arXiv subjects

Ping Koy Lam

Publications and source records attributed to Ping Koy Lam.

At least 37 records · Page 2Linked to original sources

arXiv2025

All-Gaussian State Discrimination Beyond the Coherent Helstrom Bound

A core problem in communications is the optimal discrimination of binary-phase-shift-keyed (BPSK) signals. A longstanding goal has been to reach the fundamental quantum limit, known as the Helstrom bound, for BPSK signals encoded in coherent states. However, due to technical constraints, proposals for reaching the bound remain impractical. In this letter we take an alternative approach: using only Gaussian optics - displaced squeezed states and homodyne detection - we achieve discrimination of BPSK signals with error rates below what can be achieved using coherent states and any quantum measurement.

quant-ph↗

arXiv2025

Cryogenic pressure sensing with an ultrafast Meissner-levitated microrotor

Magnetically levitated spinning rotors are key elements in important technologies such as navigation by gyroscopes, energy storage by flywheels, ultra-high vacuum generation by turbomolecular pumps, and pressure sensing for process control. However, mechanical rotors are typically macroscopic and limited to room temperature and low rotation frequencies. In particular, sensing pressure at low temperatures remains a technological challenge, while emerging quantum technologies demand a precise evaluation of pressure conditions at low temperatures to cope with quantum-spoiling decoherence. To close this gap, we demonstrate wide range pressure sensing by a spinning rotor based on a micromagnet levitated by the Meissner effect at 4.2 Kelvin. We achieve rotational speeds of up to 138 million rotations per minute, resulting in very high effective quality factors, outperforming current platforms. Beside sensing applications, we envision the use of levitated rotors for probing fundamental science including quantum mechanics and gravity, enabled by ultralow torque noise.

quant-ph↗

arXiv2025

Improving Free-Space Continuous Variable Quantum Key Distribution with Adaptive Optics

A significant performance inhibitor of free-space continuous variable quantum key distribution (CVQKD) is turbulence, which gives rise to wavefront phase and amplitude aberrations. We demonstrate that in a turbulent channel, during coherent state transmissions from a continuous-wave laser, that the interferometric visibility between the local oscillator (LO) and quantum signal decreases. A solution to this is incorporating adaptive optics at the receiver to correct phase and amplitude aberrations in the wavefronts of the received quantum signal. We demonstrate the increased interferometric visibility and decrease in its fluctuations in a 60 cm and 30 m turbulent channel when using adaptive optics through channel characterisation. In an ideal CVQKD system, we show that this leads to more precise and larger positive secret key rates, improving the performance of free-space CVQKD in turbulent channels.

quant-ph↗

arXiv2025

Counter-propagating Entangled Photon Pairs from a Monolayer

Non-phase-matched spontaneous parametric down-conversion (SPDC) in atomically thin materials provides new degrees of freedom and enhanced quantum information capacity compared to conventional phase-matched sources. These systems emerged as promising platforms for quantum computing, communication, and imaging, with the potential to support higher-order nonlinear processes. However, direct observation of photon-pair emission from a monolayer has remained experimentally challenging. In this work, we theoretically modeled SPDC emission across the full angular space from a monolayer GaSe film and experimentally validated the model through measurements of both co- and counter-propagating photon pairs. We demonstrated two-photon quantum correlations in the telecom C-band from the thinnest SPDC source reported to date. The spatially symmetric, broadband emission predicted by theory was confirmed experimentally. Furthermore, we observed high-fidelity Bell states in the counter-propagating configuration, marking the first realization of polarization-entangled photon pairs from a monolayer. Our results revealed the emission characteristics of SPDC in the deeply subwavelength, non-phase-matched regime, and introduced atomically thin, counterpropagating SPDC as a scalable and integrable platform for programmable quantum state generation, extendable via moiré superlattice engineering.

quant-ph↗

arXiv2025

Enhanced continuous-variable quantum key distribution protocol via adaptive signal processing

