High-Fidelity Deterministic Entangled Photon Source

The technology consists of an array of quantum photon sources connected via a sophisticated switching network. This system is designed to produce single pairs of entangled photons at a high rate while actively suppressing the generation of multiple pairs. The key innovation lies in its ability to detect and eliminate instances where two or more entangled photon pairs are generated, effectively reducing noise in the quantum system. The technology operates by providing a heralding pulse that notifies the external system of successful entangled photon generation. When multiple pairs are detected, they are prevented from entering the rest of the system, thereby maintaining the integrity of the quantum information. By combining multiple single-photon sources through its switching network, the technology not only reduces noise but also increases the overall single photon pair generation rate. This dual approach of noise reduction and increased generation efficiency improves qubit transmission rates, potentially by a factor of 10 to 100 over current methods. While still in the early stages of development, the source array represents a significant advancement in quantum communication systems. It addresses the critical need for high-fidelity entangled photon sources, which are essential for various quantum applications, including entangling sensor networks, quantum computer networks, and quantum key distribution for secure communications. As quantum technologies continue to evolve, this source array technology positions itself as a crucial component in the development of large-scale, efficient quantum networks, offering a solution to one of the fundamental challenges in quantum information transmission.

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.

A unique challenge in the development of a deep space optical SDR transmitter is the optimization of the ER. For a Mars to Earth optical link, an ER of greater than 33 dB may be necessary. A high ER, however, can be difficult to achieve at the low Pulse Position Modulation (PPM) orders and narrow slot widths required for high data rates. The Cascaded Offset Optical Modulator architecture addresses this difficulty by reducing the width of the PPM pulse within the optical modulation subsystem, which relieves the SDR of the high signal quality requirements imposed by the use of an MZM. With the addition of a second MZM and a variable time delay, all of the non-idealities in the electrical signal can be compensated by slightly offsetting the modulation of the laser. The pulse output is only at maximum intensity during the overlap of the two MZMs. The width of the output pulse is effectively reduced by the offset between MZMs. Measurement and analysis of the system displayed, for a 1 nanosecond pulse width, extinction ratios of of 32.5 dB, 39.1 dB, 41.6 dB, 43.3 dB, 45.8 dB, and 48.2 dB for PPM orders of 4, 16, 32, 64, 128, and 256, respectively. This approach is not limited to deep space optical communications, but can be applied to any optical transmission system that requires high fidelity binary pulses without a complex component. The system could be used as a drop-in upgrade to many existing optical transmitters, not only in free space, but also in fiber. The system could also be implemented in different ways. With an increase in ER, the engineer has the choice of using the excess ER for channel capacity, or simplifying other parts of the system. The extra ER could be traded for reduced laser power, elimination of optical amplifiers, or decreased system complexity and efficiency.

NASA Glenn's researchers have developed a means of transporting multiple radio frequency carriers through a common optical beam. In contrast to RF infrastructure systems alone, this type of hybrid RF/optical system can provide a very high data-capacity signal communication and significantly reduce power, volume, and complexity. Based on an optical wavelength division multiplexing (WDM) technique, in which optical wavelengths are generated by a tunable diode laser (TDL), the system enables multiple microwave bands to be combined and transmitted all in one unit. The WDM technique uses a different optical wavelength to carry each separate and independent high-frequency microwave band (e.g., L, C, X, Ku, Ka, Q, or higher bands). Since each RF carrier operates at a different optical wavelength, the tunable diode laser can, with the use of an electronic tunable laser controller unit, adjust the spacing wavelength and thereby minimize any crosstalk effect. Glenn's novel design features a tunable laser, configured to generate multiple optical wavelengths, along with an optical transmitter. The optical transmitter modulates each of the optical wavelengths with a corresponding RF band and then encodes each of the modulated optical wavelengths onto a single laser beam. In this way, the system can transmit multiple radio frequency bands using a single laser beam. Glenn's groundbreaking concept can greatly improve the system flexibility and scalability - not to mention the cost of - both ground and space communications.