Single Photon Detectors for Optical Quantum Information Applications

This is Xiruo’s Master thesis at the Stewart Blusson Quantum Matter Institute, UBC, under the supervision of Jeff Young PhD, PEng.

Motivation

A single photon is the known fundamental unit of electromagnetic radiation. The quantum mechanical nature of these photon states opens a whole range of novel applications in the burgeoning field of quantum information (QI). Most potential applications based on single photons, require detectors that have high sensitivity and efficiency. Superconducting nanowire single photon detectors (SNSPD) represent one promising category of such detectors that have been a topic with growing interest in the past twenty years.

Commercial single photo detection systems based on SNSPD elements currently cost on order $50-100K. In this project a circular meander design of a free-space SNSPD is fabricated and tested in house.

How It Works

As its name suggests, an SNSPD is built with the superconducting nanowire that has zero electrical resistance when kept at low temperature. The superconducting state of the nanowire material (NbTiN used in this project) can be destroyed by absorbing photons. If we can make the nanowire very thin and narrow (to nanometer scale), even a single photon can make the superconducting nanowire go normal (become resistive). With this physics principle in mind, the implementation of an SNSPD is straight forward. For example, Figure 1 is a naive implementation.

SNSPDcircuit_easy
Figure 1. A naive implementation of SNSPD

Figure 2 shows the detection of single photons. When no photon incident, the superconducting nanowire behaves as an ideal wire and the load is short-circuited, hence zero voltage readout. When a photon is absorbed, the nanowire is no longer superconducting and becomes resistive, hence non-zero voltage readout.

The actual device is of course more more complicated than the above schematics, and includes many more electronics components.

Design

My SNSPD was designed to optical fibre coupling, therefore needed to match the light spot size a single mode optical fibre. Figure 3 shows the design of the detecting area of my SNSPD (top view).

small spot
Figure 3. Top view of the SNSPD detecting area. The circular meander is designed to have a diameter of about 9 microns. Nanowires are 8 nm thin, 80 nm wide and 160 nm spaced.

The coupling scheme is shown in Figure 4.

Final Devices

The SNSPD was fabricated by myself with in-house cleanroom nano-fabrication facilities, including the processes of electron-beam lithography (EBL), physical vapor deposition (PVD), plasma etch, etc. In reality, more structures had to be added to the detecting area shown in Figure 5, for fabrication reasons. Below is an scanning electron microscopy image of the fabricated detecting area.

sem_centre
Figure 5. SEM image of fabricated SNSPD detecting area.

Figure 6 is a photo of the final chip that carries twelve SNSPDs each with a pair of gold contact pads for wire-bonding (for electrical connections).

chipXcoin
Figure 6. Final chip wire-bonded to the circuit board to be mounted to the cryostat sample tube.

Performance Test

The chip was soaked in liquid helium and the temperature was cooled down to 2.05 K (-271 Celsius). A red He-Ne laser (wavelength of 632.8 nm) was used to provide single photons for testing.

The RF signal was read by fast electronics. Figure 8 shows a single photon signal read from an fast oscilloscope.

signal
Figure 8. A single photon signal

Figure 9 shows the calibrated results of quantum efficiency (QE) and dark count rate (DCR) versus bias current (I).

The measured QEs were compared to the nanowire power absorption simulated using Lumerical FDTD Solutions, and they agreed within uncertainties. Figure 10 is the cross section of the chip for simulation, where absorption uncertainties were obtained by sweeping the aspect parameters in a reasonable range to account for possible fabrication defects.

cross
Figure 10. Cross section of the chip for simulation.

Conclusion

Although the absolute absorption efficiency of the detector is low, because it was fabricated on a substrate optimized for other applications, the measured and modelled values for both incident polarizations agree within the uncertainties. The bias current dependence is almost constant from 50% to 100% of the breakdown threshold value, which should allow operation at intrinsic dark count rates extrapolated to be < 1 Hz.

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