Quantum key distribution (QKD) provides a promising approach to secure communications, with continuous-variable QKD (CV-QKD) offering compatibility with existing telecommunication infrastructure. Despite this advantage, CV-QKD is limited by challenges such as losses in terrestrial fibres and atmospheric scintillation in free-space channels. We introduce a QKD protocol that surpasses the optimal Gaussian modulated CV-QKD (GG02) protocol by utilising probabilistic filters without known physical representation. Our approach employs a Gaussian filter at Alice's station and a non-Gaussian notch-like filter at Bob's station. Alice's filter optimises modulation variance to achieve key rates near the optimal GG02 performance, while Bob's filter adapts the effective channel conditions, which can result in higher key rates than the optimal GG02 protocol. Our security analysis avoids Gaussian extremality, accurately bounding Eve's information. The protocol dynamically optimises the secret-key rate for rapidly changing channels, such as terrestrial links and satellite-to-ground communications, and can extract keys in regions deemed non-secure by parameter estimation. Implemented at software level, our protocol requires no hardware modifications and can be integrated into existing QKD systems. Experimental results show a threefold increase in key rates over the optimal GG02 protocol, while simulations for Low Earth Orbit satellite quantum communications indicate a 400-fold increase compared to the non-optimised counterpart.

quant-ph↗

arXiv2025

Entanglement-based quantum key distribution with non-Gaussian continuous variables

Addition of single photons to two-mode-squeezed-vacuum states has the effect of distilling quantum entanglement, and, when deployed in quantum key distribution, should lead also to an increase in the secret key rate. However, the extraction of secret keys from non-Gaussian entangled states is a complex issue and is at present not fully understood. In this paper we describe a technique for adding photons to entangled states, and demonstrate how it leads to an increase in secret key rates and the maximal distance for which keys can be distributed assuming asymptotic conditions. The quantum correlations thus produced were found to be of a highly non-Gaussian character, such that the Gaussian extremity principle returns a negative keyrate and effectively kills the protocol; we have therefore developed methods of analysis that do not require prior assumptions about the state. Although it could have been that the addition of single photons would make the system more fragile, this turned out not to be the case. Rather, the addition of a single photon was found to protect the protocol against both passive and active decoherence.

quant-ph↗

arXiv2025

Polarisation and Temperature Dependence of Er$^{3+}$:CaWO$_4$ -- Towards a Solid-State Rare-Earth Ion-Doped Quantum Memory

In the endeavour of developing quantum memories, Er$^{3+}$:CaWO$_4$ has emerged as a promising rare-earth ion-doped (REID) crystal platform due to its long optical coherence times and compatibility with the 1550 nm telecommunications band. This work investigates the effects of polarisation and temperature on the absorption strength, central wavelength, and linewidth of the $Z_1\to Y_1$ and $Z_1\to Y_2$ optical transitions, with light incident along the crystal $a$ and $c$ axes. It is found that the $Z_1\to Y_1$ transition at 1532.6 nm with the incident laser along the $c$-axis at cryogenic temperatures ($\sim$3 K) is particularly favourable. The transition exhibits a stable central wavelength, narrower linewidth, polarisation independence, larger absorption cross-section, and lies within the C-band -- attributes that make it highly suitable for quantum memory applications.

quant-ph↗

arXiv2025

Levitated macroscopic rotors with 10 hours of free spin at room temperature

Low-dissipation rotors with large angular momentum are essential for precision sensing and probing macroscopic quantum phenomena. To date, low dissipation can only be achieved for micro-scale rotors. Here, we report a diamagnetically levitated millimeter-scale rotor exhibiting a measured dissipation rate as low as $3.85\,μ\mathrm{Hz}$ at room temperature, corresponding to a free spinning duration exceeding 10 hours. The rotor is levitated stably over an axisymmetric permanent magnet trap, and can be driven up to 930 RPM using contactless electrostatic actuation in high vacuum. Leveraging its low damping rate and large angular momentum, we realize a precision gyroscope with a measured sensitivity of $6.5 \times 10^{-3}\ \mathrm{^\circ/s}$ and an estimated thermal-limited stability of $5.7 \times 10^{-7}\ \mathrm{^\circ/\sqrt{h}}$. These results establish diamagnetic levitation as a promising room-temperature platform for high-performance gyroscopes.

physics.app-ph↗

arXiv2025

Machine Learning Cryptanalysis of a Quantum Random Number Generator

Random number generators (RNGs) that are crucial for cryptographic applications have been the subject of adversarial attacks. These attacks exploit environmental information to predict generated random numbers that are supposed to be truly random and unpredictable. Though quantum random number generators (QRNGs) are based on the intrinsic indeterministic nature of quantum properties, the presence of classical noise in the measurement process compromises the integrity of a QRNG. In this paper, we develop a predictive machine learning (ML) analysis to investigate the impact of deterministic classical noise in different stages of an optical continuous variable QRNG. Our ML model successfully detects inherent correlations when the deterministic noise sources are prominent. After appropriate filtering and randomness extraction processes are introduced, our QRNG system, in turn, demonstrates its robustness against ML. We further demonstrate the robustness of our ML approach by applying it to uniformly distributed random numbers from the QRNG and a congruential RNG. Hence, our result shows that ML has potentials in benchmarking the quality of RNG devices.

