22 research outputs found
Universal non-adiabatic control of small-gap superconducting qubits
Resonant transverse driving of a two-level system as viewed in the rotating
frame couples two degenerate states at the Rabi frequency, an amazing
equivalence that emerges in quantum mechanics. While spectacularly successful
at controlling natural and artificial quantum systems, certain limitations may
arise (e.g., the achievable gate speed) due to non-idealities like the
counter-rotating term. Here, we explore a complementary approach to quantum
control based on non-resonant, non-adiabatic driving of a longitudinal
parameter in the presence of a fixed transverse coupling. We introduce a
superconducting composite qubit (CQB), formed from two capacitively coupled
transmon qubits, which features a small avoided crossing -- smaller than the
environmental temperature -- between two energy levels. We control this
low-frequency CQB using solely baseband pulses, non-adiabatic transitions, and
coherent Landau-Zener interference to achieve fast, high-fidelity, single-qubit
operations with Clifford fidelities exceeding . We also perform coupled
qubit operations between two low-frequency CQBs. This work demonstrates that
universal non-adiabatic control of low-frequency qubits is feasible using
solely baseband pulses
Two-qubit spectroscopy of spatiotemporally correlated quantum noise in superconducting qubits
Noise that exhibits significant temporal and spatial correlations across
multiple qubits can be especially harmful to both fault-tolerant quantum
computation and quantum-enhanced metrology. However, a complete spectral
characterization of the noise environment of even a two-qubit system has not
been reported thus far. We propose and experimentally validate a protocol for
two-qubit dephasing noise spectroscopy based on continuous control modulation.
By combining ideas from spin-locking relaxometry with a statistically motivated
robust estimation approach, our protocol allows for the simultaneous
reconstruction of all the single-qubit and two-qubit cross-correlation spectra,
including access to their distinctive non-classical features. Only single-qubit
control manipulations and state-tomography measurements are employed, with no
need for entangled-state preparation or readout of two-qubit observables. While
our experimental validation uses two superconducting qubits coupled to a shared
engineered noise source, our methodology is portable to a variety of
dephasing-dominated qubit architectures. By pushing quantum noise spectroscopy
beyond the single-qubit setting, our work paves the way to characterizing
spatiotemporal correlations in both engineered and naturally occurring noise
environments.Comment: total: 22 pages, 7 figures; main: 13 pages, 6 figures, supplementary:
6 pages, 1 figure; references: 3 page
Generating spatially entangled itinerant photons with waveguide quantum electrodynamics
Realizing a fully connected network of quantum processors requires the ability to distribute quantum entanglement. For distant processing nodes, this can be achieved by generating, routing, and capturing spatially entangled itinerant photons. In this work, we demonstrate the deterministic generation of such photons using superconducting transmon qubits that are directly coupled to a waveguide. In particular, we generate two-photon N00N states and show that the state and spatial entanglement of the emitted photons are tunable via the qubit frequencies. Using quadrature amplitude detection, we reconstruct the moments and correlations of the photonic modes and demonstrate state preparation fidelities of 84%. Our results provide a path toward realizing quantum communication and teleportation protocols using itinerant photons generated by quantum interference within a waveguide quantum electrodynamics architecture
Characterizing and optimizing qubit coherence based on SQUID geometry
The dominant source of decoherence in contemporary frequency-tunable
superconducting qubits is 1/ flux noise. To understand its origin and find
ways to minimize its impact, we systematically study flux noise amplitudes in
more than 50 flux qubits with varied SQUID geometry parameters and compare our
results to a microscopic model of magnetic spin defects located at the
interfaces surrounding the SQUID loops. Our data are in agreement with an
extension of the previously proposed model, based on numerical simulations of
the current distribution in the investigated SQUIDs. Our results and detailed
model provide a guide for minimizing the flux noise susceptibility in future
circuits.Comment: 14 pages, 6 figure
Deep Neural Network Discrimination of Multiplexed Superconducting Qubit States
Demonstrating a quantum computational advantage will require high-fidelity
control and readout of multi-qubit systems. As system size increases,
multiplexed qubit readout becomes a practical necessity to limit the growth of
resource overhead. Many contemporary qubit-state discriminators presume
single-qubit operating conditions or require considerable computational effort,
limiting their potential extensibility. Here, we present multi-qubit readout
using neural networks as state discriminators. We compare our approach to
contemporary methods employed on a quantum device with five superconducting
qubits and frequency-multiplexed readout. We find that fully-connected
feedforward neural networks increase the qubit-state-assignment fidelity for
our system. Relative to contemporary discriminators, the assignment error rate
is reduced by up to 25% due to the compensation of system-dependent
nonidealities such as readout crosstalk which is reduced by up to one order of
magnitude. Our work demonstrates a potentially extensible building block for
high-fidelity readout relevant to both near-term devices and future
fault-tolerant systems.Comment: 18 Pages, 9 figure
Demonstration of tunable three-body interactions between superconducting qubits
Nonpairwise multi-qubit interactions present a useful resource for quantum
information processors. Their implementation would facilitate more efficient
quantum simulations of molecules and combinatorial optimization problems, and
they could simplify error suppression and error correction schemes. Here we
present a superconducting circuit architecture in which a coupling module
mediates 2-local and 3-local interactions between three flux qubits by design.
The system Hamiltonian is estimated via multi-qubit pulse sequences that
implement Ramsey-type interferometry between all neighboring excitation
manifolds in the system. The 3-local interaction is coherently tunable over
several MHz via the coupler flux biases and can be turned off, which is
important for applications in quantum annealing, analog quantum simulation, and
gate-model quantum computation.Comment: 14 pages, 11 figure