Fig. 1. System of cavity QED. (a) Energy levels of uncoupled single atom-light field system (left) and energy levels splitting of strongly coupled single atom-light field system (right); (b) schematic diagram of coupling between single atom and optical cavity in actual situtaion

Fig. 2. Schematic diagrams of single-atom laser and its energy levels^[7]. (a) Schematic diagram of single-atom laser, where single atom is trapped inside FP cavity, and Ω₃ and Ω₄ are pump light; (b) schematic diagrams of energy levels of single atom laser and coupling between pump and optical cavity

Fig. 3. Schematic diagram of photon blockade device^[60]. (a) Schematic diagram of strongly coupled single-atom cavity QED system under cavity driving or atom driving; (b) energy levels of strongly coupled cavity QED, and processes of single-photon blockade (I) and two-photon blockade (II); (c) energy level coupling under cavity driving; (d) energy level coupling under atom driving; (e) vacuum Rabi splitting under cavity driving and atom driving

Fig. 4. Schematic diagram of non-demolition measurement of single photon by strongly coupled cavity QED system^[69]

Fig. 5. Relization of atom-to-photon qubit mapping and photon-to-atom qubit mapping in quantum network by using single-atom cavity QED^[12]. (a) Atom-to-photon qubit mapping; (b) photon-to-atom qubit mapping

Fig. 6. Realization of entanglement of remote atoms in quantum network by using single-atom cavity QED[12]. (a) atom-photon entanglement is generated at the first node, and photon is written into single atom at the second node; (b) entanglement of remote atoms in quantum network

Fig. 7. Quantum logic gate between remote atoms^[15]. (a) Conceptual drawing of distributed quantum computation; (b) schematic diagram of realization of quantum logic gate between remote atoms; (c) quantum circuit corresponding to logic gate; (d) principle of atom-photon logic gate

Fig. 8. Generation of optical Schrödinger “cat state”^[78]. (a) Principle of experiment; (b) Wigner function of generated “cat state”; (c) reconstructed three-dimensional Wigner function

Fig. 9. Energy level comparison among two-atom cavity left (right), single-atom cavity QED (middle), and empty cavity (left) ^[37]

Fig. 10. Generation of heralded entanglement of two ions^[41]. (a) Schematic diagram of energy levels of ions; (b) schematic diagram of two-ion entanglement generated by detecting photons

Fig. 11. Schematic diagram of “carving” atom entanglement state by photons in two-atom cavity QED system^[42]

Fig. 12. Two-atom quantum node and its application in quantum key distribution. (a) Two-atom quantum node addressed by AOD^[44]; (b) unconditional quantum key distributionrealized by using two atoms in cavity^[45]

Fig. 13. Schematic diagram of quantum interface between nano-photonic crystal cavity and two coupled atoms^[47]. (a) Schematic diagram of quantum interface for multiple qubits based on nano-photonic crystal cavity; (b) reflective spectra of atoms in different internal states coupled with nano-photonic crystal cavity; (c) reflective photon count when two atoms are in different combinations of internal states; (d) Rabi oscillation of single atom driven by microwave

Fig. 14. Energy level splitting when different numbers of atoms are coupled with optical cavity^[84]

Fig. 15. Superradiance of single atom^[18]. (a) hot atoms enter high-precision optical cavity through nano-hole array with specific pattern; (b) relation between number of intracavity photons and number of incident atoms

Fig. 16. Quantum simulation of Dicke model^[19]. (a) No phase transition occurs when intensity of lateral pumping light is less than critical value; (b) when pump intensity is greater than critical value, superradiance occurs along with self-organization; (c) conceptual drawings of energy levels and light fields; (d) excitation path of quantum state under action of cavity and pump light

Fig. 17. Quantum simulation of spin-exchange of atoms^[21]. (a) Dispersion interaction between strontium atoms and cavity; (b) schematic diagram of spin-exchange of atoms; (c) phase distortion related to J_Z and energy gap between collective states of atoms with total spin J and J-1 caused by χJZ2

Fig. 18. Atom entanglement generation assisted by optical cavity^[22]. (a) Energy level diagram of ⁸⁷Rb, coupling of two ground states driven by microwave field, and coupling of |1> and excited state driven by optical cavity; (b) probe light is blockaded as long as one atom is on state |1>; (c) generation process of atomic entangled state expressed by Husimi-Q function on Bloch sphere; (d) distribution of Husimi-Q functions on Bloch sphere corresponding to collective spin states of different atoms

Fig. 19. Evolution of quantum Zeno dynamics in atom-cavity coupling system ^[23]. (a) Energy levels of ⁸⁷Rb; (b) evolution of collective spin state of atoms driven by microwave is restricted in smaller Hilbert space under quantum Zeno effect; (c) trajectories of atomic spin states on Bloch sphere under different microwave driving modes without measurement, trajectory I passes through south pole, and trajectory II avoids south pole; (d) when 36 atoms are coupled with cavity, evolution of Husimi-Q functions corresponding to trajectory I (upper row) and II (lower row) under non-destructive measurement

Fig. 20. Scheme of heralded entanglement with large numbers of atoms^[24]. (a) atomic entanglement is produced when |v> photon passes through cavity QED system to probe |h> photon; (b) energy levels of ⁸⁷Rb and corresponding cavity coupling modes

Fig. 21. Spin squeezing produced by nondemolition measurement^[26]. (a) atom spin state is composed of |↑> and |↓>, and Rabi splitting of cavity QED with state transition from |↑> to excited state is used to measure atomic population on state |↑>; (b) schematic diagram of experimental setup; (c) time sequence and atomic spin state on Bloch sphere

Fig. 22. Generation scheme of atomic spin squeezing with squeezing degree of 20.1 dB^[30]. (a) Atoms are trapped at antinode of 780-nm cavity mode by 1560-nm intracavity standing wave trap, 780-nm cavity mode couples |↑> , |↓> and excited state simultaneously; (b) 780-nm probe light is generated by frequency doubling of 1560-nm after frequency stabilization; (c) homodyne detection device of output light field of optical cavity and its output signal

Fig. 23. Scheme for generating atomic spin squeezed state by cavity feedback^[32]. (a) N atoms are coupled to cavity mode with equal coupling strength, free space modes b₁ and b₂ are coupled to optical cavity through cavity mirrors; (b) frequency of probe light is located at edge of cavity line; (c) optical cavity couples |↑>, |↓> and excited state |e> simultaneously; (d) schematic diagram of spin squeezed state on Bloch sphere

Fig. 24. Nonlinear ONR transmission of few photons based on multi-atom cavity QED system^[90]. (a) Optical bistability phenomenon in asymmetric cavity QED system with different light field propagation directions, shaded area represents nonlinear ONR transmission window; (b) transmission efficiency of forward transmitted light and blocking rate of reverse light corresponding to different atomic numbers