13 research outputs found
Distinguishing parity-switching mechanisms in a superconducting qubit
Single-charge tunneling is a decoherence mechanism affecting superconducting
qubits, yet the origin of excess quasiparticle excitations (QPs) responsible
for this tunneling in superconducting devices is not fully understood. We
measure the flux dependence of charge-parity (or simply, ``parity'') switching
in an offset-charge-sensitive transmon qubit to identify the contributions of
photon-assisted parity switching and QP generation to the overall
parity-switching rate. The parity-switching rate exhibits a
qubit-state-dependent peak in the flux dependence, indicating a cold
distribution of excess QPs which are predominantly trapped in the low-gap film
of the device. Moreover, we find that the photon-assisted process contributes
significantly to both parity switching and the generation of excess QPs by
fitting to a model that self-consistently incorporates photon-assisted parity
switching as well as inter-film QP dynamics
Learning-based Calibration of Flux Crosstalk in Transmon Qubit Arrays
Superconducting quantum processors comprising flux-tunable data and coupler
qubits are a promising platform for quantum computation. However, magnetic flux
crosstalk between the flux-control lines and the constituent qubits impedes
precision control of qubit frequencies, presenting a challenge to scaling this
platform. In order to implement high-fidelity digital and analog quantum
operations, one must characterize the flux crosstalk and compensate for it. In
this work, we introduce a learning-based calibration protocol and demonstrate
its experimental performance by calibrating an array of 16 flux-tunable
transmon qubits. To demonstrate the extensibility of our protocol, we simulate
the crosstalk matrix learning procedure for larger arrays of transmon qubits.
We observe an empirically linear scaling with system size, while maintaining a
median qubit frequency error below kHz
Characterization of superconducting through-silicon vias as capacitive elements in quantum circuits
The large physical size of superconducting qubits and their associated
on-chip control structures presents a practical challenge towards building a
large-scale quantum computer. In particular, transmons require a
high-quality-factor shunting capacitance that is typically achieved by using a
large coplanar capacitor. Other components, such as superconducting microwave
resonators used for qubit state readout, are typically constructed from
coplanar waveguides which are millimeters in length. Here we use compact
superconducting through-silicon vias to realize lumped element capacitors in
both qubits and readout resonators to significantly reduce the on-chip
footprint of both of these circuit elements. We measure two types of devices to
show that TSVs are of sufficient quality to be used as capacitive circuit
elements and provide a significant reductions in size over existing approaches
High-Fidelity, Frequency-Flexible Two-Qubit Fluxonium Gates with a Transmon Coupler
We propose and demonstrate an architecture for fluxonium-fluxonium two-qubit
gates mediated by transmon couplers (FTF, for fluxonium-transmon-fluxonium).
Relative to architectures that exclusively rely on a direct coupling between
fluxonium qubits, FTF enables stronger couplings for gates using
non-computational states while simultaneously suppressing the static
controlled-phase entangling rate () down to kHz levels, all without
requiring strict parameter matching. Here we implement FTF with a flux-tunable
transmon coupler and demonstrate a microwave-activated controlled-Z (CZ) gate
whose operation frequency can be tuned over a 2 GHz range, adding frequency
allocation freedom for FTF's in larger systems. Across this range,
state-of-the-art CZ gate fidelities were observed over many bias points and
reproduced across the two devices characterized in this work. After optimizing
both the operation frequency and the gate duration, we achieved peak CZ
fidelities in the 99.85-99.9\% range. Finally, we implemented model-free
reinforcement learning of the pulse parameters to boost the mean gate fidelity
up to , averaged over roughly an hour between scheduled
training runs. Beyond the microwave-activated CZ gate we present here, FTF can
be applied to a variety of other fluxonium gate schemes to improve gate
fidelities and passively reduce unwanted interactions.Comment: 23 pages, 16 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
Evolution of Flux Noise in Superconducting Qubits with Weak Magnetic Fields
The microscopic origin of magnetic flux noise in superconducting
circuits has remained an open question for several decades despite extensive
experimental and theoretical investigation. Recent progress in superconducting
devices for quantum information has highlighted the need to mitigate sources of
qubit decoherence, driving a renewed interest in understanding the underlying
noise mechanism(s). Though a consensus has emerged attributing flux noise to
surface spins, their identity and interaction mechanisms remain unclear,
prompting further study. Here we apply weak in-plane magnetic fields to a
capacitively-shunted flux qubit (where the Zeeman splitting of surface spins
lies below the device temperature) and study the flux-noise-limited qubit
dephasing, revealing previously unexplored trends that may shed light on the
dynamics behind the emergent noise. Notably, we observe an enhancement
(suppression) of the spin-echo (Ramsey) pure dephasing time in fields up to
. With direct noise spectroscopy, we further observe a
transition from a to approximately Lorentzian frequency dependence below
10 Hz and a reduction of the noise above 1 MHz with increasing magnetic field.
We suggest that these trends are qualitatively consistent with an increase of
spin cluster sizes with magnetic field. These results should help to inform a
complete microscopic theory of flux noise in superconducting circuits
Development of Magnetic Nanoparticles as Microwave-Specific Catalysts for the Rapid, Low-Temperature Synthesis of Formalin Solutions
A series of heterogeneous catalyst
materials possessing good microwave
absorption properties were investigated for their activity as oxidation
catalysts under microwave irradiation. These catalysts, a series of
nanoscale magnetic spinel oxides of the composition MCr<sub>2</sub>O<sub>4</sub> (M = Cu, Co, Fe), were irradiated in aqueous methanol
solution (1:1 MeOH:H<sub>2</sub>O v:v). This resulted in rapid conversion
of methanol to formaldehyde, directly generating aqueous formalin
solutions. The catalytic reaction occurred under relatively mild conditions
(1 atm O<sub>2</sub>, 60 °C), with irradiation times of 80 min
converting 24.5%, 17.7%, and 13.2% of the available methanol to formaldehyde
by the Cu, Fe, and Co chromite spinel catalysts, respectively. Importantly,
reactions run under identical conditions of concentration, time, and
temperature using traditional convective heating yielded dramatically
lower amounts of conversions; specifically, 1.0% and 0.21% conversions
were observed with Cu and Co spinels, and no observable thermal products
were obtained from the Fe spinels. This work provides a clear demonstration
that microwave-driven catalysis can yield enhanced reactivity and
can afford new catalytic pathways