arXiv subjects

Vinod N. Rao

Publications and source records attributed to Vinod N. Rao.

10 recordsLinked to original sources

arXiv2026

Quantum random number generation from the continuous variable payload for the SPOQC mission

The necessity of random numbers for various tasks, from simulation to cryptography, is crucial and immense. Here we demonstrate CV-QRNG using the CV payload of the SPOQC mission. The homodyne setup for QRNG uses the laser from the payload, in addition to potentially being used as detector in the case of an uplink scenario. Here we quantify the extractable secure randomness from the QRNG setup, that involves homodyne measurement of the vacuum states. The extracted randomness is tested against NIST test suite in addition to formally upper bounding the min-entropy. With the raw key length being $\approx1$ Mb in a given satellite pass, we get a total length of $\approx19.5$ Kb of certified random numbers from the 12-bit ADC.

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arXiv2026

Characterisation of a satellite-to-ground channel for continuous variable quantum key distribution protocol

In space based quantum key distribution (QKD) protocols, the quantum channel will be dynamic in nature and the channel loss will change with respect to the zenith angle. In the context of continuous variable (CV)-QKD, this will cause issues with parameter estimation and for a transmitted local oscillator in particular it will also fluctuate the shot noise. Therefore, it is vital to characterise this channel loss and the sources of this loss. In this paper the varying channel loss is characterised under practical assumptions. This is shown for various different scenarios, turbulence strengths, as well as wavelengths. This work shows, for the channel parameters considered, it is possible to generate a positive secret key if restricted Eve security assumptions are made.

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arXiv2026

Continuous variable quantum key distribution channel emulator for the SPOQC mission

In a free space optical (FSO) communication link from satellite to ground, the losses in the channel will be dynamic. Thus, the characterization of the FSO channel is of great importance and this can be emulated in the lab to evaluate the realistic performance of a satellite payload. In this work, we introduce a novel optical channel emulator capable of replicating these dynamics, especially for Low Earth Orbit based CubeSats. We demonstrate its ability to accurately emulate a satellite-to-ground optical communications channel under various atmospheric turbulence strengths, satellite trajectories, and optical ground station parameters at a given optical wavelength of interest. Our satellite channel emulator was designed to test and benchmark the performance of the continuous variable quantum key distribution payload for the Satellite Platform for Optical Quantum Communications mission - an in-orbit demonstrator for the UK's Quantum Communication Hub, to be launched in early 2026.

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arXiv2026

Energy efficient optical tracking for space quantum communication

Power consumption is a critical constraint for CubeSat based quantum communication, where tracking systems often dominate the onboard power budget. We demonstrate an energy-efficient approach that enables reliable satellite tracking at substantially reduced beacon power by treating tracking as a weak-signal estimation task. Using a closed-loop system with fine steering mirrors and higher-order Kalman filters on ground, we can maintain stable tracking at a transmitted power equivalent to 34 mW over a -60 dB satellite to ground optical channel. Our results show that the resulting penalties on QKD bit error rates and signal-to-noise ratios are negligible, allowing for more efficient power allocation to quantum payloads in CubeSat missions.

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arXiv2025

Continuous Variable Quantum Key Distribution with Single Quadrature Measurement at Arbitrary Reference Frame

We propose a simplified measurement scheme for a Gaussian modulated coherent state (GMCS) protocol for continuous variable quantum key distribution (CV-QKD), utilizing homodyne detection without quadrature switching. The reference frame of measurement is taken to be at an arbitrary angle, however, reconciliation converges the proposed scheme to GMCS with switching quadrature protocol. The arbitrary frame of measurement could also include the unknown random thermal drift within Bob's optical measurement setup. We found this scheme is advantageous for practical free-space and fibre-based GMCS protocol based CV-QKD systems as it does not require a phase modulator for random measurement selection quadrature at Bob.

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arXiv2024

Protocols for counterfactual and twin-field quantum digital signature

Quantum digital signature (QDS) is the quantum version of its classical counterpart, and can offer security against attacks of repudiation, signature forging and external eavesdropping, on the basis of quantum mechanical no-go principles. Here we propose a QDS scheme based on quantum counterfactuality, which leverages the concept of interaction-free measurement. Employing the idea behind twin-field cryptography, we show how this two-way protocol can be turned into an equivalent non-counterfactual, one-way protocol, that is both more practical and also theoretically helpful in assessing the experimental feasibility of the first protocol. The proposed QDS protocol can be experimentally implemented with current quantum technology.

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arXiv2023

Quantum counterfactuality with identical particles

Quantum self-interference enables the counterfactual transmission of information, whereby the transmitted bits involve no particles traveling through the channel. In this work, we show how counterfactuality can be realized even when the self interference is replaced by interference between identical particles. Interestingly, the facet of indistinguishability called forth here is associated with first-order coherence, and is different from the usual notion of indistinguishability associated with the (anti-)commutation relations of mode operators. From an experimental perspective, the simplest implementation of the proposed idea can be realized by slight modifications to existing protocols for differential-phase-shift quantum key distribution or interaction-free measurement.

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arXiv2021

Noiseless attack and counterfactual security of quantum key distribution

Counterfactual quantum key distribution (QKD) enables two parties to share a secret key using an interaction-free measurement. Here, we point out that the efficiency of counterfactual QKD protocols can be enhanced by including non-counterfactual bits. This inclusion potentially gives rise to the possibility of noiseless attacks, in which Eve can gain knowledge of the key bits without introducing any errors in the quantum channel. We show how this problem can be resolved in a simple way that naturally leads to the idea of "counterfactual security", whereby the non-counterfactual key bits are indicated to be secure by counterfactual detections. This method of enhancing the key rate is shown to be applicable to various existing quantum counterfactual key distribution protocols, increasing their efficiency without weakening their security.

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arXiv2021

A class of quasi-eternal non-Markovian Pauli channels and their measure

We study a class of qubit non-Markovian general Pauli dynamical maps with multiple singularities in the generator. We discuss a few easy examples involving trigonometric or other non-monotonic time dependence of the map, and discuss in detail the structure of channels which don't have any trigonometric functional dependence. We demystify the concept of a singularity here, showing that it corresponds to a point where the dynamics can be regular but the map is momentarily non-invertible, and this gives a basic guideline to construct such non-invertible non-Markovian channels. Most members of the channels in the considered family are quasi-eternally non-Markovian (QENM), which is a broader class of non-Markovian channels than the eternal non-Markovian channels. In specific, the measure of quasi-eternal non-Markovian (QENM) channels in the considered class is shown to be $\frac{2}{3}$ in the isotropic case, and about 0.96 in the anisotropic case.

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arXiv2021

Singularities, mixing and non-Markovianity of Pauli dynamical maps

Quantum non-Markovianity of channels can be produced by mixing Markovian channels, as observed recently by various authors. We consider an analogous question of whether singularities of the channel can be produced by mixing non-singular channels, i.e., ones that lack them. Here we answer the question in the negative in the context of qubit Pauli channels. On the other hand, mixing channels with a singularity can lead to the elimination of singularities in the resultant channel. We distinguish between two types of singular channels, which lead under mixing to broadly quite different properties of the singularity in the resultant channel. The connection to non-Markovianity (in the sense of completely positive indivisibility) is pointed out. These results impose nontrivial restrictions on the experimental realization of non-invertible quantum channels by a process of channel mixing.

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