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
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.
15
Mä
2015
Traveling wave parametric amplifier with Josephson junctions using minimal resonator phase matching
Josephson parametric amplifiers have become a critical tool in superconducting device physics due to their high gain and quantum-limited noise. Traveling wave parametric amplifiers
(TWPAs) promise similar noise performance while allowing for significant increases in both bandwidth and dynamic range. We present a TWPA device based on an LC-ladder transmission line of Josephson junctions and parallel plate capacitors using low-loss amorphous silicon dielectric. Crucially, we have inserted λ/4 resonators at regular intervals along the transmission line in order to maintain the phase matching condition between pump, signal, and idler and increase gain. We achieve an average gain of 12\,dB across a 4\,GHz span, along with an average saturation power of -92\,dBm with noise approaching the quantum limit.
13
Mä
2015
Simulation of quantum-mechanical supersymmetry in a Cooper-pair box shunted by a Josephson rhombus
Supersymmetries in quantum mechanics offer a way to obtain degeneracies in the excitation spectrum which do not originate from selection rules. The mechanism behind the degeneracies
is the same as the one that leads to the miraculous cancellations of divergences in supersymmetric field theories found in the high energy physics context. Even though of importance, there is up to now no realistic proposal of non-integrable systems that show level degeneracies due to a supersymmetric structure. Here, we propose an implementation of a quantum-mechanical supersymmetry in a Cooper-pair box shunted by a Josephson junction rhombus which is effectively π-periodic in the superconducting phase difference. For a characteristic ratio between the strength of the 2π- and the π-periodic junction, we find a two-fold degeneracy of all the energy levels all the way from the weak junction/charge qubit limit to the strong junction/transmon regime. We provide explicit values for the parameters of the rhombus and show that tuning in and out of the supersymmetric point is easily achieved by varying an external gate voltage. We furthermore discuss a microwave experiment to detect the supersymmetry and conclude that it could indeed be simulated with currently existing Josephson junction technology.
Searching for quantum speedup in quasistatic quantum annealers
We argue that a quantum annealer at very long annealing times is likely to experience a quasistatic evolution, returning a final population that is close to a Boltzmann distribution
of the Hamiltonian at a single (freeze-out) point during the annealing. Such a system is expected to correlate well with a proper quantum Monte Carlo simulation. It may also correlate with simulated annealing if at the freeze-out point the quantum energy eigenvalues are close to the classical ones. These correlations are just signatures of equilibration and do not mean that the evolution of the quantum annealer is classical or can be simulated by quantum Monte Carlo. The computation time extracted from such a distribution reflects the equilibrium behavior and therefore provides no information about the underlying quantum dynamics. This makes the search for quantum speedup in suboptimal quantum annealers problematic. We use an open quantum master equation with realistic parameters to illustrate quasistatic evolution in a 16 qubit quantum annealer and discuss its implication for quantum speedup.
05
Mä
2015
Single qudit realization of the Deutsch algorithm using superconducting many-level quantum circuits
Design of a large-scale quantum computer has paramount importance for science and technologies. We investigate a scheme for realization of quantum algorithms using noncomposite quantum
systems, i.e., systems without subsystems. In this framework, n artificially allocated „subsystems“ play a role of qubits in n-qubits quantum algorithms. With focus on two-qubit quantum algorithms, we demonstrate a realization of the universal set of gates using a d=5 single qudit state. Manipulation for an ancillary level in the systems allows effective implementation of operators from U(4) group via operators from SU(5) group. Using a possible experimental realization of such systems through anharmonic superconducting many-level quantum circuits, we present a blueprint for a single qudit realization of the Deutsch algorithm, which generalizes previously studied realization based on the virtual spin representation [A.R. Kessel et al., Phys. Rev. A 66, 062322 (2002)].
Interacting two-level defects as sources of fluctuating high-frequency noise in superconducting circuits
Since the very first experiments, superconducting circuits have suffered from strong coupling to environmental noise, destroying quantum coherence and degrading performance. In state-of-the-art
experiments it is found that the relaxation time of superconducting qubits fluctuates as a function of time. We present measurements of such fluctuations in a 3D-Transmon circuit and develop a qualitative model based on interactions within a bath of background two-level systems (TLS) which emerge from defects in the device material. Assuming both high- and low-frequency TLS are present, their mutual interaction will lead to fluctuations in the noise spectral density acting on the qubit circuit. This model is further supported by direct measurements of energy fluctuations in a single high-frequency TLS.
Dynamical Lamb Effect in a Tunable Superconducting Qubit-Cavity System
We suggest that a transmission line cavity coupled with a superconducting qubit can be used for the experimental investigation of the dynamical Lamb effect, which can be viewed as an
atom excitation due to the nonadiabatic modulation of atomic level Lamb shift. The qubit (artificial macroscopic atom) and resonator can be integrated in a tunable way. By varying nonadiabatically the coupling strength, it is possible to parametrically excite the qubit. This approach allows one to get rid of Casimir photons and thus to isolate the mechanism of the qubit excitation due to the dynamical Lamb effect from another mechanism due to the Casimir photons absorbtion. We evaluate a qubit excitation probability within the Jaynes-Cummings model using the perturbation theory and also numerically. We argue that the most efficient method to increase an excitation probability is a periodic driving of a qubit-resonator coupling constant. We also study a statistics of photon states and show that a significant squeezing can be obtained by using a suggested approach.