Photon pairs generated in spontaneous parametric downconversion exhibit a strong correlation in time, which has been used to synchronize spatially separated clocks in the past in quantum key distribution implementations to a high accuracy. A symmetric version of this scheme can be used to allow for an absolute clock synchronization, and the entanglement properties of photons generated in spontaneous down conversion allows to strengthen the authenticity of the received signals through entanglement witnesses. Aspects of this time synchronization technique with respect to use in optical fibers and remaining vulnerabilities are discussed.
 C. Ho, A. Lamas-Linares, and C. Kurtsiefer: Clock synchronization by remote detection of correlated photon pairs, New Journal of Physics 11,
 J. Lee, L. Shen, A. Cerè, J. Troupe, A. Lamas-Linares, C.
Kurtsiefer: Symmetrical clock synchronization with time-correlated photon pairs, Appl. Phys. Lett. 114, 101102 (2019).
Quantum key distribution (QKD) is a method for establishing secure cryptographic keys between two parties who share an optical, “quantum” channel and an authenticated classical channel. To share such keys across the globe, space-based links are required and in the near term these will take the form of trusted node, key management satellites. We consider such channels between two nanosatellite spacecraft for polarization entanglement-based QKD, and the optical channel is described in detail. Quantum channels between satellites are useful for balancing keys within constellations of trusted node QKD satellites and, in the future, may have applications in long-distance qubit exchange between quantum computers and in fundamental physics experiments. The nanosatellite mission proposed uses an optical link with 80-mm diameter optical terminals. If such a link could be maintained with 10-μrad pointing accuracy, then this would allow QKD to be performed for satellite separations up to around 400 km. A potential pointing and tracking system is also described although currently this design would likely limit the satellite separation to 100 to 150 km.
Quantum physics can provide sources of randomness that can be certified as being uncorrelated to any outside process or variable, i.e. sources of private randomness, based on a violation of a Bell inequality. Initial experimental realizations of such sources of certified randomness are based on atomic or atomic-like systems, but suffer from impractically low generation rates for most applications. High efficiency infrared photodetectors and photon pair sources permitted experimental demonstrations of loophole free violation of the Bell inequality using photons. The random bit generation rate for these setups was on the order of tens per second, where the main limitation is the fixed repetition rate of the photon pair source combined with the small violation observed.
In our experiment, we close the detection loophole with a system efficiency over 82%. The source of entangled photon pairs is based on continuously pumped spontaneous parametric down conversion. We estimate a collection efficiency of ≈90% into single mode fibers and detect photons with a transition edge sensors. Detection events are time-tagged and organized into time bins, for which we consider four possible outcomes: one or more detections at Alice’s side, one or more detections at Bob’s side, one or more detections at both Alice’s and Bob’s, and no detection in either channel. These events eventually lead to a CHSH-type Bell inequality that is violated for a range of time bin widths. With such an arrangement, we reach asymptotic device-indepentnet random bit generation rates on the order of 1000 bits per second.
We investigate alternative focusing geometries to couple near-resonant light onto a single neutral atom. In particular, we show significant light-atom interaction using the ‘4Pi-microscopy’ configuration. Performing a transmission experiment, we find a resonant extinction of 36.6(3)%. Furthermore, photon anti-bunching in the second-order correlation function of the transmitted light demonstrates nonlinear light-atom interaction at the level of single photons. Our results indicate that free-space focusing provides an alternative route to realise nonlinear optics with single photons.
We prepare single photons with a temporal envelope with rising exponential shape, resembling the time-reversed version of photons from the spontaneous decay process using a parametric conversion process in a cold atomic vapor. The mechanism is based on correlated photon pair preparation and heralding of one photon by the other one after engineering the temporal envelope of the herald.1 Such a temporal single photon profile is ideal for absorption by a two level system.2, 3 We demonstrate this in an experiment showcasing the absorption by a single Rubidium atom.4
Hong-Ou-Mandel interference between independent sources is a fundamental primitive of many quantum communication and computation protocols. We present a study of the Hong-Ou-Mandel interference of single photons generated via two different physical processes by two independent atomic systems: scattering by a single atom, and parametric generation via four-wave mixing in a cloud of cold atoms. By controlling the coherence time and central frequency of the heralded single photons generated by four-wave mixing we observe quantum beat and a varying degree of interference.
We investigate the interaction between a single atom and a light field in the strong focusing regime. Such a
configuration is subject to recent experimental work not only with atoms but also molecules and other atom-like
systems such as quantum dots. We derive the scattering probability for photons by such a microscopic object
modeled by a two-level system, starting with a Gaussian beam as the spatial mode of the light field. The focusing
by an ideal lens is modeled by adopting a field with spherical wave fronts compatible with Maxwell equations.
Using a semi-classical approach for the atom-field interaction, we predict a scattering probability of photons by a
single atom of up to 98% for realistic focusing parameters. Experimental results for different focusing strengths
are compared with our theoretical model.
We report on the implementation of a photon counting polarimeter based on a scheme known to be optimal for obtaining the polarization vector of ensembles of spin-1/2 quantum systems. We show how to use this polarimeter to estimate the complete polarization state for generic multi-photon states. State reconstruction using the polarimeter is illustrated by actual measurements on prepared ensembles of one- and two-photon systems. The rate at which the estimated polarization state converges to an asymptote state is also measured and presented.
Coding data bits in the phase or polarization state of light allows us to exploit the wave particle duality for novel communication protocols. Using this principle the first practical quantum communication systems have been built. These are the fiber and free-space quantum cryptography apparatus used for secure exchange of keys. Beyond this state of the art, various quantum communication schemes are being studied including entangled state key exchange quantum dense coding, state teleportation, and entanglement swapping. The feasibility, advantages and disadvantages of space based realisations of these novel schemes are discussed.
Quantum cryptography bases the security of key exchange on the laws of quantum physics and will become the first application of quantum information methods. Here we present the design of novel hardware components which enabled the demonstration of secure key exchange over a 23.4 km free-space link.
Multiphoton entanglement is the basis of many quantum
communication schemes, quantum cryptographic protocols, and
fundamental tests of quantum theory. Spontaneous parametric
down-conversion is the most effective source for polarization
entangled photon pairs. Here we show, that a entangled 4-photon
state can be directly created by parametric down-conversion. This
state exhibit perfect quantum correlations and a high robustness
of entanglement against photon loss. We have used this state for
four-particle test of local realistic theories. Therefore this
state can be used for new types of quantum communication. We also
report on possibilities for the experimentally realization of a
3-photon entangled state, the so called W-state, and discuss some
of its properties.