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
18
Mai
2016
Holonomic quantum computation in the ultrastrong-coupling regime of circuit QED
We present an experimentally feasible scheme to implement holonomic quantum computation in the ultrastrong-coupling regime of light-matter interaction. The large anharmonicity and the
Z2 symmetry of the quantum Rabi model allow us to build an effective three-level {\Lambda}-structured artificial atom for quantum computation. The proposed physical implementation includes two gradiometric flux qubits and two microwave resonators where single-qubit gates are realized by a two-tone driving on one physical qubit, and a two-qubit gate is achieved with a time-dependent coupling between the field quadratures of both resonators. Our work paves the way for scalable holonomic quantum computation in ultrastrongly coupled systems.
10
Mai
2016
Engineering topological materials in microwave cavity arrays
We present a scalable architecture for the exploration of interacting topological phases of photons in arrays of microwave cavities, using established techniques from cavity and circuit
quantum electrodynamics. A time-reversal symmetry breaking (non-reciprocal) flux is induced by coupling the microwave cavities to ferrites, allowing for the production of a variety of topological band structures including the α=1/4 Hofstadter model. Effective photon-photon interactions are included by coupling the cavities to superconducting qubits, and are sufficient to produce a ν=1/2 bosonic Laughlin puddle. We demonstrate by exact diagonalization that this architecture is robust to experimentally achievable levels of disorder. These advances provide an exciting opportunity to employ the quantum circuit toolkit for the exploration of strongly interacting topological materials.
09
Mai
2016
A dual approach to circuit quantization using loop charges
The conventional approach to circuit quantization is based on node fluxes and traces the motion of node charges on the islands of the circuit. However, for some devices, the relevant
physics can be best described by the motion of polarization charges over the branches of the circuit that are in general related to the node charges in a highly nonlocal way. Here, we present a method, dual to the conventional approach, for quantizing planar circuits in terms of loop charges. In this way, the polarization charges are directly obtained as the differences of the two loop charges on the neighboring loops. The loop charges trace the motion of fluxes through the circuit loops. We show that loop charges yield a simple description of the flux transport across phase-slip junctions. We outline a concrete construction of circuits based on phase-slip junctions that are electromagnetically dual to arbitrary planar Josephson junction circuits. We argue that loop charges also yield a simple description of the flux transport in conventional Josephson junctions shunted by large impedances. We show that a mixed circuit description in terms of node fluxes and loop charges yields an insight into the flux decompactification of a Josephson junction shunted by an inductor. As an application, we show that the fluxonium qubit is well approximated as a phase-slip junction for the experimentally relevant parameters. Moreover, we argue that the 0-π qubit is effectively the dual of a Majorana Josephson junction.
06
Mai
2016
Optimized Coplanar Waveguide Resonators for a Superconductor-Atom Interface
We describe the design and characterization of superconducting coplanar waveguide cavities tailored to facilitate strong coupling between superconducting quantum circuits and single
trapped Rydberg atoms. For initial superconductor-atom experiments at 4.2 K, we show that resonator quality factors above 104 can be readily achieved. Furthermore, we demonstrate that the incorporation of thick-film copper electrodes at a voltage antinode of the resonator provides a route to enhance the zero-point electric fields of the resonator in a trapping region that is 40 μm above the chip surface, thereby minimizing chip heating from scattered trap light. The combination of high resonator quality factor and strong electric dipole coupling between the resonator and the atom should make it possible to achieve the strong coupling limit of cavity quantum electrodynamics with this system.
02
Mai
2016
Introduction to Quantum-limited Parametric Amplification of Quantum Signals with Josephson Circuits
This short and opinionated review starts with a concept of quantum signals at microwave frequencies and focuses on the principle of linear parametric amplification. The amplification
process arises from the dispersive nonlinearity of Josephson junctions driven with appropriate tones. We discuss two defining characteristics of these amplifiers: the number of modes receiving the signal, idler and pump waves and the number of independent ports through which these waves enter into the circuit.
