Systems and methods for pump-based entanglement generation source enabling entanglement-assisted communication over turbulent free-space optical (FSO) channels

Abstract

Examples are disclosed of systems including a two-pumps based entanglement generation source enabling entanglement-assisted communication over turbulent free-space optical (FSO) channels. In some examples, it is proposed to employ two S- / L-band pumps, satisfying PPLN-waveguide quasi-phase-matching condition over entire C-band, providing needed flexibility in wavelength-selection for signal-idler photon pairs. By performing phase-conjugation over idler photons, the present disclosure demonstrates entanglement-assisted communication at both 1Gb / s and 10Gb / s in 1.5km FSO link operated in beyond strong turbulence regime.

Description

SYSTEMS AND METHODS FOR PUMP-BASED ENTANGLEMENT GENERATION SOURCE ENABLING ENTANGLEMENT-ASSISTED COMMUNICATION OVER TURBULENT FREE-SPACE OPTICAL (FSO) CHANNELSCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a PCT application that claims benefit to U.S. provisional application serial number 63 / 564,454 filed on March 12, 2024 which is incorporated by reference in its entirety.FIELD

[0002] The present disclosure generally relates to quantum technologies; and in particular to two-pumps based entanglement generation source enabling entanglement-assisted communication over both turbulent free-space optical (FSO) and fiber-optics channels.BACKGROUND

[0003] Entanglement represents a unique quantum information feature enabling the communications above the Shannon limit, quantum sensors with sensitivity approaching the Heisenberg limit, and secure communications with security guaranteed by the quantum information theorems. However, the quantum communication over free- space optical (FSO) channels is severely affected by absorption, diffraction, scattering, and atmospheric turbulence effects.

[0004] It is with these observations in mind, among others, that various aspects of the present disclosure were conceived and developed.SUMMARY

[0005] Aspects of the present disclosure include examples of a system (and methods) for high-speed entanglement assisted communication that operates at 10 Gb / , which performs a highly efficient, PPLN-waveguide-based, entanglementgeneration by making the simultaneous use of S- and L-band pumps. The two-pump- based entanglement generation source satisfies the quasi-phase-matching-condition over the entire C-band. To improve the system reliability, the subject system performs the phase-conjugation on idler photons in contrast to conventional ways of performing the phase-conjugation on signal photons. To study the performance of the proposed entanglement-assisted system, a 1.5 km long outdoor free-space optical (FSO) link was developed at the University of Arizona campus. Experimental results indicate that the proposed entanglement-assisted system significantly outperforms the classical counterpart at 10 Gb / s, operated in strong turbulence regime. It is also demonstrated that the traditional entanglement-assisted system performing the optical phase conjugation on signal photons at the receiver side is not operational at all in strong turbulence regime given that it is extremely difficult to perform the phase-conjugation on weak signal photons when the number of received photons is low. To improve the system performance the adaptive optics is performed on signal photons.

[0006] Examples include and single and multi-pump-based entanglement generation sources, as described herein.

[0007] The foregoing examples broadly outline various aspects, features, and technical advantages of examples according to the disclosure in order that the detailed description that follows may be better understood. It is further appreciated that the above operations described in the context of the illustrative example method, device, and computer-readable medium are not required and that one or more operations may be excluded and / or other additional operations discussed herein may be included. Additional features and advantages will be described hereinafter. The conception and specific examples illustrated and described herein may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure. Such equivalent constructions do not depart from the spirit and scope of the appended claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 is a photograph of a Google map image of the free-space optical (FSO) link established at the University of Arizona campus as described herein.

[0009] FIG. 2 is an illustration of a 10 Gb / s two-pump based entanglement assisted FSO communication system testbed.

[0010] FIG. 3 is a photograph of an optical table setup with adaptive optics at a lab.

[0011] FIG. 4 is an illustration of a S- / L-band pumps-based entanglement generation source suitable for implementation by cascaded SHG and DFG processes. The 1550 nm and 1590 nm bands are entangled. SHG: second-harmonic generation, DFG: difference frequency generation.

[0012] FIG. 5 is an illustration of a classical style laser communication step.

[0013] FIG. 6 is an illustration of an adaptive optics subsystem conceptual diagram with exemplary major components.

[0014] FIG. 7 is a photograph illustrating various components of adaptive optics subsystem as laid-out on the optical table (FSM: fast steering mirror).

