Mehul Malik

Quantum key distribution (QKD) brings the promise of communication with information-theoretic security, but is limited in practice due to its susceptibility to noise, losses, and difficulty in accounting for device imperfections. To address these challenges, we propose a robust high-dimensional (HD) one-sided device-independent QKD (1sDI-QKD) protocol whose security is certified through the violation of steering inequalities. Motivated by recent demonstrations of steering in high-dimensional systems with enhanced robustness to noise and loss [PhysRevX.12.041023], we present a systematic security analysis of HD 1sDI-QKD protocols leveraging quantum steering to certify security. We analyze the achievable secret key rates for protocols with different measurement configurations and system dimensions, combined with the reverse reconciliation scheme, which leads to significant improvements in secret key rates. Our results demonstrate two key advantages: (i) the protocols offer enhanced robustness of the key rates against noise and loss in comparison to fully device-independent QKD, and (ii) the key rate performance shows favorable scaling with increasing dimensions. Finally, we characterize the noise-loss trade-off, highlighting the feasibility of HD 1sDI-QKD in practical scenarios. We further demonstrate progress towards a proof-of-concept experimental implementation of HD 1sDI-QKD by exploring multi-outcome projective measurements across all mutually unbiased bases up to dimension 11. We observe steering violations demonstrating advantages for QKD up to dimension 7 under the fair-sampling assumption. Finally, we discuss perspectives towards a loophole-free implementation of 1sDI-QKD.

Paradigmatic QKD protocols establish secure keys between pairs of users, however when more than two parties want to communicate, recently introduced quantum conference quantum key agreement (CKA) protocols can outperform 2-party primitives in terms of resource cost. In this contribution we report an implementation of a four-user quantum CKA protocol using polarisation-encoded multi-partite GHZ states at telecom wavelength. We distribute these states over up to 50km of optical fibre and implement custom multiparty error correction and privacy amplification on the resulting raw keys. From a finite-key analysis, we establish an information-theoretic secure key of up to 1.15 × 10^6 bits, which is used to encrypt and securely share an image between the four users. Surpassing the previous maximum distance for GHZ state transmission by more than an order of magnitude, these results demonstrate the viability of network protocols relying on multi-partite-entanglement. Future applications beyond quantum CKA include entanglement-assisted remote clock-synchronization, quantum secret sharing, and GHZ-based repeater protocols.

Quantum key distribution (QKD) brings the promise of communication with information-theoretic security, but is limited in practice due to its susceptibility to noise, losses, and difficulty in accounting for device imperfections. To address these challenges, we propose a robust high-dimensional (HD) \textit{one-sided} device-independent QKD (1sDI-QKD) protocol whose security is certified \mm{via detection/demonstration of steering}. Motivated by recent demonstrations of steering in high-dimensional systems with enhanced robustness to noise and loss [PhysRevX.12.041023], we present a systematic security analysis of HD 1sDI-QKD protocols leveraging quantum steering to certify security. We analyze the achievable secret key rates for protocols with different measurement configurations and system dimensions, combined with a reverse reconciliation scheme, which leads to significant improvements in secret key rates. Our results demonstrate two main advantages: (i) the protocols offer enhanced robustness of the key rates against noise and loss in comparison to fully device-independent QKD, and (ii) the key rate performance shows favorable scaling with increasing dimensions. Finally, we characterize the noise-loss trade-off, highlighting the feasibility of HD 1sDI-QKD in practical scenarios. We further demonstrate progress towards a proof-of-concept experimental implementation of HD 1sDI-QKD by exploring multi-outcome projective measurements across all mutually unbiased bases up to dimension 11. We observe steering violations demonstrating advantages for QKD up to dimension 7 under the fair-sampling assumption. Finally, we discuss perspectives towards a loophole-free implementation of 1sDI-QKD.

Quantum key distribution (QKD) enables the exchange of cryptographic keys with information-theoretic security. However, real-world implementations of QKD are often limited by noise, losses, and imperfect devices. High-dimensional quantum systems offer a promising route to overcome these limitations, enabling denser information encoding and enhanced resilience to noise and loss compared to traditional qubit-based protocols. On the other hand, device-independent (DI) protocols can address all adversarial cases due to device imperfections; however, existing security proofs have not shown any advantage associated with higher dimensions. In this work, we present a robust high-dimensional one-sided DI-QKD (1sDI-QKD) protocol whose security is certified through violations of quantum steering inequalities. By relaxing assumptions on one of the parties while still leveraging high dimensions, this approach improves the practicality of the protocols over fully device-independent QKD, offering promising experimental implications. We investigate 1sDI-QKD utilizing high-dimensional entanglement systems based on Srivastav et al. [PhysRevX.12.041023] framework and show that reverse reconciliation leverages the inherent asymmetry of the steering scenario, resulting in significantly higher key rates in the asymptotic regime as we increase the dimensions. Furthermore, we analyze the protocol's robustness to depolarizing noise and detection inefficiencies. Our results demonstrate the enhanced noise robustness and loss tolerance of high-dimensional 1sDI-QKD. The next step will be to experimentally validate these advantages, establishing high-dimensional 1sDI-QKD as a strong candidate for secure quantum communication under realistic conditions.

Our ability to trust that a random number is truly random is essential for fields as diverse as cryptography and fundamental tests of quantum mechanics. Device-independent quantum random number generators (QRNGs) provide a means of completely trusted randomness, but are highly impractical due to their strict technological requirements, such as loophole-free quantum nonlocality. By making fixed assumptions on specific parts of the device, semi-device-independent QRNGs lower these requirements drastically. However, this {has usually been} done at the cost of limiting their flexibility and security to a specific physical implementation and level of trust. Here we propose and experimentally test a new framework for semi-device-independent randomness certification that employs a flexible set of assumptions, allowing it to be applied in a range of physical scenarios involving both quantum and classical entropy sources. At the heart of our method lies a source of trusted vacuum in the form of a signal shutter, which enables the honesty of partially trusted measurement devices to be tested and provides lower bounds on the guessing probability of their measurement outcomes. We experimentally verify our protocol with a photonic setup and generate secure random bits under three different source assumptions with varying degrees of security and resulting data rates. Our work demonstrates a simple and practical way for achieving semi-device-independent randomness generation with user-defined flexibility in terms of levels of trust and physical implementations.