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

20
Nov
2017

# Quantum simulation of the spin-boson model in a microwave circuit

We consider superconducting circuits for the purpose of simulating the spin-boson model. The spin-boson model consists of a single two-level system coupled to bosonic modes. In most

cases, the model is considered in a limit where the bosonic modes are sufficiently dense to form a continuous spectral bath. A very well known case is the Ohmic bath, where the density of states grows linearly with the frequency. In the limit of weak coupling or large temperature, this problem can be solved numerically. If the coupling is strong, the bosonic modes can become sufficiently excited to make a classical simulation impossible. Quantum simulation of this problem can be done by coupling a superconducting qubit to a specifically engineered electromagnetic environment. We discuss in detail how to build a bosonic bath using superconducting resonators and how to achieve strong couplings by additional driving. We also discuss how interesting spin dynamics with different initialization conditions can be probed by using standard techniques from circuit quantum electrodynamics.

18
Nov
2017

# Topological edge states and pumping in a chain of coupled superconducting qubits

Topological insulators have inspired the study with various quantum simulators. Exploiting the tunability of the qubit frequency and qubit-qubit coupling, we show that a superconducting

qubit chain can simulate various topological band models. When the system is restricted to the single-spin excitation subspace, the Su-Schrieffer-Heeger (SSH) model can be equivalently simulated by alternating the coupling strength between neighboring qubits. The existence of topological edge states in this qubit chain is demonstrated in the quench dynamics after the first qubit is excited. This excitation propagates along the chain where the qubit-qubit coupling is homogeneous. In contrast, in our qubit chain, the spin-up state localizes at the first qubit and the rest qubits remain in the spin-down state. We further show that the spin-up state can be transported along the chain by modulating the coupling strengths and the qubit frequencies. This demonstrates adiabatic pumping based on the Rice-Mele model. Moreover, we also discuss possible ways to construct other topological models with different topological phenomena within the current technology of superconducting qubits.

17
Nov
2017

# Dissipative stabilization of entangled cat states using a driven Bose-Hubbard dimer

We analyze a modified Bose-Hubbard model, where two cavities having on-site Kerr interactions are subject to two-photon driving and correlated dissipation. We derive an exact solution

for the steady state of this interacting driven-dissipative system, and use it show that the system permits the preparation and stabilization of pure entangled non-Gaussian states, so-called entangled cat states. Unlike previous proposals for dissipative stabilization of such states, our approach requires only a linear coupling to a single engineered reservoir (as opposed to nonlinear couplings to two or more reservoirs). Our scheme is within the reach of state-of-the-art experiments in circuit QED.

16
Nov
2017

# Cavity quantum acoustic device in the multimode strong coupling regime

We investigate an acoustical analog of circuit quantum electrodynamics that facilitates compact high-Q (>20,000) microwave-frequency cavities with dense spectra. We fabricate and characterize

a device that comprises a flux tunable transmon coupled to a 300μm long surface acoustic wave resonator. For some modes, the qubit-cavity coupling reaches 6.5MHz, exceeding the cavity loss rate (200kHz), qubit linewidth (1.1MHz), and the cavity free spectral range (4.8MHz), placing the device in both the strong coupling and strong multimode regimes. With the qubit detuned from the cavity, we show that the dispersive shift behaves according to predictions from a generalized Jaynes-Cummings Hamiltonian. Finally, we observe that the qubit linewidth strongly depends on its frequency, as expected for spontaneous emission of phonons, and we identify operating frequencies where this emission rate is suppressed.

# Quantum metrology with a transmon qutrit

Making use of coherence and entanglement as metrological quantum resources allows to improve the measurement precision from the shot-noise- or quantum limit to the Heisenberg limit.

Quantum metrology then relies on the availability of quantum engineered systems that involve controllable quantum degrees of freedom which are sensitive to the measured quantity. Sensors operating in the qubit mode and exploiting their coherence in a phase-sensitive measurement have been shown to approach the Heisenberg scaling in precision. Here, we show that this result can be further improved by operating the quantum sensor in the qudit mode, i.e., by exploiting d rather than 2 levels. Specifically, we describe the metrological algorithm for using a superconducting transmon device operating in a qutrit mode as a magnetometer. The algorithm is based on the base-3 semi-quantum Fourier transformation and enhances the quantum theoretical performance of the sensor by a factor 2. Even more, the practical gain of our qutrit-implementation is found in a reduction of the number of iteration steps of the quantum Fourier transformation by a factor log2/log3≈0.63 as compared to the qubit mode. We show, that a two-tone capacitively coupled rf-signal is sufficient for the implementation of the algorithm.

