20 research outputs found
The Phase-Contrast Imaging Instrument at the Matter in Extreme Conditions Endstation at LCLS
We describe the Phase-Contrast Imaging instrument at the Matter in Extreme
Conditions (MEC) endstation of the Linac Coherent Light Source. The instrument
can image phenomena with a spatial resolution of a few hundreds of nanometers
and at the same time reveal the atomic structure through X-ray diffraction,
with a temporal resolution better than 100 femtosecond. It was specifically
designed for studies relevant to High-Energy-Density Science and can monitor,
e.g., shock fronts, phase transitions, or void collapses. This versatile
instrument was commissioned last year and is now available to the MEC user
community
A quantum-classical co-processing protocol towards simulating nuclear reactions on contemporary quantum hardware
Quantum computers hold great promise for arriving at exact simulations of
nuclear dynamical processes (e.g., scattering and reactions) that are paramount
to the study of nuclear matter at the limit of stability and to explaining the
formation of chemical elements in stars. However, quantum simulations of the
unitary (real) time dynamics of fermionic many-body systems require a currently
prohibitive number of reliable and long-lived qubits. We propose a
co-processing algorithm for the simulation of real-time dynamics in which the
time evolution of the spatial coordinates is carried out on a classical
processor, while the evolution of the spin degrees of freedom is carried out on
a quantum processor. This hybrid algorithm is demonstrated by a quantum
simulation of the scattering of two neutrons performed at the Lawrence Berkeley
National Laboratory's Advanced Quantum Testbed. We show that, after
implementation of error mitigation strategies to improve the accuracy of the
algorithm in addition to the use of either circuit compression techniques or
tomography as methods to elucidate the onset of decoherence, this initial
demonstration validates the principle of the proposed co-processing scheme. We
anticipate that a generalization of this present scheme will open the way for
(real-time) path integral simulations of nuclear scattering.Comment: 12 pages, 10 figure
Programmable Heisenberg Interactions Between Floquet Qubits
The trade-off between robustness and tunability is a central challenge in the pursuit of quantum simulation and fault-tolerant quantum computation. In particular, quantum architectures are often designed to achieve high coherence at the expense of tunability. Many current qubit designs have fixed energy levels and consequently limited types of controllable interactions. Here by adiabatically transforming fixed-frequency superconducting circuits into modifiable Floquet qubits, we demonstrate an XXZ Heisenberg interaction with fully adjustable anisotropy. This interaction model can act as the primitive for an expressive set of quantum operations, but is also the basis for quantum simulations of spin systems. To illustrate the robustness and versatility of our Floquet protocol, we tailor the Heisenberg Hamiltonian and implement two-qubit iSWAP, CZ and SWAP gates with good estimated fidelities. In addition, we implement a Heisenberg interaction between higher energy levels and employ it to construct a three-qubit CCZ gate, also with a competitive fidelity. Our protocol applies to multiple fixed-frequency high-coherence platforms, providing a collection of interactions for high-performance quantum information processing. It also establishes the potential of the Floquet framework as a tool for exploring quantum electrodynamics and optimal control
Random-access quantum memory using chirped pulse phase encoding
Quantum memories capable of faithfully storing and recalling quantum states
on-demand are powerful ingredients in bulding quantum networks
[arXiv:0806.4195] and quantum information processors [arXiv:1109.3743]. As in
conventional computing, key attributes of such memories are high storage
density and, crucially, random access, or the ability to read from or write to
an arbitrarily chosen register. However, achieving such random access with
quantum memories [arXiv:1904.09643] in a dense, hardware-efficient manner
remains a challenge, for example requiring dedicated cavities per qubit
[arXiv:1109.3743] or pulsed field gradients [arXiv:0908.0101]. Here we
introduce a protocol using chirped pulses to encode qubits within an ensemble
of quantum two-level systems, offering both random access and naturally
supporting dynamical decoupling to enhance the memory lifetime. We demonstrate
the protocol in the microwave regime using donor spins in silicon coupled to a
superconducting cavity, storing up to four multi-photon microwave pulses and
retrieving them on-demand up to 2~ms later. A further advantage is the natural
suppression of superradiant echo emission, which we show is critical when
approaching unit cooperativity. This approach offers the potential for
microwave random access quantum memories with lifetimes exceeding seconds
[arXiv:1301.6567, arXiv:2005.09275], while the chirped pulse phase encoding
could also be applied in the optical regime to enhance quantum repeaters and
networks
Randomized compiling for scalable quantum computing on a noisy superconducting quantum processor
The successful implementation of algorithms on quantum processors relies on
the accurate control of quantum bits (qubits) to perform logic gate operations.
