Semantic categories underlying the meaning of ‘place’

This paper analyses the semantics of natural language expressions that are associated with the intuitive notion of ‘place’. We note that the nature of such terms is highly contested, and suggest that this arises from two main considerations: 1) there are a number of logically distinct categories of place expression, which are not always clearly distinguished in discourse about ‘place’; 2) the many non-substantive place count nouns (such as ‘place’, ‘region’, ‘area’, etc.) employed in natural language are highly ambiguous. With respect to consideration 1), we propose that place-related expressions should be classified into the following distinct logical types: a) ‘place-like’ count nouns (further subdivided into abstract, spatial and substantive varieties), b) proper names of ‘place-like’ objects, c) locative property phrases, and d) definite descriptions of ‘place-like’ objects. We outline possible formal representations for each of these. To address consideration 2), we examine meanings, connotations and ambiguities of the English vocabulary of abstract and generic place count nouns, and identify underlying elements of meaning, which explain both similarities and differences in the sense and usage of the various terms

From Classical State-Swapping to Quantum Teleportation

The quantum teleportation protocol is extracted directly out of a standard classical circuit that exchanges the states of two qubits using only controlled-NOT gates. This construction of teleportation from a classically transparent circuit generalizes straightforwardly to d-state systems.Comment: Missing daggers added to Figures 13, 14, and 15. Otherwise this is the version that appeared in Physical Revie

Negative entropy and information in quantum mechanics

A framework for a quantum mechanical information theory is introduced that is based entirely on density operators, and gives rise to a unified description of classical correlation and quantum entanglement. Unlike in classical (Shannon) information theory, quantum (von Neumann) conditional entropies can be negative when considering quantum entangled systems, a fact related to quantum non-separability. The possibility that negative (virtual) information can be carried by entangled particles suggests a consistent interpretation of quantum informational processes.Comment: 4 pages RevTeX, 2 figures. Expanded discussion of quantum teleportation and superdense coding, and minor corrections. To appear in Phys. Rev. Let

On the origin of noisy states whose teleportation fidelity can be enhanced through dissipation

Recently Badziag \emph{et al.} \cite{badziag} obtained a class of noisy states whose teleportation fidelity can be enhanced by subjecting one of the qubits to dissipative interaction with the environment via amplitude damping channel (ADC). We show that such noisy states result while sharing the states (| \Phi ^{\pm}> =\frac{1}{\sqrt{2}}(| 00> \pm | 11>)) across ADC. We also show that under similar dissipative interactions different Bell states give rise to noisy entangled states that are qualitatively very different from each other in the sense, only the noisy entangled states constructed from the Bell states (| \Phi ^{\pm}>) can \emph{}be made better sometimes by subjecting the unaffected qubit to a dissipative interaction with the environment. Importantly if the noisy state is non teleporting then it can always be made teleporting with this prescription. We derive the most general restrictions on improvement of such noisy states assuming that the damping parameters being different for both the qubits. However this curious prescription does not work for the noisy entangled states generated from (| \Psi ^{\pm}> =\frac{1}{\sqrt{2}}(| 01> \pm | 10>)). This shows that an apriori knowledge of the noisy channel might be helpful to decide which Bell state needs to be shared between Alice and Bob. \emph{}Comment: Latex, 18 pages: Revised version with a new result. Submitted to PR

Quantum Communication Protocol Employing Weak Measurements

We propose a communication protocol exploiting correlations between two events with a definite time-ordering: a) the outcome of a {\em weak measurement} on a spin, and b) the outcome of a subsequent ordinary measurement on the spin. In our protocol, Alice, first generates a "code" by performing weak measurements on a sample of N spins. The sample is sent to Bob, who later performs a post-selection by measuring the spin along either of two certain directions. The results of the post-selection define the "key', which he then broadcasts publicly. Using both her previously generated code and this key, Alice is able to infer the {\em direction} chosen by Bob in the post-selection. Alternatively, if Alice broadcasts publicly her code, Bob is able to infer from the code and the key the direction chosen by Alice for her weak measurement. Two possible experimental realizations of the protocols are briefly mentioned.Comment: 5 pages, Revtex, 1 figure. A second protocol is added, where by a similar set of weak measurement Alice can send, instead of receiving, a message to Bob. The security question for the latter protocol is discusse

Optimal dimensionality for quantum cryptography

We perform a comparison of two protocols for generating a cryptographic key composed from d-valued symbols: one exploiting a string of independent qubits and another one utilizing d-level systems prepared in states belonging to d+1 mutually unbiased bases. We show that the protocol based on qubits is optimal for quantum cryptography, since it provides higher security and higher key generation rate.Comment: Revtex, 4 pages, 1 eps figur

Security of the Bennett 1992 quantum-key distribution against individual attack over a realistic channel

The security of two-state quantum key distribution against individual attack is estimated when the channel has losses and noises. We assume that Alice and Bob use two nonorthogonal single-photon polarization states. To make our analysis simple, we propose a modified B92 protocol in which Alice and Bob make use of inconclusive results and Bob performs a kind of symmetrization of received states. Using this protocol, Alice and Bob can estimate Eve's information gain as a function of a few parameters which reflect the imperfections of devices or Eve's disturbance. In some parameter regions, Eve's maximum information gain shows counter-intuitive behavior, namely, it decreases as the amount of disturbances increases. For a small noise rate Eve can extract perfect information in the case where the angle between Alice's two states is small or large, while she cannot extract perfect information for intermediate angles. We also estimate the secret key gain which is the net growth of the secret key per one pulse. We show the region where the modified B92 protocol over a realistic channel is secure against individual attack.Comment: 16 pages, 15 figure

Estimates for practical quantum cryptography

In this article I present a protocol for quantum cryptography which is secure against attacks on individual signals. It is based on the Bennett-Brassard protocol of 1984 (BB84). The security proof is complete as far as the use of single photons as signal states is concerned. Emphasis is given to the practicability of the resulting protocol. For each run of the quantum key distribution the security statement gives the probability of a successful key generation and the probability for an eavesdropper's knowledge, measured as change in Shannon entropy, to be below a specified maximal value.Comment: Authentication scheme corrected. Other improvements of presentatio

Complete physical simulation of the entangling-probe attack on the BB84 protocol

We have used deterministic single-photon two qubit (SPTQ) quantum logic to implement the most powerful individual-photon attack against the Bennett-Brassard 1984 (BB84) quantum key distribution protocol. Our measurement results, including physical source and gate errors, are in good agreement with theoretical predictions for the Renyi information obtained by Eve as a function of the errors she imparts to Alice and Bob's sifted key bits. The current experiment is a physical simulation of a true attack, because Eve has access to Bob's physical receiver module. This experiment illustrates the utility of an efficient deterministic quantum logic for performing realistic physical simulations of quantum information processing functions.Comment: 4 pages, 5 figure

Nonlinear quantum state transformation of spin-1/2

A non-linear quantum state transformation is presented. The transformation, which operates on pairs of spin-1/2, can be used to distinguish optimally between two non-orthogonal states. Similar transformations applied locally on each component of an entangled pair of spin-1/2 can be used to transform a mixed nonlocal state into a quasi-pure maximally entangled singlet state. In both cases the transformation makes use of the basic building block of the quantum computer, namely the quantum-XOR gate.Comment: 12 pages, LaTeX, amssym, epsfig (2 figures included