cs.LG↗

arXiv2025

Strong cubic phase shifts on the photonic vacuum state

Addition of photons to coherent states is shown to produce effects that display remarkable similarities with cubic phase shifts acting on the vacuum state, with fidelities in excess of 90 percent. The strength of the cubic interaction is found to vary inversely with the displacement of the coherent state and the strongest interactions were one order of magnitude greater than previous observations. The interaction is non-perturbative.

quant-ph↗

arXiv2025

Attainability of quantum state discrimination bounds with collective measurements on finite copies

One of the fundamental tenets of quantum mechanics is that non-orthogonal states cannot be distinguished perfectly. When distinguishing multiple copies of a mixed quantum state, a collective measurement, which generates entanglement between the different copies of the unknown state, can achieve a lower error probability than non-entangling measurements. The error probability that can be attained using a collective measurement on a finite number of copies of the unknown state is given by the Helstrom bound. In the limit where we can perform a collective measurement on asymptotically many copies of the quantum state, the quantum Chernoff bound gives the attainable error probability. It is natural to ask at what rate does the error tend to this asymptotic limit, and whether the asymptotic limit can be attained for any finite number of copies. In this paper we address these questions. We find analytic expressions for the Helstrom bound for arbitrarily many copies of the unknown state in several simple qubit examples. Using these analytic expressions, we investigate how the attainable error rate changes as we allow collective measurements on finite numbers of copies of the quantum state. We also investigate the necessary conditions to saturate the M-copy Helstrom bound. It is known that a collective measurement on all M-copies of the unknown state is always sufficient to saturate the M-copy Helstrom bound. However, general conditions for when such a measurement is necessary to saturate the Helstrom bound remain unknown. We investigate specific measurement strategies which involve entangling operations on fewer than all M-copies of the unknown state. For many regimes we find that a collective measurement on all M-copies of the unknown state is necessary to saturate the M-copy Helstrom bound.

quant-ph↗

arXiv2025

Automating Experimental Optics with Sample Efficient Machine Learning Methods

As free-space optical systems grow in scale and complexity, troubleshooting becomes increasingly time-consuming and, in the case of remote installations, perhaps impractical. An example of a task that is often laborious is the alignment of a high-finesse optical resonator, which is highly sensitive to the mode of the input beam. In this work, we demonstrate how machine learning can be used to achieve autonomous mode-matching of a free-space optical resonator with minimal supervision. Our approach leverages sample-efficient algorithms to reduce data requirements while maintaining a simple architecture for easy deployment. The reinforcement learning scheme that we have developed shows that automation is feasible even in systems prone to drift in experimental parameters, as may well be the case in real-world applications.

physics.optics↗

arXiv2025

Quantifying total correlations in quantum systems through the Pearson correlation coefficient

Conventionally the total correlations within a quantum system are quantified through distance-based expressions such as the relative entropy or the square-norm. Those expressions imply that a quantum state can contain both classical and quantum correlations. In this work, we provide an alternative method to quantify the total correlations through the Pearson correlation coefficient. Using this method, we argue that a quantum state can be correlated in either a classical or a quantum way, i.e., the two cases are mutually exclusive. We also illustrate that, at least for the case of two-qubit systems, the distribution of the correlations among certain locally incompatible pairs of observables provides insight in regards to whether a system contains classical or quantum correlations. Finally, we show how correlations in quantum systems are connected to the general entropic uncertainty principle.

quant-ph↗

arXiv2025

Microscopic theory of a precessing ferromagnet for ultrasensitive magnetometry

Levitated systems have great potential in quantum sensing and exploring quantum effects at the macroscopic scale. Of particular interest are recent works suggesting that a levitated ferromagnet can beat the standard quantum limit of magnetometry. This work offers a theoretical model to analyze and understand critical features of the precessing dynamics of a levitated ferromagnetic needle, indeed much like a macrospin, in the presence of a weak magnetic field. The dynamics from the atomic scale reveals how the standard quantum limit is surpassed, thus verifying sensing advantages when compared with a collection of independent spins. Our theory further takes us to two additional experimental designs of immediate interest: measurement of the celebrated Berry phase with a precessing ferromagnetic needle and the use of its nutation motion to sense a low-frequency oscillating magnetic field. With a microscopic theory established for levitated ferromagnetic needles, future studies of macroscopic quantum effects and the associated quantum-classical transition also become possible.

