658 research outputs found
Tractable Simulation of Error Correction with Honest Approximations to Realistic Fault Models
In previous work, we proposed a method for leveraging efficient classical
simulation algorithms to aid in the analysis of large-scale fault tolerant
circuits implemented on hypothetical quantum information processors. Here, we
extend those results by numerically studying the efficacy of this proposal as a
tool for understanding the performance of an error-correction gadget
implemented with fault models derived from physical simulations. Our approach
is to approximate the arbitrary error maps that arise from realistic physical
models with errors that are amenable to a particular classical simulation
algorithm in an "honest" way; that is, such that we do not underestimate the
faults introduced by our physical models. In all cases, our approximations
provide an "honest representation" of the performance of the circuit composed
of the original errors. This numerical evidence supports the use of our method
as a way to understand the feasibility of an implementation of quantum
information processing given a characterization of the underlying physical
processes in experimentally accessible examples.Comment: 34 pages, 9 tables, 4 figure
Boosting Linear-Optical Bell Measurement Success Probability with Pre-Detection Squeezing and Imperfect Photon-Number-Resolving Detectors
Linear optical realizations of Bell state measurement (BSM) on two
single-photon qubits succeed with probability no higher than .
However pre-detection quadrature squeezing, i.e., quantum noise limited phase
sensitive amplification, in the usual linear-optical BSM circuit, can yield
. The ability to achieve has been found to be
critical in resource-efficient realizations of linear optical quantum computing
and all-photonic quantum repeaters. Yet, the aforesaid value of is
not known to be the maximum achievable using squeezing, thereby leaving it open
whether close-to- efficient BSM might be achievable using squeezing as a
resource. In this paper, we report new insights on why squeezing-enhanced BSM
achieves . Using this, we show that the previously-reported at single-mode squeezing strength ---for unambiguous
state discrimination (USD) of all four Bell states---is an experimentally
unachievable point result, which drops to with the slightest
change in . We however show that squeezing-induced boosting of with
USD operation is still possible over a continuous range of , with an
experimentally achievable maximum occurring at , achieving . Finally, deviating from USD operation, we explore a
trade-space between , the probability with which the BSM circuit declares
a "success", versus the probability of error , the probability of an input
Bell state being erroneously identified given the circuit declares a success.
Since quantum error correction could correct for some , this tradeoff
may enable better quantum repeater designs by potentially increasing the
entanglement generation rates with exceeding what is possible with
traditionally-studied USD operation of BSMs.Comment: 13 pages, 10 figure
The evaluation of failure detection and isolation algorithms for restructurable control
Three failure detection and identification techniques were compared to determine their usefulness in detecting and isolating failures in an aircraft flight control system; excluding sensor and flight control computer failures. The algorithms considered were the detection filter, the Generalized Likelihood Ratio test and the Orthogonal Series Generalized Likelihood Ratio test. A modification to the basic detection filter is also considered which uses secondary filtering of the residuals to produce unidirectional failure signals. The algorithms were evaluated by testing their ability to detect and isolate control surface failures in a nonlinear simulation of a C-130 aircraft. It was found that failures of some aircraft controls are difficult to distinguish because they have a similar effect on the dynamics of the vehicle. Quantitative measures for evaluating the distinguishability of failures are considered. A system monitoring strategy for implementing the failure detection and identification techniques was considered. This strategy identified the mix of direct measurement of failures versus the computation of failure necessary for implementation of the technology in an aircraft system
Fault-tolerant magic state preparation with flag qubits
Magic state distillation is one of the leading candidates for implementing
universal fault-tolerant logical gates. However, the distillation circuits
themselves are not fault-tolerant, so there is additional cost to first
implement encoded Clifford gates with negligible error. In this paper we
present a scheme to fault-tolerantly and directly prepare magic states using
flag qubits. One of these schemes uses a single extra ancilla, even with noisy
Clifford gates. We compare the physical qubit and gate cost of this scheme to
the magic state distillation protocol of Meier, Eastin, and Knill, which is
efficient and uses a small stabilizer circuit. In some regimes, we show that
the overhead can be improved by several orders of magnitude.Comment: 26 pages, 17 figures, 5 tables. Comments welcome! v2 (published
version): quantumarticle documentclass and expanded discussions on the
fault-tolerant scheme
Experimentally efficient methods for estimating the performance of quantum measurements
Efficient methods for characterizing the performance of quantum measurements
are important in the experimental quantum sciences. Ideally, one requires both
a physically relevant distinguishability measure between measurement operations
and a well-defined experimental procedure for estimating the distinguishability
measure. Here, we propose the average measurement fidelity and error between
quantum measurements as distinguishability measures. We present protocols for
obtaining bounds on these quantities that are both estimable using
experimentally accessible quantities and scalable in the size of the quantum
system. We explain why the bounds should be valid in large generality and
illustrate the method via numerical examples.Comment: 20 pages, 1 figure. Expanded details and typos corrected. Accepted
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