2,123 research outputs found
Optimal continuous-variable teleportation under energy constraint
Quantum teleportation is one of the crucial protocols in quantum information
processing. It is important to accomplish an efficient teleportation under
practical conditions, aiming at a higher fidelity desirably using fewer
resources. The continuous-variable (CV) version of quantum teleportation was
first proposed using a Gaussian state as a quantum resource, while other
attempts were also made to improve performance by applying non-Gaussian
operations. We investigate the CV teleportation to find its ultimate fidelity
under energy constraint identifying an optimal quantum state. For this purpose,
we present a formalism to evaluate teleportation fidelity as an expectation
value of an operator. Using this formalism, we prove that the optimal state
must be a form of photon-number entangled states. We further show that Gaussian
states are near-optimal while non-Gaussian states make a slight improvement and
therefore are rigorously optimal, particularly in the low-energy regime.Comment: 8 pages, 4 figures, published versio
Gaussian benchmark for optical communication aiming towards ultimate capacity
We establish the fundamental limit of communication capacity within Gaussian
schemes under phase-insensitive Gaussian channels, which employ multimode
Gaussian states for encoding and collective Gaussian operations and
measurements for decoding. We prove that this Gaussian capacity is additive,
i.e., its upper bound occurs with separable encoding and separable receivers so
that a single-mode communication suffices to achieve the largest capacity under
Gaussian schemes. This rigorously characterizes the gap between the ultimate
Holevo capacity and the capacity within Gaussian communication, showing that
Gaussian regime is not sufficient to achieve the Holevo bound particularly in
the low-photon regime. Furthermore the Gaussian benchmark established here can
be used to critically assess the performance of non-Gaussian protocols for
optical communication. We move on to identify non-Gaussian schemes to beat the
Gaussian capacity and show that a non-Gaussian receiver recently implemented by
Becerra et al. [Nat. Photon. 7, 147 (2013)] can achieve this aim with an
appropriately chosen encoding strategy.Comment: 9 pages, 6 figures, with supplemental materia
Classical capacity of Gaussian communication under a single noisy channel
A long-standing problem on the classical capacity of bosonic Gaussian
channels has recently been resolved by proving the minimum output entropy
conjecture. It is also known that the ultimate capacity quantified by the
Holevo bound can be achieved asymptotically by using an infinite number of
channels. However, it is less understood to what extent the communication
capacity can be reached if one uses a finite number of channels, which is a
topic of practical importance. In this paper, we study the capacity of Gaussian
communication, i.e., employing Gaussian states and Gaussian measurements to
encode and decode information under a single-channel use. We prove that the
optimal capacity of single-channel Gaussian communication is achieved by one of
two well-known protocols, i.e., coherent-state communication or squeezed-state
communication, depending on the energy (number of photons) as well as the
characteristics of the channel. Our result suggests that the coherent-state
scheme known to achieve the ultimate information-theoretic capacity is not a
practically optimal scheme for the case of using a finite number of channels.
We find that overall the squeezed-state communication is optimal in a
small-photon-number regime whereas the coherent-state communication performs
better in a large-photon-number regime.Comment: 9 pages, 4 figures, published versio
Steering Criteria via Covariance Matrices of Local Observables in Arbitrary Dimensional Quantum Systems
We derive steerability criteria applicable for both finite and infinite
dimensional quantum systems using covariance matrices of local observables. We
show that these criteria are useful to detect a wide range of entangled states
particularly in high dimensional systems and that the Gaussian steering
criteria for general M x N-modes of continuous variables are obtained as a
special case. Extending from the approach of entanglement detection via
covariance matrices, our criteria are based on the local uncertainty principles
incorporating the asymmetric nature of steering scenario. Specifically, we
apply the formulation to the case of local orthogonal observables and obtain
some useful criteria that can be straightforwardly computable, and testable in
experiment, with no need for numerical optimization.Comment: 6 pages with further "Remarks" and "Acknowledgement" adde
Monogamy relation in multipartite continuous-variable quantum teleportation
Quantum teleportation (QT) is a fundamentally remarkable communication
protocol that also finds many important applications for quantum informatics.
Given a quantum entangled resource, it is crucial to know to what extent one
can accomplish the QT. This is usually assessed in terms of output fidelity,
which can also be regarded as an operational measure of entanglement. In the
case of multipartite communication when each communicator possesses a part of
-partite entangled state, not all pairs of communicators can achieve a high
fidelity due to monogamy property of quantum entanglement. We here investigate
how such a monogamy relation arises in multipartite continuous-variable (CV)
teleportation particularly using a Gaussian entangled state. We show a strict
monogamy relation, i.e. a sender cannot achieve a fidelity higher than optimal
cloning limit with more than one receiver. While this seems rather natural
owing to the no-cloning theorem, a strict monogamy relation still holds even if
the sender is allowed to individually manipulate the reduced state in
collaboration with each receiver to improve fidelity. The local operations are
further extended to non-Gaussian operations such as photon subtraction and
addition, and we demonstrate that the Gaussian cloning bound cannot be beaten
by more than one pair of communicators. Furthermore we investigate a
quantitative form of monogamy relation in terms of teleportation capability,
for which we show that a faithful monogamy inequality does not exist.Comment: 10 pages, 6 figures, published versio
Continuous-variable dense coding via a general Gaussian state: Monogamy relation
We study a continuous variable (CV) dense-coding protocol, originally
proposed to employ a two-mode squeezed state, using a general two-mode Gaussian
state as a quantum channel. We particularly obtain conditions to manifest
quantum advantage by beating two well-known single-mode schemes, namely, the
squeezed-state scheme (best Gaussian scheme) and the number-state scheme
(optimal scheme achieving the Holevo bound). We then extend our study to a
multipartite Gaussian state and investigate the monogamy of operational
entanglement measured by the communication capacity under the dense-coding
protocol. We show that this operational entanglement represents a strict
monogamy relation, by means of Heisenberg's uncertainty principle among
different parties, i.e., the quantum advantage for communication can be
possible for only one pair of two-mode systems among many parties
- …