Correlated photon pairs produced by a spontaneous parametric down conversion (SPDC) process can be used for secure quantum communication over long distances including free space transmission over a link through turbulent atmosphere. We experimentally investigate the possibility to utilize the intrinsic strong correlation between the pump and output photon spatial modes to mitigate the negative targeting effects of atmospheric beam wander. Our approach is based on a demonstration observing the deflection of the beam on a spatially resolved array of single photon avalanche diodes (SPAD-array).
Correlated photon pairs produced by a spontaneous parametric down conversion (SPDC) process can be used for secure quantum communication over long distances including free space transmission over a link through turbulent atmosphere. We experimentally investigate the possibility to utilize the intrinsic strong correlation between the pump and output photon spatial modes to mitigate the negative targeting effects of atmospheric beam wander. Our approach is based on a demonstration observing the deflection of the beam on a spatially resolved array of single photon avalanche diodes (SPAD-array).
Scattering by atmospheric turbulence is one of the main challenges in creating long free-space optical links, and specifically links of entangled photons. Classical compensation methods are hard to apply to entangled photons, due to inherently low signal to noise ratios and the fragility of entanglement. We have recently shown that we can use a bright laser beam that pumps spontaneous parametric down conversion to control the spatial correlations between entangled photons for compensating their scattering. In this work, we apply the pump-shaping technique to compensate for the scrambling of correlations between entangled photons that scatter by emulated atmospheric turbulence. We use a spatial light modulator and Kolmogorov’s turbulence model to emulate atmospheric turbulence in the lab, and enhance the entangled photons’ signal by a factor of fifteen using pump optimization. We show this for both a static and dynamic emulated atmosphere, and also demonstrate the compensation of the scattering of a higher-order mode. Our results can open the door towards realizing free-space quantum links with entangled photons, used in applications such as quantum key distribution.
Turbulence presents challenges for free-space light-based protocols. In this work, we investigated the effect of turbulence on the correlations of twin photons prepared in Spontaneous Parametric Down Conversion (SPDC) by a pump beam with different transverse modes. We demonstrate that the strategy of coordinate inversion for mitigating turbulence-induced distortions on transverse coincidence profiles is effective for twin photons generated by HG/LG modes. The conservation of orbital angular momentum in SPDC enables us to encode information in the transverse degrees of freedom of the down-converted photon pairs. This information can have its correlation protected by the coordinate inversion technique against weak turbulence effects in free-space links.
We theoretically study the polarization entanglement and the non-local correlations manifested by biphotons using measures as concurrence and CHSH Bell test, generated in the spontaneous parametric down-conversion (SPDC) process for partially spatially coherent pump, affected by the spatial walk-off effects of the extraordinary beam. The study has been carried out for a well-known scheme for generating polarization entanglement utilizing non-collinear, frequency degenerate SPDC process in paired type-I uniaxial crystals. It is found that the polarization entanglement changes drastically with variation in spatial coherence of the pump, offering a possibility to realize a tunable source of polarization entanglement. The study reveals the overriding role of aperture size in compensation of the effect of spatial coherence of pump on the polarization entanglement and non-local correlations, especially in the conservation of polarization entanglement for incoherent and partially coherent pump beams. Free-space quantum secure communication utilizing these quantum states possessing maximal entanglement and partially spatially coherent properties would essentially offer robustness to the losses on propagation through atmospheric turbulence, resulting in a significant enhancement in the key rate.
A major field of research at the current time is that of implementing Quantum Key Distribution over large distances using satellites. If this protocol works with this technology, it will have huge implications on future information security. In order for a satellite to implement this idea, there are many aspects that must be taken into account. One of the big issues that comes up for this type of system is that of propagating light through the turbulent atmosphere and its effects on the acquisition, pointing and tracking system. The projects studied in this thesis study some of the effects of the atmosphere on certain detectors, try to develop pointing schemes for better accuracy as well as develop knowledge in free space propagation of other single photon experiments. \n \nIn the first experiment, I study the spatial correlations of the daughter photons created in spontaneous parametric down conversion. I look at the effect of altering the pump beam on the positions of the down converted photons and see if the pump can be manipulated in a way to control the directions of the daughter photons. I begin to utilize a deformable mirror and Shack-Hartmann wavefront sensor which are generally used in adaptive optics, but we plan to use them to alter the pump beam in the spontaneous parametric down conversion process to analyze the correlations between the pump and down converted photons. \n \nThe second experiment investigates the effects of laser scintillation on the performance of a possible tracking device that could be implemented on a satellite. This quad sensor tracks the position of a beam and a system will be developed to move the sensor to keep the beam in the center where there is a hole for the quantum single photons to stream through. In order to create the effects of scintillation, a turbulence simulator box was built and characterized. This box combines wind turbulence with a heat gradient to mimic atmospheric turbulence on a small scale. \n \nFinally, my contributions to a large scale, long distance free space quantum optics experiment are explained and the overall goal of the experiment is discussed. This experiment exposed me to actual free space transmission issues as well as many fundamental techniques for performing long distance optics experiments. In this experiment there was no correction for atmospheric turbulence, but in the future, techniques could be implemented which might increase the efficiencies of the free space links.
We demonstrate experimental generation of spatially entangled photon pairs by spontaneous parametric down-conversion (SPDC) using a partial spatially coherent pump beam. By varying the spatial coherence of the pump, we show its influence on the down-converted photon's spatial correlations and on its degree of entanglement, in excellent agreement with theory. We then exploit this property to produce pairs of photons with a specific degree of entanglement by tailoring of the pump coherence length. This work thus unravels the fundamental transfer of coherence occurring in SPDC processes, and provides a simple experimental scheme to generate photon pairs with a well-defined degree of spatial entanglement, which may be useful for quantum communication and information processing.
In practical applications to free-space quantum communications, the utilization of active beam coupling and stabilization techniques offers notable advantages, particularly when dealing with limited detecting areas or coupling into single-mode fibers(SMFs) to mitigate background noise. In this work, we introduce highly-enhanced active beam-wander-correction technique, specifically tailored to efficiently couple and stabilize beams into SMFs, particularly in scenarios where initial optical alignment with the SMF is misaligned. To achieve this objective, we implement a SMF auto-coupling algorithm and a decoupled stabilization method, effectively and reliably correcting beam wander caused by atmospheric turbulence effects. The performance of the proposed technique is thoroughly validated through quantitative measurements of the temporal variation in coupling efficiency(coincidence counts) of a laser beam(entangled photons). The results show significant improvements in both mean values and standard deviations of the coupling efficiency, even in the presence of 2.6 km atmospheric turbulence effects. When utilizing a laser source, the coupling efficiency demonstrates a remarkable mean value increase of over 50 %, accompanied by a substantial 4.4-fold improvement in the standard deviation. For the entangled photon source, a fine mean value increase of 14 % and an approximate 2-fold improvement in the standard deviation are observed. Furthermore,the proposed technique successfully restores the fidelity of the polarization-entangled state, which has been compromised by atmospheric effects in the free-space channel, to a level close to the fidelity measured directly from the source. Our work will be helpful in designing spatial light-fiber coupling system not only for free-space quantum communications but also for high-speed laser communications.