The linear-polarization correlation of pairs of photons emitted in a radiative cascade of calcium has been measured. The new experimental scheme, using two-channel polarizers (i.e. , optical analogs of Stern-Gerlach filters) is a straightforward transposition of Einstein-Podolsky-Rosen-Bohm gedankenexperiment. The present results, in excellent agreement with the quantum mechanical predictions, lead to the greatest violation of generalized Bell's inequalities ever achieved.

We have measured the linear polarization correlation of the photons emitted in a radiative atomic cascade of calcium. A high-efficiency source provided an improved statistical accuracy and an ability to perform new tests. Our

Quantum continuous variables [1] are being explored [2,3,4,5,6,7,8,9,10,11,12,13,14] as an alternative means to implement quantum key distribution, which is usually based on single photon counting [15]. The former approach is potentially advantageous because it should enable higher key distribution rates. Here we propose and experimentally demonstrate a quantum key distribution protocol based on the transmission of gaussian-modulated coherent states (consisting of laser pulses containing a few hundred photons) and shot-noise-limited homodyne detection; squeezed or entangled beams are not required [13]. Complete secret key extraction is achieved using a reverse reconciliation [14] technique followed by privacy amplification. The reverse reconciliation technique is in principle secure for any value of the line transmission, against gaussian individual attacks based on entanglement and quantum memories. Our table-top experiment yields a net key transmission rate of about 1.7 megabits per second for a loss-free line, and 75 kilobits per second for a line with losses of 3.1 dB. We anticipate that the scheme should remain effective for lines with higher losses, particularly because the present limitations are essentially technical, so that significant margin for improvement is available on both the hardware and software.

We propose several methods for quantum key distribution (QKD) based upon the generation and transmission of random distributions of coherent or squeezed states, and we show that they are are secure against individual eavesdropping attacks. These protocols require that the transmission of the optical line between Alice and Bob is larger than 50 %, but they do not rely on "non-classical" features such as squeezing. Their security is a direct consequence of the no-cloning theorem, that limits the signal to noise ratio of possible quantum measurements on the transmission line. Our approach can also be used for evaluating various QKD protocols using light with gaussian statistics.PACS numbers: 03.67. Dd, 42.50.Dv, 89.70.+c Since the experimental demonstration of quantum teleportation of coherent states [1], a lot of interest has arisen in continuous variable quantum information processing. In particular, a stimulating question is whether quantum continuous variables (QCV) may provide a valid alternative to the usual "single photon" quantum key distribution schemes [2]. Most present proposals to use QCV for QKD [3][4][5][6][7][8][9][10][11][12][13][14][15]. are based upon the use of "nonclassical" light beams, such as squeezed light, or pairs of light beams that are correlated for two different quadratures components (the so-called "EPR" beams, by analogy with the historical paper by Einstein, Podolski and Rosen [16]). But recent work on this subject [17] underlined the crucial importance of the continuous variable version of the no-cloning theorem [18], as soon as security is concerned in any exchange using QCV.In this letter, we show that there is actually no need for squeezed light : an equivalent level of security may be obtained by simply generating and transmitting random distributions of coherent states. The security of this novel protocols is related to the no-cloning theorem, that limits possible eavesdropping even though the transmitted light has no "non-classical" feature such as squeezing. We show that our analysis can be also applied to other protocols using light with gaussian statistics, i.e. squeezed or EPR beams, making thus the comparison easier. The basic tools for this analysis are the ones that have been extensively used for linearized quantum optics, including in particular optical quantum non-demolition (QND) measurements [19]. Before presenting our protocol, we will briefly review the current literature on continuous variables QKD.Here we consider security against individual attacks only, and we do not address the issue of unconditionnal security, that was demonstrated by Gottesman and Preskill [3] for squeezed states protocols (unconditional security of coherent states protocols remains an open question). Security against individual attacks was previously considered by many authors. Hillery proposed a QKD scheme based on binary modulated squeezed light [4]. Cerf et al showed it could be improved considering gaussian modulation [5,6] and described a reconciliation protocol [6,7] to implement...

