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
13
Jul
2017
Complete Bell-state analysis for superconducting-quantum-interference-device qubits with transitionless tracking algorithm
In this paper, we propose a protocol for complete Bell-state analysis for two superconducting-quantum-interference-device qubits. The Bell-state analysis could be completed by using
a sequence of microwave pulses designed by the transition- less tracking algorithm, which is an useful method in the technique of shortcut to adiabaticity. After the whole process, the information for distinguishing four Bell states will be encoded on two auxiliary qubits, while the Bell states keep unchanged. One can read out the information by detecting the auxiliary qubits. Thus the Bell-state analysis is nondestructive. The numerical simulations show that the protocol possesses high success probability of distinguishing each Bell state with current experimental technology even when decoherence is taken into account. Thus, the protocol may have potential applications for the information readout in quantum communications and quantum computations in superconducting quantum networks.
11
Jul
2017
Spatially Adiabatic Frequency Conversion in Optoelectromechanical Arrays
Faithful conversion of quantum signals between microwave and optical frequency domains is crucial for building quantum networks based on superconducting circuits. Optoelectromechanical
systems, in which microwave and optical cavity modes are coupled to a common mechanical oscillator, are a promising route towards this goal. In these systems, efficient, low-noise conversion is possible using mechanically dark mode of the fields but the conversion bandwidth is limited to a fraction of the cavity linewidth. Here, we show that an array of optoelectromechanical transducers can overcome this limitation and reach a bandwidth that is larger than the cavity linewidth. The coupling rates are varied throughout the array so that the mechanically dark mode of the propagating fields adiabatically changes from microwave to optical or vice versa. Our approach opens a new route towards frequency conversion with optomechanical systems.
07
Jul
2017
Microwave photonics with superconducting quantum circuits
In the past 20 years, impressive progress has been made both experimentally and theoretically in superconducting quantum circuits, which provide a platform for manipulating microwave
photons. This emerging field of superconducting quantum microwave circuits has been driven by many new interesting phenomena in microwave photonics and quantum information processing. For instance, the interaction between superconducting quantum circuits and single microwave photons can reach the regimes of strong, ultra-strong, and even deep-strong coupling. Many higher-order effects, unusual and less familiar in traditional cavity quantum electrodynamics with natural atoms, have been experimentally observed, e.g., giant Kerr effects, multi-photon processes, and single-atom induced bistability of microwave photons. These developments may lead to improved understanding of the counterintuitive properties of quantum mechanics, and speed up applications ranging from microwave photonics to superconducting quantum information processing. In this article, we review experimental and theoretical progress in microwave photonics with superconducting quantum circuits. We hope that this global review can provide a useful roadmap for this rapidly developing field.
Quantum interface between photonic and superconducting qubits
We show that optically active coupled quantum dots embedded in a superconducting microwave cavity can be used to realize a fast quantum interface between photonic and transmon qubits.
Single photon absorption by a coupled quantum dot results in generation of a large electric dipole, which in turn ensures efficient coupling to the microwave cavity. Using cavity parameters achieved in prior experiments, we estimate that bi-directional microwave-optics conversion in nanosecond timescales with efficiencies approaching unity is experimentally feasible with current technology. We also outline a protocol for in-principle deterministic quantum state transfer from a time-bin photonic qubit to a transmon qubit. Recent advances in quantum dot based quantum photonics technologies indicate that the scheme we propose could play a central role in connecting quantum nodes incorporating cavity-coupled superconducting qubits.
06
Jul
2017
Improving superconducting resonators in magnetic fields by reduced field-focussing and engineered flux screening
We experimentally investigate superconducting coplanar waveguide resonators in external magnetic fields and present two strategies to reduce field-induced dissipation channels and resonance
frequency shifts. One of our approaches is to significantly reduce the superconducting ground-plane areas, which leads to reduced magnetic field-focussing and thus to lower effective magnetic fields inside the waveguide cavity. By this measure, the field-induced losses can be reduced by more than one order of magnitude in mT out-of-plane magnetic fields. When these resonators are additionally coupled inductively instead of capacitively to the microwave feedlines, an intrinsic closed superconducting loop is effectively shielding the heart of the resonator from magnetic fields by means of flux conservation. In total, we achieve a reduction of the field-induced resonance frequency shift by up to two orders of magnitude. We combine systematic parameter variations on the experimental side with numerical magnetic field calculations to explain the effects of our approaches and to support our conclusions. The presented results are relevant for all areas, where high-performance superconducting resonators need to be operated in magnetic fields, e.g. for quantum hybrid devices with superconducting circuits or electron spin resonance detectors based on coplanar waveguide cavities.
