1,301 research outputs found

### Quantifying entanglement in two-mode Gaussian states

Entangled two-mode Gaussian states are a key resource for quantum information
technologies such as teleportation, quantum cryptography and quantum
computation, so quantification of Gaussian entanglement is an important
problem. Entanglement of formation is unanimously considered a proper measure
of quantum correlations, but for arbitrary two-mode Gaussian states no
analytical form is currently known. In contrast, logarithmic negativity is a
measure straightforward to calculate and so has been adopted by most
researchers, even though it is a less faithful quantifier. In this work, we
derive an analytical lower bound for entanglement of formation of generic
two-mode Gaussian states, which becomes tight for symmetric states and for
states with balanced correlations. We define simple expressions for
entanglement of formation in physically relevant situations and use these to
illustrate the problematic behavior of logarithmic negativity, which can lead
to spurious conclusions.Comment: 8 pages,3 figs; The original submission gave an analytical formula
that was claimed to give the entanglement of formation for arbitrary two-mode
Gaussian states - this was incorrect. The formula gives a lower bound of EoF
which saturates for symmetric states and for states with balanced
correlations, and is a good approximation for most other states. This error
is corrected in the revised versio

### Frequency and temporal effects in linear optical quantum computing

Typically linear optical quantum computing (LOQC) models assume that all
input photons are completely indistinguishable. In practice there will
inevitably be non-idealities associated with the photons and the experimental
setup which will introduce a degree of distinguishability between photons. We
consider a non-deterministic optical controlled-NOT gate, a fundamental LOQC
gate, and examine the effect of temporal and spectral distinguishability on its
operation. We also consider the effect of utilizing non-ideal photon counters,
which have finite bandwidth and time response.Comment: 10 pages, 9 figures, replaced with published versio

### 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

### Modeling photo-detectors in quantum optics

Photo-detection plays a fundamental role in experimental quantum optics and
is of particular importance in the emerging field of linear optics quantum
computing. Present theoretical treatment of photo-detectors is highly idealized
and fails to consider many important physical effects. We present a physically
motivated model for photo-detectors which accommodates for the effects of
finite resolution, bandwidth and efficiency, as well as dark-counts and
dead-time. We apply our model to two simple well known applications, which
illustrates the significance of these characteristics.Comment: 8 pages, 7 figure

### Quantum Metrology in the Kerr Metric

A surprising feature of the Kerr metric is the anisotropy of the speed of
light. The angular momentum of a rotating massive object causes co- and
counter-propagating light paths to move at faster and slower velocities,
respectively as determined by a far-away clock. Based on this effect we derive
ultimate quantum limits for the measurement of the Kerr rotation parameter $a$
using a interferometric set up. As a possible implementation, we propose a
Mach-Zehnder interferometer to measure the "one-way height differential" time
effect. We isolate the effect by calibrating to a dark port and rotating the
interferometer such that only the direction dependent Kerr-metric induced phase
term remains. We transform to the Zero Angular Momentum Observer (ZAMO) flat
metric where the observer see $c=1$. We use this metric and the Lorentz
transformations to calculate the same Kerr phase shift. We then consider
non-stationary observers moving with the planet's rotation, and find a method
for cancelling the additional phase from the classical relative motion, thus
leaving only the curvature induced phase.Comment: 9 pages, 7 figures, closest to published versio

### Quantifying entanglement of formation for two-mode Gaussian states: Analytical expressions for upper and lower bounds and numerical estimation of its exact value

Entanglement of formation quantifies the entanglement of a state in terms of
the entropy of entanglement of the least entangled pure state needed to prepare
it. An analytical expression for this measure exists only for special cases,
and finding a closed formula for an arbitrary state still remains an open
problem. In this work we focus on two-mode Gaussian states, and we derive
narrow upper and lower bounds for the measure that get tight for several
special cases. Further, we show that the problem of calculating the actual
value of the entanglement of formation for arbitrary two-mode Gaussian states
reduces to a trivial single parameter optimization process, and we provide an
efficient algorithm for the numerical calculation of the measure.Comment: 5 pages, 2 figures In this third version a few typos of the first and
second versions have been correcte

### Simulation of Gaussian channels via teleportation and error correction of Gaussian states

Gaussian channels are the typical way to model the decoherence introduced by
the environment in continuous-variable quantum states. It is known that those
channels can be simulated by a teleportation protocol using as a resource state
either a maximally entangled state passing through the same channel, i.e., the
Choi-state, or a state that is entangled at least as much as the Choi-state.
Since the construction of the Choi-state requires infinite mean energy and
entanglement, i.e. it is unphysical, we derive instead every physical state
able to simulate a given channel through teleportation with finite resources,
and we further find the optimal ones, i.e., the resource states that require
the minimum energy and entanglement. We show that the optimal resource states
are pure and equally entangled to the Choi-state as measured by the
entanglement of formation. We also show that the same amount of entanglement is
enough to simulate an equally decohering channel, while even more entanglement
can simulate less decohering channels. We, finally, use that fact to generalize
a previously known error correction protocol by making it able to correct noise
coming not only from pure loss but from thermal loss channels as well.Comment: 12 pages, 8 figure

### Noiseless phase quadrature amplification via electro-optic feed-forward

Theoretical results are presented which show that noiseless phase quadrature
amplification is possible, and limited experimentally only by the efficiency of
the phase detection system. Experimental results obtained using a Nd:YAG laser
show a signal gain of 10dB and a signal transfer ratio of T_s=0.9. This result
easily exceeds the standard quantum limit for signal transfer. The results also
explicitly demonstrate the phase sensitive nature of the amplification process.Comment: 8 pages, 4 figure

### 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

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