Two-axis-twisting spin squeezing by multipass quantum erasure

Many-body entangled states are key elements in quantum information science and quantum metrology. One important problem in establishing a high degree of many-body entanglement using optical techniques is the leakage of the system information via the light that creates such entanglement. We propose an all-optical interference-based approach to erase this information. Unwanted atom-light entanglement can be removed by destructive interference of three or more successive atom-light interactions, leaving behind only atom-atom entanglement. This quantum erasure protocol allows implementation of spin squeezing with Heisenberg scaling using coherent light and a cold or warm atomic ensemble. Calculations show that a significant improvement in the squeezing exceeding 10 dB is obtained compared to previous methods, and substantial spin squeezing is attainable even under moderate experimental conditions. Our method enables the efficient creation of many-body entangled states with simple setups and, thus, is promising for advancing technologies in quantum metrology and quantum information processing