495 research outputs found
Strong Violations of Bell-type Inequalities for Path-Entangled Number States
We show that nonlocal correlation experiments on the two spatially separated
modes of a maximally path-entangled number state may be performed and lead to a
violation of a Clauser-Horne Bell inequality for any finite photon number N. We
present also an analytical expression for the two-mode Wigner function of a
maximally path-entangled number state and investigate a
Clauser-Horne-Shimony-Holt Bell inequality for such states. We test other
Bell-type inequalities. Some are violated by a constant amount for any N.Comment: 6 pages, LaTex; revised version accepted for publication in PR
Estimating Fock-state linear optics evolution using coherent states
This paper presents two methods for simulating the interference of bosonic Fock states through linear interferometers using coherent states. The first method repeats the interferometer, injects coherent states in particular modes, and uses symmetric combinations of the outputs to reconstruct the state amplitudes of the Fock-state interference. The second method constructs a new interferometer that can be probed with coherent states on individual inputs to extract the required state amplitudes. The two approaches here show explicitly where the classical computational difficultly arises. In the first approach, the computational hardness is in the measurement post-processing, and in the second approach, it is within the construction of the required state evolution
Quantum Correlations in Nonlocal BosonSampling
Determination of the quantum nature of correlations between two spatially
separated systems plays a crucial role in quantum information science. Of
particular interest is the questions of if and how these correlations enable
quantum information protocols to be more powerful. Here, we report on a
distributed quantum computation protocol in which the input and output quantum
states are considered to be classically correlated in quantum informatics.
Nevertheless, we show that the correlations between the outcomes of the
measurements on the output state cannot be efficiently simulated using
classical algorithms. Crucially, at the same time, local measurement outcomes
can be efficiently simulated on classical computers. We show that the only
known classicality criterion violated by the input and output states in our
protocol is the one used in quantum optics, namely, phase-space
nonclassicality. As a result, we argue that the global phase-space
nonclassicality inherent within the output state of our protocol represents
true quantum correlations.Comment: 5 pages, 1 figure, comments are very welcome
Quantum Correlations and Global Coherence in Distributed Quantum Computing
Deviations from classical physics when distant quantum systems become
correlated are interesting both fundamentally and operationally. There exist
situations where the correlations enable collaborative tasks that are
impossible within the classical formalism. Here, we consider the efficiency of
quantum computation protocols compared to classical ones as a benchmark for
separating quantum and classical resources and argue that the computational
advantage of collaborative quantum protocols in the discrete variable domain
implies the nonclassicality of correlations. By analysing a toy model, it turns
out that this argument implies the existence of quantum correlations distinct
from entanglement and discord. We characterize such quantum correlations in
terms of the net global coherence resources inherent within quantum states and
show that entanglement and discord can be understood as special cases of our
general framework. Finally, we provide an operational interpretation of such
correlations as those allowing two distant parties to increase their respective
local quantum computational resources only using locally incoherent operations
and classical communication.Comment: Minor modifications and correction
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
Boson Sampling with efficient scaling and efficient verification
A universal quantum computer of moderate scale is not available yet, however
intermediate models of quantum computation would still permit demonstrations of
a quantum computational advantage over classical computing and could challenge
the Extended Church-Turing Thesis. One of these models based on single photons
interacting via linear optics is called Boson Sampling. Proof-of-principle
Boson Sampling has been demonstrated, but the number of photons used for these
demonstrations is below the level required to claim quantum computational
advantage. To make progress with this problem, here we conclude that the most
practically achievable pathway to scale Boson Sampling experiments with current
technologies is by combining continuous-variables quantum information and
temporal encoding. We propose the use of switchable dual-homodyne and
single-photon detections, the temporal loop technique and scattershot based
Boson Sampling. This proposal gives details as to what the required assumptions
are and a pathway for a quantum optical demonstration of quantum computational
advantage. Furthermore, this particular combination of techniques permits a
single efficient implementation of Boson Sampling and efficient verification in
a single experimental setup
Optical cluster-state generation with unitary averaging
Cluster states are the essential resource used in the implementation of
Fusion-based quantum computation (FBQC). We introduce a method to generate
high-fidelity optical cluster states by utilising the concept of unitary
averaging. This error averaging technique is entirely passive and can be
readily incorporated into the proposed PsiQuantum's FBQC architecture. Using
postselection and the redundant encoding of Fusion gates, we observe an
enhancement in the average fidelity of the output cluster state. We also show
an improvement in the linear optical Bell-state measurement (BSM) success
probability when the BSM is imperfect
A robust W-state encoding for linear quantum optics
Error-detection and correction are necessary prerequisites for any scalable
quantum computing architecture. Given the inevitability of unwanted physical
noise in quantum systems and the propensity for errors to spread as
computations proceed, computational outcomes can become substantially
corrupted. This observation applies regardless of the choice of physical
implementation. In the context of photonic quantum information processing,
there has recently been much interest in passive linear optics quantum
computing, which includes boson-sampling, as this model eliminates the
highly-challenging requirements for feed-forward via fast, active control. That
is, these systems are passive by definition. In usual scenarios, error
detection and correction techniques are inherently active, making them
incompatible with this model, arousing suspicion that physical error processes
may be an insurmountable obstacle. Here we explore a photonic error-detection
technique, based on W-state encoding of photonic qubits, which is entirely
passive, based on post-selection, and compatible with these near-term photonic
architectures of interest. We show that this W-state redundant encoding
techniques enables the suppression of dephasing noise on photonic qubits via
simple fan-out style operations, implemented by optical Fourier transform
networks, which can be readily realised today. The protocol effectively maps
dephasing noise into heralding failures, with zero failure probability in the
ideal no-noise limit. We present our scheme in the context of a single photonic
qubit passing through a noisy communication or quantum memory channel, which
has not been generalised to the more general context of full quantum
computation.Comment: Updated draft to accepted version in Quantu
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