116 research outputs found
Building logical qubits in a superconducting quantum computing system
The technological world is in the midst of a quantum computing and quantum
information revolution. Since Richard Feynman's famous "plenty of room at the
bottom" lecture, hinting at the notion of novel devices employing quantum
mechanics, the quantum information community has taken gigantic strides in
understanding the potential applications of a quantum computer and laid the
foundational requirements for building one. We believe that the next
significant step will be to demonstrate a quantum memory, in which a system of
interacting qubits stores an encoded logical qubit state longer than the
incorporated parts. Here, we describe the important route towards a logical
memory with superconducting qubits, employing a rotated version of the surface
code. The current status of technology with regards to interconnected
superconducting-qubit networks will be described and near-term areas of focus
to improve devices will be identified. Overall, the progress in this exciting
field has been astounding, but we are at an important turning point where it
will be critical to incorporate engineering solutions with quantum
architectural considerations, laying the foundation towards scalable
fault-tolerant quantum computers in the near future.Comment: 10 pages, 5 figure
Procedure for systematically tuning up crosstalk in the cross resonance gate
We present improvements in both theoretical understanding and experimental
implementation of the cross resonance (CR) gate that have led to shorter
two-qubit gate times and interleaved randomized benchmarking fidelities
exceeding 99%. The CR gate is an all-microwave two-qubit gate offers that does
not require tunability and is therefore well suited to quantum computing
architectures based on 2D superconducting qubits. The performance of the gate
has previously been hindered by long gate times and fidelities averaging
94-96%. We have developed a calibration procedure that accurately measures the
full CR Hamiltonian. The resulting measurements agree with theoretical analysis
of the gate and also elucidate the error terms that have previously limited the
gate fidelity. The increase in fidelity that we have achieved was accomplished
by introducing a second microwave drive tone on the target qubit to cancel
unwanted components of the CR Hamiltonian.Comment: 6 pages, 5 figure
Three Qubit Randomized Benchmarking
As quantum circuits increase in size, it is critical to establish scalable
multiqubit fidelity metrics. Here we investigate three-qubit randomized
benchmarking (RB) with fixed-frequency transmon qubits coupled to a common bus
with pairwise microwave-activated interactions (cross-resonance). We measure,
for the first time, a three-qubit error per Clifford of 0.106 for all-to-all
gate connectivity and 0.207 for linear gate connectivity. Furthermore, by
introducing mixed dimensionality simultaneous RB --- simultaneous one- and
two-qubit RB --- we show that the three-qubit errors can be predicted from the
one- and two-qubit errors. However, by introducing certain coherent errors to
the gates we can increase the three-qubit error to 0.302, an increase that is
not predicted by a proportionate increase in the one- and two-qubit errors from
simultaneous RB. This demonstrates three-qubit RB as a unique multiqubit
metric.Comment: 6 pages, 2 figures V2: Fixed an error in Eqn. 1 of V1 and added
supplementary informatio
Experimental demonstration of fault-tolerant state preparation with superconducting qubits
Robust quantum computation requires encoding delicate quantum information
into degrees of freedom that are hard for the environment to change. Quantum
encodings have been demonstrated in many physical systems by observing and
correcting storage errors, but applications require not just storing
information; we must accurately compute even with faulty operations. The theory
of fault-tolerant quantum computing illuminates a way forward by providing a
foundation and collection of techniques for limiting the spread of errors. Here
we implement one of the smallest quantum codes in a five-qubit superconducting
transmon device and demonstrate fault-tolerant state preparation. We
characterize the resulting codewords through quantum process tomography and
study the free evolution of the logical observables. Our results are consistent
with fault-tolerant state preparation in a protected qubit subspace
Multi-Path Interferometric Josephson Directional Amplifier for Qubit Readout
We realize and characterize a quantum-limited, directional Josephson
amplifier suitable for qubit readout. The device consists of two nondegenerate,
three-wave-mixing amplifiers that are coupled together in an interferometric
scheme, embedded in a printed circuit board. Nonreciprocity is generated by
applying a phase gradient between the same-frequency pumps feeding the device,
which plays the role of the magnetic field in a Faraday medium. Directional
amplification and reflection-gain elimination are induced via wave interference
between multiple paths in the system. We measure and discuss the main figures
of merit of the device and show that the experimental results are in good
agreement with theory. An improved version of this directional amplifier is
expected to eliminate the need for bulky, off-chip isolation stages that
generally separate quantum systems and preamplifiers in high-fidelity,
quantum-nondemolition measurement setups
Microwave-activated conditional-phase gate for superconducting qubits
