Apoorva Bisht
Author: Apoorva Bisht | Majors: Physics and Computer science
I joined Dr. Nakamura’s lab in the Physics department in the second semester of my under graduation. The goal of the lab is to eventually identify reliable single photons sources. Single photons are promising candidates for qubits which are used for example in quantum computing. However, due to the inherent probabilistic nature of quantum optics, processes that generate single photons are highly non-deterministic i.e., we cannot determine when the next single photon would be arriving. This is not ideal for utilizing single photons for practical purposes. Thus, as a first step for any single photon application, we want a deterministic source of single photons. To convert the probabilistically generated single photons into a series of single photons at determined intervals, we decided to develop a photon routing system based on a flexible computer chip called Field Programmable Gate Array (FPGA).
Background: I am starting the third of year of my under graduation in physics and computer science. Being a computer science major gave me the opportunity to work on advanced programming using FPGA. To get started, I developed a photon correlation experiment for classical light sources. This was completed in spring 2021. The FPGA application I developed, however, utilized relative time difference of photon arrival and did not record the absolute arrival time for each photon.
The goal for this project is two-fold: First, to develop an FPGA based routing system for deterministic generation of single photons. Second, to install optical systems that can deliver photons in regulated time intervals.
The first part of the summer was utilized to develop a time to digital converter which would enable us to record the absolute time stamp of photon arrival. I was able to successfully implement such TDC (time to digital conversion) application which was tested using generated sine waves of known frequency and amplitude. We intend to use such an application to time tag the single photons, a crucial step in developing the routing system using FPGAs.
The second part of the summer was utilized in establishing the physical setup for generating single photons. To this end, we use (pulsed) laser source that undergoes second harmonic generation (frequency doubling) and then spontaneous parametric down conversion using a non-linear negative uniaxial crystal like -barium borate. However, for processes like second harmonic generation and spontaneous parametric down conversion incident light should hit the crystal at a precise angle to the optic axis. This angle, called the phase matching angle can be derived by satisfying necessary phase matching conditions. We first determined the phase matching angle by theoretical calculation, and custom ordered a crystal that is cut according to the precise angle. In the meantime, we worked on optimizing the pump laser beam characteristics and identifying optical components like filters, lens, mirrors and beam splitters that would be necessary to establish the setup.
When the crystal arrived, we started optimizing the angle at which the light beam incidents the crystal, i.e., the phase matching angle. After adjustments, we were able to generate second harmonic light with efficiency of around 1 in 104. However, for this light to be useful for spontaneous parametric down conversion, we need a much higher conversion efficiency (differs by three orders of magnitude). This can be done if we use pulsed laser instead of continuous wave laser. In order to achieve a pulsed laser beam, we need to mode-lock the laser. This means that the longitudinal modes in the laser cavity are optimized to have fixed phase relationship so that extremely short laser pulses are produced, in the order of femtoseconds.
We have been optimizing the mode-locking condition and we believe that the current conditions are favorable to achieve higher conversion efficiency for second harmonic generation. Once we have second harmonic generation with reliable conversion efficiency, we will move forward to spontaneous parametric down conversion of this light.
The challenging part of the process is to optimize the conditions in a controlled manner with the available devices. Theoretically, for example, phase matching occurs at a specific angle. In practice, however, since the angle at which the light incidents the crystal needs to be adjusted manually, it is time-consuming and requires systematic approach from an experimentalist to make efficient progress. During the past few months, I learnt the greater need for a highly systematic and documented approach in any experiment, howsoever simple it may be. I also learnt the importance of extensive and exhaustive planning for any experiment. For example, had I planned for all the optical components that would be used for the experiments beforehand accounting for their lead time, I could have performed the experiment much more effectively and saved valuable time.
In all, I would say that this summer has taught me how to increase my efficiency, enjoy the preparation for experiments, and the importance of documenting the results.
Throughout the process my advisor has been a patient guide teaching me valuable techniques and honing my skills and knowledge. My fellow lab mates have been helpful in guiding me through experimental pitfalls small and big. I would like to acknowledge the support of my advisor, teachers and fellow lab mates. I would also like to thank the Honors College Research Grant for supporting my research this summer.
As stated earlier, we would like to first generate efficient second harmonic light and from this light obtain spontaneous parametric down conversion. We would then like to demonstrate the quantum nature of light, namely anti-bunching. This summer, we overcame technical difficulties and made consistent progress with our experiment and we hope to continue our work with the same spirit during the upcoming semester as well.