What is Single-Photon Source Brightness?
Single-photon source brightness refers to the efficiency with which a device emits usable single photons. It is typically defined as the probability that a single photon is emitted into a desired optical mode–such as a single-mode fiber–per excitation cycle of the device.
This parameter is essential for quantum photonic technologies, as it directly affects the computation rate in the NISQ era and the fidelity of quantum operations in fault-tolerant quantum computing. In both cases, a high single-photon source brightness is critical for the scalability of quantum photonic systems.
At Quandela, brightness is optimized through precise quantum dot fabrication, quantum control, microcavity design, and photon extraction techniques. By maximizing brightness, our single-photon sources can deliver high-performance quantum operations at high repetition rates.
Why Brightness Matters in Quantum Technologies
- Quantum Computing: Bright sources enable faster repetitions of gate operations in NISQ devices and reduce photon loss errors in fault-tolerant architectures.
- Scalability: Bright single-photon sources are essential for building large-scale photonic quantum processors and networks.
- Photon Interference: High brightness improves probability of interference between photons from separate sources—vital for generating entanglement and performing quantum logic gates between qubits embedded within each single-photon source.
- Quantum Communication: Higher brightness leads to faster secure key generation in quantum key distribution (QKD) protocols.
Frequently Asked Questions About Brightness
- How is brightness measured? Brightness is measured by dividing the count rate at a single-photon detector by the excitation laser’s repetition rate, then applying corrections for multi-photon events and optical losses to estimate the brightness at the desired location in the optical setup.
- What limits the brightness of a single-photon source? Limitations include imperfect quantum dot excitation (e.g., due to environmental interactions), inefficient photon collection, and losses introduced by filters used to reject excitation laser light.
- Can brightness be improved? Yes. Embedding quantum dots in optical cavities and operating them at cryogenic temperatures (typically 4 K) can significantly enhance brightness. Brightness also benefits from improved quantum control and reduced filtering losses through quasi-resonant excitation techniques.
- Is brightness the same as purity or indistinguishability? No. Brightness measures the likelihood of emitting at least one photon, while purity refers to the likelihood of emitting no more than one photon. Indistinguishability quantifies how similar emitted photons are to one another in terms of spectral, temporal, and spatial characteristics.