Spatial filtering is an important technique for reducing sky background noise in a satellite quantum key distribution downlink receiver. Atmospheric turbulence limits the extent to which spatial filtering can reduce sky noise without introducing signal losses. Using atmospheric propagation and compensation simulations, the potential benefit of adaptive optics (AO) to secure key generation (SKG) is quantified. Simulations are performed assuming optical propagation from a low-Earth-orbit satellite to a terrestrial receiver that includes AO. Higher-order AO correction is modeled assuming a Shack–Hartmann wavefront sensor and a continuous-face-sheet deformable mirror. The effects of atmospheric turbulence, tracking, and higher-order AO on the photon capture efficiency are simulated using statistical representations of turbulence and a time-domain wave-optics hardware emulator. SKG rates are calculated for a decoy-state protocol as a function of the receiver field of view for various strengths of turbulence, sky radiances, and pointing angles. The results show that at fields of view smaller than those discussed by others, AO technologies can enhance SKG rates in daylight and enable SKG where it would otherwise be prohibited as a consequence of background optical noise and signal loss due to propagation and turbulence effects.
The rate of secure key generation (SKG) in quantum key distribution (QKD) is adversely affected by optical noise and
loss in the quantum channel. In a free-space atmospheric channel, the scattering of sunlight into the channel can lead to
quantum bit error ratios (QBERs) sufficiently large to preclude SKG. Furthermore, atmospheric turbulence limits the
degree to which spatial filtering can reduce sky noise without introducing signal losses. A system simulation quantifies
the potential benefit of tracking and higher-order adaptive optics (AO) technologies to SKG rates in a daytime satellite
engagement scenario. The simulations are performed assuming propagation from a low-Earth orbit (LEO) satellite to a
terrestrial receiver that includes an AO system comprised of a Shack-Hartmann wave-front sensor (SHWFS) and a
continuous-face-sheet deformable mirror (DM). The effects of atmospheric turbulence, tracking, and higher-order AO
on the photon capture efficiency are simulated using statistical representations of turbulence and a time-domain waveoptics
hardware emulator. Secure key generation rates are then calculated for the decoy state QKD protocol as a
function of the receiver field of view (FOV) for various pointing angles. The results show that at FOVs smaller than
previously considered, AO technologies can enhance SKG rates in daylight and even enable SKG where it would
otherwise be prohibited as a consequence of either background optical noise or signal loss due to turbulence effects.
Transmission volume holograms are evaluated as quantum projectors operating on spatial modes of the photon in
mutually unbiased bases (MUBs). With applications to free-space quantum key distribution (QKD) in mind, state
spaces based on rectilinear and azimuthal phase modes (i.e. photon linear and orbital angular momenta) are considered.
Rectilinear phase modulation is shown to result in both greater mode densities for a receiving aperture in the far field
and better efficiency and cross-talk characteristics when volume holograms are used as de-multiplexing elements. Twoand
four-dimensional state spaces are defined via rectilinear phase modes and the complex optical fields of the MUBs
are calculated and generated with a spatial light modulator in order to record and subsequently illuminate transmission
volume holograms. Using holograms prepared in lithium niobate, diffraction efficiencies are measured for the 36
permutations associated with projecting the six MUB states of a two-dimensional state space onto the same six MUB
states. Quantum measurements associated with cascaded projection operations in a four-dimensional state space are
performed using photo-thermo-refractive glass holograms. Experimental results show approximate agreement with the
inner-product relationships that describe quantum projection probabilities.
Scatter from optical thin film coatings is a significant problem for high power laser optics. Theoretical aspects of the problem have ben well explored for scattering due to surface roughness. Over the past twenty years the surface roughness of optics has been significantly reduced. Improvements in optical surface fabrication and film deposition techniques have progressed to the point that even for complex coatings, surfaces of less than 1nm rms roughness are routinely achievable. As the surface roughness of optics decreases, bulk scatter, rather than topographic scatter, may be the major scatter source in these smooth surfaces. Atomic force microscopy can profile the surface of a coating with atomic resolution. By comparing the power spectral density (PSD) derived from the surface profile with the PSD derived from angle resolved scatter measurements, some conclusions can be reached on this question. Data from analysis of dual wavelength high reflectors deposited by reactive DC magnetron sputtered Nb2O3/SiO2 and ZrO2/SiO2 structures will be presented, allowing analysis of results for these film materials.
Angle resolved scattering from bulk fused silica substrates has been measured at .442 micrometers and .633 micrometers . The angle dependent scatter pattern and wavelength dependence are compared to Rayleigh scatter theory. The measured bulk scatter data is applied to the measurement of fused silica substrate total scatter and the resulting measurement limitations imposed by bulk scatter.
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