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
01
Apr
2015
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.
23
Mä
2015
Relativistic Motion with Superconducting Qubits
We show how the dynamical modulation of the qubit-field coupling strength in a circuit quantum electrodynamics architecture mimics the motion of the qubit at relativistic speeds. This
allows us to propose a realistic experiment to detect microwave photons coming from simulated acceleration radiation. Moreover, by combining this technique with the dynamical Casimir physics, we enhance the toolbox for studying relativistic phenomena in quantum field theory with superconducting circuits.
20
Mä
2015
YBCO microwave resonators for strong collective coupling with spin ensembles
Coplanar microwave resonators made of 330 nm-thick superconducting YBCO have been realized and characterized in a wide temperature (T, 2-100 K) and magnetic field (B, 0-7 T) range.
The quality factor Q significantly exceeds 104 below 55 K and slightly decreases for increasing fields, remaining 90% of Q(B=0) for B=7 T and T=2 K. These features allow to coherently couple resonant photons with spin ensembles at finite temperature and magnetic field. To demonstrate this, collective strong coupling regime was achieved by using the spin ensemble of a DPPH organic radical placed at the magnetic antinode of the fundamental mode: the in-plane magnetic field is used to tune the spin frequency gap across the single-mode cavity resonance at 7.78 GHz, where clear anticrossings are observed with a splitting as large as ∼82 MHz at T=2 K. The spin-cavity collective coupling rate is shown to scale as the square root of the number of active spins in the ensemble.
19
Mä
2015
Determination and correction of persistent biases in quantum annealers
Calibration of quantum computing technologies is essential to the effective utilization of their quantum resources. Specifically, the performance of quantum annealers is likely to be
significantly impaired by noise in their programmable parameters, effectively misspecification of the computational problem to be solved, often resulting in spurious suboptimal solutions. We developed a strategy to determine and correct persistent, systematic biases between the actual values of the programmable parameters and their user-specified values. We applied the recalibration strategy to two D-Wave Two quantum annealers, one at NASA Ames Research Center in Moffett Field, California, and another at D-Wave Systems in Burnaby, Canada. We show that the recalibration procedure not only reduces the magnitudes of the biases in the programmable parameters but also enhances the performance of the device on a set of random benchmark instances.
Josephson-Kondo screening cloud in circuit quantum electrodynamics
We show that the non-local polarization response in a multimode circuit-QED setup, devised from a Cooper pair box coupled to a long chain of Josephson junctions, provides an alternative
route to access the elusive Kondo screening cloud. For moderate circuit impedance, we compute analytically the universal lineshape for the decay of the charge susceptibility along the circuit, that relates to spatial entanglement between the qubit and its electromagnetic environment. At large circuit impedance, we numerically find further spatial correlations that are specific to a true many-body state.