We analyze the structure of correlations among more than two quantum systems. We introduce a classification of correlations based on the concept of nonseparability, which is different a priori from the concept of entanglement. Generalizing a result of Svetlichny [Phys. Rev. D 35, 3066 (1987)] on three-particle correlations, we find an inequality for n-particle correlations that holds under the most general separability condition and that is violated by some quantum-mechanical states.
No abstract is provided for this article.
We investigate a general class of quantum key distribution (QKD) protocols using one-way classical communication. We show that full security can be proven by considering only collective attacks. We derive computable lower and upper bounds on the secret-key rate of those QKD protocols involving only entropies of two-qubit density operators. As an illustration of our results, we determine new bounds for the Bennett-Brassard 1984, the 6-state, and the Bennett 1992 protocols. We show that in all these cases the first classical processing that the legitimate partners should apply consists in adding noise.
A long-standing tradition, largely present in both the physical and the philosophical literature, regards the advent of (special) relativity –with its block-universe picture– as the failure of any indeterministic program in physics. On the contrary, in this paper, we note that upholding reasonable principles of finiteness of information hints at a picture of the physical world that should be both relativistic and indeterministic. We thus rebut the block-universe picture by assuming that fundamental indeterminacy itself should as well be regarded as a relational property when considered in a relativistic scenario. We discuss the consequence that this view may have when correlated randomness is introduced, both in the classical case and in the quantum one.
After carrying out a protocol for quantum key agreement over a noisy quantum channel, the parties Alice and Bob must process the raw key in order to end up with identical keys about which the adversary has virtually no information. In principle, both classical and quantum protocols can be used for this processing. It is a natural question which type of protocols is more powerful. We prove for general states but under the assumption of incoherent eavesdropping that Alice and Bob share some so-called intrinsic in their classical random variables, resulting from optimal measurements, if and only if the parties' quantum systems are entangled. In addition, we provide evidence that the potentials of classical and of quantum protocols are equal in every situation. Consequently, many techniques and results from quantum theory directly apply to problems in classical theory, and vice versa. For instance, it was previously believed that two parties can carry out unconditionally secure key agreement as long as they share some intrinsic in the adversary's view. The analysis of this purely classical problem from the quantum information-theoretic viewpoint shows that this is true in the binary case, but false in general. More explicitly, entanglement, i.e., entanglement that cannot be purified by any quantum protocol, has a classical counterpart. This bound intrinsic information cannot be distilled to a secret key by any classical protocol. As another application we propose a measure for entanglement based on classical information-theoretic quantities.
No abstract is provided for this article.
The differential group delay (DGD) due to varying local birefringence and random polarization mode coupling is a statistical quantity. Its value at any wavelength fluctuates in time. At any given moment it is wavelength sensitive. Hence, polarization-mode dispersion can only be defined as a mean or rms value of the DGD. All measurements of polarization-mode dispersion involve thus an averaging over a finite sample set. Consequently, all measurements are intrinsically of limited accuracy. We present analytical formulae and numerical data characterizing this intrinsic limitation common to all measurement techniques.
I analyse a recent quantum communication protocol by Salih et al. that allows one to communicate without any particle carrying the information from the sender to the receiver. I show how this can equally be achieved using classical communication.
We present an experiment where pulses of mid-IR light are detected by converting them into 800 nm light by sum frequency generation (SFG) in a nonlinear AgGaS/sub 2/(AGS) crystal. The mid-IR optical source used for the experiment is a quantum cascade laser at 4.7 /spl mu/m and a CW 980 nm laser diode pump with a maximum power of 200 mW, based on a bound-to-continuum active region, using InGaAs/AlInAs strain-compensated on an InP substrate.
