976 publications from this institution
Quantum non-locality has been an extremely fruitful subject of research, leading the scientific revolution towards quantum information science, in particular, to device-independent quantum information processing. We argue that the time is ripe to work on another basic problem in the foundations of quantum physics, the quantum measurement problem, which should produce good physics in theoretical, mathematical, experimental and applied physics. We briefly review how quantum non-locality contributed to physics (including some outstanding open problems) and suggest ways in which questions around macroscopic quantumness could equally contribute to all aspects of physics.This article is part of a discussion meeting issue 'Foundations of quantum mechanics and their impact on contemporary society'.
Private queries allow a user, Alice, to learn an element of a database held by a provider, Bob, without revealing which element she is interested in, while limiting her information about the other elements. We propose to implement private queries based on a quantum-key-distribution protocol, with changes only in the classical postprocessing of the key. This approach makes our scheme both easy to implement and loss tolerant. While unconditionally secure private queries are known to be impossible, we argue that an interesting degree of security can be achieved by relying on fundamental physical principles instead of unverifiable security assumptions in order to protect both the user and the database. We think that the scope exists for such practical private queries to become another remarkable application of quantum information in the footsteps of quantum key distribution.
We prove that any non-product state of two-particle systems violates a Bell inequality.
The fact that not all quantum observables are jointly measurable is one of the major differences between quantum and classical theory. In the former, non-commuting observables can only be simultaneously measured with limited precision. We report on an experimental implementation of such a simultaneous measurement of two non-commuting observables based on the framework of weak von Neumann measurements. We use a photonic setup where the polarisation degree of freedom acts as the system and the two components of the transversal position correspond to the pointers of the measurement apparatuses. In addition, the theory shows that these weak measurements demonstrate a counter-intuitive non-monotonicity: weaker measurements can potentially reveal more information than stronger ones.
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.
The technological possibilities of a realistic eavesdropper are discussed. Two eavesdropping strategies taking advantage of multiphoton pulses in faint laser QKD are presented. We conclude that, as long as storage of qubits is technically impossible, faint laser QKD is not limited by this security issue, but mostly by the detector noise.
No abstract is provided for this article.
We introduce a simple approach for a tunable upconversion detector. This scheme is relevant for both single photon detection or anywhere where low light levels at telecom wavelengths need to be detected with a high degree of temporal resolution or where high count rates are desired. A system combining a periodically poled lithium niobate waveguide for the nonlinear wavelength conversion and a low jitter silicon avalanche photodiode is used in conjunction with a tunable pump source. We report more than a tenfold increase in the detectable bandwidth using this tuning scheme.
In this paper, we present a compact source of narrow-band energy–time-entangled photon pairs in the telecom regime based on a Ti-indiffused periodically poled lithium niobate (PPLN) waveguide resonator, i.e. a waveguide with end-face dielectric multi-layer mirrors. This is a monolithic doubly resonant optical parametric oscillator (OPO) far below threshold, which generates photon pairs by spontaneous parametric down-conversion (SPDC) at around 1560 nm with a 117 MHz (0.91 pm)-bandwidth. A coherence time of 2.7 ns is estimated by a time correlation measurement and a high quality of the entangled states is confirmed by a Bell-type experiment. Since highly coherent energy–time-entangled photon pairs in the telecom regime are suitable for long distance transmission and manipulation, this source is well suited to the requirements of quantum communication.
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.
A novel method for the measurement of the phase and group birefingence is described. The method is based on the evolution of the differential group delay as firnetion of the twist applied on the fiber. It gives the birefringence values for zero twist for fibers within the beatlength range of 5mm-lm. The technique is very simple and found advantageous for the study of special fibers like PCF.
We consider two different ways to encode quantum information, by parallel or antiparallel pairs of spins. We find that there is more information in the antiparallel ones. This purely quantum mechanical effect is due to entanglement, not of the states but occurring in the course of the measuring process. We also introduce a range of quantum information processing machines, such as spin flip and anticloning.
No abstract is provided for this article.
I argue against the many-world interpretation (MWI) of quantum theory by emphasizing that when everything is entangled with everything else, in one big monstrous piece, there is no room left for creativity. Since the MWI was invented, it proves itself wrong (appeared first in French in [N. Gisin, L'\'epid\'emie du multivers, in Le plus grand des hasards, p. 184, eds J.F. Dars et A. Papillault, Belin 2010]).
We show that standard nonlocal boxes, also known as Popescu-Rohrlich machines, are not sufficient to simulate any nonlocal correlations that do not allow signaling. This was known in the multipartite scenario, but we extend the result to the bipartite case. We then generalize this result further by showing that no finite set containing any finite-output-alphabet nonlocal boxes can be a universal set for nonlocality.
