Boosting Quantum Communication Efficiency

The Deterministic MPDC Source essentially runs multiple probabilistic SPDC photon sources in parallel (multiplexing) to create a deterministic output. The system pulses all photon sources simultaneously to probabilistically generate entangled pairs. When a desired four-photon emission occurs at one of the banked sources, two of these photons are directed into a polarization analyzer equipped with photon detectors. If the heralding signal successfully confirms multiple photons, the analyzer verifies that the correct two-photon N00N state has been created on the heralded pair state pathway. If fewer than two photons are detected, the system discards the emissions, effectively filtering out vacuum noise and unwanted multipair states. This automated filtering prevents noise from falsely triggering adjacent nodes in a communication chain. The successful heralding signal then triggers a controller connected to an optical switch yard. When a correct signal is detected, the switch yard turns "on" the corresponding source's pathway, routing the heralded pair into a single output mode for network injection. Delay lines are seamlessly incorporated into each source's pathway to allow the optical switches enough time to act. Ultimately, this architecture achieves a highly deterministic operating regime with an estimated 86% probability that at least one source generates a valid herald event during each pulse. The Deterministic MPDC source is available for patent licensing.

In the prior art, the systems that produced photon pairs took up a great deal of space on a laboratory table, weighed several hundred pounds, consumed tens of kilowatts of electrical power, and required cooling water. These limitations greatly restricted the utility of quantum communications systems, which rely on these photon pairs. To address this issue, Glenn's innovators developed a novel system that uses a laser source, a pair of nonlinear crystals in optical contact with each other, and a fiber coupling point configured to receive a pair of single mode fibers. Pairs of polarization-entangled photons are produced through spontaneous parametric down conversion of the laser beam and provided to the fiber coupling point. The optical signal is coded at the transmitter by modulating the inter-beam delay time between pairs of entangled photons. The inter-beam delay will determine whether the photon pairs are absorbed by a fluorescer in the receiver. When absorbed, the photon pairs cause a fluorescent optical emission that a photon detector identifies. One advantage of this system is that it eliminates the need for a coincidence counter to realize the entanglement-based secure optical communications, because the absorber acts as a coincidence counter for entangled photon pairs. In addition, this modulation spectroscopy technique is ultra-secure since the delay times are very short (femtoseconds) and unresolvable by conventional photon detectors. Finally, the system uses solid-state, monolithic construction that allows for cost-effective batch-manufacturing techniques. This technology represents a significant breakthrough in the fields of communications, optics, cryptography, and surveillance.

The core of the technology is the SAW division de-multiplexing method (LEW-19920-1). It uses a commercially available double-clad fiber optic cable with a 9um core and a 105um first cladding. By optimizing the wavelengths of the QKD photon and data transmission, a single focusing lens can create a diffraction pattern that directs the QKD photons to the 9um core and the data signal to the 105um secondary core. Key components of the system include: • SOA Driver With Wideband Current Control (LEW-19913-1): This device allows a semiconductor optical amplifier (SOA) or laser to be driven with an arbitrary current at a rate of over 100 MHz. This enables the rapid generation of sub-nanosecond laser pulses with one of four polarization states, which is necessary for QKD. • Random Bit Generator with Linear Feedback Shift Register LFSR Scrambler (LEW-20058-1): This device produces random bits by combining the output of a noise source with a pseudorandom bitstream from the LFSR. This allows a random basis set to be generated on demand for a polarization modulator. • Variable-length quantum key conversion (LEW-20224-1): Since QKD operations produce keys of varying lengths, a strategy was developed to "digest" these raw keys using a hash function, such as SHAKE256. This process generates a fixed-length output that is useful for symmetric encryption schemes like AES256. • The system also incorporates a Discretization Algorithm for Numerical Wave Optics Simulations (LEW-20119-1), which can accurately model the effects of atmospheric turbulence on the propagating optical beam.

This system takes inputs from multiple RF sources. These inputs are then processed to provide inputs to optical phase modulators. The outputs of the phase modulators are optical signals modulated by the incoming RF signal tone. Multiple signals can be optically combined to create interference fringes based on the phase differences of the incoming RF signals. Using an optical source that is single mode and linearly polarized (or any source with subsequent mode and polarization control) allows coherent interference to render the phase difference of each of the RF signals. This provides real time phase interferometric data regarding all RF sources input to the system over the range of specified frequencies. The system then interrogates the resulting interference fringe patterns to decode the phase shift vs. the reference phase source. Over time this information will establish the phase in the RF domain of each source with respect to the designated reference.

NASA Goddard Space Flight Center has developed a configurable phase mirror system that can address likely obstacles in space optical communications. Through using miniature adjustable mirrors and programmed phase delays to diffract a single communication beam, numerous diffracted beams can be sent to other satellites in various directions for communication and tracking. The initial laser beams wave profile can be dynamically regulated through a fast Fourier transform (FFT) so that when it reaches its desired destination, it forms an intended illuminated spot at the target satellite. Since all the diffracted beams share the same phase mirror, the antenna gain needed to broadcast these beams does not require a multiplied aperture.