Quantum entanglement is crucial for advancements in quantum technologies, including communication, simulation, computation, and metrology. Large-scale multipartite entanglement, while theoretically possible, is challenging to achieve due to decoherence effects. Previous experiments have demonstrated entanglement in systems containing thousands to millions of atoms, but these systems required extremely low temperatures and isolation from the environment. This research aims to overcome these limitations by exploring the possibility of generating large-scale entanglement in room-temperature conditions, utilizing billions of motional atoms. The researchers hypothesize that the information contained within a single photon can be strongly correlated with the excitation shared by a large number of atoms, stimulating multipartite entanglement. The use of a far-off-resonance spontaneous Raman scattering (SRS) scheme within the Duan-Lukin-Cirac-Zoller (DLCZ) protocol is proposed to mitigate the detrimental effects of decoherence inherent in room-temperature environments. The success of this approach would significantly expand the scale of accessible entanglement and contribute to the understanding of the quantum-to-classical transition.

The paper reviews existing work on multipartite entanglement, highlighting the generation of Greenberger-Horne-Zeilinger (GHZ) states in superconducting, photonic, and ion systems. The limitations of these approaches, including exponentially low detection efficiency for GHZ states, are discussed. The creation of twin Fock entanglement states in Bose-Einstein condensates (BECs) and W states in atomic ensembles is also reviewed. The advantages of W states, particularly their scalability due to the reduced need for coincidence measurements, are emphasized. The challenges of achieving large-scale entanglement in room-temperature conditions due to decoherence caused by atomic motion and collisions are highlighted, along with the potential of the far-off-resonance DLCZ protocol as a solution.

The researchers employed a far-off-resonance spontaneous Raman scattering (SRS) scheme within the DLCZ protocol to generate multipartite entanglement in a room-temperature cesium vapor cell. The process involved creating a correlated photon-atom pair, where the detection of a Stokes photon heralds the creation of a W state among billions of atoms. A second interrogation pulse was applied to convert the shared excitation in the W state into an anti-Stokes photon. The entanglement depth was quantified using a Hanbury Brown-Twiss interferometer to analyze the photon number statistics of the correlated Stokes and anti-Stokes photons. An M-separability witness, constructed with the correlated photon statistics, was used to certify the entanglement and determine the entanglement depth (D = N/M, where N is the total number of atoms and M is the number of separable subgroups). The dynamic evolution of entanglement depth was observed by varying the delay time between the creation and verification pulses. The number of atoms participating in the entanglement was determined by fitting the measured transmission rate of light with different frequencies using an absorption model and using coherence analysis of the collective enhancement effect. The effects of decoherence, such as the loss of photons and reduction in correlation between Stokes and anti-Stokes photons were considered in the analysis of the data, with corrective measures implemented.

The experiment successfully generated multipartite entanglement in a W state among approximately 1.77 billion motional cesium atoms at room temperature, heralded by a single photon. The entanglement depth was quantified using the M-separability witness, revealing a significant number of genuinely entangled atoms even after microseconds-level storage time. The study demonstrated the dynamic evolution of entanglement depth and the effects of decoherence. The number of separable subgroups (M) increased with storage time, indicating the degradation of entanglement due to thermal motion. The influence of different pulse energies on the entanglement depth was studied, showing that stronger pulses resulted in a higher excitation probability but potentially smaller entanglement depth.

The results demonstrate that genuine multipartite entanglement is achievable in macroscopic systems at room temperature, significantly expanding the boundary of accessible entanglement. The findings are important for understanding the quantum-to-classical transition and have implications for the development of practical quantum technologies. The ability to observe the dynamic evolution of entanglement and decoherence effects provides valuable insights into the behavior of large-scale entangled systems. The use of a room-temperature system significantly reduces the complexity and cost associated with maintaining extremely low-temperature environments. The demonstrated scalability and operational simplicity pave the way for future applications in quantum sensing and metrology.

This research successfully demonstrated multipartite entanglement in a room-temperature atomic ensemble containing billions of motional atoms. This was achieved using a far-off-resonance DLCZ protocol and a novel method for certifying and quantifying the entanglement depth. The ability to observe the dynamic evolution of entanglement depth and the influence of decoherence highlights the potential of this platform for exploring the quantum-to-classical transition and for developing future quantum technologies. Further research could focus on increasing the size of the entangled system and exploring applications in quantum sensing and metrology.

The study focuses on a specific type of entanglement (W state). The entanglement depth, while substantial, provides a lower bound, and the exact number of genuinely entangled atoms might be higher. The decoherence effects, although studied, may have other unobserved contributions. The experimental setup might limit the attainable excitation probabilities and the potential for generating even larger-scale entanglement.