonon and (optionally) to environmental particles of high density of states, called gravonons. The calculations indicate that when no gravonons are present, the current in the qubit is flipped at some time and adiabatic evolution is discontinued. The time dependent wave functional becomes a non-correctable superposition of many excited states. The results demonstrate the possibility of effectively suppressing the current flip and allowing for continued adiabatic evolution when the entanglement to gravonons is included. This adiabatic evolution is, however, a coherent evolution in high dimensional spacetime and cannot be understood as a solution of Schr\“odinger’s time dependent equation in four dimensional spacetime. Compared to Schr\“odinger’s time development, the evolution is considerably slowed down, though still adiabatic. The properties of our model reflect correctly the experimentally found behaviour of the D-Wave machine and explain the factor of 108 discrepancy between decoherence time and quantum computation time. The observation and our explanation are in analogy to the 108 discrepancy factor found, when comparing experimental results on adsorbate quantum diffusion rate with predictions of Schr\“odinger’s time dependent equation, which can also be resolved in a model with the coupling to gravonons included
Improving the lifetime of the NV center ensemble coupled with a superconducting flux qubit by applying magnetic fields
One of the promising systems to realize quantum computation is a hybrid system where a superconducting flux qubit plays a role of a quantum processor and the NV center ensemble is used
as a quantum memory. We have theoretically and experimentally studied the effect of magnetic fields on this hybrid system, and found that the lifetime of the vacuum Rabi oscillation is improved by applying a few mT magnetic field to the NV center ensemble. Here, we construct a theoretical model to reproduce the vacuum Rabi oscillations with/without magnetic fields applied to the NV centers, and we determine the reason why magnetic fields can affect the coherent properties of the NV center ensemble. From our theoretical analysis, we quantitatively show that the magnetic fields actually suppress the inhomogeneous broadening from the strain in the NV centers.
29
Mä
2015
Realization of microwave quantum circuits using hybrid superconducting-semiconducting nanowire Josephson elements
We report the realization of quantum microwave circuits using hybrid superconductor-semiconductor Josephson elements comprised of InAs nanowires contacted by NbTiN. Capacitively-shunted
single elements behave as transmon qubits with electrically tunable transition frequencies. Two-element circuits also exhibit transmon-like behavior near zero applied flux, but behave as flux qubits at half the flux quantum, where non-sinusoidal current-phase relations in the elements produce a double-well Josephson potential. These hybrid Josephson elements are promising for applications requiring microwave superconducting circuits operating in magnetic field.
28
Mä
2015
A Semiconductor Nanowire-Based Superconducting Qubit
We introduce a hybrid qubit based on a semiconductor nanowire with an epitaxially grown superconductor layer. Josephson energy of the transmon-like device („gatemon“) is
controlled by an electrostatic gate that depletes carriers in a semiconducting weak link region. Strong coupling to an on-chip microwave cavity and coherent qubit control via gate voltage pulses is demonstrated, yielding reasonably long relaxation times (0.8 {\mu}s) and dephasing times (1 {\mu}s), exceeding gate operation times by two orders of magnitude, in these first-generation devices. Because qubit control relies on voltages rather than fluxes, dissipation in resistive control lines is reduced, screening reduces crosstalk, and the absence of flux control allows operation in a magnetic field, relevant for topological quantum information.
27
Mä
2015
Optimal design for the Josephson mixer
We present an optimal design in terms of gain, bandwidth and dynamical range for the Josephson mixer, the superconducting circuit performing three-wave mixing at microwave frequencies.
In a compact all lumped-element based circuit with galvanically coupled ports, we demonstrate non degenerate amplification for microwave signals over a bandwidth up to 50 MHz for a power gain of 20 dB. The quantum efficiency of the mixer is shown to be about 70% and its dynamical range reaches 5 quanta per inverse dynamical bandwidth.