Josephson Vortex Qubit based on a Confocal Annular Josephson Junction
We report theoretical and experimental work on the development of a Josephson vortex qubit based on a confocal annular Josephson tunnel junction (CAJTJ). The key ingredient of this
geometrical configuration is a periodically variable width that generates a spatial vortex potential with bistable states. This intrinsic vortex potential can be tuned by an externally applied magnetic field and tilted by a bias current. The two-state system is accurately modeled by a one-dimensional sine-Gordon like equation by means of which one can numerically calculate both the magnetic field needed to set the vortex in a given state as well as the vortex depinning currents. Experimental data taken at 4.2K on high-quality Nb/Al-AlOx/Nb CAJTJs with an individual trapped fluxon advocate the presence of a robust and finely tunable double-well potential for which reliable manipulation of the vortex state has been classically demonstrated. The vortex is prepared in a given potential by means of an externally applied magnetic field, while the state readout is accomplished by measuring the vortex-depinning current in a small magnetic field. Our proof of principle experiment convincingly demonstrates that the proposed vortex qubit based on CAJTJs is robust and workable.
05
Jul
2017
Universal stabilization of a parametrically coupled qubit
We autonomously stabilize arbitrary states of a qubit through parametric modulation of the coupling between a fixed frequency qubit and resonator. The coupling modulation is achieved
with a tunable coupler design, in which the qubit and the resonator are connected in parallel to a superconducting quantum interference device. This allows for quasi-static tuning of the qubit-cavity coupling strength from 12 MHz to more than 300 MHz. Additionally, the coupling can be dynamically modulated, allowing for single photon exchange in 6 ns. Qubit coherence times exceeding 20 μs are maintained over the majority of the range of tuning, limited primarily by the Purcell effect. The parametric stabilization technique realized using the tunable coupler involves engineering the qubit bath through a combination of photon non-conserving sideband interactions realized by flux modulation, and direct qubit Rabi driving. We demonstrate that the qubit can be stabilized to arbitrary states on the Bloch sphere with a worst-case fidelity exceeding 80 %.
03
Jul
2017
Realization of a Λ system with metastable states of a capacitively-shunted fluxonium
We realize a Λ system in a superconducting circuit, with metastable states exhibiting lifetimes up to 7ms. We exponentially suppress the tunneling matrix elements involved in spontaneous
energy relaxation by creating a „heavy“ fluxonium, realized by adding a capacitive shunt to the original circuit design. The device allows for both cavity-assisted and direct fluorescent readout, as well as state preparation schemes akin to optical pumping. Since direct transitions between the metastable states are strongly suppressed, we utilize Raman transitions for coherent manipulation of the states.
29
Jun
2017
Simultaneous continuous measurement of non-commuting observables: quantum state correlations
We consider the temporal correlations of the quantum state of a qubit subject to simultaneous continuous measurement of two non-commuting qubit observables. Such qubit state correlators
are defined for an ensemble of qubit trajectories, which has the same fixed initial state and can also be optionally constrained by a fixed final state. We develop a stochastic path integral description for the continuous quantum measurement and use it to calculate the considered correlators. Exact analytic results are possible in the case of ideal measurements of equal strength and are also shown to agree with solutions obtained using the Fokker-Planck equation. For a more general case with decoherence effects and inefficiency, we use a diagrammatic approach to find the correlators perturbatively in the quantum efficiency. We also calculate the state correlators for the quantum trajectories which are extracted from readout signals measured in a transmon qubit experiment, by means of the quantum Bayesian state update. We find an excellent agreement between the correlators based on the experimental data and those obtained from our analytical and numerical results.
Superconducting Qubit-Resonator-Atom Hybrid System
We propose a hybrid quantum system, where an LC resonator inductively interacts with a flux qubit and is capacitively coupled to a Rydberg atom. Varying the external magnetic flux bias
controls the flux-qubit flipping and the flux qubit-resonator interface. The atomic spectrum is tuned via an electrostatic field, manipulating the qubit-state transition of atom and the atom-resonator coupling. Different types of entanglement of superconducting, photonic, and atomic qubits can be prepared via simply tuning the flux bias and electrostatic field, leading to the implementation of three-qubit Toffoli logic gate.