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Quantum Computing, Communication, and Simulation

Off-axis performance of Lyot filters in multi-access quantum communication receivers

Vladimir V. Nikulin, Rushui Fang, Stephen C. Huerster, David H. Hughes

Published Feb 22, 2018• 3 citations
Publisher

Abstract

Advanced data links (ADL) can be implemented using free-space optical channels with quantum encryption. The security features of these links are based on quantum communication protocols that rely upon the inherent properties of laser light and offer physical-layer encryption without the mathematical complexity of conventional cryptography. Traditionally, these links are organized using the point-to-point topology, but it is highly desirable to extend this approach to multi-access communication systems to service an array of ADLs connected into a hub-and-spokes network. To facilitate connectivity, the "hub" can use a single aperture to establish connections to several target platforms within its field-of-view. Multi-access high-capacity wavelength division multiplexing can be used as an enabling technology for establishing communication between hub and spatially separated spokes. The associated problem of discriminating between multiple signals arriving simultaneously from distinct spatial locations is addressed by optical circuits capable of processing non-classical features of quantum states, such as entangled photon. One of the key elements of this receiving systems is the Lyot filter that offers high selectivity while directing the signals along different optical path lengths while propagating through the quantum circuit components, which may affect their processing capabilities. In this paper, we present analysis and preliminary experimental results that demonstrate the effects of off-axis arrival on performance of the Lyot filter and the resulting limitations of this technology.

