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Optics Express

Free space quantum key distribution using modulating retro-reflectors

William S. Rabinovich, Rita Mahon, Mike S. Ferraro, Peter G. Goetz, Mark Bashkansky, Rachel E. Freeman, John F. Reintjes, James L. Murphy

Published Apr 18, 2018• 15 citations
Publisher

Abstract

Quantum key distribution (QKD) can be used to produce a cryptographic key whose security is guaranteed by quantum mechanics. The range of fiber-based QKD links is limited, by loss, to a few hundred kilometers, and cannot be used between mobile platforms. Free space QKD can, in principle, overcome these limitations. In practice, very narrow beam divergences must be used, requiring highly accurate pointing of the transmitting terminal to the receiver. This makes deployment very difficult. Here we describe the experimental implementation of a new type of free space QKD link, using modulating retro-reflectors (MRR). The MRR-QKD link eases the pointing requirements by more than three orders of magnitude, from microradians to degrees, while maintaining the narrow beam divergence necessary for long-range communication links. The system uses new, high extinction surface-normal multiple quantum well modulators with a modulation rate of 100 MHz. A laboratory-based BB84 QKD link using multiple quantum well MRRs is demonstrated, link budgets for possible applications are discussed, and security issues are considered.

DOI
10.1364/oe.26.011331

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Quantum key distribution (QKD) can be used to produce a cryptographic key whose security is guaranteed by quantum mechanics. The range of fiber-based QKD links is limited, by loss, to a few hundred kilometers, and cannot be used between mobile platforms. Free space QKD can, in principle, overcome these limitations. In practice, very narrow beam divergences must be used, requiring highly accurate pointing of the transmitting terminal to the receiver. This makes deployment very difficult. Here we describe the experimental implementation of a new type of free space QKD link, using modulating retro-reflectors (MRR). The MRR-QKD link eases the pointing requirements by more than three orders of magnitude, from microradians to degrees, while maintaining the narrow beam divergence necessary for long-range communication links. The system uses new, high extinction surface-normal multiple quantum well modulators with a modulation rate of 100 MHz. A laboratory-based BB84 QKD link using multiple quantum well MRRs is demonstrated, link budgets for possible applications are discussed, and security issues are considered.
Quantum key distribution (QKD) can be used to produce a cryptographic key whose security is unconditionally guaranteed by quantum mechanics. The range of fiber-based QKD links is limited, by loss, to a few hundred kilometers, and cannot be used between mobile platforms. Free space QKD can, in principle, overcome these limitations. In practice, very narrow beam divergences must be used, requiring highly accurate pointing of the transmitting terminal to the receiver. This makes deployment very difficult. Here we describe an entirely new type of free space QKD link, using modulating retro-reflectors (MRR). The MRR-QKD link eases the pointing requirements A laboratory-based BB84 QKD link using multiple quantum well MRRs is demonstrated, and link budgets for possible applications are discussed.
With the development of quantum computers, that can break current classical encryption schemes, unconditionally secure quantum key distribution (QKD) will become very important. Current fiber-based QKD implementations are limited to a few hundred kilometers due to optical losses in fiber and cannot be used with mobile platforms. A free space QKD system has recently been demonstrated over very large distances using entangled photons. However, due to the extremely high pointing accuracy required, the implementation of this QKD approach is very challenging and power demanding. Here we describe a new type of QKD link that uses modulating retro-reflectors. Our approach reduces pointing requirements by orders of magnitude, allowing an increase in pointing tolerance from microradians to tens of milliradians. Additionally, it reduces power requirements on the moving platform and has potential of reducing some influence of turbulence on the secure key distribution rate. Our approach relies on new, high extinction surface-normal multiple quantum well modulators with a maximum modulation rate of 100 MHz. We report on a BB84 QKD link using our system in the laboratory.
