381 research outputs found
Surface code architecture for donors and dots in silicon with imprecise and nonuniform qubit couplings
A scaled quantum computer with donor spins in silicon would benefit from a
viable semiconductor framework and a strong inherent decoupling of the qubits
from the noisy environment. Coupling neighbouring spins via the natural
exchange interaction according to current designs requires gate control
structures with extremely small length scales. We present a silicon
architecture where bismuth donors with long coherence times are coupled to
electrons that can shuttle between adjacent quantum dots, thus relaxing the
pitch requirements and allowing space between donors for classical control
devices. An adiabatic SWAP operation within each donor/dot pair solves the
scalability issues intrinsic to exchange-based two-qubit gates, as it does not
rely on sub-nanometer precision in donor placement and is robust against noise
in the control fields. We use this SWAP together with well established global
microwave Rabi pulses and parallel electron shuttling to construct a surface
code that needs minimal, feasible local control.Comment: Published version - more detailed discussions, robustness to
dephasing pointed out additionall
Using Quantum Computers for Quantum Simulation
Numerical simulation of quantum systems is crucial to further our
understanding of natural phenomena. Many systems of key interest and
importance, in areas such as superconducting materials and quantum chemistry,
are thought to be described by models which we cannot solve with sufficient
accuracy, neither analytically nor numerically with classical computers. Using
a quantum computer to simulate such quantum systems has been viewed as a key
application of quantum computation from the very beginning of the field in the
1980s. Moreover, useful results beyond the reach of classical computation are
expected to be accessible with fewer than a hundred qubits, making quantum
simulation potentially one of the earliest practical applications of quantum
computers. In this paper we survey the theoretical and experimental development
of quantum simulation using quantum computers, from the first ideas to the
intense research efforts currently underway.Comment: 43 pages, 136 references, review article, v2 major revisions in
response to referee comments, v3 significant revisions, identical to
published version apart from format, ArXiv version has table of contents and
references in alphabetical orde
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
Co-Design quantum simulation of nanoscale NMR
Quantum computers have the potential to efficiently simulate the dynamics of nanoscale NMR systems. In this work, we demonstrate that a noisy intermediate-scale quantum computer can be used to simulate and predict nanoscale NMR resonances. In order to minimize the required gate fidelities, we propose a superconducting application-specific Co-Design quantum processor that reduces the number of SWAP gates by over 90% for chips with more than 20 qubits. The processor consists of transmon qubits capacitively coupled via tunable couplers to a central co-planar waveguide resonator with a quantum circuit refrigerator (QCR) for fast resonator reset. The QCR implements the nonunitary quantum operations required to simulate nuclear hyperpolarization scenarios.The authors would like to thank Caspar Ockeloen-Korppi,
Alessandro Landra, and Johannes Heinsoo for their help in de-
veloping the idea of the star-architecture chip, Jani Tuorila for
his support in developing the gate theory, Amin Hosseinkhani
and Tianhan Liu for reviewing the manuscript, and Hen-
rikki Mäkynen and Hoang-Mai Nguyen for graphic design.
J.C. additionally acknowledges the Ramón y Cajal program
(RYC2018-025197-I). We further acknowledge support from
Atos with the Quantum Learning Machine (QLM). Finally,
the authors acknowledge financial support to BMBF through
the Q-Exa Project No. FZK: 13N16062
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
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