Zhang Tian-Cai

Elimination of the degenerate trajectory of a single atom strongly coupled to a tilted TEM{sub 10} cavity mode

We demonstrate the trajectory measurement of the single neutral atoms deterministically using a high-finesse optical micro-cavity. Single atom strongly couples to the high-order transverse vacuum TEM10 mode, instead of the usual TEM00 mode, and the parameter of the system is 10 ( , , ) / 2 (20.5,2.6,2.6) g MHz     . The atoms simply fall down freely from the magneto-optic trap into the cavity modes and the trajectories of the single atoms are linear. The transmission spectrums of atoms passing through the TEM10 mode are detected by a single photon counting modules and well fitted. Thanks to the tilted cavity transverse TEM10 mode, which is inclined to the vertical direction about 45 degrees and it helps us, for the first time, to eliminate the degenerate trajectory of the single atom falling through the cavity and get the unique atom trajectory. Atom position with high precision of 0.1m in the off-axis direction (axis y) is obtained, and the spatial resolution of 5.6m is achieved in time of 10s along the vertical direction (axis x). The average velocity of the atoms is also measured from the atom transits, which determines the temperature of the atoms in magneto-optic trap, 186K±19K. PACS number(s): 42.50.Pq, 37.10.Jk Manipulation of neutral single atoms, known as the basic system of quantum optics and atomic physics, has been extensively studied since the last two decades, either in free space [1-6] or inside a cavity [7-10]. From the early hot and stochastic atom beam to the cold and deterministic control of individual atoms, single atom is now a good system to demonstrate quantum logic gate [11] and quantum register [4, 12]. In order to get information from a single atom two predominant methods are used in most experiments. One is to detect directly the fluorescence of the atoms using a special designed light collection system and high-efficiency optical detector; another is to detect the transmitted light from a high-finesse optical cavity, which is usually strongly coupled to the atoms [13]. The cavity quantum electrodynamics (CQED) system has been used to detect single atoms as well as the atom trajectory [14]. Large coupling between single atom and cavity provides the capability of measuring the atomic trajectory through the transmission of the cavity [15, 16]. In the earlier experiments, atom beam has been used in CQED experiments [17-19] and the duration of the atom transits was so short that the detection of individual atom positions and its trajectory could not be accomplished in real time. The development of the cold atoms technology and the manipulation of single atoms [20, 21] provide the effective tools for the CQED experiments. Either through the atom free falling down or launching up to the cavity, the transit time of atoms in the cavity mode lasted more than 100μs [15, 16] and the trajectories of single atom can be measured. In 2000, Hood et al trapped single atom inside a micro-cavity for milliseconds and the 2D atom trajectories in the plane perpendicular to the cavity axis were reconstructed from the cavity transmissions and they obtained 2m of the spatial resolution in a 10s of time interval [14]. But for all these experiments mentioned above, the atom was coupled to the fundamental Hermite-Gaussian TEM00 mode. Although the coupling between atom and cavity TEM00 mode is stronger than all the other modes when the atom passes in the centre of cavity mode, the displacements of the atom along the cavity axis, or between a node and antinode, can not be determined since the spatial symmetry of the TEM00 mode causes the quadruple degeneracy of the atom trajectories in principle. Higher order transverse modes may break the spatial symmetry and reduce the degeneracy of the atom trajectories. In 2003, Puppe et al. demonstrated the single-atom trajectories in high-order transverse modes of a high-finesse optical cavity [22]. The atom trajectories were obtained according to the transmission spectrum of the cavity. However, the spatial patterns of TEM01 and TEM10 modes were oriented nearly horizontally and the atom trajectories were still degenerate, but from quadruple degeneracy to duplicate degeneracy. In this letter, a tilted spatial transverse TEM10 mode is used, which breaks the symmetry and allows eliminating the degeneracy of the atom trajectories completely. We use the strong coupled atom-cavity system to track the atomic path and determine the ballistic trajectory of a single atom uniquely. The spatial resolution of 5.6m is achieved in the time of 10s along the vertical direction (axis x), while the atom position along the horizontal direction (axis y) can be obtained with precision of 0.1m. With the help of even higher order modes and smaller mode waist of the high-finesse cavity, it is capable to obtain the trajectory of the single atom with high spatial resolution by the so-called atomic kaleidoscope [23, 24] based on the atom-cavity microscope (ACM) system [14]. The CQED system contains a cavity and an atom couples to a single mode of the electromagnetic field as shown in Fig. 1. The interaction between the atom and the single-mode field is described by the oscillatory exchange of energy (Rabi Oscillation), characterized by g. Real experimental system is an open system and both of cavity decay rate κ and atom decay rate γ must be taken in account. In the strong coupling regimes the optimum coupling constant g0 in the TEM00 mode is much lager than κ and γ. The cavity transmission in the weak-field limit of small excited state population is [25]

Experimental Study on Coherence Time of a Light Field with Single Photon Counting

The second-order degree of coherence of pseudo-thermal light and coherence time are experimentally studied via the Hanbruy-Brown-Twiss (HBT) scheme. The system consists of two non-photon-number-resolving single- photon-counting modules (SPCMs) operating in the Geiger mode. We investigate the coherence time of the incident beam for different spot sizes on a ground glass and speeds of a rotating ground glass. The corresponding coherence time can be obtained from Gaussian fitting for the measured second-order degree of coherence. The results show that the coherence time of measured pseudo-thermal light depends on the spot sizes and the rotating speeds of the ground glass. The maximum value of the second-order degree of coherence is reduced as the rotating speed decreases. This result can be well explained by the model of mixed thermal and coherent fields with different ratios.