In this era of noisy intermediate-scale quantum (NISQ) computing, systematic
miscalibrations, drift, and crosstalk in the control of qubits can lead to a
coherent form of error which has no classical analog. Coherent errors severely
limit the performance of quantum algorithms in an unpredictable manner, and
mitigating their impact is necessary for realizing reliable quantum
computations. Moreover, the average error rates measured by randomized
benchmarking and related protocols are not sensitive to the full impact of
coherent errors, and therefore do not reliably predict the global performance
of quantum algorithms, leaving us unprepared to validate the accuracy of future
large-scale quantum computations. Randomized compiling is a protocol designed
to overcome these performance limitations by converting coherent errors into
stochastic noise, dramatically reducing unpredictable errors in quantum
algorithms and enabling accurate predictions of algorithmic performance from
error rates measured via cycle benchmarking. In this work, we demonstrate
significant performance gains under randomized compiling for the four-qubit
quantum Fourier transform algorithm and for random circuits of variable depth
on a superconducting quantum processor. Additionally, we accurately predict
algorithm performance using experimentally-measured error rates. Our results
demonstrate that randomized compiling can be utilized to maximally-leverage and
predict the capabilities of modern-day noisy quantum processors, paving the way
forward for scalable quantum computing
Superstaq: Deep Optimization of Quantum Programs
We describe Superstaq, a quantum software platform that optimizes the
execution of quantum programs by tailoring to underlying hardware primitives.
For benchmarks such as the Bernstein-Vazirani algorithm and the Qubit Coupled
Cluster chemistry method, we find that deep optimization can improve program
execution performance by at least 10x compared to prevailing state-of-the-art
compilers. To highlight the versatility of our approach, we present results
from several hardware platforms: superconducting qubits (AQT @ LBNL, IBM
Quantum, Rigetti), trapped ions (QSCOUT), and neutral atoms (Infleqtion).
Across all platforms, we demonstrate new levels of performance and new
capabilities that are enabled by deeper integration between quantum programs
and the device physics of hardware.Comment: Appearing in IEEE QCE 2023 (Quantum Week) conferenc
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Noise Tailoring for Enhancing the Capabilities of Quantum Computers
The successful implementation of algorithms on quantum processors relies on the accurate control of quantum bits (qubits) to perform logic gate operations. However, qubits in the noisy intermediate-scale quantum (NISQ) era are short-lived and susceptible to a variety of errors and noise due to imperfect control signals and incomplete isolation from the surrounding environment. For example, systematic miscalibrations, unwanted entanglement, and crosstalk in the control of qubits can lead to a coherent form of error which has no classical analog. Coherent errors can severely limit the performance of quantum algorithms in an unpredictable manner on timescales shorter than the coherence times of qubits. In recent years, there has been growing interest in using methods which randomize the physical implementation of quantum gates to mitigate the impact of coherent errors, effectively tailoring them into a form of stochastic noise. In this thesis, we study one such method --- randomized compiling --- and show how gate errors under randomized compiling are accurately described by a stochastic Pauli noise model without coherent errors. We demonstrate significant performance gains under randomized compiling for various different quantum algorithms, such as the quantum Fourier transform. We further show that randomized compiling can improve the predictability of quantum algorithms, and enables unique forms of error mitigation for enhancing the performance of quantum computations in the NISQ era. Finally, we show that randomized compiling can reduce worst-case error rates by orders of magnitude, enabling the accurate characterization of quantum gates for fault tolerance. Our results demonstrate that randomized compiling can be utilized to leverage and predict the capabilities of modern-day, noisy quantum processors, paving the way forward for scalable quantum computing and fault tolerant quantum error correction
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Optimized SWAP networks with equivalent circuit averaging for QAOA
The SWAP network is a qubit routing sequence that can be used to efficiently execute the Quantum Approximate Optimization Algorithm (QAOA). Even with a minimally connected topology on an -qubit processor, this routing sequence enables operations to execute in steps. In this work, we optimize the execution of SWAP networks for QAOA through two techniques. First, we take advantage of an overcomplete set of native hardware operations [including 150-ns controlled- phase gates with up to 99.67(1)% fidelity] to decompose the relevant quantum gates and SWAP networks in a manner which minimizes circuit depth and maximizes gate cancellation. Second, we introduce equivalent circuit averaging, which randomizes over degrees of freedom in the quantum circuit compilation to reduce the impact of systematic coherent errors. Our techniques are experimentally validated at the Advanced Quantum Testbed through the execution of QAOA circuits for finding the ground state of two- and four-node Sherrington-Kirkpatrick spin-glass models with various randomly sampled parameters. We observe a average reduction in error (total variation distance) for QAOA of depth on four transmon qubits on a superconducting quantum processor