quant-ph↗

arXiv2025

Dynamic Continuous Variable Quantum Key Distribution for Securing a Future Global Quantum Network

Continuous variable quantum key distribution (CVQKD) is a developing method to secure information exchange in future quantum networks. With the recent developments in quantum technology and greater access to space, a global quantum network secured by CVQKD could be within reach. In this work, the structures of existing QKD networks are analysed, and how they can be fit into a general overarching three-layer QKD network architecture for the endeavour of a global QKD network. Such a network could comprise of different links in fibre and free-space. The finite size limit secret key rates (SKRs) with multidimensional reconciliation were calculated for the different links for which CVQKD can be used in such a network. The results show that CVQKD generally achieves longer distances and larger SKRs in inter-satellite, satellite-to-ground, fibre, and underwater links in descending order. The different links and nodes were classified and secret key distribution was studied as a graph problem. The link capacity, a routing metric for secret key distribution, which considers a dynamic SKR based on dynamic links is presented. Its use in simulated CVQKD networks is presented for the aim of spatiotemporal secret key distribution through a dynamic CVQKD network.

quant-ph↗

arXiv2024

Quantum Metrology for Gravitational Wave Astronomy

Einstein's General Theory of Relativity predicts that accelerating mass distributions produce gravitational radiation, analogous to electromagnetic radiation from accelerating charges. These gravitational waves have not been directly detected to date, but are expected to open a new window to the Universe in the near future. Suitable telescopes are kilometre-scale laser interferometers measuring the distance between quasi free-falling mirrors. Recent advances in quantum metrology may now provide the required sensitivity boost. So-called squeezed light is able to quantum entangle the high-power laser fields in the interferometer arms, and could play a key role in the realization of gravitational wave astronomy.

quant-ph↗

arXiv2024

Quasi-Phase-Matching Enabled by van der Waals Stacking

Quasi-phase matching (QPM) is a technique extensively utilized in nonlinear optics for enhancing the efficiency and stability of frequency conversion processes. However, the conventional QPM relies on periodically poled ferroelectric crystals, which are limited in availability. The 3R phase of molybdenum disulfide (3R-MoS2), a transition metal dichalcogenide (TMDc) with the broken inversion symmetry, stands out as a promising candidate for QPM, enabling efficient nonlinear process. Here, we experimentally demonstrate the QPM at nanoscale, utilizing van der Waals stacking of 3R-MoS2 layers with specific orientation to realize second harmonic generation (SHG) enhancement beyond the non QPM limit. We have also demonstrated enhanced spontaneous parametric down-conversion (SPDC) via QPM of 3R-MoS2 homo-structure, enabling more efficient generation of entangled photon pairs. The tunable capacity of 3R-MoS2 van der Waals stacking provides a platform for tuning phase-matching condition. This technique opens interesting possibilities for potential applications in nonlinear process and quantum technology.

physics.optics↗

arXiv2024

The gap persistence theorem for quantum multiparameter estimation

One key aspect of quantum metrology, measurement incompatibility, is evident only through the simultaneous estimation of multiple parameters. The symmetric logarithmic derivative Cramér-Rao bound (SLDCRB), gives the attainable precision, if the optimal measurements for estimating each individual parameter commute. When the optimal measurements do not commute, the SLDCRB is not necessarily attainable. In this regard, the Holevo Cramér-Rao bound (HCRB) plays a fundamental role, providing the ultimate attainable precisions when one allows simultaneous measurements on infinitely many copies of a quantum state. For practical purposes, the Nagaoka Cramér-Rao bound (NCRB) is more relevant, applying when restricted to measuring quantum states individually. The interplay between these three bounds dictates how rapidly the ultimate metrological precisions can be approached through collective measurements on finite copies of the probe state. We first consider two parameter estimation and prove that if the HCRB cannot be saturated with a single copy of the probe state, then it cannot be saturated for any finite number of copies of the probe state. With this, we show that it is impossible to saturate the HCRB for several physically motivated problems. For estimating any number of parameters, we provide necessary and sufficient conditions for the attainability of the SLDCRB with separable measurements. We further prove that if the SLDCRB cannot be reached with a single copy of the probe state, it cannot be reached with collective measurements on any finite number of copies of the probe state. These results together provide necessary and sufficient conditions for the attainability of the SLDCRB for any finite number of copies of the probe state. This solves a significant generalisation of one of the five problems recently highlighted by [P.Horodecki et al, Phys. Rev. X Quantum 3, 010101 (2022)].

quant-ph↗