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
27
Jun
2014
Measurement and Control of Quasiparticle Dynamics in a Superconducting Qubit
Superconducting circuits have attracted growing interest in recent years as a promising candidate for fault-tolerant quantum information processing. Extensive efforts have always been
taken to completely shield these circuits from external magnetic field to protect the integrity of superconductivity. Surprisingly, here we show vortices can dramatically improve the performance of superconducting qubits by reducing the lifetimes of detrimental single-electron-like excitations known as quasiparticles. Using a contactless injection technique with unprecedented dynamic range, we directly demonstrate the power-law decay characteristics of the canonical quasiparticle recombination process, and show quantization of quasiparticle trapping rate due to individual vortices. Each vortex in our aluminium film shows a quasiparticle „trapping power“ of 0.067±0.005 cm2/s, enough to dominate over the vanishingly weak recombination in a modern transmon qubit. These results highlight the prominent role of quasiparticle trapping in future development of quantum circuits, and provide a powerful characterization tool along the way.
06
Jun
2014
Non-Poissonian Quantum Jumps of a Fluxonium Qubit due to Quasiparticle Excitations
As the energy relaxation time of superconducting qubits steadily improves, non-equilibrium quasiparticle excitations above the superconducting gap emerge as an increasingly relevant
limit for qubit coherence. We measure fluctuations in the number of quasiparticle excitations by continuously monitoring the spontaneous quantum jumps between the states of a fluxonium qubit, in conditions where relaxation is dominated by quasiparticle loss. Resolution on the scale of a single quasiparticle is obtained by performing quantum non-demolition projective measurements within a time interval much shorter than T1, using a quantum limited amplifier (Josephson Parametric Converter). The quantum jumps statistics switches between the expected Poisson distribution and a non-Poissonian one, indicating large relative fluctuations in the quasiparticle population, on time scales varying from seconds to hours. This dynamics can be modified controllably by injecting quasiparticles or by seeding quasiparticle-trapping vortices by cooling down in magnetic field.
27
Mai
2014
Multistability of a Josephson parametric amplifier coupled to a mechanical resonator
We study the dynamics of Josephson Parametric Amplifier (JPA) coupled to a mechanical oscillator, as realised with a dc Superconducting Quantum Interference Device (SQUID) with an embedded
movable arm. We analyse this system in the regime when the frequency of the mechanical oscillator is comparable in magnitude with the plasma oscillation of the SQUID. When the nano-mechanical resonator is driven, it strongly affects the dynamics of the JPA. We show that this coupling can considerably modify the dynamics of JPA and induce its multistability rather than common bistability. This analysis is relevant if one considers a JPA for detection of mechanical motion.
23
Mai
2014
Transparency and amplification in a hybrid system of mechanical resonator and circuit QED
We theoretically study the transparency and amplification of a weak probe field applied to the cavity in hy- brid systems formed by a driven superconducting circuit QED system and a
mechanical resonator, or a driven optomechanical system and a superconducting qubit. We find that both the mechanical resonator and the su- perconducting qubit can result in the transparency to a weak probe field in such hybrid systems when a strong driving field is applied to the cavity. We also find that the weak probe field can be amplified in some parameter regimes. We further study the statistical properties of the output field via the degrees of second-order coherence. We find that the nonclassicality of the output field strongly depends on the system parameters. Our studies show that one can control single-photon transmission in the optomechanical system via a tunable artificial atom or in the circuit QED system via a mechanical resonator.
22
Mai
2014
A chemical potential for light
Photons are not conserved in interactions with other matter. Consequently, when understanding the equation of state and thermodynamics of photons, while we have a concept of temperature
for energy conservation, there is no equivalent chemical potential for particle number conservation. However, the notion of a chemical potential is crucial in understanding a wide variety of single- and many-body effects, from transport in conductors and semi-conductors to phase transitions in electronic and atomic systems. Here we show how a direct modification of the system-bath coupling via parametric oscillation creates an effective chemical potential for photons even in the thermodynamic limit. Specific implementations, using circuit-QED or optomechanics, are feasible using current technologies, and we show a detailed example demonstrating the emergence of Mott Insulator-superfluid transition in a lattice of nonlinear oscillators. Our approach paves the way for quantum simulation, quantum sources and even electron-like circuits with light.
Microwave Down-Conversion with an Impedance-Matched Λ System in Driven Circuit QED
By driving a dispersively coupled qubit-resonator system, we realize an „impedance-matched“ Λ system that has two identical radiative decay rates from the top level and
interacts with a semi-infinite waveguide. It has been predicted that a photon input from the waveguide deterministically induces a Raman transition in the system and switches its electronic state. We confirm this through microwave response to a continuous probe field, observing near-perfect (99.7%) extinction of the reflection and highly efficient (74%) frequency down-conversion. These proof-of-principle results lead to deterministic quantum gates between material qubits and microwave photons and open the possibility for scalable quantum networks interconnected with waveguide photons.
Digital Quantum Rabi and Dicke Models in Superconducting Circuits
We propose the analog-digital quantum simulation of the quantum Rabi and Dicke models using circuit quantum electrodynamics (QED). We find that all physical regimes, in particular those
which are impossible to realize in typical cavity QED setups, can be simulated via unitary decomposition into digital steps. Furthermore, we show the emergence of the Dirac equation dynamics from the quantum Rabi model when the mode frequency vanishes. Finally, we analyze the feasibility of this proposal under realistic superconducting circuit scenarios.
20
Mai
2014
Advanced Concepts in Josephson Junction Reflection Amplifiers
Low-noise amplification atmicrowave frequencies has become increasingly important for the research related to superconducting qubits and nanoelectromechanical systems. The fundamental
limit of added noise by a phase-preserving amplifier is the standard quantum limit, often expressed as noise temperature Tq=ℏω/2kB. Towards the goal of the quantum limit, we have developed an amplifier based on intrinsic negative resistance of a selectively damped Josephson junction. Here we present measurement results on previously proposed wide-band microwave amplification and discuss the challenges for improvements on the existing designs. We have also studied flux-pumped metamaterial-based parametric amplifiers, whose operating frequency can be widely tuned by external DC-flux, and demonstrate operation at 2ω pumping, in contrast to the typical metamaterial amplifiers pumped via signal lines at ω.
Tuning quantum discord in Josephson charge qubits system
A type of two qubits Josephson charge system is constructed in this paper, and properties of the quantum discord (QD) as well as the differences between thermal QD and thermal entanglement
were investigated. A detailed calculation shows that the magnetic flux ΦXk is more efficient than the voltage VXi in tuning QD. By choosing proper system parameters, one can realize the maximum QD in our two qubits Josephson charge system.
15
Mai
2014
New signatures of the dynamical Casimir effect in a superconducting circuit
We found new signatures of the dynamical Casimir effect (DCE) in the context of superconducting circuits. We show that if the recent experiment made by Wilson {it et al}, which brought
the DCE into reality for the first time, is repeated with slight modifications (for instance, different values for the capacitance of the SQUID), three remarkable results will show up, namely: {\it (i)} a quite different spectral distribution for the created particles, deviating from the typical parabolic shape; {\it (ii)} an enhancement by a factor of approximately 5×10^3 in the number of created particles with half driven frequency of the effective moving mirror and {\it (iii)} an enhancement by a factor of 3×10^2 in the particle creation rate. These results may guide the experimentalists in their search for alternative routes to observe the DCE in future experiments.