Strong light-matter interaction in confined geometries
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Abstract
With the advent of technologies, such as ultra-stable laser systems, high-speed electronics, etc. a vast range of applications of quantum technology has been explored
with a list of promising outcomes. For instance, to understand the properties of a
many-body system, a quantum simulator has become an ideal candidate. Such
systems, otherwise, are highly complex to solve with the current computational power.
Internet security is one of the key challenges in the present time as the existing communication channel is prone to eavesdropping. The laws of quantum physics suggest a mechanism where an entangled photons-pair, for example, can be an ideal candidate for a secure information exchange. The probabilistic nature of wavefunction
collapse makes these photons impossible to clone which lies in the hearts of quantum
cryptography. The use of the wave nature of the atoms surpasses the limitations of the optical intereferometry measurements as the associated wavelength can
be prepared to extremely small lengthscales. Additionally, atoms prepared in a
Rydberg state serve strong sensitivity towards external fields. These quantum
sensing aspects open up a whole new dimension to metrology. To make use of a quantum system, atoms can be prepared to represent a pure quantum state which is the basis of quantum bits or qubits. Photons, when tuned
properly, can address a pair of such quantum states hence they serve as a mediator
for exchange of quantum information in a configuration known as flying qubits. Thus,
strong atom-light interaction at single photonic level becomes an important requirement. In this thesis, an atom-optics approach is made towards creating a strongly
interacting quantum system. To start with, 87Rb atoms are cooled down using laser
cooling techniques which provides an easy manipulation of the atomic
states. To enhance the light-atom interaction strength, the ongoing probe beam is
spatially overlapped with the atomic ensemble. In a free space setting, such overlap is
limited to the Rayleigh range of the probe beam as well as the trapping potential
which relies on the far red-detuned laser beam. To overcome these limitations, in
this project a hollow-core fiber is used which serves the purpose of first, keeping the
trapping laser beam confined and second, simultaneous overlap of the probe beam
throughout the length of the atomic ensemble. This project demonstrates an optical
lattice configuration, by overlapping two trapping laser beams, to trap and a controllable guiding of cold atoms. This geometrical confinement ensures the extended overlap of the probe light with the atoms which increases the interaction probability. As a
figure of merit, an optical depth of 200 has been detected which is two orders of magnitude higher than for the same number of atoms in free space configuration. Further manipulation of the probe photons can be achieved in a three level atomic configuration using a process known as electromagnetically induced transparency (EIT).
For several interesting applications in the quantum domain, strong nonlinearity at
single photonic level becomes a key necessity which requires strong photon-photon
interactions. With the EIT process, the photonic states can be mapped on
to the atomic states. Thus, a strong photonic interaction can be
mediated via atomic interaction. To reach exactly this goal the EIT configuration is
involved with Rydberg states – atoms excited to states with large principal quantum
number. Among a range of attractive features, the atom-atom interaction scales with n^11, where n is the principal quantum number of the atomic state. Rydberg excitation near dielectric surfaces has been a challenging task due to the presence of stray electric fields. This work has demonstrated, for the first time, Rydberg EIT with cold atoms inside a hollow core fiber. Furthermore, such fibers filled with Rb atoms at room temperature show significantly lower electric fields. This possibly is due to the homogeneous charge distribution which results in cancellation of the field in the core of the fiber. These results open a novel approach to perform quantum optics experiments with neutral atoms. The fiber-atom interface presented in this project further requires a detailed understanding of the propagation of the probe beam through the extended cloud. Here, the atomic ensemble can not only impose attenuation due to the absorption but the dispersive effects can modify the actual beam path through the extended atomic cloud. The transmission lineshape gets altered due to the combined dispersion and absorption of the propagating light. To understand the mechanism, a microlensing model is presented which describes the lineshapes for both, the simple two level system and the EIT configuration.