[0015] FIG. 8A is a graph of the proposed EA communication system for different FSO channel realizations against the traditional EA with the PCR.

[0016] FIG. 8B is a graph of the proposed EA communication system against the corresponding classical counterpart in saturation regime.

[0017] FIG. 9 is a report showing uncoded BER plots comparison of two- pump entanglement assisted communication system vs. classical laser communications for different FSO channel transmissions.

[0018] FIG. 10A is a graph illustrating improvement in uncoded BER with application of adaptive optics for a 10Gb / s two-pump based entanglement assisted communication system for different FSO channel transmissions.

[0019] FIG. 10B is a histogram of received power with a Rayleigh distribution indicates the presence of strong atmospheric turbulence.

[0020] FIG. 11 is a graph illustrating adaptive optics (AO) improvements in uncoded BER comparison of two-pump entanglement assisted communication system.

[0021] FIG. 12 is a simplified block diagram of an example method associated with the inventive concept described herein.

[0022] Corresponding reference characters indicate corresponding elements among the view of the drawings. The headings used in the figures do not limit the scope of the claims.DETAILED DESCRIPTION

[0023] The present disclosure relates to inventive concepts in quantum technologies including one or multiple pumps-based entanglement generation source enabling entanglement-assisted communication over both fiber-optics and turbulent free-space optical (FSO) channels, with emphasis on FSO channels. In some examples, it is proposed to employ two S- / L-band pumps, satisfying PPLN-waveguide quasi-phase-matching condition over entire C-band, providing needed flexibility in wavelength-selection for signal-idler photon pairs. By performing phase-conjugation over idler photons, the present disclosure demonstrates entanglement-assisted communication at 1 Gb / s in 1.5km FSO link operated in beyond strong turbulence regime.

[0024] 1. Introduction

[0025] Quantum information processing (QIP) with entanglement at its core has opened new avenues for various applications including quantum communications, quantum computing, quantum metrology, and quantum networking. In particular, the entanglement is a unique QIP feature that enables communication that is above the Shannon limit, quantum metrology and sensing with sensitivity of sensors approaching the Heisenberg limit, and secure communication that is ensured by the QIP theorems (such as no-cloning theorem and theorem of indistinguishability of arbitrary quantum states) rather than unproven assumptions used in computational based cryptography. To incorporate the benefits of entanglement into these various applications that are mentioned above, it is imperative to distribute entanglement states over long distances, which has always been an overwhelming challenge because of the photon losses especially in case of free-space optical (FSO) communication. The information carrying photons beam suffer heavy losses that are due to beam wandering, scattering, atmospheric absorption, and turbulence effects, to name a few.

[0026] To overcome these problems, one inventive solution defining a new way to generate bright entangled photons pairs more efficiently can be achieved by using low-cost conventional telecom lasers, located in S-, C-, and L-bands as pump lasers rather than conventional approach that uses expensive and bulky 780 nm pump lasers.

[0027] The phase-conjugation step in an entanglement assisted communications, that is required before detection takes place, is typically performed on received signal photons. However, when the signal photons are transmitted over terrestrial FSO links, their propagation is affected by diffraction, absorption, scattering, and atmospheric turbulence effects. Under strong atmospheric turbulence effects, only a small number of weak signal photons reaches the receiver side. When the average number of received photons is «1 it is extremely difficult to perform the phaseconjugation and often render the whole process untenable.

[0028] To solve for this problem, inventive concepts herein include performing the phase-conjugation on bright idler photons instead of information carrying signal photons.

[0029] Other inventive concepts extend upon the authors’ previous work in entanglement assisted communication over terrestrial FSO links that are severely affected by the turbulence and scattering effects. In particular, one inventive concept represents an extension of the authors’ work, where it is proposed to use two pumps, located in S- and L-bands that satisfy the PPLN-waveguide quasi-phase-matching- condition to generate bright entangled-photons, wherein the signal photons wavelength can be located anywhere in the C-band. It was demonstrated the quantum advantage for 1 .5 km FSO link operated in beyond strong turbulence regime at 1 Gb / s. Further several improvements have been made in experimental setup, including the entanglement generation source and compressing telescope, to demonstrate that entanglement assisted communication at 10 Gb / s and beyond over terrestrial FSO links affected by strong turbulence regime is possible, which represents the record data rate for quantum communications. To improve the reliability of the terrestrial FSO link, adaptive optics can be used in addition to the LDPC coding.

