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Low-cost coincidence countingapparatus for quantum opticsinvestigations

Masters, Mark, Heral, Tanner, Tummala, Kakathi

Mark F. Masters, Tanner Heral, Kakathi Tummala, "Low-cost coincidencecounting apparatus for quantum optics investigations," Proc. SPIE 9793,Education and Training in Optics and Photonics: ETOP 2015, 97930V (8October 2015); doi: 10.1117/12.2223090

Event: Education and Training in Optics and Photonics: ETOP 2015, 2015,Bordeaux, France

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Low-cost coincidence counting apparatus for quantum optics investigations

Mark F. Masters, Tanner Heral, and Kakathi Tummala

Department of Physics, Indiana University-Purdue University Fort Wayne (IPFW), 2101 Coliseum

Blvd E. Fort Wayne, IN USA 46805

ABSTRACT

We have recently started investigating quantum optics for our advanced laboratory and quantum mechanics classes. For

a small department, the expenses of much of the apparatus is daunting. As such, we look for places where we can reduce

the costs while still providing benefits for our students. One of the places where there can be some cost savings are in the

coincidence counter. The coincidence counter is a critical piece of the investigation, and while not the most expensive

component, cost savings are still available. We have developed a low-cost coincidence counter (less than $50) based on a

Cypress Programmable System on a Chip (PSoC). The PSoC is quite flexible. It has a microcontroller as well as FPGA

like capabilities which enable us to build the coincidence detection and the counter. The design process and several

investigations will be presented.

1. INTRODUCTION

Quantum Optics experiments have become very popular investigations in physics teaching laboratories. A rich body of

investigations has been developed such as the Grainger experiment, Single Photon interference and the Quantum Eraser to

name a few [1-5]. These experiments allow students to learn about optics, interferometry, the fundamental nature of light,

single photon detection, counting methods and quantum mechanics. In the course of working with single photon

experiments, our students learn about different techniques for aligning optics and fiber optics as well as the fundamental

optics. The increased interest in the single photon laboratories is, in part, the availability of lower cost Single Photon

Avalanche Diode detectors (still quite expensive), lower cost coincidence detection, much more affordable laser systems

(less than $100) and affordable down conversion crystals (BBO, ~$500).

There have been a number of different coincidence detectors developed of varying degrees of simplicity and cost. Notable

are the several designs described in Branning et. al. [6, 7] which have involved both discrete logic elements and an FPGA

with a cost of $300 or a more depending on how you do the configuration. Refs[8, 9] presents designs solely based on an

FPGA with time stamp features and a cost of approximately $100. Ref [10] has several other designs.

Part of the process for us to learn about single photons is to develop our own apparatus. The focus of the present work is

to describe the development of a lower cost and simpler coincidence detection system. Our design goals were to develop

a relatively straightforward and simple system that required very little assembly but would be capable. What we came up

with has quad inputs and six counters that are each capable of collecting up to four-fold coincidences and involves very

little hardware work. Ultimately, our system costs approximately $40. It is based on a Cypress PSoC 5 microcontroller

prototyping kit CY8CKIT-059.

2. WHAT IS A PSoC?

PSoC stands for Programmable System on a Chip. The PSoC 5 is a mixed signal microcontroller system. It comes with

a 32 bit ARM Cortex M3 processor, a variety of analog components such as operational amplifiers, comparators, a 20 bit

analog to digital converter, two 12 bit analog to digital converters, four digital to analog converters, a 24 bit digital filter

system and 24 universal digital blocks (UDB). The UDB’s consist of a matrix of uncommitted Programmable Logic

Devices (PLD’s) and a digital interconnect. The PLD may be configured to be any number of digital devices from simple

logic elements to more complex digital devices such as counters, pulse width modulators and communication. Another

interesting feature of the PSoC is that almost every pin is reconfigurable for analog or digital. Furthermore, Individual

Education and Training in Optics and Photonics: ETOP 2015, edited by Eric Cormier, Laurent SargerProc. of SPIE Vol. 9793, 97930V · © 2015 SPIE, IEEE, OSA, ICO · doi: 10.1117/12.2223090

Proc. of SPIE Vol. 9793 97930V-1Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 01 Oct 2020Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

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One of the great advantages of the PSoC is that you can combine analog and digital circuitry on a single chip combined

with a relatively powerful processor which allows you to collect data and then do some processing on the data. Figure 1

shows a diagram of the PSoC 5.

