Peking University team generates switchable single-/multimode quantum light with more than 10^14 photons, setting a new record for ultra-bright quantum light sources

Recently, Professor Yunquan Liu's team at Peking University published a study in Ultrafast Science reporting a high-intensity bright squeezed vacuum quantum light source. The source produces more than 10^14 photons per pulse and can be tuned between single-mode and multimode quantum light fields, opening a new experimental platform for research at the intersection of quantum optics and attosecond strong-field physics.
Squeezed light and entangled photon pairs are core resources in modern photonic quantum technology. Frontier technologies such as quantum precision measurement, quantum imaging, and quantum communication all rely on high-quality nonclassical light fields. Conventional squeezed-light sources typically have relatively low photon numbers, while the high-intensity lasers commonly used in strong-field physics are often approximated as classical light fields, namely coherent states. How to obtain a light source that combines high photon flux with nonclassical statistical properties has become a shared question in quantum optics and strong-field physics in recent years.
To address this challenge, researchers from Peking University, Beijing University of Posts and Telecommunications, Guangdong Technion - Israel Institute of Technology, and other institutions designed an SU(1,1) interferometric optical setup based on two cascaded BBO crystals. Using a 1030 nm femtosecond laser as the pump source, they generated ultra-bright squeezed vacuum light in the 2060 nm near-infrared band through type-I high-gain spontaneous parametric down-conversion. The source reaches a pulse energy of up to 10 microjoules, with more than 10^14 photons per pulse, forming a macroscopic nonclassical quantum light field.
By changing the distance between the two nonlinear crystals, the team achieved continuous control over the spatial mode structure of the output light field. With narrowband spectral filtering, the output can approach a nearly pure single-mode squeezed vacuum, with an effective mode number of only 1.15. After removing the spectral filter, the system outputs multimode entangled quantum light with an effective mode number of 3.87, showing multimode correlation characteristics.
The team characterized the output light field using second-order correlation measurements, photon-number statistics, spectral Schmidt decomposition, and related methods. The results show clear differences between near-single-mode and multimode bright squeezed vacuum in photon-number distribution, spectral structure, and modal weights, with the experimental observations agreeing well with the corresponding theoretical models. The experiment also observed saturation of the output energy as the pump power increased, and this behavior was explained by a modified parametric-coupling model.
According to Professor Yunquan Liu, the corresponding author of the paper, this work establishes a new experimental system that combines macroscopic optical intensity with nonclassical quantum properties, indicating that China has reached an internationally leading level in ultra-bright tunable quantum light sources. The work was supported by the National Key R&D Program of China, the National Natural Science Foundation of China, and other projects.

Author Biographies:
First author: Zijian Lyu is a doctoral student in the School of Physics at Peking University. His research focuses on ultrafast quantum optics, including bright squeezed vacuum, multimode quantum light fields, quantum-state characterization, and nonlinear optical processes driven by quantum light. He is particularly interested in the generation and control of high-intensity nonclassical light fields and their applications in strong-field light-matter interactions.

Corresponding author:
Yunquan Liu is a Boya Distinguished Professor at Peking University. He received his Ph.D. from the Institute of Physics, Chinese Academy of Sciences in 2006, and conducted postdoctoral research at the Max Planck Institute for Nuclear Physics from 2006 to 2008. He joined Peking University in 2009, received support from the National Science Fund for Distinguished Young Scholars from 2012 to 2015, was appointed as a Changjiang Distinguished Professor by the Ministry of Education from 2015 to 2019, was selected for the Ministry of Science and Technology's Young and Middle-aged Science and Technology Innovation Talent Program, and was selected into the second batch of the Organization Department's National High-Level Talents Program in 2015. He has received the Wang Xuan Young Scholar Award, the Rao Yutai Basic Optics Award, the Wang Daheng Young and Middle-aged Optics Award, the Rao Yu Physics Award, and the Ye Qisun Teaching Award, among others. His research mainly focuses on ultrafast optics and ultrafast physics.