[0030] In the following sections various steps, components, modules, and techniques involved in making the subject experiment possible will be described such as two-pump based entanglement-assisted communication system, adaptive optics, followed by experimental results and concluding remarks.

[0031] 2. Detailed description of an experimental setup, a proposed two-pumps based entanglement generation source, the optical transceiver, thephase-conjugation module performing the phase-conjugation on idler photons, and an adaptive optics subsystem

[0032] 2. 1 University of Arizona campus terrestrial free-s pace optical testbed

[0033] To explore the possibilities in quantum communication over an FSO link over turbulent FSO channels, an experimental testbed was developed at the University of Arizona campus, where the information carrying signal photons are beamed outward from a Quantum Communications Lab 549 in ECE Dept, building pointed toward the rooftop of the east wing of the Meinel building (belonging to the College of Optical Sciences), where a retroreflector is placed, which is 750 m away from the ECE Lab 549. This retroreflector returns the beam back to ECE Lab 549 window and the reflected beam is captured by the periscope and compressing telescope, and overall propagation distance is 1 .5 km as shown in FIG. 1 .

[0034] The FSO link based testbed at the University of Arizona campus with its major components is depicted in FIG. 2. The line-of-sight distance between the two points is approximately 750 m and a reflected link in total is 1.5 km long. A Thorlabs 20 times zoom beam expander was utilized that expands the beam to around 8” when measured over the retroreflector at the optical sciences rooftop. The retroreflector is a 127mm (5”) clear aperture, 30 arcsec, gold retroreflector from Edmund Optics and after careful alignment a 35% efficiency link was established on a daily basis. The received beam is lowered from the window to optical table height by a periscope made up of two 8”x8” highly polished square mirrors. On the optical table the incoming beam was first collimated using a compression telescope as shown in FIG. 3. This collimated beam is then passed through the adaptive optics setup before being coupled into an optical fiber.

[0035] 2.2 Entanglement generation with pumps in S- and L-bands

[0036] At the transmitter side, the 1ststep in the experiment is the generation of entangled photon pairs. The entanglement generation source is inspired by the cascaded second-harmonic generation (SHG) and difference frequency generation (DFG) concepts introduced in Liu et al. (T. Liu, I. B. Djordjevic, et al., “Broadband wavelength converters with flattop responses based on cascaded second-harmonic generation and difference frequency generation in Bessel-chirped gratings,” Optics Express 24 (10), 10946-10955 (2016). The key difference is that we employ two pumps located in S- and L-bands satisfying the quasi-phase matching condition for the type-0 periodically poled lithium niobate (PPLN) waveguide. In the subject inventive concept, the PPLN waveguide is properly designed so that the second harmonic generation (SHG) dominates in the first half of the PPLN waveguide, while the spontaneous parametric down conversion (SPDC) process, implemented through the difference frequency generation (DFG), dominates in the second half as shown in FIG. 4. The HC Photonics has fabricated the proposed PPLN waveguides, according to example design specifications. As illustrated in FIG. 1 , we first beam-combine the two pump laser outputs running at 1527.7 nm and 1612.4 nm wavelengths, the S and L bands respectively, and then we use the 2x2 directional coupler. One of the outputs of the directional coupler is amplified using an EDFA which is then used as the input to the type-0 PPLN waveguide, which is serving the role of the entanglement generation source. The signal photons at 1550 nm and idler photons at 1590 nm get separated by the compact coarse WDM (CCWDM) demultiplexer.

[0037] Our proposed method is a stark contrast to the traditional methods where an expensive and bulky 780 nm pump laser is used to generate entangled pairs in C and L band, wherein the nonlinear conversion efficiency is much lower given large wavelength separation of pump and signal / idler photons. As we have shown in Djordjevic et al. (I. B. Djordjevic, V. Nafria, “Entanglement based detection, networking, sensing, and radars,” IEEE Photonics Journal 16 (1 ), 7300610 (Feb. 2024).) incorporated herein by reference in its entirety, we can generate several orders of magnitude higher coincidence counts by using the proposed method compared to traditional 780 nm pump-based entangled sources.