The PSoC 5 is available in either the QFN or the TQFP packages which makes them somewhat difficult to prototype since

you have to develop your own circuit board to effectively use the processor. Fortunately, Cypress has just released the

CY8CKIT-059 prototyping kit which does make this easier.

Figure 1. Diagram of a PSoC 5 showing the various devices and layout of the device. Image taken from PSoC® 5LP: CY8C58LP

Family Datasheet – Cypess

The CY8CKIT-059 prototyping kit has simplified the process of using the PSoC 5. It consists of a small circuit board with

breakout pins to allow you to access most of the GPIO ports of the PSoC5 and its capabilities. It also comes with an

integrated USB programmer which makes it very convenient to use. The programmer can also be used to communicate

serially with the computer through a virtual serial port. The board also has a micro USB port that can be used for

communication. Figure 2 shows an image of the CY8CKIT-059 prototyping kit. On the left side is the programmer which

plugs directly into a USB port (or preferably a USB extension cable). This is also used for serial communications. Once

your device is completely programmed, the programmer may be removed.


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Figure 2. Photograph of the Cypress CY8CKIT-059 prototyping kit used in this project. Image from

http://www.cypress.com/?rid=108038

Programming the PSoC involves using free and downloadable, but proprietary integrated development system from

Cypress (for windows only)- PSoC Creator [X]. PSoC Creator allows you to graphically design the digital and analog

components of the PSoC with drop and drag components (counters, PWM’s, OpAmps, A/D converters, Capacitive sensors,

etc.) and a wiring tool. Coding is completed in the environment using “c”.

Figure 3. Image of PSoC creator showing the design board for our Pseudo Random Pulse generator used later in

testing.

3. PSoC 5 COUNTER

The counter and coincidence detector that we developed has four, 50 ohm terminated input channels, six 24 bit counters,

an adjustable window (0.1 s to 10s at present) and six outputs. The coincidence detection is programmable via capacitive

buttons allowing any combination of inputs from single channels to four-fold coincidences to be counted. After each

coincidence detection, the pulses are synchronized to the clock for counting. Each of the coincidence detectors also pass

the pulses to each of six outputs which allow the counters to be to be combined to allow a greater number of inputs.


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The coincidence detection is determined by the circuit shown in Figure 4. It consists of four OR gates to choose the inputs

and an AND gate does the coincidence detection. The Control register is the interface between the processor and its

firmware and the digital circuitry. If one of the control register bits is set high as one of the inputs for an OR gate, then

the output of the OR gate will always be high and therefore that input will not matter. Our tests of the OR and AND gates

indicate that they are very fast with very short propagation times (we measured differences in propagation times of less

than 1 ns for 5 AND gates and 4 NOT gates). The largest propagation delays come from the input and output buffers.

This is on the order of 35 ns (See Figure 3).

Similar to the design in Ref [9], we did not include pulse shaping into our system in part because our detectors generate

10 ns pulses and we felt that it was not necessary. However, this is something we can revisit at a later time.

Figure 4. Section of the PSoC Counter showing the channel selection and coincidence detection circuitry. Control

registers allow the CPU to interact with logic circuits through firmware. PinA – PinD are the inputs. Pin_1 – Pin_6

are the outputs. In_Sync[5:0] is a bus.


Figure 5. The graph on the left shows the propagation time for the input (signal on the left, blue) to the output (signal

on the right, orange). The delay is approximately 35 ns. The graph on the right compares the input – then straight

through the IC to the output (orange) with the input being directed through 5 AND gates and 4 NOT. The delay is

approximately 1 ns.

We were able to use the counter by mounting the prototyping kit on an electronic breadboard. However, to make

connections more secure we designed a simple two sided circuit board which was etched in the laboratory. The circuit

board is shown in Figures 6A and 6B. The development kit plugs into this board. The board has the termination, and the

BNC connectors for input and output. The board also has capacitive buttons for controls. It also mounts the LCD screen

which provides feedback for the selection of what to count as well as showing the counts of the first four counters. It

would also have been very simple and cheaper to assemble the entire counter on a perf-board. The cost of the entire

counter is given in Table 1.

Table 1. Table of costs for assembling the counter.