Observation of single neutral atoms in a large-magnetic-gradient vapour-cell magneto-optical trap

Single caesium atoms in a large-magnetic-gradient vapour-cell magneto-optical trap have been identified. The trapping of individual atoms is marked by the steps in fluorescence signal corresponding to the capture or loss of single atoms. The typical magnetic gradient is about 29 mT/cm, which evidently reduces the capture rate of magneto-optical trap.

Continuously transferring cold atoms in caesium double magneto-optical trap

We have established a caesium double magneto-optical trap (MOT) system for cavity-QED experiment, and demonstrated the continuous transfer of cold caesium atoms from the vapour-cell MOT with a pressure of ~ 1?10?6 Pa to the ultra-high-vacuum (UHV) MOT with a pressure of ~ 8?10?8 Pa via a focused continuous-wave transfer laser beam. The effect of frequency detuning as well as the intensity of the transfer beam is systematically investigated, which makes the transverse cooling adequate before the atoms leak out of the vapour-cell MOT to reduce divergence of the cold atomic beam. The typical cold atomic flux got from vapour-cell MOT is ~ 2?107 atoms/s. About 5?106 caesium atoms are recaptured in the UHV MOT.

Sensitive Detection of Individual Neutral Atoms in a Strong Coupling Cavity QED System

We experimentally demonstrate real-time detection of individual cesium atoms by using a high-finesse optical micro-cavity in a strong coupling regime. A cloud of cesium atoms is trapped in a magneto-optical trap positioned at 5 mm above the micro-cavity center. The atoms fall down freely in gravitation after shutting off the magneto-optical trap and pass through the cavity. The cavity transmission is strongly affected by the atoms in the cavity, which enables the micro-cavity to sense the atoms individually. We detect the single atom transits either in the resonance or various detunings. The single atom vacuum-Rabi splitting is directly measured to be ? = 2? ? 23.9 MHz. The average duration of atom-cavity coupling of about 110 ?s is obtained according to the probability distribution of the atom transits.

Characterizing optical dipole trap via fluorescence of trapped cesium atoms

Optical dipole trap (ODT) is becoming an important tool of manipulating neutral atoms. In this paper ODT is realized with a far-off resonant laser beam strongly focused in the magneto-optical trap (MOT) of cesium atoms. The light shift is measured by simply monitoring the fluorescence of the atoms in the magneto-optical trap and the optical dipole trap simultaneously. The advantages of our experimental scheme are discussed, and the effect of the beam waist and power on the potential of dipole trap as well as heating rate is analyzed.

Autler–Townes doublet in novel sub-Doppler spectra with caesium vapour cell

With a coupling laser locked to caesium 6S1/2 Fg=4–6P3/2 Fe=5 cycling transition and a co-propagating probe laser scanned across 6S1/2 Fg=4–6P3/2 Fe=3, 4 and 5 transitions, a novel scheme for sub-Doppler spectra in Doppler-broadened V-type three-level system is demonstrated by detecting the transmission of the coupling laser through a caesium vapour cell. The Autler–Townes doublet in the sub-Doppler spectra of the coupling laser is clearly observed. The effects of coupling laser intensity on the splitting and linewidth of the Autler–Townes doublet are experimentally investigated and the results agree well with theoretical predictions. Taking the multiple hyperfine levels of caesium atom into account, a brief analysis is presented.

Atomic GHZ states prepared in two directly coupled cavities with virtual excitations in one step

A scheme for one-step preparation of atomic GHZ states in two directly coupled cavities via virtual excitations is proposed. In the whole procedure, the information is carried only in two ground states of Λ-type atoms, while the excited states of atoms and cavity modes are virtually excited, leading the system to be insensitive to atomic spontaneous emission and photon loss.

The duality of a single particle with an n-dimensional internal degree of freedom

The wave—particle duality of a single particle with an n-dimensional internal degree of freedom is re-examined theoretically in a Mach—Zehnder interferometer. The famous duality relation D2 + V2 ≤ 1 is always valid in this situation, where D is the distinguishability and V is the visibility. However, the sum of the particle information and the wave information, can be smaller than one for the input of a pure state if this initial pure state includes the internal degree of freedom of the particle, while the quantity D2 + V2 is always equal to one when the internal degree of freedom of the particle is excluded.

Quantum Entanglement Dynamics of Two Atoms in Two Coupled Cavities

Quantum entanglement dynamics for two atoms trapped in two coupled cavities is investigated. Numerical results show that the present of the two atomic excitations is mainly accounted for the entanglement-sudden-death (ESD) effect with the two cavities initially in the vacuum. The entanglement can also be controlled by the hopping rate and the imbalances between the two atom-cavity coupling rates.