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Journal of Optics A Pure and Applied Optics

Secure interferometric communications in free space: enhanced sensitivity for propagation in the metre range

F.J. Duarte

Published Dec 11, 2004• 9 citations
Publisher

Abstract

N-slit interferometry has recently been applied to generate a series of interferometric characters that propagate, in free space, according to well established interference and optical propagation principles. Any attempt to intercept the interferometric character leads to detectable spatial distortions. Originally, experiments were reported at distances in the centimetre range. Here, successful communications, and enhanced detection sensitivity of optical interception, at distances of over 7 m is reported. Further, we discuss conceptual countermeasures and their inapplicability to intercept, undetected, the interferometric characters.

DOI
10.1088/1464-4258/7/1/011

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N-slit interferometry has recently been applied to generate a series of interferometric characters that propagate, in free space, according to well established interference and optical propagation principles. Any attempt to intercept the interferometric character leads to detectable spatial distortions. Originally, experiments were reported at distances in the centimetre range. Here, successful communications, and enhanced detection sensitivity of optical interception, at distances of over 7 m is reported. Further, we discuss conceptual countermeasures and their inapplicability to intercept, undetected, the interferometric characters.
The N-slit laser interferometer (NSLI) was first introduced as an alternative applicable to secure free-space optical communications in 2002. Albeit its initial propagation distance in the laboratory was only 15 cm, it was envisioned as an interferometric tool ideally suited for propagation in the vacuum, or outer space. Subsequently, NSLI experiments have also been conducted over hundreds of meters, in the field, via open atmosphere The interferometric character then propagates via the intra-interferometric distance until it reaches the interferometric plane at x. In other words, the generation, propagation, and detection of the interferometric characters take place within the NSLI, thus highlighting the conceptual and configurational simplicity of the interferometric approach. The concept of interferometric characters was introduced when the NSLI was disclosed as an alternative for secure free-space optical communications. In this approach there are an infinite number of possible slit combinations that can lead to a set of interferometric characters.
The history of the probability amplitude equation |ψ〉=(|x,y〉−|y,x〉) applicable to quanta pairs, propagating in different directions with entangled polarizations, is reviewed and traced back to the 1947–1949 period. The interferometric Dirac foundations common to |ψ〉=(|x,y〉−|y,x〉) and the generalized N-slit interferometric equation, for indistinguishable quanta, are also described. The results from a series of experiments on N-slit laser interferometers, with intra interferometric propagation paths up to 527 m, are reviewed. Particular attention is given to explain the generation of interferometric characters, for secure space-to-space communications, which immediately collapse on attempts of interception. The design of a low divergence N-slit laser interferometer for low Earth orbit-low Earth orbit (LEO-LEO), and LEO-geostationary Earth orbit (LEO-GEO), secure interferometric communications is described and a weight assessment is provided.
The N-slit interferometer is demonstrated to function with an intra interferometric propagation path length of 527.33 m. Interferograms representing several interferometric characters, corresponding to N = 2, 3, 4, and 5, were recorded at the interferometric plane located at the end of an open air propagation range. Interferometric computations, based on the application of Dirac's notation, were successfully used to predict the structure and divergence of the propagating interferograms. These measurements were carried out during an unusual mild-temperature, low-humidity, summer night in northern Alabama. In the laboratory, at an intra interferometric propagation path length of 7.235 m, the N-slit interferometer was also used to successfully detect the intrusion of microscopic fibers into the intra interferometric propagation path. These experiments led to the detection of diffraction patterns superimposed over the interferograms.
Classical secure communication mainly has the following drawbacks. The first one is that we can't theoretically prove that the algorithm can't be broken. Then, with the rapid development of quantum computer, the algorithm can be cracked in a short time. In this paper, a new way of secure optical communication based on quantum entanglement is discussed. N-slit interferometry is used to generate a series of optical signals which represent some characters with specific meaning. Any intra-path perturbations will lead to the collapse of wave function. By using the Dirac's notation, propagating interferograms can be predicted successfully which carry specific information. The results show that interferometric signal particularly suitable for optical communication.
Optical communication via free-space channels is attractive for establishing long-range secure quantum networks, but is often hindered by deleterious effects on the transverse spatial mode profile of the beam, caused by propagation through the atmosphere. Additionally, interferometric measurement at the receiver becomes much more difficult with multimode profiles. This study presents a compact time-delay interferometer with high stability and interference visibility for single- and multimode spatial profiles. The results of this study are important for the future of quantum communication networks as well as a wide range of techniques for classical and quantum optical measurement.