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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Microchips for single atom detection and spin squeezing

Monika Schleier-Smith, Ian D. Leroux, Yu‐Ju Lin, Igor Teper, Vladan Vuletić

Published Jun 1, 2007• 0 citations
Publisher

Abstract

Magnetic microtraps for ultracold atoms combine tight confinement, long storage times, high degree of control over the atomic position, and good optical access. Atom detection can be implemented via fluorescence, absorption, or dispersive measurements. All three types of measurements can be enhanced by an optical resonator, where the enhancement factor is proportional to the resonator finesseF. In the limit of negligible detector counts, in a setup where the detector registers n counts per atom in a fluorescence measurement, the atom number resolution is DeltaNf= (N/n)1/2for a fluorescence measurement, and DeltaNa= (1/4N)1/2for an absorption measurement. This means that absorption and fluorescence measurements perform similarly for one atom, but that the absorption measurement is superior for N >1. However, in absorption measurements the number of atoms must remain sufficiently small for the atomic absorption not to deteriorate the resonator finesse. Larger samples can be probed by dispersive measurements, where the tuning of the resonator due to the atomic index of refraction is measured. In the absence of technical noise, the corresponding atom number resolution is independent of detuning from the atomic transition, and given by DeltaNd= (1/n)1/2. Using fluorescence detection in a resonator of finesse F=8000, we detect two counts per atom in 250 mus, enabling single-atom detection with 75% efficiency (Teper et al., 2006). Absorption detection with 3.3% transmission attenuation enables us to measure small samples with up to 10 atoms with a resolution of about one atom. Absorptive and dispersive detection are interesting in that the resolution is not decreased with higher atom number. The reason is that both rely on the forward scattered signal, where the fields emitted by different atoms add coherently. These methods therefore allow non-destructive measurements below the atom-number shot noise, provided that the optical depth of the sample exceeds unity. We have realized an optical depth of several thousand inside the resonator, which should enable trapped-atom detection significantly below the shot-noise limit. If the detection is applied in a state-selective manner to a sample of two-level atoms (Geremia et al., 2004), conditional spin squeezing results. Such squeezing of the pseudo-spin for an atomic clock transition can be used to realize a hyperfine atomic clock on a microchip (Treutlin et al., 2004) that operates below the atom number shot noise limit (standard quantum limit). We report experimental progress towards pseudo-spin squeezing for87Rb atoms trapped on the microchip.

