Complex optical photon states with entanglement shared among several modes are critical to improving our fundamental understanding of quantum mechanics and have applications for quantum information processing, imaging, and microscopy. We demonstrate that optical integrated Kerr frequency combs can be used to generate several bi- and multiphoton entangled qubits, with direct applications for quantum communication and computation. Our method is compatible with contemporary fiber and quantum memory infrastructures and with chip-scale semiconductor technology, enabling compact, low-cost, and scalable implementations. The exploitation of integrated Kerr frequency combs, with their ability to generate multiple, customizable, and complex quantum states, can provide a scalable, practical, and compact platform for quantum technologies.

Large arrays of uniform, precisely tunable, open-access optical microcavities with mode volumes as small as 2.2 μm(3) are reported. The cavities show clear Hermite-Gauss mode structure and display finesses up to 460, in addition to quality (Q) factors in excess of 10,000. The cavities are attractive for use in quantum optics applications, such as single atom detection and efficient single photon sources, and have potential to be extended for experiments in the strong coupling regime.

, J. M. (2013). Measurement of the full stress tensor in a crystal using photoluminescence from point defects: The example of nitrogen vacancy centers in diamond.

Abstract. We report the photoluminescence characteristics of a colour centre in diamond grown by plasma-assisted chemical vapour deposition. The colour centre emits with a sharp zero-phonon line at 2.330 eV (λ = 532 nm) and a lifetime of 3.3 ns, thus offering potential for a high-speed singlephoton source with green emission. It displays a vibronic emission spectrum with a Huang-Rhys parameter of 2.48 at 77 K. Hanbury-Brown and Twiss measurements reveal that the electronic level structure of the defect includes a metastable state that can be populated from the optically excited state.

We present a model describing the use of ultra-short strong pulses to control the population of the excited level of a two-level quantum system. In particular, we study an off-resonance excitation with a few cycles pulse which presents a smooth phase jump i.e. a change of the pulse's phase which is not step-like, but happens over a finite time interval. A numerical solution is given for the time-dependent probability amplitude of the excited level. The control of the excited level's population is obtained acting on the shape of the phase transient, and other parameters of the excitation pulse.Comment: 6 pages, 5 figures this version is a rewriting of the previous one, with several change

We report on the design and performance of a high stability scanning confocal microscope for optical microscopy at low temperatures. By scanning the beam in a cold objective lens system, we achieve wide fields of view without compromising image quality. Photoluminescence from single nitrogen-vacancy centers in high purity diamond is used to illustrate the imaging and stability performance of the microscope.

We show that replacing the usual sifting step of the standard quantum-key-distribution protocol BB84 [1] by a one-way reverse reconciliation procedure increases its robustness against photonnumber-splitting (pns) attacks to the level of the SARG04 protocol [2,3] while keeping the raw key-rate of BB84. This protocol, which uses the same state and detection than BB84, is the m = 4 member of a protocol-family using m polarization states which we introduce here. We show that the robustness of these protocols against pns attacks increases exponentially with m, and that the effective keyrate of optimized weak coherent pulses decreases with the transmission T like T 1+ 1 m−2 .PACS numbers: 03.67. Ac, 03.67.Dd, 03.67.Hk Over the last 25 years, quantum key distribution (QKD) has emerged as the main application of quantum information. In most experimental realizations [4], the legitimate partners -traditionally named Alice and Bob -use the BB84 protocol [1] with weak-coherentpulses (wcp), i.e. Alice sends polarized coherent states to Bob, and Bob measures their polarization to obtain the raw-key. Alice and Bob then post-select a subset of the measurement to obtain the sifted-key from which the cryptographic key is extracted. If Alice sends perfect single-photons, there is no way for an eavesdroppertraditionally named Eve -to learn anything about the sifted key without introducing errors. But, with wcps, Alice only approximates single-photon, and she sometimes sends multiphoton pulses, on which Eve can get all the information through photon-number-splitting (pns) attack [5]. SARG04 [2,3] showed that, with the same modulation and detection than BB84, one can construct a protocol more robust against pns, since Eve only gains partial information from 2 photons pulse and needs to wait for the rarer 3 photons pulses to gain the full information. However, for the same pulse intensity, SARG04's rate is the half of BB84 at low losses, because of the lower rate of it sifting. As shown in [3] SARG04's robustness can be increased by using m polarizations instead of 4, at the price of a lower sifting rate ∝ m −3 . This article shows that this price is not necessary, and that it is possible to have the best of both protocols, i.e. BB84's rate and SARG04's robustness against photon numbersplitting attacks.BB84 and SARG04 are sifting based protocols i.e. protocols where a part of the data is "sifted away" because Alice's state and Bob's measurement are not in the "same basis". We will look here at sifting-less protocols, i.e. protocols where this discussion is absent, and therefore, where the "wrong-basis" data are kept in the raw-key.Protocol description. Alice randomly choses one linear polarization and sends the corresponding phaserandomized weak coherent pulse (wcp). Let m ≥ 3 the total number of possible polarizations. To simplify the analysis, we will suppose that the polarizations are uniformly distributed along a great circle of Poincaré's sphere. Let |0 and |1 be the state of two orthogonally polarized single photons. If the ...

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