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Time-Correlated Single-Photon Counting

Rachel Westerkamp

Published Jan 1, 2018• 0 citations

Abstract

In this study of quantum optics, we want to extract information associated with quantum mechanically entangled photon pairs. Measurements involved here take advantage of the fact that the photon pairs, created at the same point in space and time, will arrive “in coincidence” at detectors placed equally far from the source. Conservation of momentum also constrains the placement of these detectors. These effects, in combination, allow us to differentiate between these entangled photons and other stray photons that may reach the detectors. Initial efforts have included building a more stable laser mount, studying parametric down conversion (the process of converting the single beam into the entangled photon pairs), and establishing a small time interval for coincidence detection. For this, we explore the consequences of moving away from a Field Programmable Gate Array (FPGA) for coincidence measurements and towards the use of a Programmable System on a Chip (PSoC), which essentially combines the fast digital logic gates of an FPGA with micro-controller functionality. We are gaining hands-on experience with some highly theoretical aspects intrinsic to quantum mechanics, expanding upon laboratory setups described in Mark Beck’s recent text on quantum mechanics, as well as materials from Exploring Quantum Physics through Hands-On Projects by David Prutchi and Shanni R. Prutchi.

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Time-correlated photons can be used to demonstrate emerging technical applications such as ghost imaging, and they can demonstrate quantum-based phenomena in order to interest undergraduate students in engineering applications involving optics and quantum mechanics. In this work, we use spontaneous parametric down-conversion to generate time-correlated photons. To achieve this, we simulated laboratory parameters with MATLAB and designed optical and electrical setups. Specifically, we used a 405 nm diode laser, various common optical components, and a nonlinear crystal designed to generate time-correlated photons, and we used single-photon detectors, time-to-amplitude converters, custom flash-conversion analog-to-digital converters, a Teensy 4.1 microcontroller, and the Arduino IDE to detect the time-correlated photons. We present two significant results. First, we measured >100,000 counts per second 5 ± 1° on either side of our input laser which was within our theoretical expectations of 4–9° and much greater than our < 2000 dark counts per second. Our second major result was finding 15.8 ± 0.7 time-correlated counts per second with an SNR of 2.7. We also outline future extensions of our work including ghost imaging and generating polarization-entangled photons.

We present results characterizing multichannel InGaAs single photon detectors utilizing gated passive quenching circuits (GPQC), self-differencing techniques, and field programmable gate array (FPGA)-based logic for both diode gating and coincidence counting. Utilizing FPGAs for the diode gating frontend and the logic counting backend has the advantage of low cost compared to custom built logic circuits and current off-the-shelf detector technology. Further, FPGA logic counters have been shown to work well in quantum key distribution (QKD) test beds. Our setup combines multiple independent detector channels in a reconfigurable manner via an FPGA backend and post processing in order to perform coincidence measurements between any two or more detector channels simultaneously. Using this method, states from a multi-photon polarization entangled source are detected and characterized via coincidence counting on the FPGA. Photons detection events are also processed by the quantum information toolkit for application testing (QITKAT).

Discrete variable quantum optics stands as one of the most prominent platform for quantum technologies with an increasing number of encouraging out-of-the-laboratory implementations [1]. The quest for competitive quantum photonic systems, compatible with future engineered systems, has promoted huge developments concerning both source and detection devices [2]. To date, a critical point lies in experiments' operation rates. Time multiplexing technics allow in principle to pump photonic sources at rates on the order of the GHz [3]. However, a strong limitation to ultra-fast operation lies in timing errors at the detection stage. Limited resolution directly affects the quality of any time-correlated single photon counting or quantum state engineering operations [3]. In particular, single photon detector's timing-jitters lead counts associated with a given optical clock cycle to appear as temporally indistinguishable from those corresponding to neighbouring ones [2].

Control of quantum signals is very robust design technique and very important in quantum signal processing (QSP). It is not easy in experimentation platforms. Field Programmable Gate Array (FPGA) is using to control very various range of devices in quantum fields through PCs. FPGA needs to run modules of components to communicate and interfacing with the PC, through decode perform commands to direct control of digital hardware’s. If programmer has a real-time control of the FPGA via USB, it can be possible to evaluate design parameters changes in real-time, without reprogramming the FPGA. That makes the proposed design platforms easier for researchers. This paper discusses experiment of control quantum signals by FPGA to control coincidence logic for Quantum Entangled Pair (QEP), that able to measure polarization correlations relationships between photons of the QEP. This FPGA helps to determine detection events at different detectors which can be attributed to a single photon pair. Also to determine the correlation time between two different beams frequencies is extremely accurate according to the delay between the beams, which is very short.

Coincidence counting is widely used in quantum optics.It could be used in measuring the pairs of polarization entangled photons and single photon interference.In quantum experiments,coincidence counting often needs more than 3 channels.In this case,it is necessary to use FPGA(Field-Programmable Gate Array) instead of discrete logic chips.FPGA also has the advantage of low cost and small size.Coincidence counter based on FPGA could support coincidence time window from 1 ns to several hundred ns.This helps to avoid noised caused by background light.This paper introduces the principle of multi-channel coincidence counter based on FPGA and gives its performance experimentally with two single photon counting modules.