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
16
Feb
2017
Observing a quantum Maxwell demon at work
In apparent contradiction to the laws of thermodynamics, Maxwell’s demon is able to cyclically extract work from a system in contact with a thermal bath exploiting the information
about its microstate. The resolution of this paradox required the insight that an intimate relationship exists between information and thermodynamics. Here, we realize a Maxwell demon experiment that tracks the state of each constituent both in the classical and quantum regimes. The demon is a microwave cavity that encodes quantum information about a superconducting qubit and converts information into work by powering up a propagating microwave pulse by stimulated emission. Thanks to the high level of control of superconducting circuits, we directly measure the extracted work and quantify the entropy remaining in the demon’s memory. This experiment provides an enlightening illustration of the interplay of thermodynamics with quantum information.
15
Feb
2017
Casimir forces in transmission-line circuits: QED and fluctuation-dissipation formalisms
Transmission-line waveguides can mediate long-range fluctuation-induced forces between neutral objects. We present two approaches for the description of these forces between electric
components embedded in transmission-line circuits. The first, following ordinary quantum electrodynamics (QED), consists of the quantization and scattering theory of voltage and current waves inside transmission lines. The second approach relies on a simple circuit analysis with additional noisy current sources due to resistors in the circuit, as per the fluctuation-dissipation theorem (FDT). We apply the latter approach to derive a general formula for the Casimir force induced by circuit fluctuations between any two impedances. The application of this formula, considering the sign of the resulting force, is discussed. While both QED and FDT approaches are equivalent, we conclude that the latter is simpler to generalize and solve.
08
Feb
2017
Tunable Superconducting Qubits with Flux-Independent Coherence
We have studied the impact of low-frequency magnetic flux noise upon superconducting transmon qubits with various levels of tunability. We find that qubits with weaker tunability exhibit
dephasing that is less sensitive to flux noise. This insight was used to fabricate qubits where dephasing due to flux noise was suppressed below other dephasing sources, leading to flux-independent dephasing times T2* ~ 15 us over a tunable range of ~340 MHz. Such tunable qubits have the potential to create high-fidelity, fault-tolerant qubit gates and fundamentally improve scalability for a quantum processor.
07
Feb
2017
Prospective two orders of magnitude enhancement in direct magnetic coupling of a single-atom spin to a circuit resonator
We report on the challenges and limitations of direct coupling of the magnetic field from a circuit resonator to an electron spin bound to a donor potential. We propose a device consisting
of a lumped-element superconducting resonator and a single donor implanted in enriched 28Si. The resonator, in contrast to coplanar waveguide resonators, includes a nano-scale spiral inductor to spatially focus the magnetic field from the photons within. The design promises approximately two orders of magnitude increase in the local magnetic field, and thus the spin to photon coupling rate g, compared to the estimated coupling rate to coplanar transmission-line resonators. We show that by using niobium (aluminum) as the resonator’s superconductor and a single phosphorous (bismuth) atom as the donor, a coupling rate of g/2π=0.24 MHz (0.39 MHz) can be achieved in the small photon limit. For this truly linear cavity quantum electrodynamic system, such enhancement in g is sufficient to enter the strong coupling regime.
06
Feb
2017
Realizing and manipulating space-time inversion symmetric topological semimetal bands with superconducting quantum circuits
We have experimentally realized novel space-time inversion (P-T) invariant Z2-type topological semimetal-bands, via an analogy between the momentum space and a controllable parameter
space in superconducting quantum circuits. By measuring the whole energy spectrum of system, we imaged clearly an exotic tunable gapless band structure of topological semimetals. Two topological quantum phase transitions from a topological semimetal to two kinds of insulators can be manipulated by continuously tuning the different parameters in the experimental setup, one of which captures the Z2 topology of the PT semimetal via merging a pair of nontrivial Z2 Dirac points. Remarkably, the topological robustness was demonstrated unambiguously, by adding a perturbation that breaks only the individual T and P symmetries but keeps the joint PT symmetry. In contrast, when another kind of PT -violated perturbation is introduced, a topologically trivial insulator gap is fully opened.