[0038] 2.3 Modulation, Transmission, and Transceiver Description

[0039] We modulate the bright 1550nm signal photons using a phase modulator with the help of an LDPC coded BPSK sequence recorded on the Keysight arbitrary waveform generator (AWG) at 120 GSa / s. The LDPC code used in experiments is girth-10, column-weight-3 quasi-cyclic (3992, 2497) code. The FEC frame starts with a header sequence of 616 bits that is also provided to receiver anddecoding subsystem to identify the start of the LDPC frame. The phase modulated signal photons are transmitted into the free space toward Meinel building via a Thorlabs 20x beam expender. After completing the round trip between the two buildings, the returning beam from the retroreflector is collected by the periscope to bring the beam to the optical bench. The beam size is reduced by the 10x compressing telescope designed and implemented in the lab. The compressed beam is passed through the adaptive optics subsystem before being coupled into an optical fiber. This received beam is then fed to one input port of the optical hybrid (not shown) before the balanced homodyne detector.

[0040] In parallel, the 2nd port of the 2x2 directional coupler (of the amplified two-pump signal) is combined with the 1590 nm idler photons and fed to the bottom type-0 PPLN waveguide serving the role of the phase-conjugation module. The phase-conjugated photons at 1550 nm are selected by the bottom CCWDM demultiplexer. The phase-conjugated photons are then passed thought the SMF serving the role of quantum memory in order to introduce a delay (optical delay line, ODL) that matches the arrival time of signal photons at balanced detector. The phase-conjugated idler photons after the optical delay line are fed to the 2ndinput port of optical hybrid, followed by the homodyne balanced detector.

[0041] The balanced detector output signal is used as the input to the Tektronix real-time scope, which is sampled at 100 GSa / s and transferred to the personal computer (PC) for offline DSP and LDPC decoding. The beginning of the FEC frame is identified by the cross-correlation method. After that the waveform is down sampled, processed, and then the LDPC decoding is performed.

[0042] To make a fair comparison we also build a classical laser-based communication system, as shown in FIG. 5 where a 1550 nm laser is modulated with same FEC frames in identical fashion, then transmitted over FSO link, and finally coupled into a fiber in the same manner. The local oscillator laser signal is mixed with received optical signal on the same optical hybrid followed by the homodyne balanced detection with the same detector.

[0043] 2.4 Adaptive optics and turbulence effects mitigation

[0044] So far, we explained that our communication system benefits greatly from quantum advantage provided by entanglement and on top of that the link quality is greatly ensured by a strong BPSK FEC LDPC methods that correct all errors occurring in FSO communication process. However, we have implemented adaptive optics (AO) at hardware level to further improve the link performance in terms of the uncoded BER. In FSO communication experiments atmospheric turbulence is the primary cause of the performance degradation, in particular due to turbulence induced beam wandering and scintillation. Our adaptive optics system tackles this very problem in a unique fashion. Adaptive optics is a highly complex system that went through several decades of development and is primarily designed and developed for astronomy applications. The starlight coming from celestial body from millions of light years has a flat wavefront but for the final few km before reaching earth’s surface it suffers wavefront distortion due to atmospheric turbulence. The atmospheric turbulence effects mainly arise from due to variability in temperature distribution and pressure that directly translate into spatial variation in air density and in turn the spatial and temporal variability of refractive indices. This AO subsystem probes the atmosphere for wavefront distortions via starlight, if it’s bright enough or a guide laser that get reflected by the sodium layer that is 80 to 100 km up in atmosphere. Now this incoming light is first made to fall on a deformable mirror (DM) as collimated beam and after that it is sampled by beam splitter to be given to a wavefront sensor (WFS). The DM comes in many types, the one we use is deformed by 144 actuators. The WFS break the wavefront into smaller sub-apertures using lenslet arrays that falls onto WFS sensor (usually CMOS or CCD arrays) forming a spot-field. This spot-field is then used to determine the deviations from what should be a flat wavefront. The wavefront information of the incoming light is then processed with the help of computations by breaking it down into a combination of various Zernike polynomials, that are ultimately used to compute the electrical signals that are needed to be given to the individual actuators of the DM. Now this solution is then electrically implemented on a deformable mirror which, as name suggests, deforms the mirror to ‘fix’ for the wavefront distortions. This process happens as a feedback loop between WFS and deformable mirror several times in a second.Further details of adaptive optics are beyond the scope of our paper. In FIG. 6 we can see the conceptual diagram of our adaptive optics setup.

