12,560 research outputs found
Physical-depth architectural requirements for generating universal photonic cluster states
Most leading proposals for linear-optical quantum computing (LOQC) use
cluster states, which act as a universal resource for measurement-based
(one-way) quantum computation (MBQC). In ballistic approaches to LOQC, cluster
states are generated passively from small entangled resource states using
so-called fusion operations. Results from percolation theory have previously
been used to argue that universal cluster states can be generated in the
ballistic approach using schemes which exceed the critical threshold for
percolation, but these results consider cluster states with unbounded size.
Here we consider how successful percolation can be maintained using a physical
architecture with fixed physical depth, assuming that the cluster state is
continuously generated and measured, and therefore that only a finite portion
of it is visible at any one point in time. We show that universal LOQC can be
implemented using a constant-size device with modest physical depth, and that
percolation can be exploited using simple pathfinding strategies without the
need for high-complexity algorithms.Comment: 18 pages, 10 figure
Spin quantum computation in silicon nanostructures
Proposed silicon-based quantum-computer architectures have attracted
attention because of their promise for scalability and their potential for
synergetically utilizing the available resources associated with the existing
Si technology infrastructure. Electronic and nuclear spins of shallow donors
(e.g. phosphorus) in Si are ideal candidates for qubits in such proposals
because of their long spin coherence times due to their limited interactions
with their environments. For these spin qubits, shallow donor exchange gates
are frequently invoked to perform two-qubit operations. We discuss in this
review a particularly important spin decoherence channel, and bandstructure
effects on the exchange gate control. Specifically, we review our work on donor
electron spin spectral diffusion due to background nuclear spin flip-flops, and
how isotopic purification of silicon can significantly enhance the electron
spin dephasing time. We then review our calculation of donor electron exchange
coupling in the presence of degenerate silicon conduction band valleys. We show
that valley interference leads to orders of magnitude variations in electron
exchange coupling when donor configurations are changed on an atomic scale.
These studies illustrate the substantial potential that donor electron/nuclear
spins in silicon have as candidates for qubits and simultaneously the
considerable challenges they pose. In particular, our work on spin decoherence
through spectral diffusion points to the possible importance of isotopic
purification in the fabrication of scalable solid state quantum computer
architectures. We also provide a critical comparison between the two main
proposed spin-based solid state quantum computer architectures, namely, shallow
donor bound states in Si and localized quantum dot states in GaAs.Comment: 14 pages. Review article submitted to Solid State Communication
Quantum Computing with Very Noisy Devices
In theory, quantum computers can efficiently simulate quantum physics, factor
large numbers and estimate integrals, thus solving otherwise intractable
computational problems. In practice, quantum computers must operate with noisy
devices called ``gates'' that tend to destroy the fragile quantum states needed
for computation. The goal of fault-tolerant quantum computing is to compute
accurately even when gates have a high probability of error each time they are
used. Here we give evidence that accurate quantum computing is possible with
error probabilities above 3% per gate, which is significantly higher than what
was previously thought possible. However, the resources required for computing
at such high error probabilities are excessive. Fortunately, they decrease
rapidly with decreasing error probabilities. If we had quantum resources
comparable to the considerable resources available in today's digital
computers, we could implement non-trivial quantum computations at error
probabilities as high as 1% per gate.Comment: 47 page
QuEST and High Performance Simulation of Quantum Computers
We introduce QuEST, the Quantum Exact Simulation Toolkit, and compare it to
ProjectQ, qHipster and a recent distributed implementation of Quantum++. QuEST
is the first open source, OpenMP and MPI hybridised, GPU accelerated simulator
of universal quantum circuits. Embodied as a C library, it is designed so that
a user's code can be deployed seamlessly to any platform from a laptop to a
supercomputer. QuEST is capable of simulating generic quantum circuits of
general single-qubit gates and multi-qubit controlled gates, on pure and mixed
states, represented as state-vectors and density matrices, and under the
presence of decoherence. Using the ARCUS Phase-B and ARCHER supercomputers, we
benchmark QuEST's simulation of random circuits of up to 38 qubits, distributed
over up to 2048 compute nodes, each with up to 24 cores. We directly compare
QuEST's performance to ProjectQ's on single machines, and discuss the
differences in distribution strategies of QuEST, qHipster and Quantum++. QuEST
shows excellent scaling, both strong and weak, on multicore and distributed
architectures.Comment: 8 pages, 8 figures; fixed typos; updated QuEST URL and fixed typo in
Fig. 4 caption where ProjectQ and QuEST were swapped in speedup subplot
explanation; added explanation of simulation algorithm, updated bibliography;
stressed technical novelty of QuEST; mentioned new density matrix suppor
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