23 research outputs found
Analysis of circuit imperfections in BosonSampling
BosonSampling is a problem where a quantum computer offers a provable speedup
over classical computers. Its main feature is that it can be solved with
current linear optics technology, without the need for a full quantum computer.
In this work, we investigate whether an experimentally realistic BosonSampler
can really solve BosonSampling without any fault-tolerance mechanism. More
precisely, we study how the unavoidable errors linked to an imperfect
calibration of the optical elements affect the final result of the computation.
We show that the fidelity of each optical element must be at least , where refers to the number of single photons in the scheme. Such
a requirement seems to be achievable with state-of-the-art equipment.Comment: 20 pages, 7 figures, v2: new title, to appear in QI
Regimes of classical simulability for noisy Gaussian boson sampling
As a promising candidate for exhibiting quantum computational supremacy,
Gaussian Boson Sampling (GBS) is designed to exploit the ease of experimental
preparation of Gaussian states. However, sufficiently large and inevitable
experimental noise might render GBS classically simulable. In this work, we
formalize this intuition by establishing a sufficient condition for approximate
polynomial-time classical simulation of noisy GBS --- in the form of an
inequality between the input squeezing parameter, the overall transmission rate
and the quality of photon detectors. Our result serves as a non-classicality
test that must be passed by any quantum computationalsupremacy demonstration
based on GBS. We show that, for most linear-optical architectures, where photon
loss increases exponentially with the circuit depth, noisy GBS loses its
quantum advantage in the asymptotic limit. Our results thus delineate
intermediate-sized regimes where GBS devices might considerably outperform
classical computers for modest noise levels. Finally, we find that increasing
the amount of input squeezing is helpful to evade our classical simulation
algorithm, which suggests a potential route to mitigate photon loss.Comment: 13 pages, 4 figures, final version accepted for publication in
Physical Review Letter
Enhancing quantum entanglement by photon addition and subtraction
The non-Gaussian operations effected by adding or subtracting a photon on the
entangled optical beams emerging from a parametric down-conversion process have
been suggested to enhance entanglement. Heralded photon addition or subtraction
is, as a matter of fact, at the heart of continuous-variable entanglement
distillation. The use of such processes has recently been experimentally
demonstrated in the context of the generation of optical coherent-state
superpositions or the verification of the canonical commutation relations.
Here, we carry out a systematic study of the effect of local photon additions
or subtractions on a two-mode squeezed vacuum state, showing that the
entanglement generally increases with the number of such operations. This is
analytically proven when additions or subtractions are restricted to one mode
only, while we observe that the highest entanglement is achieved when these
operations are equally shared between the two modes. We also note that adding
photons typically provides a stronger entanglement enhancement than subtracting
photons, while photon subtraction performs better in terms of energy
efficiency. Furthermore, we analyze the interplay between entanglement and
non-Gaussianity, showing that it is more subtle than previously expected.Comment: 10 pages, 6 figure
Simulating boson sampling in lossy architectures
Photon losses are among the strongest imperfections affecting multi-photon
interference. Despite their importance, little is known about their effect on
boson sampling experiments. In this work we show that using classical
computers, one can efficiently simulate multi-photon interference in all
architectures that suffer from an exponential decay of the transmission with
the depth of the circuit, such as integrated photonic circuits or optical
fibers. We prove that either the depth of the circuit is large enough that it
can be simulated by thermal noise with an algorithm running in polynomial time,
or it is shallow enough that a tensor network simulation runs in
quasi-polynomial time. This result suggests that in order to implement a
quantum advantage experiment with single-photons and linear optics new
experimental platforms may be needed
Quantum advantage from energy measurements of many-body quantum systems
The problem of sampling outputs of quantum circuits has been proposed as a
candidate for demonstrating a quantum computational advantage (sometimes
referred to as quantum "supremacy"). In this work, we investigate whether
quantum advantage demonstrations can be achieved for more physically-motivated
sampling problems, related to measurements of physical observables. We focus on
the problem of sampling the outcomes of an energy measurement, performed on a
simple-to-prepare product quantum state -- a problem we refer to as energy
sampling. For different regimes of measurement resolution and measurement
errors, we provide complexity theoretic arguments showing that the existence of
efficient classical algorithms for energy sampling is unlikely. In particular,
we describe a family of Hamiltonians with nearest-neighbour interactions on a
2D lattice that can be efficiently measured with high resolution using a
quantum circuit of commuting gates (IQP circuit), whereas an efficient
classical simulation of this process should be impossible. In this high
resolution regime, which can only be achieved for Hamiltonians that can be
exponentially fast-forwarded, it is possible to use current theoretical tools
tying quantum advantage statements to a polynomial-hierarchy collapse whereas
for lower resolution measurements such arguments fail. Nevertheless, we show
that efficient classical algorithms for low-resolution energy sampling can
still be ruled out if we assume that quantum computers are strictly more
powerful than classical ones. We believe our work brings a new perspective to
the problem of demonstrating quantum advantage and leads to interesting new
questions in Hamiltonian complexity.Comment: Comments are welcom
Loophole-free test of quantum non-locality using high-efficiency homodyne detectors
We provide a detailed analysis of the recently proposed setup for a
loophole-free test of Bell inequality using conditionally generated
non-Gaussian states of light and balanced homodyning. In the proposed scheme, a
two-mode squeezed vacuum state is de-gaussified by subtracting a single photon
from each mode with the use of an unbalanced beam splitter and a standard
low-efficiency single-photon detector. We thoroughly discuss the dependence of
the achievable Bell violation on the various relevant experimental parameters
such as the detector efficiencies, the electronic noise and the mixedness of
the initial Gaussian state. We also consider several alternative schemes
involving squeezed states, linear optical elements, conditional photon
subtraction and homodyne detection.Comment: 13 pages, 14 figures, RevTeX
Security of continuous-variable quantum key distribution against general attacks
We prove the security of Gaussian continuous-variable quantum key
distribution against arbitrary attacks in the finite-size regime. The novelty
of our proof is to consider symmetries of quantum key distribution in phase
space in order to show that, to good approximation, the Hilbert space of
interest can be considered to be finite-dimensional, thereby allowing for the
use of the postselection technique introduced by Christandl, Koenig and Renner
(Phys. Rev. Lett. 102, 020504 (2009)). Our result greatly improves on previous
work based on the de Finetti theorem which could not provide security for
realistic, finite-size, implementations.Comment: 5 pages, plus 11 page appendi
Experimental implementation of non-Gaussian attacks on a continuous-variable quantum key distribution system
An intercept-resend attack on a continuous-variable quantum-key-distribution
protocol is investigated experimentally. By varying the interception fraction,
one can implement a family of attacks where the eavesdropper totally controls
the channel parameters. In general, such attacks add excess noise in the
channel, and may also result in non-Gaussian output distributions. We implement
and characterize the measurements needed to detect these attacks, and evaluate
experimentally the information rates available to the legitimate users and the
eavesdropper. The results are consistent with the optimality of Gaussian
attacks resulting from the security proofs.Comment: 4 pages, 5 figure