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
30
Jan
2015
Ising interaction between capacitively-coupled superconducting flux qubits
Here, we propose a scheme to generate a controllable Ising interaction between superconducting flux qubits. Existing schemes rely on inducting couplings to realize Ising interactions
between flux qubits, and the interaction strength is controlled by an applied magnetic field On the other hand, we have found a way to generate an interaction between the flux qubits via capacitive couplings. This has an advantage in individual addressability, because we can control the interaction strength by changing an applied voltage that can be easily localized. This is a crucial step toward the realizing superconducting flux qubit quantum computation.
Digital quantum simulation of fermionic models with a superconducting circuit
Simulating quantum physics with a device which itself is quantum mechanical, a notion Richard Feynman originated, would be an unparallelled computational resource. However, the universal
quantum simulation of fermionic systems is daunting due to their particle statistics, and Feynman left as an open question whether it could be done, because of the need for non-local control. Here, we implement fermionic interactions with digital techniques in a superconducting circuit. Focusing on the Hubbard model, we perform time evolution with constant interactions as well as a dynamic phase transition with up to four fermionic modes encoded in four qubits. The implemented digital approach is universal and allows for the efficient simulation of fermions in arbitrary spatial dimensions. We use in excess of 300 single-qubit and two-qubit gates, and reach global fidelities which are limited by gate errors. This demonstration highlights the feasibility of the digital approach and opens a viable route towards analog-digital quantum simulation of interacting fermions and bosons in large-scale solid state systems.
23
Jan
2015
Interfacing microwave qubits and optical photons via spin ensembles
A protocol is discussed which allows one to realize a transducer for single photons between the optical and the microwave frequency range. The transducer is a spin ensemble, where the
individual emitters possess both an optical and a magnetic-dipole transition. Reversible frequency conversion is realized by combining optical photon storage, by means of EIT, with the controlled switching of the coupling between the magnetic-dipole transition and a superconducting qubit, which is realized by means of a microwave cavity. The efficiency is quantified by the global fidelity for transferring coherently a qubit excitation between a single optical photon and the superconducting qubit. We test various strategies and show that the total efficiency is essentially limited by the optical quantum memory: It can exceed 80% for ensembles of NV centers and approaches 99% for cold atomic ensembles, assuming state-of-the-art experimental parameters. This protocol allows one to bridge the gap between the optical and the microwave regime so to efficiently combine superconducting and optical components in quantum networks.
20
Jan
2015
A V-shape superconducting artificial atom based on two inductively coupled transmons
Circuit quantum electrodynamics systems are typically built from resonators and two-level artificial atoms, but the use of multi-level artificial atoms instead can enable promising
applications in quantum technology. Here we present an implementation of a Josephson junction circuit dedicated to operate as a V-shape artificial atom. Based on a concept of two internal degrees of freedom, the device consists of two transmon qubits coupled by an inductance. The Josephson nonlinearity introduces a strong diagonal coupling between the two degrees of freedom that finds applications in quantum non-demolition readout schemes, and in the realization of microwave cross-Kerr media based on superconducting circuits.
16
Jan
2015
Theory of microwave single-photon detection using an impedance-matched Λ system
By properly driving a qubit-resonator system in the strong dispersive regime, we implement an „impedance-matched“ Λ system in the dressed states, where a resonant single
photon deterministically induces a Raman transition and excites the qubit. Combining this effect and a fast dispersive readout of the qubit, we realize a detector of itinerant microwave photons. We theoretically analyze the single-photon response of the Λ system and evaluate its performance as a detector. We achieve a high detection efficiency close to unity without relying on precise temporal control of the input pulse shape and under a conservative estimate of the system parameters. The detector can also be reset quickly by applying microwave pulses, which allows a short dead time and a high repetition rate.
