Profile Photograph for Apoorva Bisht
Author: Apoorva Bisht | Majors: Physics and Computer Science | Semester: Spring 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 various applications of optics captured my interest. The goal of the lab is to eventually 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. This semester we worked on studying photon correlation and second harmonic generation of light. Photon correlation helps us determine the nature of light – whether it is coherent, thermal or anti-bunched (single photon level). For second harmonic generation, we use -Barium Borate, a non-linear crystal. When light is incident at a specific angle at the crystal (called the phase matching angle), the output light has double the frequency of incident light. For example, if a million photons hit the crystal then, one-twentieth of the photons result in photons having double the frequency.
Part 1: SHG light (pump for SPDC)
We first use a high-power mode-locked laser as a pump for second harmonic generation via a nonlinear crystal, BBO (β-Barium Borate). The SHG light from BBO will be used as a pump for SPDC via the second BBO crystal. The pump laser used is a frequency doubled high-repetition Ti:Sapphire laser working at 80 MHz. This laser can provide output up to 1 W. The pump beam is guided to the first BBO crystal. The BBO crystal is custom ordered to be cut at a phase-matching angle to allow SHG using NIR pump (~750 nm).
When ordinary polarized pump light hits the BBO at the phase-matching angle then extraordinary polarized SHG light is generated (o+o→e). A system of mirrors and lens to get appropriate configuration of pump at the crystal. These optical components are chosen to minimize loss and pulse broadening.
Part 2: Correlation measurements
A standalone setup for correlation measurement is very useful to determine the nature of incident light and specifically for our case, confirm the single photon generation. The first step is to identify any anomalies in the data and to make the setup reliable.
The basic correlation setup is as follows. The beam of light passes through a set of filters, wave plate and lens to correspondingly attenuate, achieve the desired polarization and focus the light near the detectors (photomultiplier tubes). This beam then hits a beam splitter that ideally splits the light 50:50 towards the two photomultiplier tubes (PMTs). The signal from the PMTs is amplified and then shaped using a discriminator. The two discriminators provide inputs (start and stop) to a time to amplitude converter. This module converts the time gap between the start and stop inputs into a voltage signal that is received by an FPGA (Field Programmable Gate Array). A multichannel pulse height analyzer on the FPGA sorts these voltage inputs into different bins. Thus, for the single photon case (anti-bunched light), the number of signals corresponding to zero time-delay would be nil.
As in any research project, this project also had challenges. One of them was to determine an efficient way of aligning the crystal with the incident pump beam. Regarding the correlation measurements, it was essential to understand the electronics well to prevent spurious signals and get reliable data. However, after crossing these hurdles the results were rewarding.
The entire process has not just helped me understand the theory and experiment better, but have also helped me evolve as a better person. I learned how to communicate better, when to ask for guidance and how to work more efficiently. Interacting with lab members has helped me understand the value of encouragement from colleagues and how to work together.
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.