DOI
10.1109/cleoe-iqec.2007.4386690

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Magnetic microtraps for ultracold atoms combine tight confinement, long storage times, high degree of control over the atomic position, and good optical access. Atom detection can be implemented via fluorescence, absorption, or dispersive measurements. All three types of measurements can be enhanced by an optical resonator, where the enhancement factor is proportional to the resonator finesseF. In the limit of negligible detector counts, in a setup where the detector registers n counts per atom in a fluorescence measurement, the atom number resolution is DeltaNf= (N/n)1/2for a fluorescence measurement, and DeltaNa= (1/4N)1/2for an absorption measurement. This means that absorption and fluorescence measurements perform similarly for one atom, but that the absorption measurement is superior for N >1. However, in absorption measurements the number of atoms must remain sufficiently small for the atomic absorption not to deteriorate the resonator finesse. Larger samples can be probed by dispersive measurements, where the tuning of the resonator due to the atomic index of refraction is measured. In the absence of technical noise, the corresponding atom number resolution is independent of detuning from the atomic transition, and given by DeltaNd= (1/n)1/2. Using fluorescence detection in a resonator of finesse F=8000, we detect two counts per atom in 250 mus, enabling single-atom detection with 75% efficiency (Teper et al., 2006). Absorption detection with 3.3% transmission attenuation enables us to measure small samples with up to 10 atoms with a resolution of about one atom. Absorptive and dispersive detection are interesting in that the resolution is not decreased with higher atom number. The reason is that both rely on the forward scattered signal, where the fields emitted by different atoms add coherently. These methods therefore allow non-destructive measurements below the atom-number shot noise, provided that the optical depth of the sample exceeds unity. We have realized an optical depth of several thousand inside the resonator, which should enable trapped-atom detection significantly below the shot-noise limit. If the detection is applied in a state-selective manner to a sample of two-level atoms (Geremia et al., 2004), conditional spin squeezing results. Such squeezing of the pseudo-spin for an atomic clock transition can be used to realize a hyperfine atomic clock on a microchip (Treutlin et al., 2004) that operates below the atom number shot noise limit (standard quantum limit). We report experimental progress towards pseudo-spin squeezing for87Rb atoms trapped on the microchip.
We explore the feasibility of single atom detection on an atom chip by using a tiny fluorescence detector mounted on the chip. Resonant fluorescence from a trapped ultracold atom will be collected with a miniature aspheric lens and taken out of a vacuum chamber through a fiber. During detection, the atom can be held at the focus of the detector with a dipole trapping beam introduced through the same fiber. We have experimentally determined the optical performance of such a detector, taking into account effects such as stray light from the dipole trapping beam and chromatic aberration. The collection efficiency for isotropically emitted radiation is experimentally obtained to be 2.5%. From this, it is estimated that the fluorescence emitted from a single Rb atom will produce a photon count rate of 4.7x10(4) Hz, which is much larger than the shot noise limited background fluctuations.
The subject of this thesis is the construction and characterization of an experimental setup for studying the interaction of an ensemble of 10^5 - 10^6 cold 87 Rb atoms held in a far off-resonance optical dipole trap with a probe detuned by about 100 natural line widths, a regime in which refractive effects are resonantly enhanced but absorption is small. An important feature of this type of atom-light interface is that it facilitates quantum non-demolition measurements which in turn have numerous applications in quantum information technology, as well as in the field of ultra-precise metrology where they allow the measurements to overcome the so-called standard quantum limit. The use of a dipole trap has several advantages over more conventional approaches in which vapor cells or magneto-optical traps (MOT) have been used. Most importantly interaction strength of comparable magnitude can be achieved with a far lower number of atoms and with longer coherence times. The elongated geometry that can be realized in a dipole trap maximizes the interaction strength. One of the complications that arise is that the achievable interaction strength is highly sensitive to the mode-matching between the trapped ensemble and the probe beam. The theoretical description of the interaction between a trapped ensemble and a freely-propagating probe beam is provided in the framework of continuous variables. The relevant quantities are a pseudo-spin, defined for an ensemble of atoms in their ground state, and the Stokes vector that characterizes the polarization state of light. In practice the experiment consists of four main blocks: the atom traps, the optical pumping setup, the probe laser setup, and the photodetection. Before loading into the dipole trap, atoms are cooled in a MOT. An existing MOT setup has been improved by implementing a new, more powerful laser system. A dedicated dipole trap, which had been constructed in collaboration with another project, has been thoroughly characterized and the results are presented in this thesis. Setups for optical pumping and for the probe have been built, characterized and incorporated into the apparatus. Several experimental techniques have been developed and tested in order to optimize the alignment of the probe with the trapped ensemble. The effect of the geometrical overlap on the achievable signal to noise ratio is discussed and illustrated using simple, intuitive models. Photo-detection is a critical part of the setup. In order to facilitate detection of non-classical (squeezed) states of the atomic (pseudo-) spin, the detection system must be shot-noise limited within the range of photon numbers used in the experiments. Such a system has been constructed and tested, and is described in detail. It employs a charge-sensitive amplifier (CSA) but, unlike earlier implementations, it does not use a pulse-shaping stage. Instead, an algorithm has been devised and implemented that derives the quantities of