We introduce a new entangling gate between two fixed-frequency qubits
statically coupled via a microwave resonator bus which combines the following
desirable qualities: all-microwave control, appreciable qubit separation for
reduction of crosstalk and leakage errors, and the ability to function as a
two-qubit conditional-phase gate. A fixed, always-on interaction is explicitly
designed between higher energy (non-computational) states of two transmon
qubits, and then a conditional-phase gate is `activated' on the otherwise
unperturbed qubit subspace via a microwave drive. We implement this
microwave-activated conditional-phase gate with a fidelity from quantum process
tomography of 87%.Comment: 5 figure
Rapid Driven Reset of a Qubit Readout Resonator
Using a circuit QED device, we demonstrate a simple qubit measurement pulse
shape that yields fast ring-up and ring-down of the readout resonator
regardless of the qubit state. The pulse differs from a square pulse only by
the inclusion of additional constant-amplitude segments designed to effect a
rapid transition from one steady-state population to another. Using a Ramsey
experiment performed shortly after the measurement pulse to quantify the
residual population, we find that compared to a square pulse followed by a
delay, this pulse shape reduces the timescale for cavity ring-down by more than
twice the cavity time constant. At low drive powers, this performance is
achieved using pulse parameters calculated from a linear cavity model; at
higher powers, empirical optimization of the pulse parameters leads to similar
performance
Reducing Spontaneous Emission in Circuit Quantum Electrodynamics by a Combined Readout/Filter Technique
Physical implementations of qubits can be extremely sensitive to
environmental coupling, which can result in decoherence. While efforts are made
for protection, coupling to the environment is necessary to measure and
manipulate the state of the qubit. As such, the goal of having long qubit
energy relaxation times is in competition with that of achieving high-fidelity
qubit control and measurement. Here we propose a method that integrates
filtering techniques for preserving superconducting qubit lifetimes together
with the dispersive coupling of the qubit to a microwave resonator for control
and measurement. The result is a compact circuit that protects qubits from
spontaneous loss to the environment, while also retaining the ability to
perform fast, high-fidelity readout. Importantly, we show the device operates
in a regime that is attainable with current experimental parameters and provide
a specific example for superconducting qubits in circuit quantum
electrodynamics.Comment: 9 pages, 6 figures, 1 tabl
Characterization of hidden modes in networks of superconducting qubits
We present a method for detecting electromagnetic (EM) modes that couple to a
superconducting qubit in a circuit-QED architecture. Based on
measurement-induced dephasing, this technique allows the measurement of modes
that have a high quality factor (Q) and may be difficult to detect through
standard transmission and reflection measurements at the device ports. In this
scheme the qubit itself acts as a sensitive phase meter, revealing modes that
couple to it through measurements of its coherence time. Such modes are
indistinguishable from EM modes that do not couple to the qubit using a vector
network analyzer. Moreover, this technique provides useful characterization
parameters including the quality factor and the coupling strength of the
unwanted resonances. We demonstrate the method for detecting both high-Q
coupling resonators in planar devices as well as spurious modes produced by a
3D cavity.Comment: 4 pages, 2 figures; updated to published versio
Hardware-efficient Variational Quantum Eigensolver for Small Molecules and Quantum Magnets
Quantum computers can be used to address molecular structure, materials
science and condensed matter physics problems, which currently stretch the
limits of existing high-performance computing resources. Finding exact
numerical solutions to these interacting fermion problems has exponential cost,
while Monte Carlo methods are plagued by the fermionic sign problem. These
limitations of classical computational methods have made even few-atom
molecular structures problems of practical interest for medium-sized quantum
computers. Yet, thus far experimental implementations have been restricted to
molecules involving only Period I elements. Here, we demonstrate the
experimental optimization of up to six-qubit Hamiltonian problems with over a
hundred Pauli terms, determining the ground state energy for molecules of
increasing size, up to BeH2. This is enabled by a hardware-efficient
variational quantum eigensolver with trial states specifically tailored to the
available interactions in our quantum processor, combined with a compact
encoding of fermionic Hamiltonians and a robust stochastic optimization
routine. We further demonstrate the flexibility of our approach by applying the
technique to a problem of quantum magnetism. Across all studied problems, we
find agreement between experiment and numerical simulations with a noisy model
of the device. These results help elucidate the requirements for scaling the
method to larger systems, and aim at bridging the gap between problems at the
forefront of high-performance computing and their implementation on quantum
hardware.Comment: 6 pages, 4 figures in main text. 18 pages, 9 figures in supplementary
informatio
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