976 publications from this institution
A nonlinear stochastic process is presented that, for each realization and for large times, reproduces L\"uder's projection postulate. The corresponding density operator undergoes a linear evolution reproducing von Neumann's projection postulate. The violation of the Bell inequality, for instance, is described with the two apparatus acting independently on the composed system.
We construct a Bell inequality for coincidence probabilities on a three three-dimensional (qutrit) system. We show that this inequality is violated when each observer measures two noncommuting observables, defined by the so-called unbiased six-port beam splitter, on a maximally entangled state of two qutrits. The strength of the violation agrees with the numerical results presented by Kaszlikowski et al, quant-ph/0202019. It is proven that the inequality defines facets of the polytope of local variable models.
We analyze the security of quantum cryptography schemes for d-level systems using 2 or d+1 maximally conjugated bases, under individual eavesdropping attacks based on cloning machines and measurement after the basis reconciliation. We consider classical advantage distillation protocols, that allow to extract a key even in situations where the mutual information between the honest parties is smaller than the eavesdropper's information. In this scenario, advantage distillation protocols are shown to be as powerful as quantum distillation: key distillation is possible using classical techniques if and only if the corresponding state in the entanglement based protocol is distillable.
We review the long history of nonlocality in physics with special emphasis on the conceptual breakthroughs over the last few years. For the first time it is possible to study "nonlocality without signaling" {\it from the outside}, that is without all the quantum physics Hilbert space artillery. We emphasize that physics has always given a nonlocal description of Nature, except during a short 10 years gap. We note that the very concept of "nonlocality without signaling" is totally foreign to the spirit of relativity, the only strictly local theory.
We give the complete list of 175 facets of the local polytope for the case where Alice and Bob each choose their measurements from a set of four binary outcome measurements. For each inequality we compute the maximum quantum violation for qubits, the resistance to noise, and the minimal detection efficiency required for closing the detection loophole with maximally entangled qubit states, in the case where both detectors have the same efficiency (symmetric case).
Quantum mechanics is nonlocal. Classical mechanics is local. Consequently classical mechanics can not explain all quantum phenomena. Conversely, it is cumbersome to use quantum mechanics to describe classical phenomena. Not only are the computations more complex, but - and this is the main point - it is conceptually more difficult: one has to argue that nonlocality, entanglement and the principle of superposition can be set aside when crossing the "quantum $\rightarrow$ classical" border. Clearly, nonlocality, entanglement and the principle of superposition should become irrelevant in the classical limit. But why should one argue? Shouldn't it just come out of the equations? Does it come out of the equations? This contribution is about the last question. And the answer is: "it depends on which equation".
We consider a spin chain extending from Alice to Bob with next neighbors interactions, initially in its ground state. Assuming that Bob measures the last spin of the chain, the energy of the spin chain has to increase, at least on average, due to the measurement disturbance. Presumably, the energy is provided by Bob's measurement apparatus. Assuming now that, simultaneously to Bob's measurement, Alice measures the first spin, we show that either energy is not conserved, - implausible - or the projection postulate doesn't apply, and that there is signalling. An explicit measurement model shows that energy is conserved (as expected), but that the spin chain energy increase is not provided by the measurement apparatus(es), that the projection postulate is not always valid - illustrating the Wigner-Araki-Yanase (WAY) theorem - and that there is signalling, indeed. The signalling is due to the non-local interaction Hamiltonian. This raises the question of a suitable quantum information inspired model of such non-local Hamiltonians.
Pseudo-telepathy is the most recent form of rejection of locality. Many of its properties have already been discovered: for instance, the minimal entanglement, as well as the minimal cardinality of the output sets, have been characterized. This paper contains two main results. First, we prove that no bipartite pseudo-telepathy game exists, in which one of the partners receives only two questions; as a corollary, we show that the minimal "input cardinality", that is, the minimal number of questions required in a bipartite pseudo-telepathy game, is 3 × 3. Second, we study the Bell-type inequality derived from the pseudo-telepathy game known as the Magic Square game: we demonstrate that it is a tight inequality for 3 inputs and 4 outputs on each side and discuss its weak resistance to noise.
Several classes of state-dependent quantum cloners for three-level systems are investigated. These cloners optimally duplicate some of the four maximally-conjugate bases with an equal fidelity, thereby extending the phasecovariant qubit cloner to qutrits. Three distinct classes of qutrit cloners can be distinguished, depending on whether two, three, or four maximally-conjugate bases are cloned as well (the latter case simply corresponds to the universal qutrit cloner). These results apply to symmetric as well as asymmetric cloners, so that the balance between the fidelity of the two clones can also be analysed.
We relate the nonlocal properties of noisy entangled states to Grothendieck's constant, a mathematical constant appearing in Banach space theory. For two-qubit Werner states ${\ensuremath{\rho}}_{p}^{W}=p\ensuremath{\mid}{\ensuremath{\psi}}^{\ensuremath{-}}⟩⟨{\ensuremath{\psi}}^{\ensuremath{-}}\ensuremath{\mid}+(1\ensuremath{-}p)\mathbb{1}∕4$, we show that there is a local model for projective measurements if and only if $p\ensuremath{\leqslant}1∕{K}_{G}(3)$, where ${K}_{G}(3)$ is Grothendieck's constant of order 3. Known bounds on ${K}_{G}(3)$ prove the existence of this model at least for $p\ensuremath{\lesssim}0.66$, quite close to the current region of Bell violation, $p\ensuremath{\sim}0.71$. We generalize this result to arbitrary quantum states.
A quantum cryptography set-up is described. It uses polarized photons to code the key. The photons remain guided from the semiconductor laser diode until the photon counter module. The feasibility of establishing a key over more than 1 km by this method has been experimentally demonstrated.
Quantum memories based on the photon-echo principle (with controlled reversible inhomogeneous broadening) allow in principle perfect reconstruction of the stored light. In the retrieval process, the envelope of the absorbed wave packet is reversed in time, but the evolution of the phase of the carrier wave is unchanged. We discuss the consequences of this fact for the relative phase of pulses with a certain time delay, and thus for the storage of time-bin qubits. As an illustration, we show that the combination of photon-echo-based memories and unbalanced interferometers leads to a counterintuitive interference effect, allowing one to measure a path length difference $\ensuremath{\Delta}L$ using pulses that are much shorter than $\ensuremath{\Delta}L$.
Quantum cryptography, quantum teleportation and experiments on entanglement is reviewed with special emphasize on the state of the art and the challenges of the 5-10 next years
We consider alternative models to quantum mechanics, that have been proposed in the recent years in order to explain the EPR correlations between two particles. These models allow in principle local hidden variables produced at the source, and some superluminal “hidden communication” (or “influences”) to reproduce the non-local correlations. Moving to the case of three particles, we show that these alternative models lead to signaling when “hidden communication” alone is considered as the origin of the correlations.
