Quantum Tomography Of Single Photons From Cold Atomic Ensemble

Single photons are a kind of flying qubits.It has the characteristics of fast motion,weak interaction with the external environment,suitable for long-distance transmission,and easy manipulation of quantum states.Therefore,it has crucial application significance in quantum technology,and it will also play an essential role in the future of quantum networks technology.In addition,single photons,like qubits,have necessary scientific research value and the technical application potential in quantum communication and quantum precision measurement.The quantum entanglement exchange between the atomic ensembles based on the single-photon source and the teleportation of the single-photon state has already come true in the atomic ensemble.The cold atomic ensemble,a medium platform combined with laser cooling and trapping technology,has many advantages in preparation and storage,and manipulation of single photons.The main content of this thesis is to use the interaction of light and cold atomic ensemble to generate heralded single photons through the four-wavemixing process,and to explore the quantum tomography of single-photon time mode quantum states and build qubits on this basis.Regarding the research of temporal mode single photons,this thesis mainly completes the following three tasks:1.Based on the cold atomic ensemble,the entangled photon pairs are obtained from the spontaneous four-wave mixing process of embedded electromagnetically induced transparency effect,thereby obtaining the heralded single photon.Through the combination of balanced homodyne detection and heterodyne detection,the heralded single photon has been characterized in the time domain,and the density matrix of the single-photon state in the time bin space is reconstructed.Comprehensively obtain the time wave function of a single photon state,including its function amplitude and phase information.2.By manipulating the classical light field,we can control the amplitude and phase of the wave function of the single-photon state in the time-frequency space,so that we can construct the orthogonal basis of the single-photon state in the time dimension.It demonstrates that the single-photon state in the time-frequency dimension can be used to encode information and has potential applications in quantum communication and quantum computing.3.Using the quantum tomography scheme,we reconstruct the Fock state singlephoton density matrix and its Wigner function in phase space.The single photons are generated from the cold atomic ensemble with electromagnetically-induced transparency,which selecting the mode of the generated photons.In conclusion,the work of this paper fully characterizes the time wave function of a single photon and the purity of a single photon state of Fock state experimentally,paving the way for the application of single-photon temporal mode.In the timefrequency space,the wave function of a single photon can be used as a qubit to encode and be applied in the fields of quantum communication and quantum computing.