We describe the implementation of a system for studying light-matter interactions using an ensemble of ${10}^{6}$ cold rubidium 87 atoms trapped in a single-beam optical dipole trap. In this configuration the elongated shape of the atomic cloud increases the strength of the collective light-atom coupling. Trapping all optically allows for long storage times in a low decoherence environment. We are able to perform several thousands of measurements on one atomic ensemble with little destruction. We report results on paramagnetic Faraday rotations from a macroscopically polarized atomic ensemble. Our results confirm that strong light-atom coupling is achievable in this system which makes it attractive for single-pass quantum information protocols.
We describe the implementation of a system for studying light-matter interactions using an ensemble of 106 cold rubidium 87 atoms trapped in a single-beam optical dipole trap. In this configuration the elongated shape of the atomic cloud increases the strength of the collective light-atom coupling. Trapping all optically allows for long storage times in a low decoherence environment. We are able to perform several thousands of measurements on one atomic ensemble with little destruction. We report results on paramagnetic Faraday rotations from a macroscopically polarized atomic ensemble. Our results confirm that strong light-atom coupling is achievable in this system which makes it attractive for single-pass quantum information protocols.
Atomic ensembles have gained a lot of interest in the last decade for their prospective use as quantum memories, in precision measurements and other applications in quantum information processing. The atomic part of the interface presented here is an all-optically trapped atomic ensemble of 87Rb at a temperature of 25 muK. The ensemble consists of 106atoms and has an elongated shape with an aspect ratio of about 250:1. The resonant optical depth is around 50. The light part is an off-resonant polarized laser beam which dispersively probes the atomic state. The detection is a shot-noise limited polarization measurement of light pulses of a few mus and 3 x 105photons.
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.
We study the collective radiation properties of cold, trapped ensembles of atoms. We consider the high-density regime with the mean interatomic distance being comparable to, or smaller than, the wavelength of the resonant optical radiation emitted by the atoms. We find that the emission rate of a photon from an excited atomic ensemble is strongly enhanced for an elongated cloud. We analyze collective single-excitation eigenstates of the atomic ensemble and find that the absorption-emission spectrum is broadened and shifted to lower frequencies as compared to the noninteracting (low-density) or single-atom spectrum. We also analyze the spatial and temporal profile of the emitted radiation. Finally, we explore how to efficiently excite the collective superradiant states of the atomic ensemble from a long-lived storage state in order to implement matter-light interfaces for quantum computation and communication applications.
The main objective of this research is to study a particular case of light-matter interaction: the scattering of light in dense atomic samples.The regime of high atomic densities is represented by samples where the distance between neighboring atoms is on the order of the wavelength associated with the atomic transition.In this regime, short-range atomic interactions cannot be neglected, and in these systems, we expect to observe significant modifications in collective effects such as subradiance or superradiance.Overall, the propagation of light through this type of system is not well-known to the scientific community, and the existing models work to describe very specific situations.To pave the way for achieving our overall goal, this work proposes an experimental setup to obtain a dense cloud of 88 Sr, which is the chemical species we work with in our laboratory.For this purpose, atoms previously trapped using a Magneto-Optical Trap are transferred to a Crossed Optical Dipole Trap.During the optimization and characterization of the optical trap, problems were encountered in producing an image with quantitative information of the atomic ensemble and in transferring a large number of atoms in a very small trapping volume.These problems were resolved with the implementation of a Phase Contrast Imaging system and the implementation of an Optical Molasses as an additional cooling step.Under these conditions, we obtained a cloud that can be suitable for conducting studies on collective effects, transitioning from the dilute to the dense regime.The initial experiments, once our atomic ensemble was prepared, involve measuring the coherent transmission of a low-intensity beam through the cloud.For this purpose, incident light near the resonance of a dipolar transition of the type J = 0 ↔ J = 1 of 88 Sr is used, interacting with a homogeneous region of the sample.Simulations of this type of process can be performed using the Coupled Dipole Model, which describes the overall response of light scattering by the sample considering dipole-dipole interactions mediated by light.With this work, we aim to advance the understanding of density effects by comparing the coherent optical response of the dense cloud with the theory based on the Coupled Dipole Model.
We present an experimentally feasible protocol for the complete storage and retrieval of arbitrary light states in an atomic quantum memory using the Faraday interaction between light and matter. Our protocol relies on multiple passages of a single light pulse through the atomic ensemble without the impractical requirement of kilometer-long delay lines between the passages. A time-dependent interaction strength enables the storage and retrieval of states with arbitrary pulse shapes. The fidelity approaches unity exponentially without squeezed or entangled initial states, as illustrated by calculations for a photonic qubit.
We investigate the interplay between Zeeman and light shifts in the transmission spectrum of an optically trapped, spin-polarized Rubidium atom. The spectral shape of the transmission changes from multiple, broad resonances to a single, narrow Lorentzian with a high resonant extinction value when we increase the magnetic field strength and lower the depth of the dipole trap. We present an experimental configuration well-suited for quantum information applications that enables not only efficient light-atom coupling, but also a long coherence time between ground state hyperfine levels.