13 research outputs found
Interacting Qubit-Photon Bound States with Superconducting Circuits
Qubits strongly coupled to a photonic crystal give rise to many exotic
physical scenarios, beginning with single and multi-excitation qubit-photon
dressed bound states comprising induced spatially localized photonic modes,
centered around the qubits, and the qubits themselves. The localization of
these states changes with qubit detuning from the band-edge, offering an avenue
of in situ control of bound state interaction. Here, we present experimental
results from a device with two qubits coupled to a superconducting microwave
photonic crystal and realize tunable on-site and inter-bound state
interactions. We observe a fourth-order two photon virtual process between
bound states indicating strong coupling between the photonic crystal and
qubits. Due to their localization-dependent interaction, these states offer the
ability to create one-dimensional chains of bound states with tunable and
potentially long-range interactions that preserve the qubits' spatial
organization, a key criterion for realization of certain quantum many-body
models. The widely tunable, strong and robust interactions demonstrated with
this system are promising benchmarks towards realizing larger, more complex
systems of bound states
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
Enhanced Quantum State Transfer and Bell State Generation over Long-Range Multimode Interconnects via Superadiabatic Transitionless Driving
Achieving high-fidelity direct two-qubit gates over meter-scale long quantum
interconnects is challenging in part due to the multimode nature of such
systems. One alternative scheme is to combine local operations with remote
quantum state transfer or remote entanglement. Here, we study quantum state
transfer and entanglement generation for two distant qubits, equipped with
tunable interactions, over a common multimode interconnect. We employ the
SuperAdiabatic Transitionless Driving (SATD) solutions for adiabatic passage
and demonstrate various favorable improvements over the standard protocol. In
particular, by suppressing leakage to a select (resonant) interconnect mode,
SATD breaks the speed-limit relation imposed by the qubit-interconnect
interaction , where instead the operation time is limited by leakage to the
adjacent modes, i.e. free spectral range of the interconnect,
allowing for fast operations even with weak . Furthermore, we identify a
multimode error mechanism for Bell state generation using such adiabatic
protocols, in which the even/odd modal dependence of qubit-interconnect
interaction breaks down the dark state symmetry, leading to detrimental
adiabatic overlap with the odd modes growing as . Therefore,
adopting a weak coupling, imposed by a multimode interconnect, SATD provides a
significant improvement in terms of operation speed and consequently
sensitivity to incoherent error.Comment: 14 pages, 12 figures, 4 appendice
Beyond Strong Coupling in a Massively Multimode Cavity
The study of light-matter interaction has seen a resurgence in recent years,
stimulated by highly controllable, precise, and modular experiments in cavity
quantum electrodynamics (QED). The achievement of strong coupling, where the
coupling between a single atom and fundamental cavity mode exceeds the decay
rates, was a major milestone that opened the doors to a multitude of new
investigations. Here we introduce multimode strong coupling (MMSC), where the
coupling is comparable to the free spectral range (FSR) of the cavity, i.e. the
rate at which a qubit can absorb a photon from the cavity is comparable to the
round trip transit rate of a photon in the cavity. We realize, via the circuit
QED architecture, the first experiment accessing the MMSC regime, and report
remarkably widespread and structured resonance fluorescence, whose origin
extends beyond cavity enhancement of sidebands. Our results capture complex
multimode, multiphoton processes, and the emergence of ultranarrow linewidths.
Beyond the novel phenomena presented here, MMSC opens a major new direction in
the exploration of light-matter interactions.Comment: 14 pages, 11 figures. References added, typos correcte
Encoding a magic state with beyond break-even fidelity
We distill magic states to complete a universal set of fault-tolerant logic
gates that is needed for large-scale quantum computing. By encoding better
quality input states for our distillation procedure, we can reduce the
considerable resource cost of producing magic states. We demonstrate an
error-suppressed encoding scheme for a two-qubit input magic state, that we
call the CZ state, on an array of superconducting qubits. Using a complete set
of projective logical Pauli measurements, that are also tolerant to a single
circuit error, we propose a circuit that demonstrates a magic state prepared
with infidelity . Additionally, the yield of
our scheme increases with the use of adaptive circuit elements that are
conditioned in real time on mid-circuit measurement outcomes. We find our
results are consistent with variations of the experiment, including where we
use only post-selection in place of adaptive circuits, and where we interrogate
our output state using quantum state tomography on the data qubits of the code.
Remarkably, the error-suppressed preparation experiment demonstrates a fidelity
exceeding that of the preparation of the same unencoded magic-state on any
single pair of physical qubits on the same device.Comment: 10 pages, 7 figures, comments welcom
Demonstration of quantum volume 64 on a superconducting quantum computing system
We improve the quality of quantum circuits on superconducting quantum
computing systems, as measured by the quantum volume, with a combination of
dynamical decoupling, compiler optimizations, shorter two-qubit gates, and
excited state promoted readout. This result shows that the path to larger
quantum volume systems requires the simultaneous increase of coherence, control
gate fidelities, measurement fidelities, and smarter software which takes into
account hardware details, thereby demonstrating the need to continue to
co-design the software and hardware stack for the foreseeable future.Comment: Fixed typo in author list. Added references [38], [49] and [52