Broadcasting of three qubit entanglement via local copying and entanglement swapping

In this work,We investigate the problem of secretly broadcasting of three-qubit entangled state between two distant partners. The interesting feature of this problem is that starting from two particle entangled state shared between two distant partners we find that the action of local cloner on the qubits and the measurement on the machine state vector generates three-qubit entanglement between them. The broadcasting of entanglement is made secret by sending the measurement result secretly using cryptographic scheme based on orthogonal states. Further we show that this idea can be extended to generate three particle entangled state between three distant partners.Comment: 18 pages, 4 figures, Accepted in Physical Review

The Parity Bit in Quantum Cryptography

An $n$-bit string is encoded as a sequence of non-orthogonal quantum states. The parity bit of that $n$-bit string is described by one of two density matrices, $\rho_0^{(n)}$ and $\rho_1^{(n)}$, both in a Hilbert space of dimension $2^n$. In order to derive the parity bit the receiver must distinguish between the two density matrices, e.g., in terms of optimal mutual information. In this paper we find the measurement which provides the optimal mutual information about the parity bit and calculate that information. We prove that this information decreases exponentially with the length of the string in the case where the single bit states are almost fully overlapping. We believe this result will be useful in proving the ultimate security of quantum crytography in the presence of noise.Comment: 19 pages, RevTe

The display of spatial information and visually guided behavior

The basic informational elements of spatial orientation are attitude and position within a coordinate system. The problem that faces aeronautical designers is that a pilot must deal with several coordinate systems, sometimes simultaneously. The display must depict unambiguously not only position and attitude, but also designate the relevant coordinate system. If this is not done accurately, spatial disorientation can occur. The different coordinate systems used in aeronautical tasks and the problems that occur in the display of spatial information are explained

Thermodynamics and the Measure of Entanglement

We point out formal correspondences between thermodynamics and entanglement. By applying them to previous work, we show that entropy of entanglement is the unique measure of entanglement for pure states.Comment: 8 pages, RevTeX; edited for clarity, additional references, to appear as a Rapid Communication in Phys. Rev.

Entanglement Swapping Chains for General Pure States

We consider entanglement swapping schemes with general (rather than maximally) entangled bipartite states of arbitary dimension shared pairwise between three or more parties in a chain. The intermediate parties perform generalised Bell measurements with the result that the two end parties end up sharing a entangled state which can be converted into maximally entangled states. We obtain an expression for the average amount of maximal entanglement concentrated in such a scheme and show that in a certain reasonably broad class of cases this scheme is provably optimal and that, in these cases, the amount of entanglement concentrated between the two ends is equal to that which could be concentrated from the weakest link in the chain.Comment: 18 pages, 5 figure

EPR Studies on the Mono- and Dicobalt(II)-Substituted Forms of the Aminopeptidase from \u3cem\u3eAeromonas proteolytica\u3c/em\u3e. Insight into the Catalytic Mechanism of Dinuclear Hydrolases

The structure and function of the prototypical dinuclear hydrolase, namely, the aminopeptidase from Aeromonas proteolytica (AAP), was probed by EPR spectroscopy of the mono- and dicobalt(II)-substituted derivatives. A new systematic protocol for the interpretation of Co(II) EPR spectra is described and the S = 3/2 spin states of the Co(II)-substituted forms of the enzyme have been characterized. This protocol allows the simulation of line shape using theoretically allowed geff values corresponding to an isotropic greal value. In addition, the gross distortion of EPR spectra of high-spin S = 3/2 Co(II) ions has been investigated, and the effects of saturation on the line shapes and on simulation-derived spectral parameters are discussed. For [Co-(AAP)], a distinctive EPR signal was observed in which the hyperfine pattern due to 59Co was not centered on the low-field absorption feature, and the signal could not be simulated as a single species. Subtraction of EPR spectra recorded at different temperatures revealed that two species were, in fact, present in samples of [Co-(AAP)]. The first species was a relatively featureless axial signal with geff values of 5.75, 4.50, and 2.50. These values correspond to an Ms = |±1/2〉 ground-state transition with greal = 2.57 and E/D = 0.08. The second species had geff values of 6.83, 2.95, and 1.96 and exhibited a characteristic eight-line 59Co hyperfine pattern with Az = 7.2 mT. The observed 59Co hyperfine lines were simulated in both line width as well as signal intensity for the first time. These parameters correspond to the Ms = |±1/2〉 ground-state transition with greal = 2.57; however, the signal exhibited marked rhombicity (E/D = 0.28), consistent with a highly distorted tetrahedral Co(II) species. The possibility that the spectrum could be due to contributions from the Ms = |±1/2〉 and Ms = |±3/2〉 doublets of a single spin system was investigated, but subtraction of spectra recorded at various temperatures clearly indicated that the features at g = 2.95 and g = 1.96 were correlated with the feature at g = 6.83. In addition, at temperatures above 15 K, the signal intensity rapidly decreases and the signal is lost. The EPR spectrum of [CoCo(AAP)] reveals a relatively featureless signal that was simulated as a single species with geff(1,2,3) values of 5.10, 3.85, and 2.19; Ms = |±1/2〉; greal = 2.25; E/D = 0.095. The intensity of the observed signal for [CoCo(AAP)] corresponded to 0.13 spin/mol of Co(II). These data strongly suggest that the two Co(II) ions in the active site of AAP experience significant spin−spin interaction and are either antiferromagnetically or ferromagnetically coupled. Perpendicular mode EPR titration of apo-AAP with Co(II) revealed a low-field signal extending out of zero-field in samples with more than 1 equiv of Co(II) added. This type of EPR absorption is indicative of an integral spin system. Coincident with the appearance of the low-field perpendicular mode signal was the appearance of a parallel mode EPR signal with g ∼ 12. These data represent the first definitive evidence for ferromagnetic coupling between two high-spin S = 3/2 Co(II) ions in a dinuclear center. The effect of pH, added peroxide, and the coordination of the competitive inhibitor 1-butaneboronic acid (BuBA) on the signal both confirm the origin of the signal and provide important mechanistic information for this novel dicobalt(II) active site cluster. Based on the present study and the available literature data, a detailed mechanism of action is proposed for AAP

