How are single photon sensors used in quantum computing?

Single-photon sensors, also known as single-photon detectors (SPDs), enable the precise detection and manipulation of individual photons, which can serve as quantum information carriers, or qubits, in photonic quantum computers (PQCs). PQCs are a type of measurement-based quantum computing (MBQC) that promises highly scalable systems and operates at room temperature.

Instead of using quantum gates, computation in PQCs relies on local measurements of a highly entangled quantum state, called a cluster state. That requires single-photon sensors. In addition to operating at room temperature, computations in PQCs are expected to be simpler to implement compared with complex multi-qubit gates.

Single-photon sensors support several operations in PQCs, including:

Detecting the correlated behavior of photons to verify their entangled state.
Reading the quantum state of a qubit encoded in a photon.
Using quantum teleportation and entangled photons to transfer information between qubits.

A major challenge for PQC researchers is developing single-photon detectors that can operate at room temperature. Superconducting nanowire single-photon detectors (SNSPDs) are currently the most common due to their high efficiency and sensitivity. Avalanche photodiodes (APDs) are also used. APDs can operate at room temperature, but most lack the sensitivity needed for PQC applications. Work is underway to develop new approaches to APDs that enhance performance while maintaining room-temperature operation.

A block diagram of a typical SNSPD is shown in Figure 1. The polarization controller forwards polarized photons, the attenuator controls the average number of photons, and a cryocooler is needed to support single-photon detection. On the output side of the SNSPD, the bias tee passes the DC bias current to the device and the AC signal from the device to the amplifier that feeds the results into the photon counter.

Figure 1. Block diagram of a typical SNSPD. (Image: ScienceDirect)

SNSPDs are typically fabricated using niobium nitride (NbN) and operate at a temperature below 4 K. They offer several important performance advantages compared with current APD designs including GHz rate counts (rate at which photons can be registered), high detection efficiency of over 90% (actual detection rate compared to the number of incident photons), low dark count rate of 10⁻⁴ Hz (false detection events that occur without the presence of a photon), small jitter of under 5 ps (variation in the timing when the detector registers a photon after its arrival), and very fast 10 ps reset time (lag between successive photon detection events).

Room temperature photon sensor

A normal-incidence germanium-silicon (GeSi) single-photon avalanche photodiode (SPAD) operating at 300 K has been demonstrated. It is expected to support room-temperature PQC operation using shortwave infrared (SWIR) photons. The device has been optimized for integration with photonic integrated circuits (PICs) and is intended to support future generations of PQCs.

The SPAD uses Si-wafer-based Si-on-insulator (SOI) and Ge-on-Si (GOS) technologies, commonly employed for silicon photonics (SiPh) devices, to achieve complementary metal-oxide-semiconductor (CMOS) fabrication compatibility. The waveguide GeSi SPAD at 300 K performed as well as an SNSPD at 4 K.

The waveguide GeSi SPAD has a straightforward structure, as shown in Figure 2, and can be fabricated using either a top-down or bottom-up process. In either case, the Al back mirror is fabricated by first forming a trench at the end of the waveguide GeSi SPAD through oxide etching, followed by the deposition and patterning of Al to serve as the back mirror.

Figure 2. Room-temperature waveguide GeSi SPAD structure and materials. (Image: APL Quantum)

Summary

PQCs using SPDs are a type of MBQC based on cluster states and are expected to be simpler to implement compared with quantum computers using multi-qubit gates. SPDs support quantum detection, reading, and information transfer needed to implement PQC. Current NbN-based SNSPDs deliver high performance but must be operated at temperatures below 4 K. Room-temperature GeSi SPADs are being developed that are compatible with SiPh devices and deliver performance comparable to that of low-temperature SNSPDs.