Quantum optics  can be harnessed to implement cryptographic protocols that are verifiably immune against any conceivable attack . Even quantum computers, that will break most current public keys [3, 4], cannot harm quantum encryption. Based on these intriguing quantum features, metropolitan quantum networks have been implemented around the world [5-15]. However, the long-haul link between metropolitan networks is currently missing . Existing fiber infrastructure is not suitable for this purpose since classical telecom repeaters cannot relay quantum states . Therefore, optical satellite-to-ground communication [17-22] lends itself to bridge intercontinental distances for quantum communication [23-40].
The effect of the quantum properties of light on nonlinear processes has been well studied theoretically. It has been shown that the efficiency of n-photon nonlinear processes in many cases scales as the normalized n-th order correlation function. For light with high intensity correlation function, the efficiency of the n-th harmonic generation will be considerably higher than for coherent light. The experimental observation of this effect remained difficult until recently, because of the absence of bright sources with strong and fast intensity fluctuations.
For the experimental demonstration of statistical effects in optical harmonic generation we use as a pump the radiation of high-gain parametric down conversion. Such light shows quantum properties (e.g. quadrature or two-mode squeezing) and has large number of photons in one mode. The normalized n-th order correlation function for this light is (2n - 1)!!, which makes it more attractive for nonlinear processes than both coherent and thermal light.
For the generation of optical harmonics we used broadband parametric down conversion around frequency-degeneracy (1600 nm) produced in 1cm BBO crystal from Ti:Sapphire laser (800 nm, 1.6ps, 5kHz, 3W mean intensity). Due to spectral filtering and post-selection technique we could vary the statistics of light from coherent to super-bunched, which allowed us to demonstrate the efficiency enhancement for second-, third-, and fourth-harmonic generation. The obtained experimental results show a good agreement with the theory.
Continuous-variable quantum key distribution is a practical application of quantum information theory that is aimed at generation of secret cryptographic key between two remote trusted parties and that uses multi-photon quantum states as carriers of key bits. Remote parties share the secret key via a quantum channel, that presumably is under control of of an eavesdropper, and which properties must be taken into account in the security analysis.
Well-studied fiber-optical quantum channels commonly possess stable transmittance and low noise levels, while free-space channels represent a simpler, less demanding and more flexible alternative, but suffer from atmospheric effects such as turbulence that in particular causes a non-uniform transmittance distribution referred to as fading. Nonetheless free-space channels, providing an unobstructed line-of-sight, are more apt for short, mid-range and potentially long-range (using satellites) communication and will play an important role in the future development and implementation of QKD networks.
It was previously theoretically shown that coherent-state CV QKD should be in principle possible to implement over a free-space fading channel, but strong transmittance fluctuations result in the significant modulation-dependent channel excess noise. In this regime the post-selection of highly transmitting sub-channels may be needed, which can even restore the security of the protocol in the strongly turbulent channels. We now report the first proof-of-principle experimental test of coherent state CV QKD protocol using different levels Gaussian modulation over a mid-range (1.6-kilometer long) free-space atmospheric quantum channel. The transmittance of the link was characterized using intensity measurements for the reference but channel estimation using the modulated coherent states was also studied.
We consider security against Gaussian collective attacks, that were shown to be optimal against CV QKD protocols . We assumed a general entangling cloner collective attack (modeled using data obtained from the state measurement results on both trusted sides of the protocol), that allows to purify the noise added in the quantum channel . Our security analysis of coherent-state protocol also took into account the effect of imperfect channel estimation, limited post-processing efficiency and finite data ensemble size on the performance of the protocol. In this regime we observe the positive key rate even without the need of applying post-selection. We show the positive improvement of the key rate with increase of the modulation variance, still remaining low enough to tolerate the transmittance fluctuations.
The obtained results show that coherent-state CV QKD protocol that uses real free-space atmospheric channel can withstand negative influence of transmittance fluctuations, limited post-processing efficiency, imperfect channel estimation and other finite-size effects, and be successfully implemented. Our result paves the way to the full-scale implementation of the CV QKD in real free-space channels at mid-range distances.