22
Apr
2016
A coaxial line architecture for integrating and scaling 3D cQED systems
Numerous loss mechanisms can limit coherence and scalability of planar and 3D-based circuit quantum electrodynamics (cQED) devices, particularly due to their packaging. The low loss
and natural isolation of 3D enclosures make them good candidates for coherent scaling. We introduce a coaxial transmission line device architecture with coherence similar to traditional 3D cQED systems. Measurements demonstrate well-controlled external and on-chip couplings, a spectrum absent of cross-talk or spurious modes, and excellent resonator and qubit lifetimes. We integrate a resonator-qubit system in this architecture with a seamless 3D cavity, and separately pattern a qubit, readout resonator, Purcell filter and high-Q stripline resonator on a single chip. Device coherence and its ease of integration make this a promising tool for complex experiments.
16
Apr
2016
Signatures of topological quantum phase transitions in driven and dissipative qubit-arrays
We study photonic signatures of symmetry broken and topological phases in a driven, dissipative circuit QED realization of spin-1/2 chains. Specifically, we consider the transverse-field
XY model and a dual model with 3-spin interactions. The former has a ferromagnetic and a paramagnetic phase, while the latter features, in addition, a symmetry protected topological phase. Using the method of third quantization, we calculate the non-equilibrium steady-state of the open spin chains for arbitrary system sizes and temperatures. We find that the bi-local correlation function of the spins at both ends of the chain provides a sensitive measure for both symmetry-breaking and topological phase transitions of the systems, but no universal means to distinguish between the two types of transitions. Both models have equivalent representations in terms of free Majorana fermions, which host zero, one and two topological Majorana end modes in the paramagnetic, ferromagnetic, and symmetry protected topological phases, respectively. The correlation function we study retains its bi-local character in the fermionic representation, so that our results are equally applicable to the fermionic models in their own right. We propose a photonic realization of the dissipative transverse-field XY model in a tunable setup, where an array of superconducting transmon qubits is coupled at both ends to a photonic microwave circuit.
14
Apr
2016
Topological quantum fluctuations and travelling wave amplifiers
It is now well-established that photonic systems can exhibit topological energy bands; similar to their electronic counterparts, this leads to the formation of chiral edge modes which
can be used to transmit light in a manner that is protected against back-scattering. Most topological photonic states are completely analogous to their electronic counterpart, as they are based on single-particle physics: the topological invariants and edge states are identical in both the bosonic and fermionic case. Here, we describe a new kind of topological photonic state which has no electronic analogue. In our system, a non-zero topological invariant guarantees the presence of a parametrically-unstable chiral edge mode in a system with boundaries, even though there are no bulk-mode instabilities. We show that by stabilizing these unstable edge modes via coupling waveguides, one realizes a topologically protected, quantum-limited travelling-wave parametric amplifier. The device is protected against both internal losses and back-scattering; the latter feature is in stark contrast to standard travelling wave amplifiers. We show that the unstable edge mode also naturally serves as a topologically-protected source for non-classical squeezed light.
13
Apr
2016
Quantum coherence between cavity and artificial atom in a superconducting circuit QED ladder system
We have created a quantum three-level ladder system with the cavity dispersive energy level in a superconducting circuit quantum electrodynamics system consisting of a transmon qubit
and a cavity, and have directly observed the Autler-Townes splitting eff?ect instead of representing it by the probability of the qubit being at each level. A coupler tone is applied on the transition between the second excited state of transmon and cavity dispersive level, while the cavity spectrum is probed. A doublet transmission and anormalous dispersion spectrum of the cavity level is clearly shown. The inverse Fourier transform of cavity spectrum indicates that there is a quantum coherence Rabi oscillation of the populations between cavity and qubit.
11
Apr
2016
A universal gate for fixed-frequency qubits via a tunable bus
A challenge for constructing large circuits of superconducting qubits is to balance addressability, coherence and coupling strength. High coherence can be attained by building circuits
from fixed-frequency qubits, however, leading techniques cannot couple qubits that are far detuned. Here we introduce a method based on a tunable bus which allows for the coupling of two fixed-frequency qubits even at large detunings. By parametrically oscillating the bus at the qubit-qubit detuning we enable a resonant exchange (XX+YY) interaction. We use this interaction to implement a 183ns two-qubit iSWAP gate between qubits separated in frequency by 854MHz with a measured average fidelity of 0.9823(4) from interleaved randomized benchmarking. This gate may be an enabling technology for surface code circuits and for analog quantum simulation.