35 research outputs found
Quantum trajectories and their statistics for remotely entangled quantum bits
We experimentally and theoretically investigate the quantum trajectories of
jointly monitored transmon qubits embedded in spatially separated microwave
cavities. Using nearly quantum-noise limited superconducting amplifiers and an
optimized setup to reduce signal loss between cavities, we can efficiently
track measurement-induced entanglement generation as a continuous process for
single realizations of the experiment. The quantum trajectories of transmon
qubits naturally split into low and high entanglement classes corresponding to
half-parity collapse. The distribution of concurrence is found at any given
time and we explore the dynamics of entanglement creation in the state space.
The distribution exhibits a sharp cut-off in the high concurrence limit,
defining a maximal concurrence boundary. The most likely paths of the qubits'
trajectories are also investigated, resulting in three probable paths,
gradually projecting the system to two even subspaces and an odd subspace. We
also investigate the most likely time for the individual trajectories to reach
their most entangled state, and find that there are two solutions for the local
maximum, corresponding to the low and high entanglement routes. The theoretical
predictions show excellent agreement with the experimental entangled qubit
trajectory data.Comment: 11 pages and 4 figure
Extremely Large Area (88 mm X 88 mm) Superconducting Integrated Circuit (ELASIC)
Superconducting integrated circuit (SIC) is a promising "beyond-CMOS" device
technology enables speed-of-light, nearly lossless communications to advance
cryogenic (4 K or lower) computing. However, the lack of large-area
superconducting IC has hindered the development of scalable practical systems.
Herein, we describe a novel approach to interconnect 16 high-resolution deep UV
(DUV EX4, 248 nm lithography) full reticle circuits to fabricate an extremely
large (88mm X 88 mm) area superconducting integrated circuit (ELASIC). The
fabrication process starts by interconnecting four high-resolution DUV EX4 (22
mm X 22 mm) full reticles using a single large-field (44 mm X 44 mm) I-line
(365 nm lithography) reticle, followed by I-line reticle stitching at the
boundaries of 44 mm X 44 mm fields to fabricate the complete ELASIC field (88
mm X 88 mm). The ELASIC demonstrated a 2X-12X reduction in circuit features and
maintained high-stitched line superconducting critical currents. We examined
quantum flux parametron (QFP) circuits to demonstrate the viability of common
active components used for data buffering and transmission. Considering that no
stitching requirement for high-resolution EX4 DUV reticles is employed, the
present fabrication process has the potential to advance the scaling of
superconducting quantum devices
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
Realization of high-fidelity CZ and ZZ-free iSWAP gates with a tunable coupler
High-fidelity two-qubit gates at scale are a key requirement to realize the
full promise of quantum computation and simulation. The advent and use of
coupler elements to tunably control two-qubit interactions has improved
operational fidelity in many-qubit systems by reducing parasitic coupling and
frequency crowding issues. Nonetheless, two-qubit gate errors still limit the
capability of near-term quantum applications. The reason, in part, is the
existing framework for tunable couplers based on the dispersive approximation
does not fully incorporate three-body multi-level dynamics, which is essential
for addressing coherent leakage to the coupler and parasitic longitudinal
() interactions during two-qubit gates. Here, we present a systematic
approach that goes beyond the dispersive approximation to exploit the
engineered level structure of the coupler and optimize its control. Using this
approach, we experimentally demonstrate CZ and -free iSWAP gates with
two-qubit interaction fidelities of % and %,
respectively, which are close to their limits.Comment: 28 pages, 32 figure
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
Broadband Squeezed Microwaves and Amplification with a Josephson Traveling-Wave Parametric Amplifier
Squeezing of the electromagnetic vacuum is an essential metrological
technique used to reduce quantum noise in applications spanning gravitational
wave detection, biological microscopy, and quantum information science. In
superconducting circuits, the resonator-based Josephson-junction parametric
amplifiers conventionally used to generate squeezed microwaves are constrained
by a narrow bandwidth and low dynamic range. In this work, we develop a
dual-pump, broadband Josephson traveling-wave parametric amplifier that
combines a phase-sensitive extinction ratio of 56 dB with single-mode squeezing
on par with the best resonator-based squeezers. We also demonstrate two-mode
squeezing at microwave frequencies with bandwidth in the gigahertz range that
is almost two orders of magnitude wider than that of contemporary
resonator-based squeezers. Our amplifier is capable of simultaneously creating
entangled microwave photon pairs with large frequency separation, with
potential applications including high-fidelity qubit readout, quantum
illumination and teleportation
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