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
Mä
2016
Landau-Zener-Stückelberg-Majorana lasing in circuit QED
We demonstrate amplification (and attenuation) of a probe signal by a driven two-level quantum system in the Landau-Zener regime. In the experiment, a superconducting qubit was strongly
coupled to a microwave cavity, the conventional arrangement of circuit quantum electrodynamics. Two different types of flux qubits show a similar result, lasing at the points where amplification takes place. The experimental data are explained by the interaction of the probe signal with Rabi-like oscillations. The latter are created by constructive interference of Landau-Zener-St\“{u}ckelberg-Majorana (LZSM) transitions during the driving period of the qubit. A detailed description of the occurrence of these oscillations and a comparison of obtained data with both analytic and numerical calculations are given.
29
Feb
2016
Entanglement concentration of microwave photons based on Kerr effect in circuit QED
Microwave photons are interesting qubits for quantum information processing. Here, we present the first scheme for the entanglement concentration on microwave photons, resorting to
the cross-Kerr effect in circuit quantum electrodynamics (QED). Two superconducting transmission line resonators (TLRs) coupled to two transmon qubits with the N-type level structure induce the effective cross-Kerr effect for realizing the quantum nondemolition (QND) measurement on microwave photons in entanglement concentration. With this device, we present a two-qubit polarization parity QND detector on the photon states of the superconducting TLRs, which can be used to concentrate efficiently the non-maximally entangled states of microwave photons assisted by several linear microwave elements. This scheme may be useful for solid-state quantum information processing.
25
Feb
2016
Dissipative optomechanical preparation of macroscopic quantum superposition states
The transition from quantum to classical physics remains an intensely debated question even though it has been investigated for more than a century. Further clarifications could be
obtained by preparing macroscopic objects in spatial quantum superpositions and proposals for generating such states for nano-mechanical devices either in a transient or a probabilistic fashion have been put forward. Here we introduce a method to deterministically obtain spatial superpositions of arbitrary lifetime via dissipative state preparation. In our approach, we engineer a double-well potential for the motion of the mechanical element and drive it towards the ground state, which shows the desired spatial superposition, via optomechanical sideband cooling. We propose a specific implementation based on a superconducting circuit coupled to the mechanical motion of a lithium-decorated monolayer graphene sheet, introduce a method to verify the mechanical state by coupling it to a superconducting qubit, and discuss its prospects for testing collapse models for the quantum to classical transition.
19
Feb
2016
Recurrent delocalization and quasi-equilibration of photons in coupled circuit QED systems
We explore the photon population dynamics in two coupled circuit QED systems. For a sufficiently weak inter-cavity photon hopping, as the photon-cavity coupling increases, the dynamics
undergoes double transitions first from a delocalized to a localized phase and then from the localized to another delocalized phase. The latter delocalized phase is distinguished from the former one; instead of oscillating between the two cavities, the photons rapidly quasi-equilibrate over the two cavities. These intrigues are attributed to an interplay between two qualitatively distinctive nonlinear behaviors of the circuit QED systems in the utrastrong coupling regime, whose distinction has been widely overlooked.
18
Feb
2016
Inhibition of ground-state superradiance and light-matter decoupling in circuit QED
We study effective light-matter interactions in a circuit QED system consisting of a single LC resonator, which is coupled symmetrically to multiple superconducting qubits. Starting
from a minimal circuit model, we demonstrate that in addition to the usual collective qubit-photon coupling the resulting Hamiltonian contains direct qubit-qubit interactions, which prevent the otherwise expected superradiant phase transition in the ground state of this system. Moreover, these qubit-qubit interactions are responsible for an opposite mechanism, which at very strong couplings completely decouples the photon mode and projects the qubits into a highly entangled ground state. These findings shed new light on the controversy over the existence of superradiant phase transitions in cavity and circuit QED systems, and show that the physics of ultrastrong light-matter interactions in two- or multi-qubit settings differ drastically from the more familiar one qubit case.
