83 research outputs found
Improving wafer-scale Josephson junction resistance variation in superconducting quantum coherent circuits
Quantum bits, or qubits, are an example of coherent circuits envisioned for
next-generation computers and detectors. A robust superconducting qubit with a
coherent lifetime of (100 s) is the transmon: a Josephson junction
functioning as a non-linear inductor shunted with a capacitor to form an
anharmonic oscillator. In a complex device with many such transmons, precise
control over each qubit frequency is often required, and thus variations of the
junction area and tunnel barrier thickness must be sufficiently minimized to
achieve optimal performance while avoiding spectral overlap between neighboring
circuits. Simply transplanting our recipe optimized for single, stand-alone
devices to wafer-scale (producing 64, 1x1 cm dies from a 150 mm wafer)
initially resulted in global drifts in room-temperature tunneling resistance of
30%. Inferring a critical current variation from this
resistance distribution, we present an optimized process developed from a
systematic 38 wafer study that results in 3.5% relative standard deviation
(RSD) in critical current () for 3000 Josephson junctions (both single-junctions and
asymmetric SQUIDs) across an area of 49 cm. Looking within a 1x1 cm moving
window across the substrate gives an estimate of the variation characteristic
of a given qubit chip. Our best process, utilizing ultrasonically assisted
development, uniform ashing, and dynamic oxidation has shown = 1.8% within 1x1 cm, on average,
with a few 1x1 cm areas having 1.0% (equivalent to 0.5%). Such stability would drastically improve the yield of
multi-junction chips with strict critical current requirements.Comment: 10 pages, 4 figures. Revision includes supplementary materia
Measurement Of Quasiparticle Transport In Aluminum Films Using Tungsten Transition-Edge Sensors
We report new experimental studies to understand the physics of phonon
sensors which utilize quasiparticle diffusion in thin aluminum films into
tungsten transition-edge-sensors (TESs) operated at 35 mK. We show that basic
TES physics and a simple physical model of the overlap region between the W and
Al films in our devices enables us to accurately reproduce the experimentally
observed pulse shapes from x-rays absorbed in the Al films. We further estimate
quasiparticle loss in Al films using a simple diffusion equation approach.Comment: 5 pages, 6 figures, PRA
Nonlinear Optimal Filter Technique For Analyzing Energy Depositions In TES Sensors Driven Into Saturation
We present a detailed thermal and electrical model of superconducting transition edge sensors (TESs) connected to quasiparticle (qp) traps, such as the W TESs connected to Al qp traps used for CDMS (Cryogenic Dark Matter Search) Ge and Si detectors. We show that this improved model, together with a straightforward time-domain optimal filter, can be used to analyze pulses well into the nonlinear saturation region and reconstruct absorbed energies with optimal energy resolution
Quantum Information Scrambling in a Superconducting Qutrit Processor
The theory of quantum information provides a common language which links
disciplines ranging from cosmology to condensed-matter physics. For example,
the delocalization of quantum information in strongly-interacting many-body
systems, known as quantum information scrambling, has recently begun to unite
our understanding of black hole dynamics, transport in exotic non-Fermi
liquids, and many-body analogs of quantum chaos. To date, verified experimental
implementations of scrambling have dealt only with systems comprised of
two-level qubits. Higher-dimensional quantum systems, however, may exhibit
different scrambling modalities and are predicted to saturate conjectured speed
limits on the rate of quantum information scrambling. We take the first steps
toward accessing such phenomena, by realizing a quantum processor based on
superconducting qutrits (three-level quantum systems). We implement two-qutrit
scrambling operations and embed them in a five-qutrit teleportation algorithm
to directly measure the associated out of-time-ordered correlation functions.
Measured teleportation fidelities, Favg = 0.568 +- 0001, confirm the occurrence
of scrambling even in the presence of experimental imperfections. Our
teleportation algorithm, which connects to recent proposals for studying
traversable wormholes in the laboratory, demonstrates how quantum information
processing technology based on higher dimensional systems can exploit a larger
and more connected state space to achieve the resource efficient encoding of
complex quantum circuits
Imaging the oblique propagation of electrons in germanium crystals at low temperature and low electric field
Excited electrons in the conduction band of germanium collect into four energy minima, or valleys, in momentum space. These local minima have highly anisotropic mass tensors which cause the electrons to travel in directions which are oblique to an applied electric field at sub-Kelvin temperatures and low electric fields, in contrast to the more isotropic behavior of the holes. This experiment produces a full two-dimensional image of the oblique electron and hole propagation and the quantum transitions of electrons between valleys for electric fields oriented along the [0,0,1] direction. Charge carriers are excited with a focused laser pulse on one face of a germanium crystal and then drifted through the crystal by a uniform electric field of strength between 0.5 and 6 V/cm. The pattern of charge density arriving on the opposite face is used to reconstruct the trajectories of the carriers. Measurements of the two-dimensional pattern of charge density are compared in detail with Monte Carlo simulations developed for the Cryogenic Dark Matter Search (SuperCDMS) to model the transport of charge carriers in high-purity germanium detectors
Multipartite Entanglement in Rabi Driven Superconducting Qubits
Exploring highly connected networks of qubits is invaluable for implementing
various quantum algorithms and simulations as it allows for entangling qubits
with reduced circuit depth. Here, we demonstrate a multi-qubit STAR (Sideband
Tone Assisted Rabi driven) gate. Our scheme is inspired by the ion qubit
M{\o}lmer-S{\o}rensen gate and is mediated by a shared photonic mode and
Rabi-driven superconducting qubits, which relaxes restrictions on qubit
frequencies during fabrication and supports scalability. We achieve a two-qubit
gate with maximum state fidelity of 0.95 in 310 ns, a three-qubit gate with
state fidelity 0.905 in 217 ns, and a four-qubit gate with state fidelity 0.66
in 200 ns. Furthermore, we develop a model of the gate that show the four-qubit
gate is limited by shared resonator losses and the spread of qubit-resonator
couplings, which must be addressed to reach high-fidelity operations.Comment: 16 pages, 14 figure
Monitoring Fast Superconducting Qubit Dynamics Using A Neural Network
Weak measurements of a superconducting qubit produce noisy voltage signals that are weakly correlated with the qubit state. To recover individual quantum trajectories from these noisy signals, traditional methods require slow qubit dynamics and substantial prior information in the form of calibration experiments. Monitoring rapid qubit dynamics, e.g., during quantum gates, requires more complicated methods with increased demand for prior information. Here, we experimentally demonstrate an alternative method for accurately tracking rapidly driven superconducting qubit trajectories that uses a long short-term memory (LSTM) artificial neural network with minimal prior information. Despite few training assumptions, the LSTM produces trajectories that include qubit-readout resonator correlations due to a finite detection bandwidth. In addition to revealing rotated measurement eigenstates and a reduced measurement rate in agreement with theory for a fixed drive, the trained LSTM also correctly reconstructs evolution for an unknown drive with rapid modulation. Our work enables new applications of weak measurements with faster or initially unknown qubit dynamics, such as the diagnosis of coherent errors in quantum gates
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