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Position and Temperature Measurements of a Single Atom via Resonant Fluorescence

Richard Wagner

Published Sep 18, 2019• 1 citation
OpenAlex

Abstract

The magneto-optical trap (MOT) has been an important tool in quantum optics research for three decades. MOTs allow for hundreds of thousands to millions of atoms to be cooled to micro-Kelvin temperatures for use in a wide variety of experiments. For nearly as long, MOTs with just a single atom have been of some interest to the research community. We have developed an algorithm, based on Bayesian statistics, to carefully measure small numbers of atoms in a MOT.\n\nMany techniques have been developed to measure the temperature of atoms in a MOT, including some that can translate to single atoms. We propose a new technique to measure the temperature of a single atom without releasing the atom from the MOT. Temporal modulations in a spatially dependent magnetic field encode information about the position of an atom through associated variation in its fluorescence rate. Measuring this variation reveals the atom's position distribution and therefore its temperature. The technique is examined for a variety of MOT parameters. Measurements with the technique are an order of magnitude larger than predicted by theory and potential routes for future study are offered.

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The magneto-optical trap (MOT) has been an important tool in quantum optics research for three decades. MOTs allow for hundreds of thousands to millions of atoms to be cooled to micro-Kelvin temperatures for use in a wide variety of experiments. For nearly as long, MOTs with just a single atom have been of some interest to the research community. We have developed an algorithm, based on Bayesian statistics, to carefully measure small numbers of atoms in a MOT.\n\nMany techniques have been developed to measure the temperature of atoms in a MOT, including some that can translate to single atoms. We propose a new technique to measure the temperature of a single atom without releasing the atom from the MOT. Temporal modulations in a spatially dependent magnetic field encode information about the position of an atom through associated variation in its fluorescence rate. Measuring this variation reveals the atom's position distribution and therefore its temperature. The technique is examined for a variety of MOT parameters. Measurements with the technique are an order of magnitude larger than predicted by theory and potential routes for future study are offered.
We discuss a simple time of flight technique for measurement of temperature of a cold cloud in a magneto-optical trap (MOT). The technique is based on spatiotemporal fluorescence imaging of the cloud that is allowed to undergo one-dimensional expansion in the presence of the orthogonal two-dimensional configuration of laser beams by temporal modulation of a pair of counterpropagating trapping beams in the MOT. We show that, in the time scale 0< or =t<5 ms, the expansion of the cloud is ballistic and the temperature can be extracted from the time variation of the rms size of the cloud in the expansion direction. The reliability of the technique has been established by comparing the results with release and recapture method, and also by fitting them to the known temperature scaling law.
In our experiment, firstly, we carry out the loading of single atoms in the magneto-optical trap (MOT) by increasing the quadrupole magnetic field gradient, improving the background vacuum, and reducing the diameters of the cooling and trapping laser beams. Secondly, we get the single atomic fluorescence signal of a high signal-to-background ratio in the MOT by means of reducing the detuning of cooling light and increasing its intensity appropriately, and using the polarization spectroscopy locking technique to suppress the fluctuations of cooling laser. In addition, with the real-time feedback on quadrupole magnetic field gradient, we demonstrate a probability of loading single atoms in the MOT as high as 98%. We also measure the statistical properties of the single atomic fluorescence which is excited by continuous light in the MOT; the measured second-order correlation is g(2)( = 0) = 0.09.
By recording the fluorescence fraction of the cold atoms remaining in the magneto-optical trap (MOT) as a function of the release time, the release-and-recapture (R&R) method is utilized to evaluate the effective temperature of the cold atomic ensemble. We prepare a single atom in a large-magnetic-gradient MOT and then transfer the trapped single atom into a 1064-nm microscopic optical tweezer. The energy of the single atom trapped in the tweezer is further reduced by polarization gradient cooling (PGC) and the effective temperature is evaluated by extending the R&R technique to a single atom tweezer. The typical effective temperature of a single atom in the tweezer is improved from about 105 μK to about 17 μK by applying the optimum PGC phase.
Rydberg atoms are those which have their valence electron excited to high principal quantum number n. Atoms in the Rydberg state are the research focus of the experiment from which this paper derives. Rydberg atoms are fragile, and thermal collisions are enough to ionize and destroy the Rydberg state. A magneto-optical trap (MOT) serves the purpose of "ultra-cooling" atoms to temperatures around 140 micro-Kelvin and dramatically reduces the impact of thermal collisions. This paper details the theory and equipment necessary to realize a MOT of rubidium-85. It will then discuss the characterization of the MOT we implemented by a technique called low intensity absorption imaging. The result of the measurements by the absorption imaging gave an atom count of N=8.87 +/- (0.45)stat + (0.14)sys X 105 atoms and a peak atomic density of n0=1.57 +/- (0.14)stat + (0.02)sys X 1011 cm-3, where "stat" and "sys" denote statistical and systematic uncertainties. The MOT is of sufficient size and density to continue with the long term goals of the experiment [1][2].
We experimentally demonstrate a reliable method based on a nanofiber to optimize the number of cold atoms in a magneto–optical trap (MOT) and to monitor the MOT in real time. The atomic fluorescence is collected by a nanofiber with subwavelength diameter of about 400 nm. The MOT parameters are experimentally adjusted in order to match the maximum number of cold atoms provided by the fluorescence collected by the nanofiber. The maximum number of cold atoms is obtained when the intensities of the cooling and re-pumping beams are about 23.5 mW/cm2 and 7.1 mW/cm2, respectively; the detuning of the cooling beam is −13.0 MHz, and the axial magnetic gradient is about 9.7 Gauss/cm. We observe a maximum photon counting rate of nearly (4.5±0.1) × 105 counts/s. The nanofiber–atom system can provide a powerful and flexible tool for sensitive atom detection and for monitoring atom–matter coupling. It can be widely used from quantum optics to quantum precision measurement.
Recording the fluorescence of a magneto-optical trap (MOT) is a standard tool for measuring atom numbers in experiments with ultracold atoms. When trapping few atoms in a small MOT, the emitted fluorescence increases with the atom number in discrete steps, which allows one to measure the atom number with single-particle resolution. Achieving such single-particle resolution requires stringent minimization of stray light from the MOT beams, which is very difficult to achieve in experimental setups that require in-vacuum components close to the atoms. Here, we present a modified scheme that addresses this issue: Instead of collecting the fluorescence on the MOT (D2) transition, we scatter light on an additional probing (D1) transition and collect this fluorescence with a high-resolution microscope while filtering out the intense MOT light. Using this scheme, we are able to reliably distinguish up to 17 40K^{40}\mathrm{K}40K atoms with classification fidelities of $\ensuremath{\sim}98%$ for up to 5 atom numbers and fidelities of more than 85% for up to 17 atoms.