Qubit connectivity is an important property of a quantum processor, with an ideal processor having random access — the ability of arbitrary qubit pairs to interact directly. Here,we implement a random access superconducting quantum information processor, demonstrating universal operations on a nine-bit quantum memory, with a single transmon serving as the central processor. The quantum memory uses the eigenmodes of a linear array of coupled superconducting resonators. The memory bits are superpositions of vacuum and single-photon states, controlled by a single superconducting transmon coupled to the edge of the array. We selectively stimulate single-photon vacuum Rabi oscillations between the transmon and individual eigenmodes through parametric flux modulation of the transmon frequency, producing sidebands resonant with the modes. Utilizing these oscillations for state transfer, we perform a universal set of single- and two-qubit gates between arbitrary pairs of modes, using only the charge and flux bias of the transmon. Further, we prepare multimode entangled Bell and GHZ states of arbitrary modes. The fast and flexible control, achieved with efficient use of cryogenic resources and control electronics, in a scalable architecture compatible with state-of-the-art quantum memories is promising for quantum computation and simulation.
Condensed matter physics has been driven forward by significant experimental and theoretical progress in the study and understanding of equilibrium phase transitions based on symmetryand topology. However, nonequilibrium phase transitions have remained a challenge, in part due to their complexity in theoretical descriptions and the additional experimental difficulties in systematically controlling systems out of equilibrium. Here, we study a one-dimensional chain of 72 microwave cavities, each coupled to a superconducting qubit, and coherently drive the system into a nonequilibrium steady state. We find experimental evidence for a dissipative phase transition in the system in which the steady state changes dramatically as the mean photon number is increased. Near the boundary between the two observed phases, the system demonstrates bistability, with characteristic switching times as long as 60 ms — far longer than any of the intrinsic rates known for the system. This experiment demonstrates the power of circuit QED systems for studying nonequilibrium condensed matter physics and paves the way for future experiments exploring nonequilbrium physics with many-body quantum optics.
Microwave photons inside lattices of coupled resonators and superconducting qubits can exhibit surprising matter-like behavior. Realizing such open-system quantum simulators presentsan experimental challenge and requires new tools and measurement techniques. Here, we introduce Scanning Defect Microscopy as one such tool and illustrate its use in mapping the normal-mode structure of microwave photons inside a 49-site Kagome lattice of coplanar waveguide resonators. Scanning is accomplished by moving a probe equipped with a sapphire tip across the lattice. This locally perturbs resonator frequencies and induces shifts of the lattice resonance frequencies which we determine by measuring the transmission spectrum. From the magnitude of mode shifts we can reconstruct photon field amplitudes at each lattice site and thus create spatial images of the photon-lattice normal modes.
In charge-coupled circuit QED systems, transition amplitudes and dispersive shifts are governed by the matrix elements of the charge operator. For the fluxonium circuit, these matrixelements are not limited to nearest-neighbor energy levels and are conveniently tunable by magnetic flux. Previously, their values were largely obtained numerically. Here, we present analytical expressions for the fluxonium charge matrix elements. We show that new selection rules emerge in the asymptotic limit of large Josephson energy and small inductive energy. We illustrate the usefulness of our expressions for the qualitative understanding of charge matrix elements in the parameter regime probed by previous experiments.
The Jaynes-Cummings model describes the coupling between photons and a single
two-level atom in a simplified representation of light-matter interactions. In
circuit QED, this modelis implemented by combining microwave resonators and
superconducting qubits on a microchip with unprecedented experimental control.
Arranging qubits and resonators in the form of a lattice realizes a new kind of
Hubbard model, the Jaynes-Cummings-Hubbard model, in which the elementary
excitations are polariton quasi-particles. Due to the genuine openness of
photonic systems, circuit QED lattices offer the possibility to study the
intricate interplay of collective behavior, strong correlations and
non-equilibrium physics. Thus, turning circuit QED into an architecture for
quantum simulation, i.e., using a well-controlled system to mimic the intricate
quantum behavior of another system too daunting for a theorist to tackle
head-on, is an exciting idea which has served as theorists‘ playground for a
while and is now also starting to catch on in experiments. This review gives a
summary of the most recent theoretical proposals and experimental efforts in
this context.
The intriguing appeal of circuits lies in their modularity and ease of
fabrication. Based on a toolbox of simple building blocks, circuits present a
powerful framework for achievingnew functionality by combining circuit
elements into larger networks. It is an open question to what degree modularity
also holds for quantum circuits — circuits made of superconducting material,
in which electric voltages and currents are governed by the laws of quantum
physics. If realizable, quantum coherence in larger circuit networks has great
potential for advances in quantum information processing including topological
protection from decoherence. Here, we present theory suitable for quantitative
modeling of such large circuits and discuss its application to the fluxonium
device. Our approach makes use of approximate symmetries exhibited by the
circuit, and enables us to obtain new predictions for the energy spectrum of
the fluxonium device which can be tested with current experimental technology.
We assess experimentally the suitability of coupled transmission line
resonators for studies of quantum phase transitions of light. We have measured
devices with low photon hoppingrates t/2pi = 0.8MHz to quantify disorder in
individual cavity frequencies. The observed disorder is consistent with small
imperfections in fabrication. We studied the dependence of the disorder on
transmission line geometry and used our results to fabricate devices with
disorder less than two parts in 10^4. The normal mode spectrum of devices with
a high photon hopping rate t/2pi = 31MHz shows little effect of disorder,
rendering resonator arrays a good backbone for the study of condensed matter
physics with photons.