Item Cost Vendor

CY8CKIT-059 PSoC5 prototyping kit $10.00 http://cypress.com

4x20 serial LCD $8.00 http://ebay.com

Double sided circuit board material $6.99 http://circuitspecialists.com

BNC connectors (10) $12.70 http://mouser.com

51 Ω Termination Resistors (4) $0.40 http://mouser.com

Total $38.09


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Figure 6A. Bottom layer of the circuit board for the PSoC Counter. Artwork is shown 1:1

Figure 6B. Top layer of the circuit board for the PSoC Counter. Artwork is shown 1:1

Choosing what combination of input channels will be counted is set using capacitive buttons which are etched on the

circuit board as shown in Figure 6B and Figure 7B. The capacitive buttons have the advantage of being fairly easy to

interface directly to the microcontroller and have no moving parts. All of the counts can be sent through the serial

connection to the computer. We found when using our SPAD’s that the output is below TTL thresholds. We set the PSoC

input to be LVTTL, which is functional. However, as a future alternative, we will actually set the input voltage level on

the prototyping kit using VDDIO.

The finished counter prototype is shown in Figure 7. All files, PSoC Creator files, Eagle Cad circuit board files, and

interface software are available at Ref [13].


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Figure 7. On the left, the completed counter system. On the right, we see the circuit board. The capacitive buttons

are on the right hand side. The inputs are at the top. And the six outputs are on the bottom of the image. .

4. COUNTER AND COINCIDENCE TESTS.

To test the counter we initially used a pulse generator which allowed us to use pulses of 5 ns or longer. We used a 50MHz

counter to compare the PSoC counter with measured counts. A 200 MHz oscilloscope was used to examine the pulses in

and out of the PSoC counter. Figure 8 shows the counts for 10 ns pulses using the PSOC counter vs. the counts measured

using the 50MHz counter as a function of input frequency. Exceeding 25 MHz, the PSoC counter is unable to keep up and

starts to miss counts making it inaccurate. We do not consider it usable for more than 20MHz. We also found that for

pulses shorter than 10 ns, the counter would also miss counts. Therefore, the limitations we place on the device are a

minimum pulse length of 10 ns and a maximum count rate of 2 × 107 cps. Since we are using 24 bit counters, the window

needs to be adjusted to match the count rate since the maximum counters for the 24 bit counter is 224 counts or 1.678 ×

107 counts. For low count rates we can use longer windows.

As a preliminary test of the counter, we assembled a dual channel pseudo random sequence (PRS) generator on a second

PSoC 5 prototyping kit so that we could examine the response of the system using random inputs. As in Ref 2 and 6, the

coincidences in two random events has been shown to follow the form of 𝐶 = 𝜏 × 𝑅1𝑅2, where C is the number of

coincidences and 𝑅1 is the counts on channel 1 and 𝑅2 is the counts on channel 2. 𝜏 is the detection window. The data is

shown in Figure 9 and yields a detection window of approximately 21.5 ns which is consistent with the pulse duration

from our PRS.

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Figure 8. PSoC 5 Counter response vs. Commercial Counter. This indicates the counter is fairly well calibrated.

Figure 9. PSoC 5 Counter response to the two channel PRS. Analysis of this data indicates that the pulses generated

by the PRS generator were approximately 21.5 ns since we do not make use of pulse shaping on the PSoC.

5. EXPERIMENTAL TESTS

As a final test of this counter, we put it into our single photon system. Since this is our first foray into quantum optics we

are showing several simple tests as presented in Ref 2. Our experimental system is similar to the one described by Mark

Beck [14] with fiber coupled detectors and RG 780 filters in the path to protect the detectors.

In experiment 1, we use a small incandescent bulb (a random process) with ND filters and look at random coincidences

between two detectors which allows us to determine the coincidence window. For the random thermal process we would

expect the anticorrelation coefficient, 𝛼2𝑑 =𝑅𝐴𝐵

𝑅𝐴𝑅𝐵𝜏 to be 1 (just as it was for the pseudo random pulse generator). In this

expression, 𝑅𝐴𝐵 is the number of coincident counts per unit time, 𝑅𝐴 and 𝑅𝐵 are the counts per unit time of each of the

detectors, and 𝜏 is the coincidence window. The 2d superscript indicates we are using two detectors as shown in Figure

10.