Low-noise amplification and frequency conversion with a multiport microwave optomechanical device
High-gain amplifiers of electromagnetic signals operating near the quantum limit are crucial for quantum information systems and ultrasensitive quantum measurements. However, the existing
techniques have a limited gain-bandwidth product and only operate with weak input signals. Here we demonstrate a two-port optomechanical scheme for amplification and routing of microwave signals, a system that simultaneously performs high-gain amplification and frequency conversion in the quantum regime. Our amplifier, implemented in a two-cavity microwave optomechanical device, shows 41 dB of gain and has a high dynamic range, handling input signals up to 1013 photons per second, three orders of magnitude more than corresponding Josephson parametric amplifiers. We show that although the active medium, the mechanical resonator, is at a high temperature far from the quantum limit, only 4.6 quanta of noise is added to the input signal. Our method can be readily applied to a wide variety of optomechanical systems, including hybrid optical-microwave systems, creating a universal hub for signals at the quantum level.
A Reconfigurable Cryogenic Platform for the Classical Control of Scalable Quantum Computers
Recent advances in solid-state qubit technology are paving the way to fault-tolerant quantum computing systems. However, qubit technology is limited by qubit coherence time and by the
complexity of coupling the quantum system with a classical electronic infrastructure.
We propose an infrastructure, enabling to read and control qubits, that is implemented on a field-programmable gate array (FPGA). The FPGA platform supports functionality required by several qubit technologies and can operate physically close to the qubits over a temperature range from 4K to 300K. Extensive characterization of the platform over this temperature range revealed all major components (such as LUTs, MMCM, PLL, BRAM, IDELAY2) operate correctly and the logic speed is very stable. The stability is finally concretized by operating an integrated ADC with relatively stable performance over temperature.
17
Feb
2016
Emulating the one-dimensional Fermi-Hubbard model by a double chain of qubits
The Jordan-Wigner transformation maps a one-dimensional spin-1/2 system onto a Fermionic model without spin degree of freedom. Here we show that a double chain of qubits with XX and
ZZ couplings of neighboring qubits along and between the chains, respectively, can be mapped on a spin-full 1D Fermi-Hubbard model. The qubit system can thus be used to emulate the quantum properties of this model. We analyze physical implementations of such analog quantum simulators, including one based on transmon qubits, where the ZZ interaction arises due to an inductive coupling and the XX interaction due to a capacitive interaction. We propose protocols to gain confidence in the results of the simulation through measurements of local operators.
Using Spontaneous Emission of a Qubit as a Resource for Feedback Control
Persistent control of a transmon qubit is performed by a feedback protocol based on continuous weak measurement of its fluorescence. By driving the qubit and cavity with microwave signals
whose amplitudes depend linearly on the instantaneous values of the quadratures of the measured fluorescence field, we demonstrate the permanent stabilization of the qubit in any direction of the Bloch sphere. Using a Josephson mixer as a phase-preserving amplifier, it was possible to reach a total measurement efficiency η=35%, leading to a maximum of 59% of excitation and 44% of coherence for the stabilized states. The experiment demonstrates multiple-input multiple-output (MIMO) analog markovian feedback in the quantum regime.
16
Feb
2016
Engineered dissipative reservoir for microwave light using circuit optomechanics
Dissipation can significantly affect the quantum behaviour of a system and even completely suppress it. Counterintuitively, engineered dissipation enables the preparation of quantum
states as well as their stabilization. In cavity electro- and optomechanics, the control over mechanical oscillators relies on a dissipation hierarchy in which the electromagnetic energy decay rate significantly exceeds that of the mechanical oscillator. In contrast, recent theoretical work has considered the opposite regime in which the mechanical oscillator dissipation dominates and provides a cold dissipative reservoir to the electromagnetic degree of freedom. This novel regime allows to manipulate the electromagnetic mode and enables a new class of dissipative interactions. Here, we report on the experimental realization of this reversed dissipation regime in a microwave cavity optomechanical system. We directly evidence the preparation of a quasi-instantaneous, cold reservoir for a microwave field by on-demand decreasing or increasing the damping rate of the microwave mode, that corresponds to amplification and de-amplification of the microwave field. Moreover, we observe the onset of parametric instability, i.e. stimulated emission of microwaves (masing). The dissipative interaction additionally enables to operate the system as a low-noise, large-gain phase-preserving amplifier. Realizing a dissipative reservoir for microwave light is a requirement for the dissipative coupling of multiple cavity modes, which in turn forms the basis of dissipative quantum phase transitions, microwave entanglement schemes, and electromechanical quantum-limited amplifiers. Equally importantly, this interaction underpins recently predicted non-reciprocal devices, which would extend the available toolbox of quantum-limited microwave manipulation techniques.