Figure 2 gives an overview of the optical table. Note that positioning and alignment of the various components must be done with great precision. (The overlap of paths must be within a few wavelengths of light, a precision on the order of microns!) Due to time constraints, the optical components have been aligned on the table for you. You will perform only a limited number of manipulations during the experiment.

Figure 2: Layout of components on the optical table.

For simplicity we can break the whole setup down in to three parts based on function – a photon source, and two separate experimental setups. Let us look at each part and describe the components within each section.

The optical components are described in more detail on this page.

The APD signals are collected and the piezo voltage is controlled by a computer data acquisition (DAQ) system. A National Instruments PCIe-6341 multifunction DAQ card (NI-DAQ), installed in the computer tower, connects to an SCB-68A interface as shown in Fig. 6.

Figure 6: Computer data acquisition system.

Signals from the APDs and to the ThorLabs piezo driver connect to the SCB-68A through a BNC connector plate as shown schematically in Fig. 7.

Figure 7a: Layout of interface connections.

Figure 7b: Wiring diagram for making connections to the NI interface.

The DAQ card is capable of both generating and collecting a variety of analog and digital signals. For this experiment we use one analog voltage output to control the piezo voltage, and four digital counter inputs to record pulses from the APDs. The card is controlled by a LabView program called SP_DAQ.

SP_DAQ has three operating modes selectable by tabs in the interface window; Singles Rates, Coincidence Rates and Coincidence Scan.

In Singles Rates mode the interface displays an analog and digital rate meter for each counter on the DAQ card. There is an input box for the user to set the Count Time which is the time, in seconds, for which the DAQ will count pulses from the APDs. Clicking the Start button initiates a measurement. While running the analog meters display the pulse rate for each counter. At the end of the measurement the average event rate and the total number of counts recorded are displayed along with the actual elapsed run time. The Abort button stops a measurement in progress.

In _Coincidence Rates _mode the interface displays analog and digital rate meters for Idler singles rates, Idler/Port A coincidence rate, and Idler/Port B coincidence rate. This mode operates just like the singles rates mode.

In Coincidence Scan mode the interface displays controls and readings for two photon coincidence counting while scaning over a range of piezo voltages. The user can set a Starting Piezo Voltage, an Ending Piezo Voltage, and the Voltage Step size for the piezo scan using the sliders. The Run Length can be set which determines how long the software spends counting at each voltage step. Clicking the Start button initiates a scan. The Abort button stops a scan in progress. A Save button allows the user to save the measurements from the most recent scan to a text file.

In order to count coincident hits on two different APD's we utilize the counters on the NI-DAQ card in a start/stop mode as shown in Fig. 8. In this mode each pulse from the Idler APD is split into two signals, one (prompt) going to the start input of a counter, the other (delayed) going through an additional 100 ns of delay before arriving at the stop input of the same counter. The signal from a second APD runs through a 50 ns delay before connecting to the counter input. Thus a photon detected by the Idler APD will cause the counter to run for a 100 ns interval of time. If a photon strikes the APD connected to the input of that counter within this 100 ns interval it will be recorded. If both APDs detect photons simultaneously, the signal from the second APD will arrive at the counter input 50 ns after the counter has been started by the signal from the Idler APD and will thus be counted.

Figure 8: NI-DAQ card counter connections.

Used this way, the counter increments only when the two APD's are hit within ±50 ns of one another.

Note that when counting coincidences between two detectors where events arrive randomly but at a well defined average rate, there will always be some probability that the two detectors are struck within the coincidence time window due to purely random chance.  If the rate of detections at each detector is known ($R_1$ and $R_2$) and the coincidence window is given as $\Delta t$, then the rate of accidental coinsidences $R_{acc}$ is given by $R_{acc} = R_1 * R_2 * \Delta t$.

The BNC side of the interface connector panel is labeled as shown in Fig. 7a. Verify that the connections are correct.

The four APD's should all have 3 ft long BNC cables coming from the outputs on the APD modules. The APDs from Ports A and B each connect to an additional 50 ns long BNC cable before connecting to the interface connector panel. The 3 ft long BNC from the idler APD connects to the Idler Prompt input on the interface connector panel with a “T” adapter. The other end of the “T” adapter connects to a 100 ns long BNC cable the other end of which connects to the Idler Delay input.

The Piezo voltage output on the interface connector panel should be connected to the voltage input for the y-axis of the ThorLabs 3-Axis Piezo Controller. Use the manual knob on the Piezo controller to set the y-axis voltage to 50 V.