03
Feb
2017
Demonstrating a gyrator operation using Josephson mixers
Nonreciprocal microwave devices such as circulators are useful in routing quantum signals in quantum networks and protecting quantum systems against noise coming from the detection
chain. However, commercial, cryogenic circulators, used nowadays, are unsuitable for scalable superconducting quantum architectures due to their appreciable size, loss, and inherent magnetic field. In this work, we report on the measurement of a key nonreciprocal element, i.e., the gyrator, which can be used to realize a circulator. Unlike state-of-the-art gyrators, which use a magneto-optic effect to induce a phase shift of π between transmitted signals in opposite directions, our device uses the phase nonreciprocity of a Josephson-based three-wave mixing device. By coupling two of these mixers and operating them in noiseless frequency conversion mode, we show that the device acts as a nonreciprocal phase shifter, whose phase shift is controlled by the phase difference of the microwave tones driving the mixers. Such a device could be used to realize a lossless, on-chip, superconducting circulator suitable for quantum information processing applications.
02
Feb
2017
3-Wave Mixing Josephson Dipole Element
Parametric conversion and amplification based on three-wave mixing are powerful primitives for efficient quantum operations. For superconducting qubits, such operations can be realized
with a quadrupole Josephson junction element, the Josephson Ring Modulator (JRM), which behaves as a loss-less three-wave mixer. However, combining multiple quadrupole elements is a difficult task so it would be advantageous to have a pure three-wave dipole element that could be tessellated for increased power handling and/or information throughput. Here, we present a novel dipole circuit element with third-order nonlinearity, which implements three-wave mixing while minimizing harmful Kerr terms present in the JRM. Experimental results for a non-degenerate amplifier based on the proposed pure third-order nonlinearity are reported.
Analogue Cosmological Particle Generation in a Superconducting Circuit
We propose the use of a waveguide-like transmission line based on direct-current superconducting quantum interference devices (dc-SQUID) and demonstrate that the node flux in this transmission
line behaves in the same way as quantum fields in an expanding (or contracting) universe. We show how to detect the analogue cosmological particle generation and analyze its feasibility with current circuit quantum electrodynamics (cQED) technology. Our setup in principle paves a new way for the exploration of analogue quantum gravitational effects.
Quantum Nonlinear Optics without Photons
Spontaneous parametric down-conversion is a well-known process in quantum nonlinear optics in which a photon incident on a nonlinear crystal spontaneously splits into two photons. Here
we propose an analogous physical process where one excited atom directly transfers its excitation to a pair of spatially-separated atoms with probability approaching one. The interaction is mediated by the exchange of virtual rather than real photons. This nonlinear atomic process is coherent and reversible, so the pair of excited atoms can transfer the excitation back to the first one: the atomic analogue of sum-frequency generation of light. The parameters used to investigate this process correspond to experimentally-demonstrated values in ultrastrong circuit quantum electrodynamics. This approach can be extended to realize other nonlinear inter-atomic processes, such as four-atom mixing, and is an attractive architecture for the realization of quantum devices on a chip. We show that four-qubit mixing can efficiently implement quantum repetition codes and, thus, can be used for error-correction codes.
27
Jan
2017
Cutoff-free Circuit Quantum Electrodynamics
Any quantum-confined electronic system coupled to the electromagnetic continuum is subject to radiative decay and renormalization of its energy levels. When coupled to a cavity, these
quantities can be strongly modified with respect to their values in vacuum. Generally, this modification can be accurately captured by including only the closest resonant mode of the cavity. In the circuit quantum electrodynamics architecture, where the coupling strengths can be substantial, it is however found that the radiative decay rates are strongly influenced by far off-resonant modes. A multimode calculation accounting for the infinite set of cavity modes leads to divergences unless a cutoff is imposed. It has so far not been identified what the source of divergence is. We show here that unless gauge invariance is respected, any attempt at the calculation of circuit QED quantities is bound to diverge. We then present a theoretical approach to the calculation of a finite spontaneous emission rate and the Lamb shift that is free of cutoff.