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.

We have shown how to conduct QKD vulnerability assessment in practice, based on a sound methodology inherited from Common Criteria. Taking a running CV-QKD system as a reference platform, we have experimentally tested and rated two different attack paths exploiting a common threat: detector saturation. Our results illustrate the importance of rating attacks in order to prioritize the implementation of countermeasures and to steer the design and engineering of practical QKD systems towards the highest possible security standards, paving the way to their security certification.

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.

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.

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.

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.

Abstract A Two-Mode Squeezed Vacuum (TMSV) is a quantum resource proven useful in several aplications in Quantum Technology, one of them being Quantum Key Distribution (QKD). Here we report the building of a TMSV source for use in QKD. Our system will comprise of two OPO, with its squeezed vacuum outputs combined in a balanced beam splitters. Active controls are employed for cavities stabilization, squeezing phase lock and relative phase lock between squeezed fields. The new cavity for the first OPO was designed and is in operation. Our target is to obtain 13 dB of corrected squeezing for the amplitude quadrature and a combined Duan inequality violation of up to 10 dB. We will show the status and our more recent results towards those goals.

We consider passive eavesdropping in continuous-variable quantum key distribution (CV-QKD) over atmospheric turbulence channels. We study the effect of turbulence in creating independent channels from Alice to Bob and Eve, and examine the performance of transmitted local oscillator (TLO) and local local oscillator (LLO) based CV-QKD system.

Continuous variable quantum key distribution (CV-QKD) uses shot-noise limited detection for measuring the quadratures of the signal sent by Alice over the quanutm channel. Typically, the signal transmission rate is limited by the bandwidth of the detection. One of the drawbacks of CV-QKD systems is low secure key generation rate at longer transmission distances. High bandwidth detectors—which allow higher signal transmission rates—cannot improve the key generation rate as these increase the electronic noise variance. Electronic noise variance in CV-QKD systems is considered as a trusted noise source and so theoretically, it does not have a great impact on the final key rate. However, from the practical point of view, there is impact on the performance of error correction codes—such as low density parity codes (LDPC). Increased electronic noise decreases the signalto-noise ratio (SNR). Constructing LDPC at lower SNR is a bottleneck for achieving long distance CV-QKD. Multi-mode signals can improve the SNR of CV-QKD system to a significant extent. In this work, we consider a multi-mode signal based CV-QKD system where signal modes are jointly measured in order to reduce the impact of electronic noise on the SNR.

Quantum Key Distribution (QKD) over a free-space channel is challenging due to inefficient coupling of received signals to the detection system. A narrow detector cross-section, such as with fibre coupling to a telescope, reduces the field of view (FOV) and thus increases the loss due to atmospheric turbulence. A larger detector cross-section increases the FOV; however, the corresponding increase in background noise also increases the QBER associated with a Discrete Variable (DV) QKD system. On the contrary, Continuous Variable (CV) QKD systems are highly tolerant to background noise, but their performance is still limited by the channel loss. The FOV of the receiver is defined as the area expressed in solid angle from which the detector can accepts signal. A simple geometrical analysis reveals FOV as $\Theta = 2\tan^{-1}(d/2F)$, where $d$ is the detector diameter and $F$ is the effective focal length of the receiver telescope. With a large-area detector, one can design the signal collection optics, such that the FOV of the telescope with a given aperture can be larger. In this work, we present a larger FOV receiver system for CVQKD which reduces the channel loss due to beam wandering and atmospheric turbulence. We will describe the performance of the receiver system in terms of shot-noise sensitivity, loss reduction and enhancement in secure key rate, compared to a typical fibre-coupled receiver system.