15
Jan
2015
Flux noise in superconducting circuits: Electron versus nuclear spins
Superconducting Quantum Interference Devices (SQUIDs) and other superconducting circuits are limited by intrinsic flux noise whose origin is believed to be due to spin impurities. We
present a flux vector model for the interaction of spins with thin-film superconducting wires, and show how measurements of flux as a function of the direction of an external magnetic field applied in the plane of the wires can reveal the value of impurity spin quantum number and the nature of its interaction with the circuit. We describe a method to accurately calculate the flux produced by spin impurities in realistic superconducting thin-film wires, and show that the flux produced by each spin is much larger than anticipated by former calculations. Remarkably, flux noise power due to electron spins at the thin edge surface of the wires is found to be of similar magnitude as the one at the wide top surface. In addition, flux noise due to lattice nuclear spins in the bulk of the wires is found to be approximately 5% of the total noise power. We discuss the relative importance of electron and nuclear spin species in determining flux noise, and propose strategies to design low noise SQUIDs.
14
Jan
2015
Proposal for a flux qubit in a dc SQUID with the 4π period Josephson effect
Constructing qubits which are suitable for quantum computation remains a notable challenge. Here, we propose a superconducting flux qubit in a dc SQUID structure, formed by a conventional
insulator Josephson junction and a topological nanowire Josephson junction with Majorana bound states. The zero energy Majorana bound states transport 4π period Josephson currents in the nanowire junction. The interplay between this 4π period Josephson effect and the convectional 2π period Josephson effect in the insulator junction induces a double-well potential energy landscape in the SQUID. As a result, the two lowest energy levels of the SQUID are isolated from other levels. These two levels show contradicting circulating supercurrents, thus can be used as a flux qubit. We reveal that this flux qubit has the merits of stability to external noises, tolerance to the deviation of system parameters, and scalability to large numbers. Furthermore, we demonstrate how to couple this flux qubit with the Majorana qubit by tuning the junction parameters, and how to use this coupling to manipulate the Majorana qubit.
13
Jan
2015
Dipolar Spin Models with Arrays of Superconducting Qubits
We propose a novel platform for quantum many body simulations of dipolar spin models using current circuit QED technology. Our basic building blocks are 3D Transmon qubits where we
use the naturally occurring dipolar interactions to realize interacting spin systems. This opens the way toward the realization of a broad class of tunable spin models in both two- and one-dimensional geometries. We illustrate the potential offered by these systems in the context of dimerized Majumdar-Ghosh-type phases, archetypical examples of quantum magnetism, showing how such phases are robust against disorder and decoherence, and could be observed within state-of-the-art experiments.
07
Jan
2015
Microwave multimode memory with an Er3+:Y2SiO5 spin ensemble
Interfacing photonic and solid-state qubits within a hybrid quantum architecture offers a promising route towards large scale distributed quantum computing. In that respect, hybrid
quantum systems combining circuit QED with ions doped into solids are an attractive platform. There, the ions serve as coherent memory elements and reversible conversion elements of microwave to optical qubits. Among many possible spin-doped solids, erbium ions offer the unique opportunity of a coherent conversion of microwave photons into the telecom C-band at 1.54μm employed for long distance communication. In our work, we perform a time-resolved electron spin resonance study of an Er3+:Y2SiO5 spin ensemble at milli-Kelvin temperatures and demonstrate multimode storage and retrieval of up to 16 coherent microwave pulses. The memory efficiency is measured to be 10−4 at the coherence time of T2=5.6μs.
Two-photon lasing by a superconducting qubit
We study the response of a magnetic-field-driven superconducting qubit strongly coupled to a superconducting coplanar waveguide resonator. We observed a strong amplification/damping
of a probing signal at different resonance points corresponding to a one and two-photon emission/absorption. The sign of the detuning between the qubit frequency and the probe determines whether amplification or damping is observed. The larger blue detuned driving leads to two-photon lasing while the larger red detuning cools the resonator. Our experimental results are in good agreement with the theoretical model of qubit lasing and cooling at the Rabi frequency.