Rupesh Kumar

The security of prepare-and-measure satellite-based quantum key distribution (QKD), under restricted eavesdropping scenarios, is addressed. We particularly consider cases where the eavesdropper, Eve, has limited access to the transmitted signal by Alice, and/or Bob’s receiver station. For instance, Eve can only receive an attenuated version of the transmitted signals. This results in settings where an uncharacterized bypass channel, inaccessible to Eve, can also carry signals to Bob. We obtain generic bounds on the key rate in the presence of bypass channels and apply them to continuous-variable QKD protocols with Gaussian encoding as well as to the family of BB84 protocols. We find regimes of operation in which the above restrictions on Eve can considerably improve system performance. Our work opens up new security frameworks for spaceborne quantum communications systems.

Continuous-Variable Quantum Key Distribution (CV-QKD) uses continuous variables of the electromagnetic field, such as amplitude and phase, to transmit quantum information between two users, Alice and Bob. In this regime, there are two standard practical implementations: the transmitted local oscillator (TLO) and the local-local oscillator (LLO), where the LLO has recently reached 100km in laboratory-based fibre. Despite this success, the LLO also suffers major disadvantages like reduced quantum signal bandwidth capacity. Typically, only 50% of the transmission is for exchanging common phase references between Alice and Bob using reference signals. Subsequently, the key rate is reduced by half over all the transmission distances. In this work, we propose using a machine learning algorithm such as Long Short-Term Memory(LSTM) to predict the reference phase. The algorithm is trained with a sufficient number of phase reference data. Then, it predicts the phase drift with minimal usage of the reference signals, thereby reducing the reference signal bandwidth and increasing the key rate.

The future quantum internet will leverage existing communication infrastructures, including deployed optical fibre networks, to enable novel applications that outperform current information technology. In this scenario, we perform a feasibility study of quantum communications over an industrial 224 km submarine optical fibre link deployed between Southport in the United Kingdom (UK) and Portrane in the Republic of Ireland (IE). With a characterisation of phase drift, polarisation stability and the arrival time of entangled photons, we demonstrate the suitability of the link to enable international UK–IE quantum communications for the first time.

Development of Quantum Key Distribution (QKD) over long horizontal distances has provided both potential use cases for horizontal links within future quantum networks and testbeds to test protocols for satellite QKD. However, the majority of these implementations have used the polarisation of light as encoding scheme, with little work performed on phase-encoded schemes. Given the advantages that recent phase-based protocols such as ‘twin-field’ (TF) QKD have within fibre, it is possible the same distance-rate benefits can be found with free-space phase-based protocols.

Continuous variable quantum key distribution (CV-QKD) uses modulation of amplitude and phase of electromagnetic fields to encode information and shot noise limited detectors for decoding. The shot noise limited detection shows superior tolerance to noise and therefore a promising candidate for space-to-ground quantum communications, especially in daylight conditions. The parameters that affect the secure key generation rate of CV-QKD systems are the channel parameters- transmittance and uncalibrated noise (excess noise). On a typical static channel, such as an optical fibre link, the channel transmittance stays constant over a long period of signal transmission. On a dynamic channel, such as a Low Earth Orbit (LEO) based satellite to ground optical link, the transmittance of the channel varies concerning the zenith angle of the satellite.

In this talk, I will present the research and development of a quantum payload constructed for the Satellite Platform for Optical Quantum Communications (SPOQC) - a CubeSat mission by the Quantum Communication Hub, UK, scheduled for launch in 2025. The payload generates amplitude and phase-modulated coherent states for performing Continuous Variable Quantum Key Distribution (CVQKD) with Gaussian modulated coherent state protocol with a transmitter local oscillator based design. The payload is also equipped with a shot noise-limited homodyne detector, which will act as a Quantum Random Number Generator (QRNG), as well as a shot noise-limited optical receiver unit for ground-to-space uplink communication. We have also constructed an optical receiver unit for the ground station, based on single quadrature homodyne detection with shot noise sensitivity. The talk will disclose many interesting features of the payload and receiver, such as amplitude modulation without bias control, tolerance to polarization rotation, and a large-area detector unit that avoids the use of adaptive optics, among others.

This poster presentation demonstrates the quantum random number generation (QRNG) from a shotnoise-limited homodyne detector. The detector is part of the continuous variable quantum key distribution (CVQKD) payload developed for the Satellite Platform for Optical Quantum Communications (SPOQC) mission. We also show how the onboard homodyne detector works as a CVQKD receiver on the satellite.