Microwave cavities coupled to superconducting qubits have been demonstrated to be a promising platform for quantum information processing. A major challenge in this setup is to realizeuniversal control over the cavity. A promising approach are selective number-dependent arbitrary phase (SNAP) gates combined with cavity displacements. It has been proven that this is a universal gate set, but a central question remained open so far: how can a given target operation be realized efficiently with a sequence of these operations. In this work, we present a practical scheme to address this problem. It involves a hierarchical strategy to insert new gates into a sequence, followed by a co-optimization of the control parameters, which generates short high-fidelity sequences. For a broad range of experimentally relevant applications, we find that they can be implemented with 3 to 4 SNAP gates, compared to up to 50 with previously known techniques.
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 whichcan 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.
Extensive efforts have been expended in developing hybrid quantum systems to overcome the short coherence time of superconducting circuits by introducing the naturally long-lived spindegree of freedom. Among all the possible materials, single-crystal yttrium iron garnet has shown up very recently as a promising candidate for hybrid systems, and various highly coherent interactions, including strong and even ultra-strong coupling, have been demonstrated. One distinct advantage of these systems is that the spins are in the form of well-defined magnon modes, which allows flexible and precise tuning. Here we demonstrate that by dissipation engineering, a non-Markovian interaction dynamics between the magnon and the microwave cavity photon can be achieved. Such a process enables us to build a magnon gradient memory to store information in the magnon dark modes, which decouple from the microwave cavity and thus preserve a long life-time. Our findings provide a promising approach for developing long-lifetime, multimode quantum memories.
We study several dynamical properties of a recently proposed implementation
of the quantum transverse-field Ising chain in the framework of circuit QED.
Particular emphasis is placedon the effects of disorder on the nonequilibrium
behavior of the system. We show that small amounts of fabrication-induced
disorder in the system parameters do not jeopardize the observation of
previously-predicted phenomena. Based on a numerical extraction of the mean
free path of the system, we also provide a simple quantitative estimate for
certain disorder effects on the nonequilibrium dynamics of the circuit QED
quantum simulator. We discuss the transition from weak to strong disorder,
characterized by the onset of Anderson localization of the system’s wave
functions, and the qualitatively different dynamics it leads to.
We show how a quantum Ising spin chain in a time-dependent transverse
magnetic field can be simulated and experimentally probed in the framework of
circuit QED with current technology.The proposed setup provides a new platform
for observing the nonequilibrium dynamics of interacting many-body systems. We
calculate its spectrum to offer a guideline for its initial experimental
characterization. We demonstrate that quench dynamics and the propagation of
localized excitations can be observed with the proposed setup and discuss
further possible applications and modifications of this circuit QED quantum
simulator.