image: Figure 1 | Working principle and experimental setup of the squeezed-light array. a, simplified atomic level scheme for polarization squeezing and a Poincaré-sphere representation of the polarization-squeezed state. b, experimental setup in which a diffractive optical element generates multiple spatially separated beams that propagate through a single paraffin-coated rubidium vapor cell. Thermal atomic motion transports and homogenizes long-lived ground-state coherence throughout the cell, thereby mediating global dissipative coupling among the optical channels. c, schematic illustration of aligned squeezing ellipses across the array, representing nearly identical squeezing levels and squeezing directions enabled by the global coupling.
Credit: Dongdong Hao et al.
Squeezed light is a crucial non-classical resource for precision measurement, quantum imaging and quantum information processing. In a squeezed state, quantum fluctuations in one optical quadrature can be reduced below the shot-noise level of a coherent state, while fluctuations in the conjugate quadrature increase accordingly. As quantum technologies move toward larger and more spatially distributed systems, an important challenge is to generate many individually addressable squeezed-light beams while keeping their quantum-noise properties closely matched. Replicating independent squeezing devices increases hardware, pump power and stabilization requirements, while time- or frequency-multiplexed architectures do not directly provide large numbers of spatially addressable outputs.
In a new paper published in Light: Science & Applications, a team led by Professor Yanhong Xiao at Fudan University, together with collaborators from the Weizmann Institute of Science and Shanxi University, reports a different route. The researchers generated a 30-channel spatial array of polarization-squeezed light in a single paraffin-coated rubidium vapor cell, with a maximum measured squeezing of 2 dB. Rather than behaving as 30 independent squeezing sources, the optical channels are collectively regulated through a common ground-state atomic coherence.
The key mechanism is the transport of atomic coherence by thermal motion. A diffractive optical element forms a two-dimensional array from a single 795 nm laser beam. In each illuminated region, atomic-coherence-enhanced polarization self-rotation, equivalently described as a degenerate four-wave-mixing process, generates squeezed vacuum in the polarization orthogonal to the pump. At the same time, all input beams collectively create long-lived ground-state coherence. Through their thermal motion, atoms continuously carry this coherence from illuminated regions into the dark volume of the cell and subsequently between spatially separated optical channels. Because the paraffin coating preserves the coherence through thousands of atom-wall collisions, atomic motion transports and homogenizes the collective coherence throughout the vapor cell. The dark volume therefore acts as a shared, dynamically evolving reservoir, producing global dissipative coupling among spatially separated optical channels.
This shared reservoir changes the operating regime of the array. At a power of about 1 mW per channel, a single beam does not generate squeezing, whereas the 30-beam array produces about 1.5 dB of squeezing. Measurements on randomly selected channels also show closely matched squeezing levels and squeezing angles. These improvements arise from collective atomic dynamics, although they are not unlimited: at fixed power per channel the total optical power rises with array size, and eventually finite optical depth, optical pumping and spontaneous-emission noise drive the system into saturation.
The clearest evidence for synchronization comes from deliberately introducing a channel with a different pump power. When operated alone, this channel has a squeezing level distinct from the other beams. Once it is embedded in the coupled array, its squeezing is drawn toward the collective value, reducing the inter-channel difference from 0.79 dB to 0.19 dB. Time-resolved measurements show that the mismatched channel also relaxes much more rapidly toward the collective steady state when the array is present. Importantly, this synchronization cannot be attributed to motional averaging alone. Thermal atomic motion transports the shared coherence and provides global dissipative coupling between spatial channels, while nonlinear saturation in the atom-light interaction helps drive the channels toward a common steady state. This mechanism is analogous to passive mode locking, in which intensity-dependent loss from a saturable absorber favors a collectively selected low-loss dynamical state. In the present experiment, synchronization is manifested in the measurable squeezing properties, particularly the squeezing level and the orientation of the squeezing ellipse.
The work shows that atomic motion and a coherence-preserving environment can be used as resources for regulating a many-channel quantum light source, rather than being treated only as sources of cross-talk or decoherence. Because the output beams are spatially separated and resonant with an atomic transition, the platform is naturally suited for interfaces with rubidium-based memories and sensors and for spatially resolved quantum sensing and imaging. The demonstrated beams are not mutually entangled, but subsequent linear-optical processing could convert independently squeezed inputs into multipartite continuous-variable entangled states. Larger vapor cells, more thermally stable anti-relaxation coatings and optimized atom-light interactions could further increase the channel number and squeezing performance, opening routes toward distributed quantum sensing, multiparameter measurement and scalable quantum information processing.
Journal
Light: Science & Applications
Article Title
Large-scale array of squeezed light and synchronization using atomic vapor