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Hou-Man Chin

Publications and source records attributed to Hou-Man Chin.

13 recordsLinked to original sources
arXiv2025

Practical continuous-variable quantum key distribution with squeezed light

Continuous-variable quantum key distribution (CV-QKD) has gathered significant interest for its potential to achieve high secret key rates and seamless integration with existing optical communication infrastructure. State-of-the-art CV-QKD systems primarily use coherent states for simplicity. However, squeezed states of light have been theoretically shown to offer significant advantages, including higher secret key rates, greater resilience to excess noise, and reduced requirements on information reconciliation efficiency. In this work, we experimentally verify these theoretical predictions and propose and demonstrate a practical squeezed-state CV-QKD system based on modern local-local oscillator and digital-signal-processing techniques. Operating over fibre channels and considering finite-size security against collective attacks we show the advantages of our system over its coherent state counterpart. Our work paves the way for squeezed states to become practical resources for quantum key distribution and other quantum information protocols.

quant-ph↗
arXiv2024

Digital reconstruction of squeezed light for quantum information processing

Squeezed light plays a vital role in quantum information processing. By nature, it is highly sensitive, which presents significant practical challenges, particularly in remote detection, traditionally requiring complex systems such as active phase locking, clock synchronization, and polarization control. Here, we propose and demonstrate an asynchronous detection method for squeezed light that eliminates the need for these complex systems. By employing radio-frequency heterodyne detection with a locally generated local oscillator and applying a series of digital unitary transformations, we successfully reconstruct squeezed states of light. We validate the feasibility of our approach in two key applications: the distribution of squeezed light over a 10 km fiber channel, and secure quantum key distribution between two labs connected via deployed fiber based on continuous variables using squeezed vacuum states without active modulation. This demonstrates a practical digital reconstruction method for squeezed light, opening new avenues for practical distributed quantum sensing networks and high-performance and long-distance quantum communication using squeezed states and standard telecom technology.

quant-ph↗
arXiv2024

Future proofing network encryption technology with continuous-variable quantum key distribution

We demonstrate a proof-of-concept establishment of quantum-secure data transfer links in field trials at two locations in Denmark: on the campus of Technical University of Denmark in Lyngby and between power grid nodes owned and operated by Energinet in Odense. Several different links, implemented physically using optical ground wires, underground fibers as well as their combinations, were investigated. Coherent `quantum' states at 1550 nm, prepared and measured using a semi-autonomous continuous-variable quantum key distribution (CVQKD) prototype, were multiplexed in wavelength with `classical' 100Gbps encrypted data traffic from a pair of commercial layer-2 network encryption devices operating at around 1300 nm. Under the assumptions of real-time data processing, we estimate average secret key rates of $434.8, 148.6$, and $78.3$ kbps in the asymptotic limit for diverse channels with losses (at 1550 nm) of 4.1, 5.5, and 6.7 dB, respectively. The demonstrations permit an evaluation of the prototype's tolerance to harsh field conditions and showcase that CVQKD can serve as an additional layer to protect sensitive network traffic propagating on insecure channels.

quant-ph↗
arXiv2023

Long-distance continuous-variable quantum key distribution over 100 km fiber with local local oscillator

Quantum key distribution (QKD) enables two remote parties to share encryption keys with security based on the laws of physics. Continuous variable (CV) QKD with coherent states and coherent detection integrates well with existing telecommunication networks. However, thus far, long-distance CV-QKD has only been demonstrated using a highly complex scheme where the local oscillator is transmitted, opening security loopholes for eavesdroppers and limiting its potential applications. Here, we report a long-distance CV-QKD experiment with a locally generated local oscillator over a 100 km fiber channel with a total loss of 15.4 dB. This record-breaking distance is achieved by controlling the phase-noise-induced excess noise through a machine-learning framework for carrier recovery and optimizing the modulation variance. We implement the full CV-QKD protocol and demonstrate the generation of keys secure against collective attacks in the finite-size regime. Our results mark a significant milestone for realizing CV quantum access networks with a high loss budget, and pave the way for large-scale deployment of secure QKD.

quant-ph↗
arXiv2022

Modulation leakage-free continuous-variable quantum key distribution

Distributing cryptographic keys over public channels in a way that can provide information-theoretic security is the holy grail for secure communication. This can be achieved by exploiting quantum mechanical principles in so-called quantum key distribution (QKD). Continuous-variable (CV) QKD based on coherent states, in particular, is an attractive scheme for secure communication since it requires only standard telecommunication technology that can operate at room temperature. However, a recently discovered side-channel created in the process of state preparation leads to a leakage of information about the transmitted quantum state, opening a security loophole for eavesdroppers to compromise the security of the CVQKD system. Here, we present a CVQKD system without this modulation leakage vulnerability. Our implementation is based on a baseband modulation approach, and uses an in-phase and quadrature (IQ) modulator for state preparation and radio frequency heterodyne detection together with carefully designed digital signal processing for state measurement. We consider practical aspects in the implementation and demonstrate the generation of a composable secret key secure against collective attacks. This work is a step towards protecting CVQKD systems against practical imperfections of physical devices and operational limitations without performance degradation.

