50 research outputs found
Multipartite Nonlocality without Entanglement in Many Dimensions
We present a generic method to construct a product basis exhibiting
Nonlocality Without Entanglement with parties each holding a system of
dimension at least . This basis is generated via a quantum circuit made of
control-Discrete Fourier Transform gates acting on the computational basis. The
simplicity of our quantum circuit allows for an intuitive understanding of this
new type of nonlocality. We also show how this circuit can be used to construct
Unextendible Product Bases and their associated Bound Entangled States. To our
knowledge, this is the first method which, given a general Hilbert space
with , makes it possible to
construct (i) a basis exhibiting Nonlocality Without Entanglement, (ii) an
Unextendible Product Basis, and (iii) a Bound Entangled state.Comment: 8 pages, 4 figure
Experimentally feasible quantum erasure-correcting code for continuous variables
We devise a scheme that protects quantum coherent states of light from
probabilistic losses, thus achieving the first continuous-variable quantum
erasure-correcting code. If the occurrence of erasures can be probed, then the
decoder enables, in principle, a perfect recovery of the original light states.
Otherwise, if supplemented with postselection based on homodyne detection, this
code can be turned into an efficient erasure-filtration scheme. The
experimental feasibility of the proposed protocol is carefully addressed
Tight bounds on the concurrence of quantum superpositions
The entanglement content of superpositions of quantum states is investigated
based on a measure called {\it concurrence}. Given a bipartite pure state in
arbitrary dimension written as the quantum superposition of two other such
states, we find simple inequalities relating the concurrence of the state to
that of its components. We derive an exact expression for the concurrence when
the component states are biorthogonal, and provide elegant upper and lower
bounds in all other cases. For quantum bits, our upper bound is tighter than
the previously derived bound in [Phys. Rev. Lett. 97, 100502 (2006).]Comment: 7 pages, 2 figure
A No-Go Theorem for Gaussian Quantum Error Correction
It is proven that Gaussian operations are of no use for protecting Gaussian
states against Gaussian errors in quantum communication protocols.
Specifically, we introduce a new quantity characterizing any single-mode
Gaussian channel, called entanglement degradation, and show that it cannot
decrease via Gaussian encoding and decoding operations only. The strength of
this no-go theorem is illustrated with some examples of Gaussian channels.Comment: 4 pages, 2 figures, REVTeX
Quantum optical coherence can survive photon losses: a continuous-variable quantum erasure correcting code
A fundamental requirement for enabling fault-tolerant quantum information
processing is an efficient quantum error-correcting code (QECC) that robustly
protects the involved fragile quantum states from their environment. Just as
classical error-correcting codes are indispensible in today's information
technologies, it is believed that QECC will play a similarly crucial role in
tomorrow's quantum information systems. Here, we report on the first
experimental demonstration of a quantum erasure-correcting code that overcomes
the devastating effect of photon losses. Whereas {\it errors} translate, in an
information theoretic language, the noise affecting a transmission line, {\it
erasures} correspond to the in-line probabilistic loss of photons. Our quantum
code protects a four-mode entangled mesoscopic state of light against erasures,
and its associated encoding and decoding operations only require linear optics
and Gaussian resources. Since in-line attenuation is generally the strongest
limitation to quantum communication, much more than noise, such an
erasure-correcting code provides a new tool for establishing quantum optical
coherence over longer distances. We investigate two approaches for
circumventing in-line losses using this code, and demonstrate that both
approaches exhibit transmission fidelities beyond what is possible by classical
means.Comment: 5 pages, 4 figure
Experimental Proof of Quantum Nonlocality without Squeezing
It is shown that the ensemble where is a Gaussian distribution of finite variance and is a
coherent state can be better discriminated with an entangled measurement than
with any local strategy supplemented by classical communication. Although this
ensemble consists of products of quasi-classical states, it exhibits some
quantum nonlocality. This remarkable effect is demonstrated experimentally by
implementing the optimal local strategy together with a joint nonlocal strategy
that yields a higher fidelity.Comment: 4 pages, 2 figure
A framework for bounding nonlocality of state discrimination
We consider the class of protocols that can be implemented by local quantum
operations and classical communication (LOCC) between two parties. In
particular, we focus on the task of discriminating a known set of quantum
states by LOCC. Building on the work in the paper "Quantum nonlocality without
entanglement" [BDF+99], we provide a framework for bounding the amount of
nonlocality in a given set of bipartite quantum states in terms of a lower
bound on the probability of error in any LOCC discrimination protocol. We apply
our framework to an orthonormal product basis known as the domino states and
obtain an alternative and simplified proof that quantifies its nonlocality. We
generalize this result for similar bases in larger dimensions, as well as the
"rotated" domino states, resolving a long-standing open question [BDF+99].Comment: 33 pages, 7 figures, 1 tabl
Protecting Quantum Information with Entanglement and Noisy Optical Modes
We incorporate active and passive quantum error-correcting techniques to
protect a set of optical information modes of a continuous-variable quantum
information system. Our method uses ancilla modes, entangled modes, and gauge
modes (modes in a mixed state) to help correct errors on a set of information
modes. A linear-optical encoding circuit consisting of offline squeezers,
passive optical devices, feedforward control, conditional modulation, and
homodyne measurements performs the encoding. The result is that we extend the
entanglement-assisted operator stabilizer formalism for discrete variables to
continuous-variable quantum information processing.Comment: 7 pages, 1 figur
The state of professionalisation of midwifery in Belgium: A discussion paper.
AIM: To describe the state of the professionalisation of midwifery in Belgium, and to formulate recommendations for advancing the midwifery profession. METHODS: A descriptive overview of maternity care in Belgium and the professionalisation of midwifery through an analysis of relevant policy and academic texts, underpinned by Greenwood's sociological criteria for a profession: (1) own body of knowledge, (2) recognised authority, (3) broader community sanctions, (4) own code of ethics and (5) professional culture sustained by formal professional associations. From these insights, recommendations for advancing the midwifery profession in Belgium are formulated. FINDINGS: Current strengths of the professionalisation of midwifery in Belgium included unified midwifery education programmes, progress in midwifery research and overarching national documents for guiding midwifery education, practice and regulation. In contrast however challenges, such as the limited recognition of midwives' roles by its clientele, limitations of midwives' competencies and autonomy, lacking development of advanced roles in maternity care practice and a lack of unity of the organisation and its members, were also identified. Based on these, recommendations are made to strengthen Belgian midwifery. CONCLUSIONS: Recommendations for advancing the midwifery profession in Belgium includes in particular increasing public awareness of midwives' roles and competencies, implementing the full scope of midwifery practice and monitoring and advancing this practice. Thus, professional autonomy over both midwifery practice and working conditions should be enhanced. United midwifery organisations, together with women's groups, other maternity care professionals and policy-makers as equal partners are key to bring about changes in the Belgian maternity care landscape