234 research outputs found
Bandwidth-limited and noisy pulse sequences for single qubit operations in semiconductor spin qubits
Spin qubits are very valuable and scalable candidates in the area of quantum
computation and simulation applications. In the last decades, they have been
deeply investigated from a theoretical point of view and realized on the scale
of few devices in the laboratories. In semiconductors, spin qubits can be built
confining the spin of electrons in electrostatically defined quantum dots.
Through this approach it is possible to create different implementations:
single electron spin qubit, singlet-triplet spin qubit, or a three electrons
architecture, e. g. the hybrid qubit. For each qubit type, we study the single
qubit rotations along the principal axis of Bloch sphere including the
mandatory non-idealities of the control signals that realize the gate
operations. The realistic transient of the control signal pulses are obtained
by adopting an appropriate low-pass filter function. In addition the effect of
disturbances on the input signals is taken into account by using a Gaussian
noise model.Comment: 11 pages, 9 figures; Accepted for publications in Entropy - Special
Issue "Quantum Information Processing
Implications of Electronics Constraints for Solid-State Quantum Error Correction and Quantum Circuit Failure Probability
In this paper we present the impact of classical electronics constraints on a
solid-state quantum dot logical qubit architecture. Constraints due to routing
density, bandwidth allocation, signal timing, and thermally aware placement of
classical supporting electronics significantly affect the quantum error
correction circuit's error rate. We analyze one level of a quantum error
correction circuit using nine data qubits in a Bacon-Shor code configured as a
quantum memory. A hypothetical silicon double quantum dot quantum bit (qubit)
is used as the fundamental element. A pessimistic estimate of the error
probability of the quantum circuit is calculated using the total number of
gates and idle time using a provably optimal schedule for the circuit
operations obtained with an integer program methodology. The micro-architecture
analysis provides insight about the different ways the electronics impact the
circuit performance (e.g., extra idle time in the schedule), which can
significantly limit the ultimate performance of any quantum circuit and
therefore is a critical foundation for any future larger scale architecture
analysis.Comment: 10 pages, 7 figures, 3 table
Phenomenological Study of Decoherence in Solid-State Spin Qubits due to Nuclear Spin Diffusion
We present a study of the prospects for coherence preservation in solid-state
spin qubits using dynamical decoupling protocols. Recent experiments have
provided the first demonstrations of multipulse dynamical decoupling sequences
in this qubit system, but quantitative analyses of potential coherence
improvements have been hampered by a lack of concrete knowledge of the relevant
noise processes. We present simulations of qubit coherence under the
application of arbitrary dynamical decoupling pulse sequences based on an
experimentally validated semiclassical model. This phenomenological approach
bundles the details of underlying noise processes into a single experimentally
relevant noise power spectral density. Our results show that the dominant
features of experimental measurements in a two-electron singlet-triplet spin
qubit can be replicated using a noise power spectrum associated
with nuclear-spin-flips in the host material. Beginning with this validation we
address the effects of nuclear programming, high-frequency nuclear-spin
dynamics, and other high-frequency classical noise sources, with conjectures
supported by physical arguments and microscopic calculations where relevant.
Our results provide expected performance bounds and identify diagnostic metrics
that can be measured experimentally in order to better elucidate the underlying
nuclear spin dynamics.Comment: Updated References. Related articles at:
http://www.physics.usyd.edu.au/~mbiercuk/Publications.htm
Adiabatic two-qubit gates in capacitively coupled quantum dot hybrid qubits
The ability to tune qubits to flat points in their energy dispersions ("sweet
spots") is an important tool for mitigating the effects of charge noise and
dephasing in solid-state devices. However, the number of derivatives that must
be simultaneously set to zero grows exponentially with the number of coupled
qubits, making the task untenable for as few as two qubits. This is a
particular problem for adiabatic gates, due to their slower speeds. Here, we
propose an adiabatic two-qubit gate for quantum dot hybrid qubits, based on the
tunable, electrostatic coupling between distinct charge configurations. We
confirm the absence of a conventional sweet spot, but show that controlled-Z
(CZ) gates can nonetheless be optimized to have fidelities of 99% for a
typical level of quasistatic charge noise (1
eV). We then develop the concept of a dynamical sweet spot (DSS), for
which the time-averaged energy derivatives are set to zero, and identify a
simple pulse sequence that achieves an approximate DSS for a CZ gate, with a
5 improvement in the fidelity. We observe that the results depend on
the number of tunable parameters in the pulse sequence, and speculate that a
more elaborate sequence could potentially attain a true DSS.Comment: 14 pages, 9 figure
Quantum Computing
Quantum mechanics---the theory describing the fundamental workings of
nature---is famously counterintuitive: it predicts that a particle can be in
two places at the same time, and that two remote particles can be inextricably
and instantaneously linked. These predictions have been the topic of intense
metaphysical debate ever since the theory's inception early last century.
However, supreme predictive power combined with direct experimental observation
of some of these unusual phenomena leave little doubt as to its fundamental
correctness. In fact, without quantum mechanics we could not explain the
workings of a laser, nor indeed how a fridge magnet operates. Over the last
several decades quantum information science has emerged to seek answers to the
question: can we gain some advantage by storing, transmitting and processing
information encoded in systems that exhibit these unique quantum properties?
Today it is understood that the answer is yes. Many research groups around the
world are working towards one of the most ambitious goals humankind has ever
embarked upon: a quantum computer that promises to exponentially improve
computational power for particular tasks. A number of physical systems,
spanning much of modern physics, are being developed for this task---ranging
from single particles of light to superconducting circuits---and it is not yet
clear which, if any, will ultimately prove successful. Here we describe the
latest developments for each of the leading approaches and explain what the
major challenges are for the future.Comment: 26 pages, 7 figures, 291 references. Early draft of Nature 464, 45-53
(4 March 2010). Published version is more up-to-date and has several
corrections, but is half the length with far fewer reference
Towards a realistic GaAs-spin qubit device for a classical error-corrected quantum memory
Based on numerically-optimized real-device gates and parameters we study the
performance of the phase-flip (repetition) code on a linear array of Gallium
Arsenide (GaAs) quantum dots hosting singlet-triplet qubits. We first examine
the expected performance of the code using simple error models of circuit-level
and phenomenological noise, reporting, for example, a circuit-level
depolarizing noise threshold of approximately 3%. We then perform
density-matrix simulations using a maximum-likelihood and minimum-weight
matching decoder to study the effect of real-device dephasing, read-out error,
quasi-static as well as fast gate noise. Considering the trade-off between
qubit read-out error and dephasing time (T2) over measurement time, we identify
a sub-threshold region for the phase-flip code which lies within experimental
reach.Comment: 22 page
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