15
Nov
2017

# Quantum non-demolition detection of an itinerant microwave photon

Photon detectors are an elementary tool to measure electromagnetic waves at the quantum limit and are heavily demanded in the emerging quantum technologies such as communication, sensing,

and computing. Of particular interest is a quantum non-demolition (QND) type detector, which projects the quantum state of a photonic mode onto the photon-number basis without affecting the temporal or spatial properties. This is in stark contrast to conventional photon detectors which absorb a photon to trigger a `click‘ and thus inevitably destroy the photon. The long-sought QND detection of a flying photon was recently demonstrated in the optical domain using a single atom in a cavity. However, the counterpart for microwaves has been elusive despite the recent progress in microwave quantum optics using superconducting circuits. Here, we implement a deterministic entangling gate between a superconducting qubit and a propagating microwave pulse mode reflected by a cavity containing the qubit. Using the entanglement and the high-fidelity qubit readout, we demonstrate a QND detection of a single photon with the quantum efficiency of 0.84, the photon survival probability of 0.87, and the dark-count probability of 0.0147. Our scheme can be a building block for quantum networks connecting distant qubit modules as well as a microwave photon counting device for multiple-photon signals.

14
Nov
2017

# General method for extracting the quantum efficiency of dispersive qubit readout in circuit QED

We present and demonstrate a general 3-step method for extracting the quantum efficiency of dispersive qubit readout in circuit QED. We use active depletion of post-measurement photons

and optimal integration weight functions on two quadratures to maximize the signal-to-noise ratio of non-steady-state homodyne measurement. We derive analytically and demonstrate experimentally that the method robustly extracts the quantum efficiency for arbitrary readout conditions in the linear regime. We use the proven method to optimally bias a Josephon traveling-wave parametric amplifier and to quantify the different noise contributions in the readout amplification chain.

11
Nov
2017

# Heisenberg-Langevin Formalism For Open Circuit-QED Systems

We present a Heisenberg-Langevin formalism to study the effective dynamics of a superconducting qubit coupled to an open multimode resonator, without resorting to the rotating wave,

two level, Born or Markov approximations. Our effective equations are derived by eliminating resonator degrees of freedom while encoding their effect in the Green’s function of the electromagnetic background. We account for the openness of the resonator exactly by employing a spectral representation for the Green’s function in terms of a set of non-Hermitian modes. A well-behaved time domain perturbation theory is derived to systematically account for the nonlinearity of weakly nonlinear qubits like transmon. We apply this method to the problem of spontaneous emission, capturing accurately the non-Markovian features of the qubit dynamics, valid for any qubit-resonator coupling strength. Any discrete-level quantum system coupled to the electromagnetic continuum is subject to radiative decay and renormalization of its energy levels. When inside a cavity, these quantities can be strongly modified with respect to vacuum. Generally, this modification can be captured by including only the closest resonant cavity mode. In circuit-QED architecture, with substantial coupling strengths, it is however found that such rates are strongly influenced by far off-resonant modes. A multimode calculation over the infinite set of cavity modes leads to divergences unless an artificial cutoff is imposed. Previous studies have not pointed out what the source of this divergence is. We show that unless the effect of A2 is accounted for up to all orders exactly, any multimode calculations of circuit-QED quantities is bound to diverge. Subsequently, we present the calculation of finite radiative corrections to qubit properties that is free of an artificially introduced high frequency cut-off.

08
Nov
2017

# Optimal operation of a Josephson parametric amplifier for vacuum squeezing

A Josephson parametric amplifier (JPA) can create squeezed states of microwave light, lowering the noise associated with certain quantum measurements. We experimentally study how the

JPA’s pump influences the phase-sensitive amplification and deamplification of a coherent tone’s amplitude when that amplitude is commensurate with vacuum fluctuations. We predict and demonstrate that by operating the JPA with a pump power greater than the value that maximizes gain, the amplifier distortion is reduced and consequently squeezing is improved. Optimizing the JPA’s operation in this fashion, we directly observe 3.87±0.03 dB of vacuum squeezing.

# Real-time quantum state estimation in circuit QED via Bayesian approach

Using a circuit QED device, we present a theoretical study of real-time quantum state estimation via quantum Bayesian approach. Suitable conditions under which the Bayesian approach

can accurately update the density matrix of the qubit are analyzed. We also consider the correlation between some basic and physically meaningful parameters of the circuit QED and the performance of the Bayesian approach. Our results advance the understanding of quantum Bayesian approach and pave the way to study quantum feedback control and adaptive control.