Apoorva Bisht

Author: Apoorva Bisht | Majors: Physics and Computer Science | Semester: Fall 2022

I started my research project in physics in the spring semester of my first year in Dr. Nakamura’s lab. I joined the lab as the combination of optics and its applications captured my interest. The goal of the project of which the current work is a sub-part, is to deterministically generate single photons and identify reliable single photon sources. Single photons are promising candidates for qubits (quantum bits) which have applications in quantum computing. The first steps in the process for generating reliable single photons is to produce probabilistic single photons and I spent the last year working towards this.

The first part involves using a mode-locked 740 nm laser to generate second harmonic light which in turn will generate 740 nm photons via down conversion. Both processes, SHG (Second Harmonic Generation) and SPDC (Spontaneous Parametric Down Conversion) are sensitive to the phase-matching angle that is, the angle of the incident light and the optic axis. This angle is first theoretically determined and crystal (β-barium borate, a non-linear crystal) for the corresponding angle is custom-ordered.

Since the mode-locked laser in the lab is utilized by various projects, it was essential that the design used minimum space and was modular. At the same time, we wanted the experiment to be at a table with minimum vibrations to prevent disturbance to the phase-matching angle. A periscope guided the light from the mode-locked laser to the setup. The following details of the setup are shown in figure 1. Second harmonic generation had already been achieved in the previous setup, but the angles had to be re-aligned for this new setup. After successfully tweaking the phase matching angle and the focusing of the pump laser at the crystal, a conversion efficiency of around 3% was achieved.

The next step involved tweaking the phase matching angle for parametric down conversion of the generated SHG light. This has been more challenging since SPDC conversion efficiency is very low which means that the potential 740 nm light generated cannot be observed by naked eye. In addition, the pump to SPDC power would be very high which poses a challenge to suppressing the pump enough such that the SPDC photons can be visualized. There are three aspects which were focused on to optimize the conditions for SPDC: 1. Focusing of the pump beam at second BBO crystal. Beam waist should be at the center of the crystal and the spot size at the crystal should be small enough. The former is mainly attained by translating the focusing lens before the crystal and the latter by using a lens of small enough focal length. 2. Suppressing the pump in relation to the potential SPDC photons. For the detectors to be able to detect the SPDC photons, it is important that the pump intensity at the detector should not mask the SPDC photons. A combination of filters is used to achieve this. Since UV light can cause fluorescence in glass which can lead to further noise, the filters were arranged in a “Z” fashion to dump the majority of the discarded pump away from the main path as shown in figure 1. 3. Detector. The SPDC photons generated are expected to have very low power (around few nW) which means that standard power meters cannot be utilized. Thus, high efficiency detector was utilized in our efforts to image the SPDC photons.

Once probabilistic single photons are generated, the next part will involve temporal multiplexing for deterministic generation of single photons. Our vision is to demonstrate easy and robust single photon generation system to expand the application of single photon source as well as to provide means to perform interesting quantum-entanglement experiment even suitable for undergraduate-level labs.

I am grateful towards my advisor, Dr. Nakamura for patiently explaining the theoretical aspects of the project and also for the invaluable experimental skills. His constant support and guidance have allowed me to develop as a learner.