Free-space quantum key distribution (QKD) has attracted considerable attention due to its lower channel loss and link flexibility. It allows two participants share theoretical unconditional secure keys, and can potentially be applied to air-to-ground quantum communication to establish a global quantum network. Free-space QKD using modulating retro-reflectors (MRR-QKD) significantly reduces the pointing requirement and simplifies the structure of the mobile terminal, therefore making it suitable for lightweight aircraft such as unmanned aerial vehicle and Cubesat, etc. Based on intensity modulation of two non-orthogonal states and the B92 protocol, we proposed a scheme to improve the previous work (Optics Express 2018, 26, 11331). Our scheme simplifies the optical structure and shows more robustness in equipment imperfection. The analysis and simulation show that the number of multiple quantum well modulators needed in our scheme decreases from eight to three with similar performance. Additionally, while the previous scheme cannot work due to low modulator extinction ratio or high optical misalignment, our scheme can still operate.
Modulating retro-reflector (MRR), originally introduced to support laser communication, relieves most of the weight, power, and pointing requirements to the ground station. In this paper, a plug-and-play measurement device independent quantum key distribution (MDI-QKD) scheme with MRR is proposed not only to eliminate detector side channels and allow an untrusted satellite relay between two users, but also to simplify the requirements set-ups in practical flexible moving scenarios. The plug-and-play architecture compensates for the polarization drift during the transmission to provide superior performance in implementing the MDI-QKD on a free-space channel, and the MRR device is adopted to relax the requirements on both communication terminals. A double-pass correlated turbulent channel model is presented to investigate the complex and unstable channel characteristics caused by the atmospheric turbulence. Furthermore, the security of the modified MDI-QKD scheme is analyzed under some classical attacks and the simulation results indicate the feasibility under the situation that the system performance deteriorates with the increase of fading correlation coefficient and the turbulence intensity, which provides a meaningful step towards an MDI-QKD based on the moving platforms to join a dynamic quantum network with untrusted relays.
Unmanned aerial vehicles (UAVs)-based free-space optics (FSO)/quantum key distribution (QKD) systems require high-precision pointing mechanisms. This increases system complexity and limits rapid deployment for lightweight and energy-constrained UAVs. This paper proposes a modulating retroreflector (MRR)-equipped UAV architecture for BB84-QKD systems that enables simplified yet accurate tracking while relaxing pointing requirements. A realistic quantum channel model is developed, for which we newly derive the channel probability distribution of transmittance (PDT). Capitalizing on the derived channel PDT, several QKD performance metrics are analytically obtained. Numerical results verify the feasibility of the proposed MRR-aided UAV for practical QKD deployment, highlight its effectiveness over conventional UAV-ground systems, and validate the accuracy of the developed analytical framework.
To enable practical real-time quantum key distribution (QKD) over free-space links, such as for satellite-to-ground secure communications, the encoding rate of the system should be sufficiently high to distribute long keys (>106bits) in a short period of time (< 300 seconds) [1]. Here, we report an optical assembly and driving electronics designed to form a low-size}, weight and power (SWaP) QKD transmitter, which can operate at gigahertz clock rates. The QKD transmitter employs multiple vertical cavity surface emission lasers (VCSELs), which provide a low power and high polarization extinction ratio (PER) source of weak coherent pulses of light. These light pulses are combined using an intrinsically stable optical combining module utilising polarization-maintaining fiber. In the experiments presented we employ discrete variable QKD via the decoy-state BB84 protocol to achieve high secure key rates over large channel losses [2]. We characterise the source using a free-space polarization decoder over short free-space links within the laboratory and explore the performance of the system with commercially available single photon detectors. Fig. 1 Plot showing the effect of varying the channel loss over the free-space link. We show the measured quantum bit error rate (blue circles), sifting rate (green triangles) and secure key rate (red squares) alongside the rates simulated using the measured system parameters (solid lines). We characterise the system up to ~38 db of channel loss, revealing positive key rates at relatively high channel loss.