Quantum optical communication system. Bit modulation coding: for state “0,” the coincident (A,B) polarizing state is H1H2, with the liquid-crystal phase retarder (LCPR) set at zero; for state “1,” the coincident (A,C) polarizing state is V1V2, with LCPR set at λ/2. APD, avalanche photodiode.
Long description of figure.
A quantum communications system for the demonstration of optical communication at ultra-low-power levels was developed this year at the NASA Glenn Research Center. This system is meant as a precursor for a micro-optical transmitter that will work with microwatts or less of power. The experimental implementation of this architecture is shown schematically in the figure. Here, the beam from a 351-nm (<150-mW) single-frequency argon-ion laser passes through two thin (0.69-mm), optically contacted, orthogonally aligned Type-I beta barium borate crystals to generate two beams of polarized entangled photons through spontaneous parametric down conversion. One of the beams then passes through a digital-logic-controllable liquid-crystal phase modulator set to act as a bistate half-wave plate (0 or λ/2) that encodes information onto the beam via polarization rotation, either horizontal H or vertical V.
For the receiver, photon detectors A and B are set to detect only H polarization photons, whereas detector C is set to detect only V polarization photons through the use off a polarizing filter and a polarizing beam splitter. Binary data modulate a variable-phase retarder, keeping the H and V photons intact under state “1” and changing the H photons into V and the V photons into H under state “0.” As a result, photon detectors A and B trigger H photon coincidences in coincidence detector 1 when state “1” is transmitted, and detectors A and C trigger V photon coincidences in coincidence detector 2 when state “0” is transmitted. Decisions are made by comparing the number of coincidences per data bit from the two coincidence detectors. If the count from coincidence detector 1 is greater than that for coincidence detector 2, the receiver records a “1”; if not, the receiver records a “0.” Accidental coincidences due to ambient light and unentangled photons from the laser source have been found to be much smaller than signal coincidences, affording reliable communications under very low signal-to-noise ratios.
Through the use of quantum-entangled photons, we have shown that reliable communication is possible at remarkably low received-energy levels (on the order of 1 picojoule per bit in the presence of an overwhelming background light, resulting in exceptionally low optical signal-to-noise ratios. With this system, we have demonstrated communication with the coincident-photon count to single-photon count ratio at -26.6 dB.
Glenn contacts: John D. Lekki, 216-433-5650, John.D.Lekki@nasa.gov; Dr. Quang-Viet Nguyen, 216-433-3574, QuangViet.Nguyen-1@nasa.gov; Binh V. Nguyen, 216-433-8338, Binh.V.Nguyen@nasa.gov; and Tom P. Bizon, 216-433-8121, Thomas.P.Bizon@nasa.govLast updated: October 6, 2006
Responsible NASA Official:
Gynelle.C.Steele@nasa.gov
216-433-8258
Point of contact for NASA Glenn's Research & Technology reports:
Cynthia.L.Dreibelbis@nasa.gov
216-433-2912
SGT, Inc.
Web page curator:
Nancy.L.Obryan@nasa.gov
216-433-5793
Wyle Information Systems, LLC
