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Journal of the Optical Society of America B

Proposal for almost noise-free optical communication under conditions of high background

L. Mandeļ

Published Mar 1, 1984• 25 citations
Publisher

Abstract

By making use of the fact that photons are created in pairs in the process of parametric downconversion and by detecting them in coincidence, we propose a scheme for communication by light that should make it possible to achieve high discrimination against background. Weak light beams that are greatly overshadowed by background light should therefore be usable for an optical channel.

DOI
10.1364/josab.1.000108

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By making use of the fact that photons are created in pairs in the process of parametric downconversion and by detecting them in coincidence, we propose a scheme for communication by light that should make it possible to achieve high discrimination against background. Weak light beams that are greatly overshadowed by background light should therefore be usable for an optical channel.
Photons emitted in the process of spontaneous parametric downconversion of pump light in a nonlinear crystal are made the basis of a new optical communication scheme, which depends on the fact that two downconverted photons are always produced simultaneously. If the pump light is modulated and the downconverted photons are transmitted in two separate beams which fall on two separate detectors, registering their simultaneous arrival allows great discrimination against background and noise to be achieved. Communication is possible with relatively few photons and even when the signal is ordinarily lost in the noise, so that the communication channel can be made virtually interception proof. The feasibility of the scheme is demonstrated by a pilot experiment.
In the process of parametric downconversion in a nonlinear crystal, incident photons split up into two lower frequency photons that emerge in different directions. Because the downconverted photons appear simultaneously, an optical communication channel based on transmission and simultaneous detection of these photon pairs should be able to achieve great discrimination against background. Indeed, it should be able to operate even in conditions in which the light signal would otherwise be lost in the noise. Such a communication channel requires relatively few photons per second and could be almost interception proof. The feasibility of the idea has been explored experimentally, and the results involving photoelectric coincidence counting measurements are presented.
A proposed method of suppressing the effect of background noise in an optical communication system would exploit the transmission and reception of correlated photons at the receiver. The method would not afford any advantage in a system in which performance is limited by shot noise. However, if the performance of the system is limited by background noise (e.g., sunlight in the case of a free-space optical communication system or incoherently scattered in-band photons in the case of a fiber-optic communication system), then the proposed method could offer an advantage: the proposed method would make it possible to achieve a signal-to-noise ratio (S/N) significantly greater than that of an otherwise equivalent background- noise-limited optical communication system based on the classical transmission and reception of uncorrelated photons. The figure schematically depicts a classical optical-communication system and a system according to the proposed method. In the classical system, a modulated laser beam is transmitted along an optical path to a receiver, the optics of which include a narrow-band-pass filter that suppresses some of the background noise. A photodetector in the receiver detects the laser-beam and background photons, most or all of which are uncorrelated. In the proposed system, correlated photons would be generated at the transmitter by making a modulated laser beam pass through a nonlinear parametric down-conversion crystal. The sum of frequencies of the correlated photons in each pair would equal the frequency of the incident photon from which they were generated. As in the classical system, the correlated photons would travel along an optical path to a receiver, where they would be band-pass filtered and detected. Unlike in the classical system, the photodetector in the receiver in this system would be one that intrinsically favors the detection of pairs of correlated photons over the detection of uncorrelated photons. Even though there would be no way of knowing the precise location and time of creation of a given pair of correlated signal photons in the nonlinear down-conversion crystal, the fact that the photons are necessarily created at the same time and place makes it possible to utilize conventional geometrical imaging optics to reunite the photons in coincidence in the receiving photodetector. Because most or all of the signal photons would be correlated while most or all of the noise photons would be uncorrelated, the S/N would be correspondingly enhanced in the photodetector output. An additional advantage to be gained by use of a correlated-photon detector is that it could be capable of recovering the signal even in the presence of background light so bright that a classical uncorrelated-photon detector would be saturated. A blocked-impurity-band (BIB) photodetector that preferentially detects pairs of correlated photons over uncorrelated ones and that operates at a quantum efficiency of 88 percent is commercially available. This detector must be cooled to the temperature of liquid helium to obtain the desired low-noise performance. It is planned to use this detector in a proof-of-principle demonstration. In addition, it may be possible to develop GaN-based photodetectors that could offer the desired low-noise performance at room temperature.
A limitation of free-space optical communications is the ease with which the information can be intercepted. Overcoming this limitation is possible by hiding the information within the background optical noise that is present in all real-world situations. We demonstrate the limitations of our experimental system for transferring images over free-space using a photon-pair source emitting two correlated beams. The system uses spontaneous parametric down-conversion to create photon-pairs, where one photon contains the spatial information and the other the heralding information.
A limitation of free-space optical communications is the ease with which the information can be intercepted. This limitation can be overcome by hiding the information within background optical noise. We demonstrate the transfer of images over free-space using a photon-pair source emitting two correlated beams. One of these beams contains image information, to which noise is added, and the other correlated beam is used as a heralding trigger so that the intended recipient can differentiate this image signal from the background noise. The system uses spontaneous parametric down-conversion to create photon-pairs with a wide spectral bandwidth and a gated intensified camera to extract the image from the background noise. The high-dimensionality of the image space means that the information content can be many bits per detected photon, whereas the heralding photon can be restricted to a single spatial-mode within a secure fiber which itself could be protected against interception by traditional low-dimensionality quantum key protocols.