881 research outputs found
Layered architecture for quantum computing
We develop a layered quantum computer architecture, which is a systematic
framework for tackling the individual challenges of developing a quantum
computer while constructing a cohesive device design. We discuss many of the
prominent techniques for implementing circuit-model quantum computing and
introduce several new methods, with an emphasis on employing surface code
quantum error correction. In doing so, we propose a new quantum computer
architecture based on optical control of quantum dots. The timescales of
physical hardware operations and logical, error-corrected quantum gates differ
by several orders of magnitude. By dividing functionality into layers, we can
design and analyze subsystems independently, demonstrating the value of our
layered architectural approach. Using this concrete hardware platform, we
provide resource analysis for executing fault-tolerant quantum algorithms for
integer factoring and quantum simulation, finding that the quantum dot
architecture we study could solve such problems on the timescale of days.Comment: 27 pages, 20 figure
Cluster-based architecture for fault-tolerant quantum computation
We present a detailed description of an architecture for fault-tolerant
quantum computation, which is based on the cluster model of encoded qubits. In
this cluster-based architecture, concatenated computation is implemented in a
quite different way from the usual circuit-based architecture where physical
gates are recursively replaced by logical gates with error-correction gadgets.
Instead, some relevant cluster states, say fundamental clusters, are
recursively constructed through verification and postselection in advance for
the higher-level one-way computation, which namely provides error-precorrection
of gate operations. A suitable code such as the Steane seven-qubit code is
adopted for transversal operations. This concatenated construction of verified
fundamental clusters has a simple transversal structure of logical errors, and
achieves a high noise threshold ~ 3 % for computation by using appropriate
verification procedures. Since the postselection is localized within each
fundamental cluster with the help of deterministic bare controlled-Z gates
without verification, divergence of resources is restrained, which reconciles
postselection with scalability.Comment: 16 pages, 34 figure
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
qTorch: The Quantum Tensor Contraction Handler
Classical simulation of quantum computation is necessary for studying the
numerical behavior of quantum algorithms, as there does not yet exist a large
viable quantum computer on which to perform numerical tests. Tensor network
(TN) contraction is an algorithmic method that can efficiently simulate some
quantum circuits, often greatly reducing the computational cost over methods
that simulate the full Hilbert space. In this study we implement a tensor
network contraction program for simulating quantum circuits using multi-core
compute nodes. We show simulation results for the Max-Cut problem on 3- through
7-regular graphs using the quantum approximate optimization algorithm (QAOA),
successfully simulating up to 100 qubits. We test two different methods for
generating the ordering of tensor index contractions: one is based on the tree
decomposition of the line graph, while the other generates ordering using a
straight-forward stochastic scheme. Through studying instances of QAOA
circuits, we show the expected result that as the treewidth of the quantum
circuit's line graph decreases, TN contraction becomes significantly more
efficient than simulating the whole Hilbert space. The results in this work
suggest that tensor contraction methods are superior only when simulating
Max-Cut/QAOA with graphs of regularities approximately five and below. Insight
into this point of equal computational cost helps one determine which
simulation method will be more efficient for a given quantum circuit. The
stochastic contraction method outperforms the line graph based method only when
the time to calculate a reasonable tree decomposition is prohibitively
expensive. Finally, we release our software package, qTorch (Quantum TensOR
Contraction Handler), intended for general quantum circuit simulation.Comment: 21 pages, 8 figure
Scalable Designs for Quasiparticle-Poisoning-Protected Topological Quantum Computation with Majorana Zero Modes
We present designs for scalable quantum computers composed of qubits encoded
in aggregates of four or more Majorana zero modes, realized at the ends of
topological superconducting wire segments that are assembled into
superconducting islands with significant charging energy. Quantum information
can be manipulated according to a measurement-only protocol, which is
facilitated by tunable couplings between Majorana zero modes and nearby
semiconductor quantum dots. Our proposed architecture designs have the
following principal virtues: (1) the magnetic field can be aligned in the
direction of all of the topological superconducting wires since they are all
parallel; (2) topological -junctions are not used, obviating possible
difficulties in their fabrication and utilization; (3) quasiparticle poisoning
is abated by the charging energy; (4) Clifford operations are executed by a
relatively standard measurement: detection of corrections to quantum dot
energy, charge, or differential capacitance induced by quantum fluctuations;
(5) it is compatible with strategies for producing good approximate magic
states.Comment: 34 pages, 17 figures; v4: minor changes, final versio
Experimental demonstration of a graph state quantum error-correction code
Scalable quantum computing and communication requires the protection of
quantum information from the detrimental effects of decoherence and noise.
Previous work tackling this problem has relied on the original circuit model
for quantum computing. However, recently a family of entangled resources known
as graph states has emerged as a versatile alternative for protecting quantum
information. Depending on the graph's structure, errors can be detected and
corrected in an efficient way using measurement-based techniques. In this
article we report an experimental demonstration of error correction using a
graph state code. We have used an all-optical setup to encode quantum
information into photons representing a four-qubit graph state. We are able to
reliably detect errors and correct against qubit loss. The graph we have
realized is setup independent, thus it could be employed in other physical
settings. Our results show that graph state codes are a promising approach for
achieving scalable quantum information processing
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