333 research outputs found
Observation of a dissipative phase transition in a one-dimensional circuit QED lattice
Condensed matter physics has been driven forward by significant experimental
and theoretical progress in the study and understanding of equilibrium phase
transitions based on symmetry and 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
Imaging Photon Lattice States by Scanning Defect Microscopy
Microwave photons inside lattices of coupled resonators and superconducting
qubits can exhibit surprising matter-like behavior. Realizing such open-system
quantum simulators presents an 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
Engineering Dynamical Sweet Spots to Protect Qubits from 1/ Noise
Protecting superconducting qubits from low-frequency noise is essential for
advancing superconducting quantum computation. Based on the application of a
periodic drive field, we develop a protocol for engineering dynamical sweet
spots which reduce the susceptibility of a qubit to low-frequency noise. Using
the framework of Floquet theory, we prove rigorously that there are manifolds
of dynamical sweet spots marked by extrema in the quasi-energy differences of
the driven qubit. In particular, for the example of fluxonium biased slightly
away from half a flux quantum, we predict an enhancement of pure-dephasing by
three orders of magnitude. Employing the Floquet eigenstates as the
computational basis, we show that high-fidelity single- and two-qubit gates can
be implemented while maintaining dynamical sweet-spot operation. We further
confirm that qubit readout can be performed by adiabatically mapping the
Floquet states back to the static qubit states, and subsequently applying
standard measurement techniques. Our work provides an intuitive tool to encode
quantum information in robust, time-dependent states, and may be extended to
alternative architectures for quantum information processing
Simple master equations for describing driven systems subject to classical non-Markovian noise
Driven quantum systems subject to non-Markovian noise are typically difficult
to model even if the noise is classical. We present a systematic method based
on generalized cumulant expansions for deriving a time-local master equation
for such systems. This master equation has an intuitive form that directly
parallels a standard Lindblad equation, but contains several surprising
features: the combination of driving and non-Markovianity results in effective
time-dependent dephasing rates that can be negative, and the noise can generate
Hamiltonian renormalizations even though it is classical. We analyze in detail
the highly relevant case of a Rabi-driven qubit subject to various kinds of
non-Markovian noise including fluctuations, finding an excellent
agreement between our master equation and numerically-exact simulations over
relevant timescales. The approach outlined here is more accurate than commonly
employed phenomenological master equations which ignore the interplay between
driving and noise.Comment: 12+4 pages, 6+4 figure
Thermopower of Single-Molecule Devices
We investigate the thermopower of single molecules weakly coupled to metallic
leads. We model the molecule in terms of the relevant electronic orbitals
coupled to phonons corresponding to both internal vibrations and to
oscillations of the molecule as a whole. The thermopower is computed by means
of rate equations including both sequential-tunneling and cotunneling
processes. Under certain conditions, the thermopower allows one to access the
electronic and phononic excitation spectrum of the molecule in a
linear-response measurement. In particular, we find that the phonon features
are more pronounced for weak lead-molecule coupling. This way of measuring the
excitation spectrum is less invasive than the more conventional current-voltage
characteristic, which, by contrast, probes the system far from equilibrium.Comment: 13 pages, 7 figures included; minor changes, version published in PR
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