arXiv subjects

Yukai Lu

Publications and source records attributed to Yukai Lu.

11 recordsLinked to original sources

arXiv2026

Static-Field Shielding of Bosonic Molecules: Evaporation to Degeneracy and Self-Bound Droplets

The strong, tunable dipolar interactions of ultracold molecules make them a powerful platform for quantum many-body physics, but reaching degeneracy by evaporative cooling requires suppressing inelastic collisions. Microwave collisional shielding has enabled the preparation of degenerate Fermi gases and Bose-Einstein condensates of polar molecules, whereas static electric field shielding has been limited to fermionic species, which are less prone to inelastic loss. We demonstrate static-field F\"{o}rster shielding of bosonic $^{23}$Na$^{87}$Rb molecules, suppressing two-body loss by up to four orders of magnitude. Within a bound-state-free electric-field window, three-body loss is also strongly suppressed, enabling efficient evaporation. Evaporating with an efficiency of 2.09(9), we increase the phase-space density of the gas by two orders of magnitude, reaching degeneracy with 7200(1000) molecules. We observe self-bound droplets at the end of evaporation over a wide range of field strengths, emerging from either degenerate or non-degenerate parent gases. We perform Path Integral Monte Carlo simulations, which suggest that the observed droplets are filamentary in nature, and find good agreement with the experimentally observed droplet formation temperatures. Our results establish F\"{o}rster shielding as a single-field route to prepare degenerate gases and self-bound droplets of bosonic polar molecules.

cond-mat.quant-gas↗

arXiv2026

Creating and Probing Spin-Squeezed States of Molecules

Polar molecules are a promising platform for quantum-enhanced sensing and precision tests of fundamental physics, owing to their strong long-range dipolar interactions, broad sensitivity to electromagnetic fields, and sensitivity to potential physics beyond the Standard Model. However, the creation of metrologically useful entangled states in molecular systems has remained elusive. Here, we report the first observation of a class of metrologically useful entangled states - spin-squeezed states - in polar CaF molecules trapped in an optical tweezer array. The spin degree of freedom is encoded in rotational levels which are directly coupled by dipolar exchange interactions. By harnessing appropriate dynamical decoupling schemes we observe up to 3.0(3)dB of metrological gain, (2.2(3)dB without measurement correction) from direct exchange interactions. Using Floquet engineering, we further realize richer Hamiltonians that preserve spin squeezing while enabling the development of longer-range quantum correlations. Using site- and spin-resolved measurements we demonstrate that these entangled states enhance sensitivity to both homogeneous and spatially varying fields, and reveal strong non-classical correlations, including bipartite entanglement and Einstein-Podolsky-Rosen steering. Finally, we transfer the spin-squeezed states into long-lived and non-interacting hyperfine states, where the metrological enhancement persists for up to 100ms. Our results establish molecular optical tweezer arrays as a scalable platform for generating, controlling, characterizing, and storing entangled states of molecules, opening new opportunities for quantum-enhanced sensing and precision tests of fundamental physics.

physics.atom-ph↗

arXiv2026

Probing Coherent Many-Body Spin Dynamics in a Molecular Tweezer Array Quantum Simulator

Models of interacting quantum spins are used in many areas of physics ranging from the study of magnetism and strongly correlated materials to quantum sensing. In this work, we study coherent many-body dynamics of interacting spin models realized using polar molecules trapped in rearrangeable optical tweezer arrays. Specifically, we encode quantum spins in long-lived rotational states and use the electric dipolar interaction between molecules, together with Floquet Hamiltonian engineering, to realize $1/r^3$ XXZ and XYZ models. We microscopically probe several types of coherent dynamics in these models, including quantum walks of single spin excitations, the emergence of magnon bound states, and coherent creation and annihilation of magnon pairs. Our results establish molecular tweezer arrays as a new quantum simulation platform for interacting quantum spin models.