quant-ph↗
arXiv2021

Practical continuous-variable quantum key distribution with composable security

A quantum key distribution (QKD) system must fulfill the requirement of universal composability to ensure that any cryptographic application (using the QKD system) is also secure. Furthermore, the theoretical proof responsible for security analysis and key generation should cater to the number $N$ of the distributed quantum states being finite in practice. Continuous-variable (CV) QKD based on coherent states, despite being a suitable candidate for integration in the telecom infrastructure, has so far been unable to demonstrate composability as existing proofs require a rather large $N$ for successful key generation. Here we report the first Gaussian-modulated coherent state CVQKD system that is able to overcome these challenges and can generate composable keys secure against collective attacks with $N \lesssim 3.5\times10^8$ coherent states. With this advance, possible due to novel improvements to the security proof and a fast, yet low-noise and highly stable system operation, CVQKD implementations take a significant step towards their discrete-variable counterparts in practicality, performance, and security.

quant-ph↗
arXiv2021

Modulation leakage vulnerability in continuous-variable quantum key distribution

Flaws in the process of modulation, or encoding of key bits in the quadratures of the electromagnetic light field, can make continuous-variable quantum key distribution systems susceptible to leakage of secret information. Here, we report such a modulation leakage vulnerability in a system that uses an optical in-phase and quadrature modulator to implement a single sideband encoding scheme. The leakage arises from the limited suppression of a quantum-information-carrying sideband during modulation. Based on the results from a proof-of-concept experiment, we theoretically analyse the impact of this vulnerability. Our results indicate that the leakage reduces the range over which a positive secret key can be obtained, and can even lead to a security breach if not properly taken into account. We also study the effectiveness of additional trusted noise as a countermeasure to this vulnerability.

quant-ph↗
arXiv2020

Thermodynamics of a Quantum Annealer

The D-wave processor is a partially controllable open quantum system which exchanges energy with its surrounding environment (in the form of heat) and with the external time dependent control fields (in the form of work). Despite being rarely thought as such, it is a thermodynamic machine. Here we investigate the properties of the D-Wave quantum annealers from a thermodynamical perspective. We performed a number of reverse-annealing experiments on the D-Wave 2000Q via the open access cloud server Leap, with the aim of understanding what type of thermal operation the machine performs, and quantifying the degree of dissipation that accompanies it, as well as the amount of heat and work that it exchanges. The latter is a challenging task in view of the fact that one can experimentally access only the overall energy change occurring in the processor, (which is the sum of heat and work it receives). However, recent results of non-equilibrium thermodynamics(namely, the fluctuation theorem and the thermodynamic uncertainty relations), allow to calculate lower bounds on the average entropy production (which quantifies the degree of dissipation) as well as the average heat and work exchanges. The analysis of the collected experimental data shows that 1) in a reverse annealing process the D-Wave processor works as a thermal accelerator and 2) its evolution involves an increasing amount of dissipation with increasing transverse field.

quant-ph↗
arXiv2020

Machine learning aided carrier recovery in continuous-variable quantum key distribution

The secret key rate of a continuous-variable quantum key distribution (CV-QKD) system is limited by excess noise. A key issue typical to all modern CV-QKD systems implemented with a reference or pilot signal and an independent local oscillator is controlling the excess noise generated from the frequency and phase noise accrued by the transmitter and receiver. Therefore accurate phase estimation and compensation, so-called carrier recovery, is a critical subsystem of CV-QKD. Here, we explore the implementation of a machine learning framework based on Bayesian inference, namely an unscented Kalman filter (UKF), for estimation of phase noise and compare it to a standard reference method. Experimental results obtained over a 20 km fibre-optic link indicate that the UKF can ensure very low excess noise even at low pilot powers. The measurements exhibited low variance and high stability in excess noise over a wide range of pilot signal to noise ratios. This may enable CV-QKD systems with low implementation complexity which can seamlessly work on diverse transmission lines.

quant-ph↗
arXiv2018

Effect of filter shape on excess noise performance in continuous variable quantum key distribution with Gaussian modulation

An attractive implementation for quantum cryptography is the continuous variable variation, as it relies on standard telecommunication components. Modulating the quantum signal using a Gaussian format is attractive since it has been proven to be secure. This work investigates the effect of the roll-off of a root raised cosine pulse shaping and matched filter on the excess noise performance of a Gaussian modulated quantum key distribution system in a simulated back to back configuration. Contrary to intuition, it is found that the roll-off parameter does not significantly impact the performance of the system.

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