arXiv subjects

X. T. Yan

Publications and source records attributed to X. T. Yan.

6 recordsLinked to original sources

arXiv2026

Bell inequality violation with momentum-entangled massive particles

Bell's theorem revealed the fundamental incompatibility between the predictions of quantum mechanics and local realism. Bell inequality violations have since demonstrated quantum nonlocality using photons and internal states of massive particles, but never using their motional states. Here we report the first Bell inequality violation in the motional states of massive particles. Using momentum-entangled pairs of metastable helium atoms manipulated by matter-wave interferometry, we measure a Clauser-Horne-Shimony-Holt (CHSH) Bell parameter of $S = 2.52 \pm 0.17$, violating the CHSH-Bell inequality ($S \le 2$). Our work completes a long-standing objective in quantum atom optics by extending Bell tests from internal quantum variables to the external degrees of freedom of massive particles, opening a new regime for exploring quantum nonlocality in matter waves and for investigating the interplay between quantum mechanics and gravity.

cond-mat.quant-gas↗

arXiv2026

A framework for separating dephasing from decoherence in matter-wave Bell interferometers

Matter-wave Bell interferometers provide a sensitive probe of mass-dependent decoherence in entangled quantum systems. The degree of entanglement is obtained from the Bell-correlation amplitude of this interferometer. For observing potential mass-dependent decoherence, a reliable interpretation of any observed reduction in the Bell correlation amplitude is required, which depends on three factors: geometric dephasing, environmental decoherence, and technical dilution from source and detection statistics. In this work, we present a framework based on the Schwinger SU(2) mapping to separate these contributions into local unitaries or dissipative channels. We show that by evaluating the Bell correlation at zero interferometer path difference, it is possible to extract a source-distribution-independent Bell correlation amplitude reduction. When this framework is extended to involve atoms of different mass, we show that the known differential decoherence channels are negligible at current sensitivity. This yields a concrete bound at which a dual-species Bell interferometer would begin to signal differential decoherence beyond the known systematics, opening the way for such systems to probe new physics, such as mass-dependent decoherence mechanisms.

cond-mat.quant-gas↗

arXiv2026

Pulse Breathing Dynamics in a Mode-Locked Laser measured via SHG autocorrelation

Pulse-to-pulse fluctuations in mode-locked lasers fundamentally limit applications from optical frequency combs to supercontinuum generation. While timing jitter has been extensively characterized, pulse amplitude and width fluctuations remain less accessible experimentally. We present a statistical autocorrelation method that demonstrates pulse breathing dynamics through Fano factor analysis of second-harmonic generation autocorrelation. This reveals a characteristic W-shape in the enhanced Fano profile, a signature of pulse shape dynamics that is invisible to time-averaged fluctuations. Applying this method to two commercially available passively mode-locked oscillators operating at 1030 nm and 1045 nm, with different performance specifications, we measure pulse width fluctuations of 3.2(1)\,fs and 2.86(2)\,fs respectively. The two independent instruments serve as a cross-validation of the technique across different laser platforms. This diagnostic capability opens the door to identifying and suppressing specific breathing mechanisms, paving the way for the design of ultra-stable oscillators required for precision frequency metrology.

physics.optics↗

arXiv2025

Bell correlations between momentum-entangled pairs of $^4\text{He}^*$ atoms

Nonlocal entanglement between pair-correlated particles is a highly counter-intuitive aspect of quantum mechanics, where measurement on one particle can instantly affect the other, regardless of distance. While the rigorous Bell's inequality framework has enabled the demonstration of such entanglement in photons and atomic internal states, no experiment has yet involved motional states of massive particles. Here we report the experimental observation of Bell correlations in motional states of momentum-entangled ultracold helium atoms. Momentum-entangled pairs are generated via $s$-wave collisions. Using a Rarity-Tapster interferometer and a Bell-test framework, we observe atom-atom correlations required for violation of a Bell inequality. This result shows the potential of ultracold atoms for fundamental tests of quantum mechanics and opens new avenues to studying gravitational effects in quantum states.

cond-mat.quant-gas↗

arXiv2024

Proposal for a Bell Test with Entangled Atoms of Different Mass

We propose a Bell test experiment using momentum-entangled atom pairs of different masses, specifically metastable helium isotopes 3He* and 4He*, though the method extends to other atom species. Entanglement is generated via collisions, after which the quantum states are manipulated using two independent atom interferometers, enabling precise phase control over each species. Numerical simulations predict a significant violation of Bell's inequality under realistic conditions. This proposal opens a new paradigm to study the intersection of quantum mechanics and gravity.

quant-ph↗

arXiv2024

Measurement of the $s$-wave scattering length between metastable helium isotopes

We report the first experimental determination of the interspecies $s$-wave scattering length\,($a_{34}$) between the $2\,^3S_1\,(F=3/2,m_F=3/2)$ state of $^3$He$^*$ and the $2\,^3S_1\,(m_J=1)$ state of $^4$He$^*$. We determine $a_{34}$ by inducing oscillations in a trapped Bose-Einstein condensate of $^4$He$^*$ and measuring the damping rate of these oscillations due to the presence of $^3$He$^*$ atoms. The deduced value of $a_{34}=29\pm3$\,nm is in good agreement with theoretical predictions. The knowledge of this scattering length is important for many fundamental experiments between these helium isotopes.

cond-mat.quant-gas↗