1,979 research outputs found
Nearly Deterministic Bell Measurement for Multiphoton Qubits and Its Application to Quantum Information Processing
We propose a Bell measurement scheme by employing a logical qubit in
Greenberger-Horne-Zeilinger (GHZ) entanglement with an arbitrary number of
photons. Remarkably, the success probability of the Bell measurement as well as
teleportation of the GHZ entanglement can be made arbitrarily high using only
linear optics elements and photon on-off measurements as the number of photons
increases. Our scheme outperforms previous proposals using single photon qubits
when comparing the success probabilities in terms of the average photon usages.
It has another important advantage for experimental feasibility that it does
not require photon number resolving measurements. Our proposal provides an
alternative candidate for all-optical quantum information processing.Comment: 7 pages (including supplementary material), 2 figures, to be
published in Phys. Rev. Let
Unconditional measurement-based quantum computation with optomechanical continuous variables
Universal quantum computation encoded over continuous variables can be
achieved via Gaussian measurements acting on entangled non-Gaussian states.
However, due to the weakness of available nonlinearities, generally these
states can only be prepared conditionally, potentially with low probability.
Here we show how universal quantum computation could be implemented
unconditionally using an integrated platform able to sustain both linear and
quadratic optomechanical-like interactions. Specifically, considering cavity
opto- and electro-mechanical systems, we propose a realisation of a
driven-dissipative dynamics that deterministically prepares the required
non-Gaussian cluster states --- entangled squeezed states of multiple
mechanical oscillators suitably interspersed with cubic-phase states. We next
demonstrate how arbitrary Gaussian measurements on the cluster nodes can be
performed by continuously monitoring the output cavity field. Finally, the
feasibility requirements of this approach are analysed in detail, suggesting
that its building blocks are within reach of current technology.Comment: 5 pages + 9 pages supplementary materia
Error models for mode-mismatch in linear optics quantum computing
One of the most significant challenges facing the development of linear
optics quantum computing (LOQC) is mode-mismatch, whereby photon
distinguishability is introduced within circuits, undermining quantum
interference effects. We examine the effects of mode-mismatch on the parity (or
fusion) gate, the fundamental building block in several recent LOQC schemes. We
derive simple error models for the effects of mode-mismatch on its operation,
and relate these error models to current fault tolerant threshold estimates.Comment: 6 pages, 7 figure
What can quantum optics say about computational complexity theory?
Considering the problem of sampling from the output photon-counting
probability distribution of a linear-optical network for input Gaussian states,
we obtain results that are of interest from both quantum theory and the
computational complexity theory point of view. We derive a general formula for
calculating the output probabilities, and by considering input thermal states,
we show that the output probabilities are proportional to permanents of
positive-semidefinite Hermitian matrices. It is believed that approximating
permanents of complex matrices in general is a #P-hard problem. However, we
show that these permanents can be approximated with an algorithm in BPP^NP
complexity class, as there exists an efficient classical algorithm for sampling
from the output probability distribution. We further consider input
squeezed-vacuum states and discuss the complexity of sampling from the
probability distribution at the output.Comment: 5 pages, 1 figur
Estimating space-time parameters with a quantum probe in a lossy environment
We study the problem of estimating the Schwarzschild radius of a massive body
using Gaussian quantum probe states. Previous calculations assumed that the
probe state remained pure after propagating a large distance. In a realistic
scenario, there would be inevitable losses. Here we introduce a practical
approach to calculate the Quantum Fisher Informations (QFIs) for a quantum
probe that has passed through a lossy channel. Whilst for many situations loss
means coherent states are optimal, we identify certain situations for which
squeezed states have an advantage. We also study the effect of the frequency
profile of the wavepacket propagating from Alice to Bob. There exists an
optimal operating point for a chosen mode profile. In particular, employing a
smooth rectangular frequency profile significantly improves the error bound on
the Schwarzschild radius compared to a Gaussian frequency profile.Comment: 14 pages, 18 figure
Entanglement-free certification of entangling gates
Not all quantum protocols require entanglement to outperform their classical
alternatives. The nonclassical correlations that lead to this quantum advantage
are conjectured to be captured by quantum discord. Here we demonstrate that
discord can be explicitly used as a resource: certifying untrusted entangling
gates without generating entanglement at any stage. We implement our protocol
in the single-photon regime, and show its success in the presence of high
levels of noise and imperfect gate operations. Our technique offers a practical
method for benchmarking entangling gates in physical architectures in which
only highly-mixed states are available.Comment: 5 pages, 2 figure
Gaussian Post-selection for Continuous Variable Quantum Cryptography
We extend the security proof for continuous variable quantum key distribution
protocols using post selection to account for arbitrary eavesdropping attacks
by employing the concept of an equivalent protocol where the post-selection is
implemented as a series of quantum operations including a virtual distillation.
We introduce a particular `Gaussian' post selection and demonstrate that the
security can be calculated using only experimentally accessible quantities.
Finally we explicitly evaluate the performance for the case of a noisy Gaussian
channel in the limit of unbounded key length and find improvements over all
pre-existing continuous variable protocols in realistic regimes.Comment: 4+4 pages. arXiv admin note: substantial text overlap with
arXiv:1106.082
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