We theoretically study a circuit QED architecture based on a superconducting flux qubit directly coupled to the center conductor of a coplanar waveguide transmission-line resonator. As already shown experimentally [Abdumalikov et al. Phys. Rev. B 78, 180502 (2008)], the strong coupling regime of cavity QED can readily be achieved by optimizing the local inductance of the resonator in the vicinity of the qubit. In addition to yielding stronger coupling with respect to other proposals for flux qubit based circuit QED, this approach leads to a qubit-resonator coupling strength g which does not scale as the area of the qubit but is proportional to the total inductance shared between the resonator and the qubit. Strong coupling can thus be attained while still minimizing sensitivity to flux noise. Finally, we show that by taking advantage of the the large kinetic inductance of a Josephson junction in the center conductor of the resonator can lead to coupling energies of several tens of percent of the resonator frequency, reaching the ultrastrong coupling regime of cavity QED where the rotating-wave approximation breaks down. This should allow an on-chip implementation of the E x B Jahn-Teller model.Comment: 9 pages, 7 figures. Published version with minor changes and correction

We show how to realize fast and high-fidelity quantum nondemolition qubit readout using longitudinal qubit-oscillator interaction. This is accomplished by modulating the longitudinal coupling at the cavity frequency. The qubit-oscillator interaction then acts as a qubit-state dependent drive on the cavity, a situation that is fundamentally different from the standard dispersive case. Single-mode squeezing can be exploited to exponentially increase the signal-to-noise ratio of this readout protocol. We present an implementation of this longitudinal parametric readout in circuit quantum electrodynamics and a possible multiqubit architecture.

We provide a general method to find the Hamiltonian of a linear circuit in the presence of a nonlinearity. Focussing on the case of a Josephson junction embedded in a transmission-line resonator, we solve for the normal modes of the system by taking into account exactly the effect of the quadratic (i.e. inductive) part of the Josephson potential. The nonlinearity is then found to lead to self and cross-Kerr effects, as well as beam-splitter type interactions between modes. By adjusting the parameters of the circuit, the Kerr coefficient K can be made to reach values that are weak (K < κ), strong (K > κ) or even very strong (K ≫ κ) with respect to the photon-loss rate κ. In the latter case, the resonator+junction circuit corresponds to an in-line version of the transmon. By replacing the single junction by a SQUID, the Kerr coefficient can be tuned in-situ, allowing for example the fast generation of Schrödinger cat states of microwave light. Finally, we explore the maximal strength of qubit-resonator coupling that can be reached in this setting.

We propose a novel protocol for quantum illumination: a quantum-enhanced noise radar. A two-mode squeezed state, which exhibits continuous-variable entanglement between so-called signal and idler beams, is used as input to the radar system. Compared to existing proposals for quantum illumination, our protocol does not require joint measurement of the signal and idler beams. This greatly enhances the practicality of the system by, for instance, eliminating the need for a quantum memory to store the idler. We perform a proof-of-principle experiment in the microwave regime, directly comparing the performance of a two-mode squeezed source to an ideal classical noise source that saturates the classical bound for correlation. We find that, even in the presence of significant added noise and loss, the quantum source outperforms the classical source by as much as an order of magnitude.Quantum illumination has recently gained attention as a possible avenue to improve the sensitivity of radar and other target detection technologies. 1,2 The approach takes advantage of strong signal correlations that can be created in electromagnetic beams using quantum processes. These quantum correlations, a form of entanglement, can be stronger than anything allowed by classical physics giving a "quantum advantage" to the detection process. A number of proposals exist to use these correlations in a wide range of quantum sensing applications with the goal of making precision measurements beyond the standard quantum limit. 3,4 Most of these applications require that the entire sensor system be low-noise and have negligible loss in order to maintain entanglement. Notably, quantum illumination seems to be very robust to the presence of background noise and loss, suggesting that it may have broader practical applications.In this Letter, we present measurements demonstrating the potential of a novel quantum illumination protocol that implements a form of noise radar. Noise radar has been studied in the classical regime because of, among other reasons, the inherent difficulty in detecting the noisy probe beam against the ambient thermal background noise. 5,6 As discussed below, our protocol relaxes a challenging requirement of existing protocols, namely, joint measurement. This greatly increases the practicality of our scheme compared to others. In a proof-ofprinciple experiment, we use the protocol to demonstrate a quantum enhancement in the detected signal-to-noise ratio of an order of magnitude when comparing the performance of an entangled-photon source to an ideal classical noise source that saturates the classical bound for correlation.At the heart of quantum illumination (QI) is a nonlinear quantum process known as parametric downconversion (PDC). In PDC, a strong pump beam with a high frequency, f p , is incident on a nonlinear medium, resulting in the production of two lower frequency beams, commonly referred to as the signal and idler, such that the frequencies of the produced beams, f s and f i , satisfy the relation f p = f s + f i ...