Microcontroller-based time interval and correlation measurement for quantum optics experiments

We describe the design, implementation and performance of a compact microcontroller-based time interval/correlation measurement device suitable for characterizing light in quantum optics experiments. It is also suitable for measuring the lifetime of atoms excited by a laser pulse. This correlator is based on measuring the time between the arrival of two consecutive photons and generating a histogram of time between photons. The interferometer is capable of measuring time difference with a resolution of at least 110 ps. The two-photon correlator discussed here was built using an analogue feature within a 16 bit PIC microcontroller whereby the charge on a capacitor is measured. The arrival of the first photon starts the charging and the second photon stops it. The voltage on the capacitor is then proportional to the time difference in the arrival of the two photons. This feature within the microcontroller is called a charge time measurement unit. The system consists of three parts. The first part, made up of two microcontrollers, performs time-to-amplitude conversions using the charge time measurement units and keeps a count of the total number of photons detected by the single-photon detectors. The second part consists of separate circuitry to detect coincidence events using logic gates. The third part consists of a control gateway in the form of another microcontroller between the time measurement microcontrollers and the control software running on a user PC. The system was tested in a lab using simulated signals from a signal generator and in a live quantum optics experiment where our system was compared with a commercial product.