With growing demand on transmission capacity, spectral-efficient multilevel modulation formats such as quadrature amplitude modulation (QAM) become of great interest. One of their weaknesses is high sensitivity to noise accumulation, especially in long-haul transmission systems. Our investigations have shown the possibility of all-optical regeneration of multiple amplitude and phase states. Processing of amplitude noise in several amplitude states is based on periodicity of interference conditions in modified nonlinear fiber Sagnac interferometers. Their staircase-like power transfer characteristic can be used for phase-preserving amplitude regeneration of multiple amplitude states. Processing of QAM with up to three non-zero amplitude states, e.g. 16QAM, has been demonstrated in numerical simulations. Furthermore, simultaneous amplitude regeneration of a star-8QAM format with two amplitude states was performed experimentally. Recently, it has also been shown that phase-sensitive amplification for multiple phase states can be realized in fiber optical parametric amplifiers using four-wave mixing (FWM) with a high-order idler. Our numerical simulations and experimental results for star-8QAM revealed that with some modifications, this approach can be used not only for reduction of phase noise in multilevel phase-shift keying but also for signals with multiple amplitude states. The transmission improvement using a cascade of these two regenerator types has been demonstrated in numerical simulations and experiments. Numerical investigations confirm also the possibility to combine both approaches in a single device by using the highly nonlinear fiber in the Sagnac interferometer loop simultaneously for phase-sensitive amplification in one propagation direction.
We report several vulnerabilities found in Clavis2, the flagship quantum key distribution (QKD) system from ID Quantique. We show the hacking of a calibration sequence run by Clavis2 to synchronize the Alice and Bob devices before performing the secret key exchange. This hack induces a temporal detection efficiency mismatch in Bob that can allow Eve to break the security of the cryptosystem using faked states. We also experimentally investigate the superlinear behaviour in the single-photon detectors (SPDs) used by Bob. Due to this superlinearity, the SPDs feature an actual multi-photon detection probability which is generally higher than the theoretically-modelled value. We show how this increases the risk of detector control attacks on QKD systems (including Clavis2) employing such SPDs. Finally, we review the experimental feasibility of Trojan-horse attacks. In the case of Clavis2, the objective is to read Bob's phase modulator to acquire knowledge of his basis choice as this information suffices for constructing the raw key in the Scarani-Acin-Ribordy-Gisin 2004 (SARG04) protocol. We work in close collaboration with ID Quantique and for all these loopholes, we notified them in advance. Wherever possible, we or ID Quantique proposed countermeasures and they implemented suitable patches and upgrade their systems.
The application of a Nonlinear Amplifying Loop Mirror (NALM) as a nonlinear phase-shift compensator (NPSC) in
phase-shift keyed transmission is investigated with emphasis on Differential Quadrature Phase-Shift Keying (DQPSK).
The origin of the effective negative nonlinear phase shift in a NALM and the effects of the NALM parameters on the
phase-shift compensation are discussed. Two possible application modes of the NALM as NPSC have been found: sole
nonlinear phase-shift compensation up to 3 rad, and phase-shift compensation in a smaller range with simultaneous
amplitude equalization. The points of operation for both modes and the limitations in their implementation are presented.
Results show that the use of a NALM, optimized for phase-shift compensation, seems to be very promising, especially
for the application as post-compensator in differential phase-shift-keyed transmission systems.
To evaluate the performance of a NALM as post-compensator a simplified model of the NALM and a DQPSK
transmission system was used. The BER with and without the NALM was calculated in dependence on the average
nonlinear phase shift and on the input noise. The results show that with a NALM as NPSC the amount of nonlinear phase
shift in a preceding transmission system can be approximately doubled for the same BER.
A nonlinear optical loop mirror with a bidirectional attenuator has been used for regeneration of return-to-zero
differential phase-shift-keyed (RZ-DPSK) signals. A 2.5 ps, 10 Gb/s signal with amplitude fluctuations of 28 % was
regenerated with a negative power penalty of 2 dB practically back to the quality of the undistorted reference signal.
Parameters limiting system performance and optimization possibilities will be discussed.
Interferometric surface tests of stigmatic aspherics can be carried out in a null test configuration. Null tests require
reference null elements either plane or spherical surfaces or both. A parabolic reflector transforms a plane into a spherical
wave which converges to the focus of the paraboloid. Therefore, a spherical ball lens or a steel ball can be placed into the
focus enabling a double-pass geometry for the null test. Here a Fizeau interferometer geometry has been selected in order
to guarantee invariance against polarization distortions under the assumption that radially polarized laser light is used for
the interferometer. Radial polarized light is necessary to mimic a Hertzian dipole field. Due to the extreme solid angle
produced by the paraboloid the alignment of the setup is very critical and needs auxiliary systems for the control.
Aberrations caused by misalignments are removed via fitting of suitable functionals provided through ray-trace
simulations. It turned out that the usual vector approximations fail under these extreme circumstances. Test results are
given for a paraboloid with 2mm focal length transforming a plane wave into a near dipole wave comprising a solid angle
of about 3,4π.
We present theoretical and experimental studies of the reflected field in the vicinity of sub λ structures. Rigorous numerical calculations and measurements were performed to get high-precision information of certain object parameters which go beyond the limits of classical microscopy. We show that the polarization of the illumination plays a key role for the field distribution, which is reflected from the examined objects. Furthermore, we present a simplified model, which is able to qualitatively predict the behavior of the phase singularities correctly.