DOI
10.1117/12.2287073

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Advanced data links (ADL) can be implemented using free-space optical channels with quantum encryption. The security features of these links are based on quantum communication protocols that rely upon the inherent properties of laser light and offer physical-layer encryption without the mathematical complexity of conventional cryptography. Traditionally, these links are organized using the point-to-point topology, but it is highly desirable to extend this approach to multi-access communication systems to service an array of ADLs connected into a hub-and-spokes network. To facilitate connectivity, the "hub" can use a single aperture to establish connections to several target platforms within its field-of-view. Multi-access high-capacity wavelength division multiplexing can be used as an enabling technology for establishing communication between hub and spatially separated spokes. The associated problem of discriminating between multiple signals arriving simultaneously from distinct spatial locations is addressed by optical circuits capable of processing non-classical features of quantum states, such as entangled photon. One of the key elements of this receiving systems is the Lyot filter that offers high selectivity while directing the signals along different optical path lengths while propagating through the quantum circuit components, which may affect their processing capabilities. In this paper, we present analysis and preliminary experimental results that demonstrate the effects of off-axis arrival on performance of the Lyot filter and the resulting limitations of this technology.
Free-space optical communication channels offer unsurpassed advantages over traditional radio frequency systems. They can be implemented in a point-to-point topology and, if necessary, augmented with additional security features for privacy-critical applications. The adoption of quantum mechanical principles in optical communication networks became a foundation for quantum communication protocols that offer added security without the mathematical complexity of traditional cryptography. Encryption can be achieved at the physical layer by using quantized values of photons, which makes exploitation of such communication links extremely difficult. Additional challenges are encountered when the focus is shifted from point-to-point links to multi-access communication systems arranged in a hub-and-spokes topology. To facilitate connectivity, the “hub” can use a single aperture to establish connection to a target platform within its fieldof- view. Polarization entanglement is proposed for data encoding, and additional degrees of freedom can be achieved for each quantum state by using hyper-entanglement. For example, if carrier waves arriving at the same time from multiple transmitters (spokes) are assigned specific frequencies in the 200 GHz ITU grid, their messages can be processed simultaneously by a receiver (hub) that uses a hyperspectral quantum optical circuit. In this paper, we present analysis of the components in the optical trains and propagation of hyper-entangled states in the quantum circuits. The results obtained in this project can be used as the first step before physical implementation of the quantum communication systems.
Lyot filters have been designed for multi-access high capacity wavelength division multiplexing between a communications hub and spatially separated spokes. Reported here are modifications to a basic classical communications Lyot filter stage [1], but using a hyper-entangled bipartite input state possessing non-degenerate frequencies for advanced quantum communications. Lyot filters and pre-shared linking knowledge can be used to better ensure efficiency and efficacy in manifesting entanglement correlations between polarization degrees of freedom in non-rotationally invariant measurement bases. Lyot filters also allow entangled photon routing, a function of entanglement between incident frequencies and polarization.
This paper discusses high-performance quantum optical circuits for processing polarization entanglement between correlated photon pairs. The primary application of this technology is secure communication links with quantum encryption that rely on the inherent properties of laser light and physical-layer processes in optical components. It can be used for generating identical pairs of encryption keys in quantum key distribution applications, for a variety of encryption protocols ranging from single-photon to multiple-photon, keyed communication in quantum noise, but we see a potential spectrum extending further to include multi-access systems. Success in these applications can be achieved by introducing additional degrees-of-freedom in each single-particle quantum state. We see hyper-entanglement as a very promising approach to do this. The concept is visualized by having multiple entangled photons with specifically assigned frequencies in the 100 GHz ITU grid and arriving simultaneously from multiple sources. Processing of such signals requires hyperspectral optical circuits capable of responding to non-classical features of quantum states. The function of these circuits is to direct the arriving signals along different processing paths to single photon detectors, which can be realized by a combination of different technologies, such as highly selective Lyot filters, dense wavelength division multiplexing, and others. Processing capabilities of these quantum circuits, while seemingly straight-forward in theory, still present a great implementation challenge. Practical operating conditions and characteristics of optical components must be taken into consideration to address the underlying design problems and make this technology feasible. This constitutes the main focus of our paper.
Secure free-space data links can be established using conventional laser communication technology or, if necessary, they can be further enhanced with quantum encryption. Security features of these systems are based on the protocols that make use of the inherent properties of laser light. In this case, encryption does not rely on complex mathematical algorithms that add overhead to the communication stream, but instead it is based on physical-layer processes in the laser sources and other modulating components. One promising approach is based on polarization entanglement between correlated photon pairs to achieve data encryption in quantum communication systems. The foundation of security lies in the response of photons to polarization measurements. Additional degrees of freedom can be added to each “singleparticle” state by using hyper-entanglement. The situation can be visualized when several carrier waves are assigned specific frequencies in the 100 GHz International Telecommunication Union (ITU) grid. The two technologies that can be eventually integrated to achieve this task include hyperspectral quantum circuits and the entangled pair source and detection systems. This results in frequency/polarization hyper-entanglement, which can be processed with additional wavelength-division multiplexing (WDM) components to achieve efficient separation of the signals. It is important to understand that most of the previous work is theoretical and assumes ideal properties of all optical parts. In reality, many non-ideal features of the quantum circuits and their components can change the way the quantum states are processed, and this constitutes the main focus of our paper.
The primary focus of this paper is high-performance quantum communication systems that facilitate secure data transfer via free-space links. We consider an approach that uses correlated photon pairs generated in such a way that their polarizations are entangled and can be used to support quantum encryption protocols. However, when deployed in free space, these links can be affected by channel distortion, primarily via the spatial and temporal fields of the refractive index along the propagation path. In classical links, these fields alter the optical wave front characteristics; however, this mechanism does not directly apply to the quantum states utilized in single-photon or entangled photon protocols. Transmitting signals with quantum-based encryption creates a realm of problems, not related to wave front distortions, but rather to integrity of the quantum states after the signals propagate over free-space channels. We study these phenomena by implementing a laboratory testbed capable of creating a turbulent environment using atmospheric chambers developed by the AFRL. It is then used for experimental investigation of quantum entanglement after photon pairs are propagated both collinearly and via separate paths.
Secure optical data links can be implemented using quantum communication (QC) protocols that offer physical-layer encryption without the mathematical complexity of traditional cryptography. Data encoding can be achieved using polarization entanglement and it is further proposed to obtain additional degrees of freedom for each single particle quantum state by using hyper-entanglement. For example, multiple carrier signals arriving at the same time can be assigned specific frequencies in the 100 GHz International Telecommunication Union (ITU) grid and be processed simultaneously by a receiver that uses a hyperspectral quantum optical circuit. In this paper, we address preservation of hyper-entanglement as the signals propagate through optical components of the quantum circuits and encounter their realistic properties.