Publications

Security of quantum key distribution with entangled qutrits

The study of quantum cryptography and quantum non-locality have traditionnally been based on two-level quantum systems (qubits). In this paper we consider a generalisation of Ekert's cryptographic protocol [Ekert] where qubits are replaced by qutrits. The security of this protocol is related to non-locality, in analogy with Ekert's protocol. In order to study its robustness against the optimal individual attacks, we derive the information gained by a potential eavesdropper applying a cloning-based attack.

Single-photon space-like antibunching

We use heralded single photons to perform an antibunching experiment in which the clicks at the detectors are spacelike separated events. The idea of such experiment dates back to the 5th Solvay conference, when it was proposed by Einstein as an expression of his concerns about quantum theory.

Heralded photon amplification for quantum communication

Heralded noiseless amplification based on single-photon sources and linear optics is ideally suited for long-distance quantum communication tasks based on discrete variables. We experimentally demonstrate such an amplifier, operating at telecommunication wavelengths. Coherent amplification is performed with a gain of G=1.98+/-0.2, for a state with a maximum expected gain G=2. We also demonstrate that there is no need for a stable phase reference between the initial signal state and the local auxiliary photons used by the amplifier. These results highlight the potential of heralded quantum amplifiers for long-distance quantum communication, and bring device-independent quantum key distribution one step closer.

Polarization Properties of Single-Moded, Large-Mode Area Photonic Crystal Fibers

We report on an experimental analysis of the polarization properties of novel large-core photonic crystal fibers. The results show very low birefringence indicating a highly symmetrical hole structure over a long length

Quantum Chance

No abstract is provided for this article.

Quantum Cryptography Using Entangled Photons in Energy-Time Bell States

We present a setup for quantum cryptography based on photon pairs in energy-time Bell states and show its feasibility in a laboratory experiment. Our scheme combines the advantages of using photon pairs instead of faint laser pulses and the possibility to preserve energy-time entanglement over long distances. Moreover, using four-dimensional energy-time states, no fast random change of bases is required in our setup: Nature itself decides whether to measure in the energy or in the time base, thus rendering eavesdropper attacks based on "photon number splitting" less efficient.

Letter Optical quantum random number generator

No abstract is provided for this article.

Device-independent quantum key distribution secure against collective attacks

Device-independent quantum key distribution (DIQKD) represents a relaxation of the security assumptions made in usual quantum key distribution (QKD). As in usual QKD, the security of DIQKD follows from the laws of quantum physics, but contrary to usual QKD, it does not rely on any assumptions about the internal working of the quantum devices used in the protocol. In this paper, we present in detail the security proof for a DIQKD protocol introduced in Acín et al (2008 Phys. Rev. Lett. 98 230501). This proof exploits the full structure of quantum theory (as opposed to other proofs that exploit only the no-signaling principle), but only holds against collective attacks, where the eavesdropper is assumed to act on the quantum systems of the honest parties independently and identically in each round of the protocol (although she can act coherently on her systems at any time). The security of any DIQKD protocol necessarily relies on the violation of a Bell inequality. We discuss the issue of loopholes in Bell experiments in this context.

Une memoire quantique ideale

Quantum teleportation over the Swisscom telecommunication network

We present a quantum teleportation experiment in the quantum relay configuration using the installed telecommunication network of Swisscom. In this experiment, the Bell-state measurement occurs well after the entanglement has been distributed, at a point where the photon upon which data are teleported is already far away, and the entangled qubits are photons created from a different crystal and laser pulse than the teleported qubit. A raw fidelity of 0.93±0.04 has been achieved using a heralded single-photon source.

