Single photon emitters: quantum light | Lidia Lozano

Why Quantum Light Matters

Classical sources such as sunlight, bulbs, LEDs, and lasers generally produce light containing many photons. Quantum technologies can encode information in individual photons, with potential applications in:

• Quantum key distribution (QKD): Attempts to intercept or copy quantum information can be detected. The no-cloning theorem prevents an unknown quantum state from being copied perfectly.
• Photonic quantum computing: Photons can serve as qubits.
• Quantum teleportation: Information can be transferred between particles.
• Quantum sensing: Photons can help detect very small changes in fields or forces.

These applications require sources that produce individual photons reliably and on demand.

What Distinguishes a Single-Photon Source

An ideal single-photon emitter produces photons one at a time rather than in pairs or bunches. This behavior is called antibunching and is associated with sub-Poissonian statistics, in which photon-number fluctuations are lower than in classical light.

A laser has Poissonian statistics, so even a weak laser has some probability of emitting multiple photons at once. Attenuated lasers can approximate single-photon sources, but they are not true single-photon emitters.

How Single-Photon Behavior Is Measured

The video describes a Hanbury Brown–Twiss (HBT) setup:

1. Light from the source is sent to a beam splitter.
2. The beam splitter directs photons toward one of two single-photon detectors.
3. The arrival time of each detection is recorded.
4. The times recorded by the two detectors are compared to determine how often photons arrive at different time intervals.
5. These measurements are used to calculate the second-order correlation function, g²(τ).
6. The value at zero time delay, g²(0), is examined:
   • A low value indicates that simultaneous photon emissions are unlikely.
   • For an ideal single-photon source, it approaches zero. The closer it is to zero, the stronger the source’s single-photon character.
   • The correlation function also provides information about the source’s emission rate.

The HBT arrangement was originally developed to measure stellar diameters and is now widely used to characterize photon sources in quantum experiments.

Example Emitters

Quantum Dots

Quantum dots are tiny semiconductor crystals that confine electrons and holes. An excited electron–hole pair, called an exciton, can recombine and emit a single photon. Quantum dots can be excited optically or electrically.

They are often called “artificial atoms” because quantum confinement gives them discrete energy levels. Their emission wavelength can be tuned through their design. The video also highlights their brightness, efficiency, potential integration into optical circuits, and improving performance at different temperatures.

Nitrogen-Vacancy Centers

Nitrogen-vacancy (NV) centers are defects in diamond consisting of a nitrogen atom beside a missing carbon atom. They act as isolated quantum systems and emit light when excited by a laser.

The video describes creating NV centers by introducing nitrogen and vacancies into diamond, then using high-temperature processing so vacancies can move and pair with nitrogen atoms. NV centers can operate at room temperature, be controlled with magnetic fields, and remain stable. In addition to quantum communication, they can be used for precision measurements, such as sensing magnetic fields.

The video also mentions atoms and single molecules as possible single-photon sources.

Remaining Challenges and Outlook

Single-photon sources still need to become smaller, cheaper, and easier to scale. They also need to emit at wavelengths suitable for fiber-optic networks and integrate efficiently with optical circuits.

Researchers are exploring hybrid materials and on-chip sources to help build scalable quantum networks and bring quantum technologies closer to practical devices.

Speaker and Sources Featured

• Speaker: Lidia Lozano, the presenter and narrator identified in the video title.
• Photon-source types discussed: Quantum dots, nitrogen-vacancy centers, atoms, single molecules, and attenuated lasers as an imperfect approximation.