interest directly from the output signal of the CSA. This novel approach avoids the technical problems that arise in systems using hardware-based pulse shaping. It also makes the detector more flexible in terms of the shape and timing of the probe pulses that can be used. The atomic states produced in the setup are intrinsically sensitive to stray magnetic fields. Therefore an efficient cancellation of stray fields is essential. On the other hand having the possibility to apply a fully controlled, external magnetic field makes the experimental setup more versatile. Both goals can be achieved with a magnetic field control unit that has been built during this project. It facilitates active cancellation of stray magnetic fields at the frequency of the mains and in addition allows to apply an arbitrary waveform magnetic fields along all three axes. One of the main results reported in this thesis is the observation of polarization rotation caused by population imbalance created in the process of optical pumping and, in a separate series of measurements, due to an applied magnetic field. These measurements have allowed us to assess the strength of the atom-light interaction available in the current setup. They indicate that with the necessary modifications to the optical pumping scheme the system should prove successful in implementing quantum information protocols, in particular quantum memory for light. An interesting off-shoot has been the spectroscopic characterization of the atomic state in the dipole trap. In these measurements Doppler free spectra are recorded in a very simple setup without the need to use nonlinear techniques. Apart from providing a method to characterize the efficiency of the optical pumping into the hyperfine level of interest, the acquired spectra are a very direct illustration of the power of the laser cooling techniques developed in the recent years and without which this work would not have been possible.
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.
In this paper, we describe the preparation and detection of ultracold atoms on a microchip with single-atom sensitivity. The detection scheme is based on multi-photon ionization of atoms and the subsequent guiding of the generated ions by ion optics to a channel electron multiplier. We resolve single atoms with a detection efficiency above 60%. The detector is suitable for real-time observations of static and dynamic processes in ultracold quantum gases. Although the ionization is destructive, sampling a small subset of the atomic distribution is sufficient for the determination of the desired information. We take full high-resolution spectra of ultracold atoms by ionizing only 5% of the atoms. Using an additional microwave near 6.8 GHz, the detection scheme becomes energy, position and state selective. This can be used for in situ determination of the energy distribution and temperature of atom clouds inside the trap and applied for future correlation measurements.
At the beginning of the 20th century measurement precision was related exclusively to the capabilities of the experimenter and the apparatus. Any imprecision was attributed to imperfect devices for the measurement. Later on, when the quantumness of objects like photons, electrons, or atoms, was recognized, it became clear that the measurement and the object to be measured could no longer be separated. Both follow the laws of quantum physics which impose limits on the accuracy of any measurement. In the last three decades, a lot of research has been dedicated to find strategies to overcome these limitations. This new field of quantum metrology is where this work is placed. This thesis describes the generation of interparticle entanglement in form of a squeezed spin state in a cloud of cold 87Rb atoms by performing nondestructive spin measurements. Towards this goal, we have implemented a shot-noise limited polarization probing and detection system, atom counting by means of absorption imaging, spin state preparation via optical pumping and nondestructive spin state detection. The quantum noise limited spin readout with polarized light is characterized in two complementary ways in terms of sensitivity and all possible quantum and classical noise sources are measured. To address the rich opportunities provided in the multilevel structure of 87Rb with dispersive probing, we have developed and implemented a frequency offsetlock which allows a large tuning range of several GHz for the dispersive probing. Working with a magnetically sensitive atomic system triggered the development of a magnetic field imaging technique. This applies the elongated atomic sample and internal atomic states as a way of measuring magnetic fields over several millimeters with micrometer resolution and sensitivities down to a few tens of pT/ Hz. We developed a new method for tomographic measurements of the atomic density matrix. Based on our ability of shot noise limited detection of polarization rotations we can in principle, characterize the whole atomic spin state. First, measurements are shown and possible improvements are discussed. An in depth characterization of the light-atom interaction between polarized pulses of light and collective atomic states are the basis for measurements at the quantum level. On one side, we apply classical measurements of spin polarized atoms to measure the strength of the lightatom interaction. On the other side, we use quantum measurements of unpolarized atomic states to characterize all noise sources both technical and quantum. One of the major results of this thesis is the realization of quantum nondemolition measurements in a largespin system. We realized limitations to the performance of the QND measurement implied by higher order light shifts in multilevel atoms, which are often neglected for these kind of systems. We develop a technique which suppresses the detrimental effects of higher order light shifts and recovers an ideal QND measurement. The thesis culminates in the demonstration of spin squeezing in the magnetically sensitive ground state of 87Rb. We achieve a quantum noise reduction by 3 dB, where metrological sensitivity is improved by 2 dB beyond the standard quantum limit of a coherent spin state with the same spin polarization. We address the question how spatial and temporal inhomogeneities both in light and atoms influence the preparation of nonclassical states. An existing covariance matrix approach is extended and we apply it to study effects of detector time resolution, spatial inhomogeneities and atomic motion.