[0045] The subject inventive FSO communication system essentially operates few 10s of feet above the ground at almost horizontal to the ground. The beam travels over various surfaces and structures such as building’s rooftop, asphalt road, reflective shed, and AC vents, to mention few. All these surfaces radiate heat at different rate, at different time of the day and under different weather conditions. This radiated heat, heats up the air right above these structures variably all over the optical path that our information carrying beam traverses. Now interestingly similar to astronomy problem the atmospheric turbulence is from these temperature variations that leads to air density differences that ultimately leads to spatial and temporal variabilities in refractive indices throughout the optical path. In short, the atmospheric turbulence is responsible for Azimuthal phase distortion in our FSO channel. Our adaptive optics system, as shown in FIG. 7, addresses this very problem. We have developed our adaptive optics system with a Shack-Hartmann wavefront sensor (WFS) from Applied Optical Systems (AOS) and a 144-actuator deformable mirror from Boston Micromachine (BMC). After the beam returns from the FSO round trip we lower the beam with the help of a periscope from window to the optical table level. We collimate the incoming light into a narrow beam with the help of lenses acting as a compression telescope. This collimated beam falls onto a fast-steering mirror (FSM) and then sampled (8%) for a quad detector, both are responsible for beam steering. Afterwards this collimated beam falls onto the BMC deformable mirror and after that it is further sampled, using a 92:8 beam splitter (BS), for the WFS before finally coupled into a large core (62.5 pm) multi-mode optical fiber. Thanks to the employment of the constant amplitude modulation format we don’t need the guide laser, but the portion of the received beam is used to run the AO subsystem.

[0046] For computations, lab-made software solutions were implemented that were developed using SDK and drivers from National Instrument, Boston Micromachines, and AOS. At the time of this experiment, adaptive optics computation / s were running on a PC built on Intel i9 9900k CPU, running at 5GHz, enabling the subject AO subsystem to make over 350 AO corrections per second under medium tostrong turbulence. Along with this, we constantly monitor the atmospheric turbulence in terms of received power plotted as histogram.

[0047] 3. Experimental Results

[0048] Various experiments were performed to validate the performance of the subject two-pump entanglement assisted communication method and tested against a phase conjugated receiver (PCR) and also against classical lightwave communications. We tested performance at different data rates ranging from 1 Gb / s to 10 Gb / s. FIG. 8A shows 1 Gb / s results when the test was conducted under extreme turbulence condition with launch power as low as 4.5 mW against a PCR. FIG. 8B shows two-pump EA communication results against the classical laser communication along with plot for improvements made from adaptive optics at 5.4 mW launch power, again while operating in extreme turbulence as indicated by near-exponential histogram of received power.

[0049] The results from the experiment at 10 Gb / s are summarized in FIG. 9, for different FSO channel transmissions. On the left half of the graph the blue curve shows BER results of the two-pump-based entanglement assisted system operated in strong turbulence regime significantly outperforming the classical laser communication system operated in medium turbulence regime, which is shown by the red curve. On the right-half of FIG. 9, the blue histogram of the received power for the entanglement assisted cases shows a Rayleigh distribution, indicating presence of strong turbulence. The red histogram is tailored towards higher mean values with distribution between lognormal and Rician distribution thus indicating the medium turbulence regime. The classical communication link was not operational in strong turbulence regime at all. This represents a clear quantum advantage. In both cases LDPC decoder was able to correct all errors introduced by the FSO channel.

[0050] In FIGS. 10A-10B, the experimental results are summarized related to the BER improvements of entanglement assisted communication coming from the adaptive optics, for the same launch power as in FIG. 8. In FIG. 10A, the BER results of applying adaptive optics are shown with a blue curve against the BER plot, shown in red curve, for cases when no adaptive optics is applied. As it is evident from these results, adaptive optics proved to be an effective method for wavefront correction to make BERimprovement to mitigate the information loss due to atmospheric turbulence. The histogram in FIG. 10B has a Rayleigh distribution, which indicates that the experiment was performed in a strong atmospheric turbulent regime.

[0051] Similar experimental results are shown in FIG. 11 , summarized. Here, again we see that the BER improvements made by adaptive optics-based azimuthal phase compensation method in a two-pump based entanglement assisted communication system, operating at 10Gb / s, follow similar trend as in FIG. 10.

[0052] 4. Example Conclusions

[0053] In this disclosure, improvements over the authors’ previous work on two-pump-based entanglement assisted communication system are described by various design changes that helped to achieve a 10-fold improvement in data rate of up to 10 Gb / s. The major changes include the improvements in entanglement photon generation efficiency by utilizing more efficient pump wavelengths, improvement in beam launching method, we improved overall FSO link efficiency to 35% from 15% previously, and improved compressing telescope to better collimate the beam that help us not just in improving coupling efficiency but also improving the adaptive optics performance. The improved two-pump-based entanglement assisted scheme operated in strong turbulence regime, significantly outperformed corresponding classical counterpart operated in medium turbulence thus representing the clear quantum advantage.