We report on two experiments using an atomic cascade as a light source, and a triggered detection scheme for the second photon of the cascade. The first experiment shows a strong anticorrelation between the triggered detections on both sides of a beam splitter. This result is in contradiction with any classical wave model of light, but in agreement with a quantum description involving single-photon states. The same source and detection scheme were used in a second experiment, where we have observed interferences with a visibility over 98%.

We present a detailed experimental analysis of a free-propagating light pulse prepared in a "Schrödinger kitten" state, which is defined as a quantum superposition of "classical" coherent states with small amplitudes. This kitten state is generated by subtracting one photon from a squeezed vacuum beam, and it clearly presents a negative Wigner function. The predicted influence of the experimental parameters is in excellent agreement with the experimental results. The amplitude of the coherent states can be amplified to transform our "Schrödinger kittens" into bigger Schrödinger cats, providing an essential tool for quantum information processing.

The dipole blockade between Rydberg atoms has been proposed as a basic tool in quantum information processing with neutral atoms. Here we demonstrate experimentally the Rydberg blockade of two individual atoms separated by 4 µm. Moreover, we show that, in this regime, the single atom excitation is enhanced by a collective two-atom behavior associated with the excitation of an entangled state. This observation is a crucial step towards the deterministic manipulation of entanglement of two or more atoms using the Rydberg dipole interaction.PACS numbers: 32.80. Rm, 03.67.Lx, 32.80.Pj, 42.50.Ct A large experimental effort is nowadays devoted to the production of entanglement, that is quantum correlations, between individual quantum objects such as atoms, ions, superconducting circuits, spins, or photons. Entangled states are important in many areas of physics such as quantum information and quantum metrology, the study of strongly correlated systems in many-body physics, and more fundamentally in the understanding of quantum physics.There are several ways to engineer entanglement in a quantum system. Here, we focus on a method that relies on a blockade mechanism where the strong interaction between different parts of a system prevents their simultaneous excitation by the same driving pulse. Single excitation is still possible, but it is delocalized over the whole system, and results in the production of an entangled state. This approach to entanglement is deterministic and can be used to realize quantum gates [1] or to entangle mesoscopic ensembles, provided that the blockade is effective over the whole sample [2]. Blockade effects have been observed in systems where interactions are strong such as systems of electrons using the Coulomb force [3] or the Pauli effective interaction [4], as well as with photons and atoms coupled to an optical cavity [5]. Recently, atoms held in the ground state of the wells of an optical lattice have been shown to exhibit interaction blockade, due to s-wave collisions [6].An alternative approach uses the comparatively strong interaction between two atoms excited to Rydberg states, which have very large dipole moments. This strong interaction gives rise to the so-called Rydberg blockade, which has been observed in clouds of cold atoms [7,8,9,10,11,12] as well as in a Bose condensate [13]. A collective behavior associated with the blockade has been reported in an ultra-cold atomic cloud [14]. Recently, an experiment demonstrated the blockade between two atoms 10 µm apart, by showing that when one atom is excited to a Rydberg state, the excitation of the second one is greatly suppressed [15].In the present work, we study two individual atoms, held at a distance of ∼ 4 µm by two optical tweezers. We demonstrate that under this condition, the atoms are in the Rydberg blockade regime since only one atom can be excited. Furthermore, we show that the single atom excitation is enhanced by a collective two-atom behavior, associated with the production of a two-atom entangled state between th...

We report the generation of entanglement between two individual 87Rb atoms in hyperfine ground states |F=1,M=1> and |F=2,M=2> which are held in two optical tweezers separated by 4 microm. Our scheme relies on the Rydberg blockade effect which prevents the simultaneous excitation of the two atoms to a Rydberg state. The entangled state is generated in about 200 ns using pulsed two-photon excitation. We quantify the entanglement by applying global Raman rotations on both atoms. We measure that 61% of the initial pairs of atoms are still present at the end of the entangling sequence. These pairs are in the target entangled state with a fidelity of 0.75.