cond-mat.quant-gas↗

arXiv2026

Extended Rydberg Lifetimes in a Cryogenic Atom Array

We report on the realization of a $^{133}$Cs optical tweezer array in a cryogenic blackbody radiation (BBR) environment. By enclosing the array within a 4K radiation shield, we measure long Rydberg lifetimes, up to $406 (36)\,\mu$s for the $55 P_{3/2}$ Rydberg state, a factor of 3.3(3) longer than the room-temperature value. We employ single-photon coupling for coherent manipulation of the ground-Rydberg qubit. We measure a small differential dynamic polarizability of the transition, beneficial for reducing dephasing due to light intensity fluctuations. Our results pave the path for advancing neutral-atom two-qubit gate fidelities as their error budgets become increasingly dominated by $T_1$ relaxation of the ground-Rydberg qubit.

physics.atom-ph↗

arXiv2024

Demonstration of Erasure Conversion in a Molecular Tweezer Array

Programmable optical tweezer arrays of molecules are an emerging platform for quantum simulation and quantum information science. For these applications, reducing and mitigating errors that arise during initial state preparation and subsequent evolution remain major challenges. In this paper, we present work on site-resolved detection of internal state errors and quantum erasures, which are qubit errors with known locations. First, using a new site-resolved detection scheme, we demonstrate robust and enhanced tweezer array preparation fidelities. This enables creating molecular arrays with low defect rates, opening the door to high-fidelity simulation of quantum many-body systems. Second, for the first time in molecules, we demonstrate mid-circuit detection of erasures using a composite detection scheme that minimally affects error-free qubits. We also demonstrate mid-circuit conversion of blackbody-induced errors into detectable erasures. Our demonstration of erasure conversion, which has been shown to significantly reduce overheads for fault-tolerant quantum error correction, could be useful for quantum information processing in molecular tweezer arrays.

quant-ph↗

arXiv2023

A Blue-Detuned Magneto-Optical Trap of CaF Molecules

A key method to produce trapped and laser-cooled molecules is the magneto-optical trap (MOT), which is conventionally created using light red-detuned from an optical transition. In this work, we report a MOT for CaF molecules created using blue-detuned light. The blue-detuned MOT (BDM) achieves temperatures well below the Doppler limit, and provides the highest densities and phase-space densities reported to date in CaF MOTs. We observe short BDM lifetimes at high magnetic field gradients, preventing magnetic compression as a means to increase densities. By directly measuring the BDM restoring force, we find that the short lifetimes are explained by low effective trap depths. Notably, we find sub-mK depths at typical magnetic gradients, in contrast to $\sim 50\,\text{mK}$ depths in red molecular MOTs and $\sim0.5\,\text{K}$ depths in red atomic MOTs.

physics.atom-ph↗

arXiv2023

Raman Sideband Cooling of Molecules in an Optical Tweezer Array

Ultracold molecules, because of their rich internal structures and interactions, have been proposed as a promising platform for quantum science and precision measurement. Direct laser-cooling promises to be a rapid and efficient way to bring molecules to ultracold temperatures. For trapped molecules, laser-cooling to the quantum motional ground state remains an outstanding challenge. A technique capable of reaching the motional ground state is Raman sideband cooling, first demonstrated in trapped ions and atoms. In this work, we demonstrate for the first time Raman sideband cooling of molecules. Specifically, we demonstrate 3D Raman cooling for single CaF molecules trapped in an optical tweezer array, achieving average radial (axial) motional occupation as low as $\bar{n}_r=0.27(7)$ ($\bar{n}_z=7.0(10)$). Notably, we measure a 1D ground state fraction as high as 0.79(4), and a motional entropy per particle of $s = 4.9(3)$, the lowest reported for laser-cooled molecules to date. These lower temperatures could enable longer coherence times and higher fidelity molecular qubit gates desirable for quantum information processing and quantum simulation. With further improvements, Raman cooling could also be a new route towards molecular quantum degeneracy applicable to many laser-coolable molecular species including polyatomic ones.