Algebraic and geometric properties of local transformations

Some properties of physical systems can be characterized from their correlations. In that framework, subsystems are viewed as abstract devices that receive measurement settings as inputs and produce measurement outcomes as outputs. The labeling convention used to describe these inputs and outputs does not affect the physics; and relabelings are easily implemented by rewiring the input and output ports of the devices. However, a more general class of operations can be achieved by using correlated preprocessing and postprocessing of the inputs and outputs. In contrast to relabelings, some of these operations irreversibly lose information about the underlying device. Other operations are reversible, but modify the number of cardinality of inputs and/or outputs. In this work, we single out the set of deterministic local maps as the one satisfying two equivalent constructions: an operational definition from causality, and an axiomatic definition reminiscent of the definition of quantum completely positive trace-preserving maps. We then study the algebraic properties of that set. Surprisingly, the study of these fundamental properties has deep and practical applications. First, the invariant subspaces of these transformations directly decompose the space of correlations/Bell inequalities into nonsignaling, signaling and normalization components. This impacts the classification of Bell and causal inequalities, and the construction of assemblages/witnesses in steering scenarios. Second, the left and right invertible deterministic local operations provide an operational generalization of the liftings introduced by Pironio [J. Math. Phys., 46(6):062112 (2005)]. Not only Bell-local, but also causal inequalities can be lifted; liftings also apply to correlation boxes in a variety of scenarios.

Storage of hyperentanglement in a solid-state quantum memory

Two photons can simultaneously share entanglement between several degrees of freedom such as polarization, energy-time, spatial mode and orbital angular momentum.This resource is known as hyperentanglement, and it has been shown to be an important tool for optical quantum information processing.Here we demonstrate the quantum storage and retrieval of photonic hyperentanglement in a solid-state quantum memory.A pair of photons entangled in polarization and energy-time is generated such that one photon is stored in the quantum memory, while the other photon has a telecommunication wavelength suitable for transmission in optical fibre.We measured violations of a Clauser-Horne-Shimony-Holt (CHSH) Bell inequality for each degree of freedom, independently of the other one, which proves the successful storage and retrieval of the two bits of entanglement shared by the photons.Our scheme is compatible with long-distance quantum communication in optical fibre, and is in particular suitable for linear-optical entanglement purification for quantum repeaters.

Limits on Correlations in Networks for Quantum and No-Signaling Resources

A quantum network consists of independent sources distributing entangled states to distant nodes which can then perform entangled measurements, thus establishing correlations across the entire network. But how strong can these correlations be? Here we address this question, by deriving bounds on possible quantum correlations in a given network. These bounds are nonlinear inequalities that depend only on the topology of the network. We discuss in detail the notably challenging case of the triangle network. Moreover, we conjecture that our bounds hold in general no-signaling theories. In particular, we prove that our inequalities for the triangle network hold when the sources are arbitrary no-signaling boxes which can be wired together. Finally, we discuss an application of our results for the device-independent characterization of the topology of a quantum network.

Quantum State Diffusion: from Foundations to Applications

Deeper insight leads to better practice. We show how the study of the foundations of quantum mechanics has led to new pictures of open systems and to a method of computation which is practical and can be used where others cannot. We illustrate the power of the new method by a series of pictures that show the emergence of classical features in a quantum world. We compare the development of quantum mechanics and of the theory of (biological) evolution.

Experimental investigation of the polarization properties of a hollow core photonic bandgap fiber for 1550 nm

The properties of a hollow core photonic bandgap fiber designed for 1.55 um transmission are investigated with special emphasis on polarization issues.Large and strongly wavelength dependent phase and group delays are found.At the same time the principle states of polarization move strongly and erratically as a function of wavelength, leading to strong mode coupling.Wavelength regions with high polarization dependent loss coincide with depolarization due to a polarization dependent coupling to surface modes at these wavelengths.

PMD emulators and signal distortion in 2.48-Gb/s IM-DD lightwave systems

Effects of polarization mode dispersion (PMD) on 2.48 Gb/s IM-DD lightwave systems are experimentally observed by using PMD emulators made with segments of highly birefringent fiber with random-mode coupling splices. Random birefringence between mode couplings was also included in the emulator. The measurements indicate that the eye diagram is corrupted by PMD values above 10 ps.

The relativity of indetermininacy

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.

Transmission gratings for chromatic dispersion compensation

It is theoretically demonstrated that a succession of four unchirped gratings, ~20 mm long, induces a negative chromatic dispersion in transmission of -100 ps/nm over a 0.2-nm bandwidth, with a power loss below 1 dB. The stability of the proposed arrangement is also considered and found to be within today's technology.