arXiv subjects

Nam Hun Park

Publications and source records attributed to Nam Hun Park.

4 recordsLinked to original sources

arXiv2026

Time-spectral control of accidental coincidences in daylight entanglement-based free-space QKD

Daylight entanglement-based free-space quantum key distribution (QKD) is limited by accidental coincidences from receiver-admitted background light. We develop and experimentally validate a receiver-level framework anchored to measured singles rates and reference coincidence components, linking receiver bandwidth, accepted temporal width, and background-noise density to Bob singles, sifted-key rate, error rate, and quantum bit error rate (QBER) in telecom-wavelength BBM92 QKD. Indoor sweeps show that the sifted-key rate saturates near the source-matched bandwidth, whereas broader bandwidth or higher background mainly increases accidental contamination. Increasing the accepted temporal width leaves Bob singles nearly unchanged but directly raises QBER by enlarging the random-overlap probability. A two-dimensional design map calculated from the model shows that the temporal-window margin contracts rapidly with increasing background-to-signal ratio, while the bandwidth margin remains comparatively broad near source-matched filtering. A rooftop experiment demonstrates daylight operation over a 10 m free-space link, yielding a mean sifted-key rate of 2,811 cps and a mean QBER of 4.43%. This framework provides a quantitative basis for choosing receiver bandwidth and temporal acceptance to meet a target QBER under specified background conditions.

quant-ph↗

arXiv2026

Recon?figurable measurement scheme compatible with MDI-QKD and BB84 protocols using single-laser sideband generation

A quantum network can be operated efficiently by switching between measurement-device independent quantum key distribution (MDI-QKD) and BB84 according to the trust level of an intermediate node, since MDI-QKD is immune to all detector side-channel attacks while generally yielding a lower secret-key rate. Here, we present a proof-of-principle demonstration of a recon?figurable measurement scheme compatible with both protocols using a single laser. Two mutually independent weak coherent states (WCSs) are generated from a shared continuous-wave laser via electro-optic phase modulation and subsequent ?first-order sideband fi?ltering. Channel indistinguishability is verifi?ed using two complementary Hong-Ou-Mandel (HOM) measurements: time-resolved coincidence measurements and polarization mismatch scans, both yielding HOM visibilities close to the theoretical limit of 0.5 for WCSs. The measurement module performs a partial Bell-state measurement for MDI-QKD, while rotating a single half-wave plate to 22.5 degree reconfi?gures the same module into a dual-basis polarization analyzer for BB84. The quantum bit error rates measured in both confi?gurations agree well with theoretical expectations, verifying the operational feasibility of the reconfi?gurable receiver at the physical layer. By eliminating the need for active frequency locking while retaining independent sideband tunability, this approach provides a practical foundation for flexible and cost-effective quantum networks.

quant-ph↗

arXiv2024

True image construction in quantum-secured single-pixel imaging under spoofing attack

In this paper, we introduce a quantum-secured single-pixel imaging (QS-SPI) technique designed to withstand spoofing attacks, wherein adversaries attempt to deceive imaging systems with fake signals. Unlike previous quantum-secured protocols that impose a threshold error rate limiting their operation, even with the existence of true signals, our approach not only identifies spoofing attacks but also facilitates the reconstruction of a true image. Our method involves the analysis of a specific mode correlation of a photon-pair, which is independent of the mode used for image construction, to check security. Through this analysis, we can identify both the targeted image region by the attack and the type of spoofing attack, enabling reconstruction of the true image. A proof-of-principle demonstration employing polarization-correlation of a photon-pair is provided, showcasing successful image reconstruction even under the condition of spoofing signals 2000 times stronger than the true signals. We expect our approach to be applied to quantum-secured signal processing such as quantum target detection or ranging.

quant-ph↗

arXiv2023

Gaussian Quantum Illumination via Monotone Metrics

Quantum illumination is to discern the presence or absence of a low reflectivity target, where the error probability decays exponentially in the number of copies used. When the target reflectivity is small so that it is hard to distinguish target presence or absence, the exponential decay constant falls into a class of objects called monotone metrics. We evaluate monotone metrics restricted to Gaussian states in terms of first-order moments and covariance matrix. Under the assumption of a low reflectivity target, we explicitly derive analytic formulae for decay constant of an arbitrary Gaussian input state. Especially, in the limit of large background noise and low reflectivity, there is no need of symplectic diagonalization which usually complicates the computation of decay constants. First, we show that two-mode squeezed vacuum (TMSV) states are the optimal probe among pure Gaussian states with fixed signal mean photon number. Second, as an alternative to preparing TMSV states with high mean photon number, we show that preparing a TMSV state with low mean photon number and displacing the signal mode is a more experimentally feasible setup without degrading the performance that much. Third, we show that it is of utmost importance to prepare an efficient idler memory to beat coherent states and provide analytic bounds on the idler memory transmittivity in terms of signal power, background noise, and idler memory noise. Finally, we identify the region of physically possible correlations between the signal and idler modes that can beat coherent states.

quant-ph↗