physics.atom-ph↗

arXiv2022

On-Demand Entanglement of Molecules in a Reconfigurable Optical Tweezer Array

Entanglement is crucial to many quantum applications including quantum information processing, simulation of quantum many-body systems, and quantum-enhanced sensing. Molecules, because of their rich internal structure and interactions, have been proposed as a promising platform for quantum science. Deterministic entanglement of individually controlled molecules has nevertheless been a long-standing experimental challenge. Here we demonstrate, for the first time, on-demand entanglement of individually prepared molecules. Using the electric dipolar interaction between pairs of molecules prepared using a reconfigurable optical tweezer array, we realize an entangling two-qubit gate, and use it to deterministically create Bell pairs. Our results demonstrate the key building blocks needed for quantum information processing, simulation of quantum spin models, and quantum-enhanced sensing. They also open up new possibilities such as using trapped molecules for quantum-enhanced fundamental physics tests and exploring collisions and chemical reactions with entangled matter.

cond-mat.quant-gas↗

arXiv2022

Bichromatic Imaging of Single Molecules in an Optical Tweezer Array

We report on a novel bichromatic fluorescent imaging scheme for background-free detection of single CaF molecules trapped in an optical tweezer array. By collecting fluorescence on one optical transition while using another for laser-cooling, we achieve an imaging fidelity of 97.7(2)% and a non-destructive detection fidelity of 95.5(6)%. We characterize loss mechanisms of our scheme, many of which are generically relevant to the fluorescent detection of trapped molecules, including two-photon decay and admixtures of higher excited states that are induced by the trapping light.

physics.atom-ph↗

arXiv2021

Molecular Laser-Cooling in a Dynamically Tunable Repulsive Optical Trap

Recent work with laser-cooled molecules in attractive optical traps has shown that the differential AC Stark shifts arising from the trap light itself can become problematic, limiting collisional shielding efficiencies, rotational coherence times, and laser-cooling temperatures. In this work, we explore trapping and laser-cooling of CaF molecules in a ring-shaped repulsive optical trap. The observed dependences of loss rates on temperature and barrier height show characteristic behavior of repulsive traps and indicate strongly suppressed average AC Stark shifts. Within the trap, we find that $Λ$-enhanced gray molasses cooling is effective, producing similar minimum temperatures as those obtained in free space. By combining in-trap laser cooling with dynamical reshaping of the trap, we also present a method that allows highly efficient and rapid transfer from molecular magneto-optical traps into conventional attractive optical traps, which has been an outstanding challenge for experiments to date. Notably, our method could allow nearly lossless transfer over millisecond timescales.

physics.atom-ph↗

arXiv2020

Synthesizing Optical Spectra using Computer-Generated Holography Techniques

Experimental control and detection of atoms and molecules often rely on optical transitions between different electronic states. In many cases, substructure such as hyperfine or spin-rotation structure leads to the need for multiple optical frequencies spaced by MHz to GHz. The task of creating multiple optical frequencies -- optical spectral engineering -- becomes challenging when the number of frequencies becomes large, a situation that one could encounter in complex molecules and atoms in large magnetic fields. In this work, we present a novel method to synthesize arbitrary optical spectra by modulating a monochromatic light field with a time-dependent phase generated through computer-generated holography techniques. Our method is compatible with non-linear optical processes such as sum frequency generation and second harmonic generation. Additional requirements that arise from the finite lifetimes of excited states can also be satisfied in our approach. As a proof-of-principle demonstration, we generate spectra suitable for cycling photons on the X-B transition in CaF, and verify via Optical Bloch Equation simulations that one can achieve high photon scattering rates, which are important for fluorescent detection and laser cooling. Our method could offer significant simplifications in future experiments that would otherwise be prohibitively complex.

physics.atom-ph↗