27 research outputs found
Planar multilayer circuit quantum electrodynamics
Experimental quantum information processing with superconducting circuits is
rapidly advancing, driven by innovation in two classes of devices, one
involving planar micro-fabricated (2D) resonators, and the other involving
machined three-dimensional (3D) cavities. We demonstrate that circuit quantum
electrodynamics can be implemented in a multilayer superconducting structure
that combines 2D and 3D advantages. We employ standard micro-fabrication
techniques to pattern each layer, and rely on a vacuum gap between the layers
to store the electromagnetic energy. Planar qubits are lithographically defined
as an aperture in a conducting boundary of the resonators. We demonstrate the
aperture concept by implementing an integrated, two cavity-modes, one
transmon-qubit system
Direct microwave measurement of Andreev-bound-state dynamics in a proximitized semiconducting nanowire
The modern understanding of the Josephson effect in mesosopic devices derives
from the physics of Andreev bound states, fermionic modes that are localized in
a superconducting weak link. Recently, Josephson junctions constructed using
semiconducting nanowires have led to the realization of superconducting qubits
with gate-tunable Josephson energies. We have used a microwave circuit QED
architecture to detect Andreev bound states in such a gate-tunable junction
based on an aluminum-proximitized InAs nanowire. We demonstrate coherent
manipulation of these bound states, and track the bound-state fermion parity in
real time. Individual parity-switching events due to non-equilibrium
quasiparticles are observed with a characteristic timescale . The of a topological nanowire
junction sets a lower bound on the bandwidth required for control of Majorana
bound states
Continuous monitoring of a trapped, superconducting spin
Readout and control of fermionic spins in solid-state systems are key
primitives of quantum information processing and microscopic magnetic sensing.
The highly localized nature of most fermionic spins decouples them from
parasitic degrees of freedom, but makes long-range interoperability difficult
to achieve. In light of this challenge, an active effort is underway to
integrate fermionic spins with circuit quantum electrodynamics (cQED), which
was originally developed in the field of superconducting qubits to achieve
single-shot, quantum-non-demolition (QND) measurements and long-range
couplings. However, single-shot readout of an individual spin with cQED has
remained elusive due to the difficulty of coupling a resonator to a particle
trapped by a charge-confining potential. Here we demonstrate the first
single-shot, cQED readout of a single spin. In our novel implementation, the
spin is that of an individual superconducting quasiparticle trapped in the
Andreev levels of a semiconductor nanowire Josephson element. Due to a
spin-orbit interaction inside the nanowire, this "superconducting spin"
directly determines the flow of supercurrent through the element. We harnessed
this spin-dependent supercurrent to achieve both a zero-field spin splitting as
well as a long-range interaction between the quasiparticle and a
superconducting microwave resonator. Owing to the strength of this interaction
in our device, measuring the resultant spin-dependent resonator frequency
yielded QND spin readout with 92% fidelity in 1.9 s and allowed us to
monitor the quasiparticle's spin in real time. These results pave the way for
new "fermionic cQED" devices: superconducting spin qubits operating at zero
magnetic field, devices in which the spin has enhanced governance over the
circuit, and time-domain measurements of Majorana modes
Coherent manipulation of an Andreev spin qubit
Two promising architectures for solid-state quantum information processing
are electron spins in semiconductor quantum dots and the collective
electromagnetic modes of superconducting circuits. In some aspects, these two
platforms are dual to one another: superconducting qubits are more easily
coupled but are relatively large among quantum devices ,
while electrostatically-confined electron spins are spatially compact () but more complex to link. Here we combine beneficial aspects
of both platforms in the Andreev spin qubit: the spin degree of freedom of an
electronic quasiparticle trapped in the supercurrent-carrying Andreev levels of
a Josephson semiconductor nanowire. We demonstrate coherent spin manipulation
by combining single-shot circuit-QED readout and spin-flipping Raman
transitions, finding a spin-flip time and a spin
coherence time . These results herald a new spin qubit
with supercurrent-based circuit-QED integration and further our understanding
and control of Andreev levels -- the parent states of Majorana zero modes -- in
semiconductor-superconductor heterostructures
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