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IEEE Globecom Workshops (GC Wkshps)

Polarization Calibration Scheme for a Practical Handheld Free Space Quantum Key Distribution Link

Lai Zhou, David Lowndes, Vincent Lee, Indranil Mitra, Sai Gopal T., John Rarity, Grahame E. Faulkner, Dominic C. O'Brien

Published Dec 1, 2019• 3 citations
Publisher

Abstract

Free space Quantum Key Distribution (QKD) links between terminals that move relative to each other pose a number of challenges. The beam-steering components (typically mirrors) and the relative movement lead to variable birefringence, and for particular configuration there will also be fixed birefringence. The paper proposes a polarization calibration scheme suitable for real-time operation. Simulation and experiment are performed to verify the proposed scheme.

DOI
10.1109/gcwkshps45667.2019.9024509

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Free space Quantum Key Distribution (QKD) links between terminals that move relative to each other pose a number of challenges. The beam-steering components (typically mirrors) and the relative movement lead to variable birefringence, and for particular configuration there will also be fixed birefringence. The paper proposes a polarization calibration scheme suitable for real-time operation. Simulation and experiment are performed to verify the proposed scheme.
The field of quantum key distribution (QKD) has seen many advancements since its conception in 1984. Recently there have been developments towards establishing free space optical links between satellites and earth to enable global QKD networks. Polarization encoded photos are commonly utilized as qubits for these free space links, placing polarization distortion constrains on the optics used. \n \nTwo polarization distortion studies were conducted in this thesis. The first is a simulation of stress induced birefringence in the Window Observational Research Facility window on board the International Space Station (ISS). Atmospherics pressure inside the ISS elicits stress on window leading to optical birefringence, which in turn can distort the polarization of photons as they pass through the window. The model studies the birefringent effects for incoming optical beams over different polarizations, diameters and incoming angles. \n \nAs part of free space optical quantum links, it can be beneficial to utilize beam steering optics to manage the spatial positioning of various components in the system. Implementing beam steering optics can aid in achieving spatial and maneuverability requirements for ground stations and satellites as they track each other. However, these optics can have an adverse effect on incoming photon polarization states. Because of this, the effects of various different beam steering configurations were characterized experimentally, and compared to a corresponding simulated model. \n \nThe studies presented in this thesis have revealed that common optics such as windows and mirrors can distort polarization based quantum signals. The polarization distortion of the studied optics have shown high dependencies on the angular orientation of the incoming optical beam. However, the distortions measured can be noticeably reduced by implementing compensation optics. As a result, their distortions are not detrimental to a quantum link that utilizes them. The studies presented in this thesis are critical to the design of polarization based quantum receivers as they shed light on potential sources of link distortions. This work is part of a large collection of developments bringing the implementation of global quantum key distribution networks closer to reality.
Quantum key distribution (QKD) is moving from research laboratories towards applications. As computing becomes more mobile, cashless as well as cardless payment solutions are introduced. A possible route to increase the security of wireless communications is to incorporate QKD in a mobile device. Handheld devices present a particular challenge as the orientation and the phase of a qubit will depend on device motion. This problem is addressed by the reference frame independent (RFI) QKD scheme. The scheme tolerates an unknown phase between logical states that vary slowly compared to the rate of particle repetition. Here we experimentally demonstrate the feasibility of RFI QKD over a free-space link in a prepare and measure scheme using polarization encoding. We extend the security analysis of the RFI QKD scheme to be able to deal with uncalibrated devices and a finite number of measurements. Together these advances are an important step towards mass production of handheld QKD devices.
Free-space quantum key distribution (QKD) is attractive for the establishment of future global-scale quantum networks. However, it can be quite difficult for dynamic polarization control required in continuous-variable QKD systems to work properly in the presence of channel fading. Here we propose a dynamic polarization control scheme and verify its validity via simulations and an experiment performed over a 150 m free-space channel. The results indicate the capability of the scheme to effectively control the states of polarization for free-space continuous-variable quantum communication.
In polarization-encoding free-space Quantum Key Distribution (QKD), unwanted variations in the states of polarization may cause errors, leading to an increase in the quantum bit error rate (QBER), and a reduction of the Secure Key Rate (SKR). Optical elements that reflect or transmit the quantum signal at the transmitter and/or receiver are the main cause of polarization variations in the quantum states. Although most of these optical elements affect polarization in a constant manner, requiring relatively simple compensation techniques, mobile elements such as steering mirrors, cause time-varying polarization variations that are more complex to characterize and correct. In this work, we simulate the effect of steering mirrors’ typical values of angular rotation on the QBER. In particular, we show that, for most cases, an active correction of the polarization reference frame and a passive correction of the phase shift between the electric field components is sufficient to obtain QBER values due to the steering mirrors below 0.2%. This avoids the need of a fully active correction strategy, which increases the complexity, weight, and cost of the implementation. Furthermore, we present different design considerations to further reduce the steering mirrors’ contribution to the QBER (less than 0.05%).
Satellite quantum key distribution is a promising technique that overcomes the limited transmission distance in optical-fiber-based systems. The polarization tracking technique is one of the key techniques in the satellite quantum key distribution. With free-space quantum key distribution between an optical ground station and a satellite, the photon polarization state will be changed according to the satellite movement. To enable polarization based quantum key distribution between mobile terminals, we developed a polarization-basis tracking scheme allowing a common frame of reference to be shared. It is possible to orient two platforms along a common axis by detecting the reference optical signal only on the receiver side with no prior information about the transmitter's orientation. We developed a prototype system for free-space quantum key distribution with the polarization-basis tracking scheme. Polarization tracking performance was 0.092° by conducting quantum key distribution experiments over a 1 km free space between two buildings in a Tokyo suburb.
We present a miniaturised free-space quantum key distribution (QKD) system which allows key exchange between a handheld transmitter and a fixed terminal. The QKD system requires to be optically aligned emphasising the need of a beamsteering unit for later applications. To maintain within the size, weight and power restrictions, the active beamsteering hardware is exclusively located inside the receiver. Our target is consumer use so we present rigorous characterisation against a range of background light levels to show anticipated performance outside of a laboratory environment. Experimental results show a reduction in the raw count rate commensurate with the transmission of the added components (74.5%) and a small degradation of the error rate (0.5 percentage points) due to the worse signal-to-noise ratio. These combine to a 50% reduction in estimated secret key rate of the system with the additional components for beam steering.