Figure 10. Setup with an incandescent light bulb to measure the coincidence window width which was 10.5 ns, in

agreement with the pulse width from our SPAD’s.

Finally, we performed a dual channel investigation using the nominally single photon source from a down conversion

system. The experimental set up is shown in Figure 11. We would hope to measure the anti-correlation coefficient with

a value much greater than 1. However, to date we have only measured it to be 1.2, which is only two standard deviations

above 1 and is rather disappointing. We currently attribute this failing to the pump laser which has a horrible beam shape

(doughnut) and are in the process of replacing it was a different system.

Figure 11. Setup for single photon generation using spontaneous down conversion

6. CONCLUSIONS

We have demonstrated a fairly simple counter which can be assembled for approximately $40. Tests indicate that it is

functional. While it presently is limited to four-fold detections, we have the ability to expand the number of inputs and

outputs. What we cannot do is expand the number of counters without decreasing counter resolution to 16 bit. This would

then greatly reduce the counting windows.

There are improvements to be made to the system. A different circuit board design would place the capacitive buttons on

the bottom of the circuit board and use the board as the lid to a box rather than an open frame system. It would also be

helpful to utilize the onboard USB port rather than use the programmer for data communication. Data communication

could be improved by better support software with graphical display – either through LabView or with a custom Python

program. We also would like to add an SD card reader so that data can be stored directly on an SD card without the need

for a computer.

At present we make little use of the processor on the microcontroller other than to read the counters, display the counts

and then to send those to the computer. With a little effort we can have it collect statistics and report these.

Finally, we are considering developing our own circuit board, rather than depend on the prototyping kit because there are

some advantages such as the ability to control the input levels of different ports. This would still be a one chip solution,

however, the complexity of such a system would be quite a bit higher, requiring a more complex custom circuit board.

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REFERENCES

[1] Thorn, J.J., Neel, M.S., Donato, V.W. , Bergreen, G.S. , Davies, R.E. and Beck, M.; “Observing the quantum behavior

of light in an undergraduate laboratory”; Am. J. Phys., 72, 1210-1219 (2004).

[2] B.J. Pearson and D. Jackson; “A hands-on introduction to single photons and quantum mechanics for undergraduates”;

Am. J. Phys., 78, 471-484 (2010).

[3] W. Rueckner and J. Peidle; Youngs double-slit experiment with single photons and quantum eraser”; Am. J. Phys., 81,

951-958 (2013).

[4] D. Dehlinger and M.W. Mitchel; “Entangled photon apparatus for the undergraduate laboratory”; Am. J. Phys., 70,

898-902 (2002).

[5] Beck, M. , [Quantum Mechanics: Theory and Experiment], Oxford University Press, Oxford & New York, (2012).

[6] Branning, D., S. Bhandari, and Beck, M.; “Low-cost coincidence-counting electronics for undergraduate quantum

Optics”; Am. J. Phys., 77, 667-670 (2009).

[7] Branning, D., Khanal, S., Shin, Y.H., Clary, B., and Beck, M.; “Note: Scalable multiphoton coincidence-counting

electronics”, RSI, 882, 016102 (2011).

[8] B. Gamari, D. Zhang, R.E. Buckman, P. Milas, J.S. Denker, H. Chen, H. Li, and L. Goldner; ”Inexpensive electronics

and software for photon statistics and correlation spectroscopy”, Am. J. Phys., 82, 713-722 (2014)

[9] Peters, J.K., Polyakov, S, Midgal, A. and Nam, S.W.; “Simple and Inexpensive FPGA-based Fast Multichannel

Acquisition Board”; <http://www.nist.gov/pml/div684/grp03/multicoincidence.cfm> (January 2015)

[10] Stummer, A; < http://www.physics.utoronto.ca/~astummer/pub/mirror/Projects/index.html> (2015).

[11] Information on the CY8CKIT-059 prototyping kit <http://www.cypress.com/?rid=108038>

[12] Information and download of PSoC Creator software <http://www.cypress.com/psoccreator/>

[13] Download for PSoC Creator files, interface software and Eagle Cad files for circuit boards for both the PSoC 5

counter and also for the PRS Generator. <http://users.ipfw.edu/masters>.

[14] Beck, M., <http://people.whitman.edu/~beckmk/QM/>


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