The recently-developed notion of 'parity-time (PT) symmetry' in optical systems with a controlled gain-loss interplay has spawned an intriguing way of achieving optical behaviors that are presently unattainable with standard arrangements. In most experimental studies so far, however, the implementations rely highly on the advances of nanotechnologies and sophisticated fabrication techniques to synthesize solid-state materials. Here, we report the first experimental demonstration of optical anti-PT symmetry, a counterpart of conventional PT symmetry, in a warm atomic-vapor cell. By exploiting rapid coherence transport via flying atoms, our scheme illustrates essential features of anti-PT symmetry with an unprecedented precision on phase-transition threshold, and substantially reduces experimental complexity and cost. This result represents a significant advance in non-Hermitian optics by bridging a firm connection with the field of atomic, molecular and optical physics, where novel phenomena and applications in quantum and nonlinear optics aided by (anti-)PT symmetry could be anticipated.Canonical quantum mechanics postulates Hermitian Hamiltonians to describe closed physical systems so that the system energy is conserved with real eigenvalues and the orthogonality between eigenstates with different eigen-energy is ensured. For systems with open boundaries, non-Hermitian Hamiltonians with complex eigenvalues and non-orthogonal eigen-functions are commonly expected. However, a counterintuitive discovery by Bender and Boettcher in 1998 1 has radically challenged this cognition and evoked considerable efforts on extending canonical quantum theory to non-Hermitian Hamiltonian systems 2,3,4 . In their pioneering work 1 , they showed that below some phase-transition point, a wide class of non-Hermitian Hamiltonians (̂) display entirely real spectra if they are invariant under the anti-linear parity-time (PT) operator (̂̂), [̂,̂̂] = 0 . In non-relativistic quantum mechanics, governed by Schrӧdinger equation, a necessary but not sufficient condition 1 for PT symmetry to hold mandates the complex potential involved to satisfy ( ) = * (− ), which implies its real (imaginary) part is an even (odd) function of position. Even more exciting is the occurrence of a sharp, symmetry-breaking transition once a non-Hermitian parameter crosses an exceptional point. As such, the Hamiltonian and PT operator no longer share the same set of eigen-functions and the real eigen-spectra of the system start to become complex.Despite the ramifications of these theoretical developments 1,2,3,4 , unfortunately, quantum mechanics is by nature an Hermitian theory and thus any attempt to observe PT symmetry in such systems is out of reach. On the other hand, due to the presence of gain and loss, optics has been recognized as a fertile ground for experimental investigations of PT symmetry. Given such optical settings, they can be fully realized without introducing any conflict with the Hermiticity of standard quantum mechanics. Thanks to the mathemat...

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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.

Spatially splitting nonclassical light beams is in principle prohibited due to noise contamination during beam splitting. We propose a platform based on thermal motion of atoms to realize spatial multiplexing of squeezed light. Light channels of separate spatial modes in an anti-relaxation coated vapor cell share the same long-lived atomic coherence jointly created by all channels through the coherent diffusion of atoms which in turn enhances individual channel's nonlinear process responsible for light squeezing. Consequently, it behaves as squeezed light in one optical channel transferring to other distant channels even with laser powers below the threshold for squeezed light generation. An array of squeezed light beams was created with low laser power ∼ mW. This approach holds great promise for applications in multi-node quantum network and quantum enhanced technologies such as quantum imaging and sensing. * Electronic address: heng.shen@physics.ox.ac.uk † Electronic address: yxiao@fudan.edu.cn

The linewidth of electromagnetically induced transparency (EIT) in a coated Rb vapor cell was studied under a magnetic field gradient. The nonlinear broadening of the EIT linewidth with the magnetic field gradient was observed. It was found that the motional averaging of the field gradient was more pronounced at higher laser intensities and larger beam sizes. In the same regime, there was a small linewidth decrease with the increasing magnetic field gradient. We have established a Monte-Carlo model, which gave results in good qualitative agreement with our experiment. Physics pictures for the above phenomena were also suggested. These results provide an understanding of the EIT linewidth behavior under motional averaging, and should be useful for applications in quantum optics and metrology based on coated vapor cells.