Comment on "Quantum dense key distribution"

In this Comment we question the security of recently proposed by Degiovanni et al. [Phys. Rev. A 69 (2004) 032310] scheme of quantum dense key distribution

Spectroscopically Distinct Cobalt(II) Sites in Heterodimetallic Forms of the Aminopeptidase from \u3cem\u3eAeromonas proteolytica\u3c/em\u3e: Characterization of Substrate Binding

The Co(II)Zn(II)- and Zn(II)Co(II)-substituted derivatives of the aminopeptidase from Aeromonas proteolytica (AAP) were probed by EPR spectroscopy. EPR spectra of the high-spin S = 3/2 Co(II) ions in [CoZn(AAP)] and [ZnCo(AAP)] indicated that each metal binding site provides a spectroscopically distinct signature. For [CoZn(AAP)], subtraction of EPR spectra recorded at pH 7.5 and 10 revealed that two species were present and that the relative contributions to each of the experimental spectra were pH-dependent. The first EPR species, predominant at lower pH values, was simulated as a relatively featureless axial signal with geff values of 2.20, 3.92, and 5.23 which correspond to an Ms = |±1/2〉 ground state transition with a greal of 2.29 and an E/D of 0.1. The second species, predominant at high pH, was simulated with geff values of 1.80, 2.75, and 6.88 and exhibited a characteristic eight-line 59Co hyperfine pattern with an Az(59Co) of 7.0 mT. These parameters correspond to an Ms = |±1/2〉 ground state transition with a greal of 2.54; however, the signal exhibited marked rhombicity (E/D = 0.32) indicative of an asymmetric tetrahedral or five-coordinate Co(II) ion. Summation of these two species provided an excellent simulation of the observed [CoZn(AAP)] EPR spectrum. The EPR spectrum of [ZnCo(AAP)] also contained two species, at least one of which also exhibited 59Co hyperfine features. However, this signal exhibited little pH dependence, and individual species could not be isolated. The addition of the competitive inhibitor 1-butaneboronic acid (BuBA) to [CoZn(AAP)] resulted in a distinct change in the EPR spectrum; however, addition of BuBA to [ZnCo(AAP)] left the EPR spectrum completely unperturbed. These data indicate that BuBA binds only to the first metal binding site in AAP and does not interact with the second site. On the basis of the X-ray crystallographic data for the transition state analog-inhibited complexes of AAP and the aminopeptidase from bovine lens, BuBA was reclassified as a substrate analog inhibitor rather than a transition state analog inhibitor as previously suggested [Baker, J. O., & Prescott, J. M. (1983) Biochemistry 22, 5322−5331]. From difference spectroscopy and from the simulation of the [CoZn(AAP)] EPR spectrum, a third signal appearing upon BuBA binding was isolated. This signal was simulated with geff values of 2.08, 3.15, and 6.15 which correspond to an Ms = |±1/2〉 ground state transition with a greal of 2.41 and an E/D of 0.22. This simulation also invoked an eight-line unresolved 59Co hyperfine pattern with an Az(59Co) value of 4.0 mT. Summation of the these three species provided an excellent simulation of the observed [CoZn(AAP)] + BuBA EPR spectrum at both pH values. This work establishes that substrate binds only to the first metal binding site in AAP and thus substantiates the first step in catalysis in the recently proposed mechanism of action for AAP [Bennett, B., & Holz, R. C. (1997) J. Am. Chem. Soc. 119, 1923−1933; Chen, G., et al. (1997) Biochemistry 36, 4278−4286]

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