[0054] The improved two-pump based quantum entanglement assisted communication system is capable of communicating over much longer distance and with much higher data rate yet reigning quantum advantage over the classical laser based FSO communication system, especially in power limited FSO links and also in FSO channels that are severely affected by the losses from wavefront and azimuthal phase distortion due to atmospheric turbulence effects.

[0055] To summarize, from our experimental results in terms of the uncoded BERs we were able demonstrate the clear quantum advantage of the proposed entanglement assisted communication system compared to the classical lightwave communication system, especially in strong turbulence regime when theclassical communication system was not operational. The AO provided better tolerance to turbulence effects and was operational even in beyond the strong turbulence regime.

[0056] It should be understood from the foregoing that, while particular embodiments have been illustrated and described, various modifications can be made thereto without departing from the spirit and scope of the invention as will be apparent to those skilled in the art. Such changes and modifications are within the scope and teachings of this invention as defined in the claims appended hereto.

Claims

CLAIMSWhat is claimed is:

1. A method, comprising: generating a first laser signal by a first pump laser; generating a second laser signal by a second pump laser; combining the first laser and the second laser signals and using them as the input of the periodically poled lithium niobate (PPLN) waveguide that generates entangled signal-idler photon pairs by a cascaded second- harmonic generation (SHG) and difference frequency generation (DFG) processes-based entanglement generation source, generating an entangled signal-idler pair at the output of the PPLN waveguide device in operative association with a wavelength division multiplexing (WDM) demultiplexer; and applying a wavelength conversion operation and an optical phase-conjugation (OPC) operation to the idler photons resulting in phase-conjugated idler photons having a wavelength that matches a wavelength of the signal photons.

2. The method of claim 1 , the PPLN waveguide device based entanglement generation source is based on a cascaded second-harmonic generation and difference frequency generation processes, wherein the SHG process dominates in the first half of the waveguide while the DFG process dominates in the second half of the PPLN waveguide.

3. The method of claim 1 , further comprising: selecting a first wavelength of the first laser and a second wavelength of the second laser such that the resultant signal photons and idler photons’ wavelengths are located within a targeted frequency band, including C- band, L-band, 2.1 pm, 3.9p, and 9-10 pm telecommunications windows.

4. The method of claim 1 , further comprising: modulating the signal photons by a phase modulator; constructing an expanded beam that carries the signal photons; transmitting the expanded beam to a destination over a Free-Space Optical (FSO) channel.

5. The method of claim 1 , further comprising: collecting a received beam over the FSO channel that includes the signal photons; mixing signal photons of the received beam with the phase-conjugated idler photons stored within an optical delay line by an optical hybrid; and applying a low-cost homodyne-balanced detection operation to the signal photons mixed with the phase-conjugated idler photons.

6. The method of claim 1 , other optical transmission media are used such as fiberoptics medium, underwater medium, satellite link, and deep-space link.

7. The method of claim 1 , further comprising applying adaptive optics on the signal photons to correct for the wavefront distortions and to improve the tolerance to turbulence effects.

8. The method of claim 7, wherein the adaptive optics includes a wavefront sensor, a deformable mirror and a control unit.

9. The method of claim 8, further comprising measuring, by the wavefront sensor, the distortion on the received wavefront of signal photons.

10. The method of claim 8, wherein the adaptive optics includes a control unit (FPGA-, ASIC-, GPU-, or CPU-based) configured to run the optimization procedure to determine the optimum strokes to be applied on actuators of the deformable mirror.11 . The method of claim 8, wherein the deformable mirror is deformed by actuators configured to undo action of a turbulent channel.

12. The method of claim 1 , further comprising utilizing constant modulation formats, such as phase-shift keying, so that the portion of the received signal photons is tapped out to operate the adaptive optics thus reducing the system cost as the beacon laser signal is not used.

13. The method of claim 1 , further comprising implementing a fast steering mirror and quad-detector to compensate for turbulence induced beam wandering effects.1 . The method of claim 1 , wherein the signaling rates transmitted over signal photons are at least 10 Giga symbols / s.

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