Light is composed of discrete energy packets called photons. A single photon emitter (SPE) is a specialized quantum device engineered to reliably generate and release exactly one photon at a time, on demand. This capability stands in stark contrast to conventional light sources, such as light bulbs or standard lasers, which emit light in massive, random bursts containing billions of photons. Even a heavily dimmed laser follows a principle called Poissonian statistics, meaning it often releases zero, two, or more photons in a single pulse, rather than the required single particle. The ability to isolate and precisely control these individual quanta of light is a foundational requirement for the current technological revolution in information science. This precise control allows scientists to harness the unique properties of quantum mechanics, paving the way for advancements that are impossible with classical light sources.
Why Single Photon Control is Essential for Quantum Technology
The controlled generation of single photons provides the necessary non-classical light required to manipulate quantum states. Technologies like quantum computing and secure communication depend entirely on maintaining the delicate quantum phenomena of superposition and entanglement. Classical light, characterized by its uncontrolled, multi-photon pulses, instantly destroys these fragile quantum properties upon interaction, making meaningful quantum information processing impossible.
A single photon, however, can carry information encoded in its quantum state, such as its polarization or phase, effectively acting as a photonic qubit. For these qubits to be useful, they must be indistinguishable, meaning they need to be perfectly identical in their wavelength, temporal profile, and frequency. If two photons are not perfectly uniform, the quantum interference required for logic operations or communication protocols will fail.
The signature of a true single photon source is a phenomenon called anti-bunching, which is verified when the probability of detecting two photons simultaneously is near zero. This anti-bunching effect confirms that the light source is truly non-classical and is emitting photons one after the other. Reliable, anti-bunched emission transforms the theory of quantum mechanics into a functional technology platform. This achievement is the first step toward building the complex devices that rely on the precise interaction of individual light particles.
Materials Enabling Reliable Single Photon Emission
Scientists have focused on isolating single quantum systems within solid-state materials to create practical single photon emitters. These systems generally involve a localized defect or nanostructure that can be excited to a higher energy state, then relax by emitting a single photon. The material platform chosen dictates the emitter’s properties, such as its operating temperature, brightness, and ease of integration.
One of the most widely studied platforms is the Nitrogen-Vacancy (NV) center in diamond, which is a point defect where a single nitrogen atom substitutes a carbon atom next to a vacant lattice site. The unique electronic structure of the NV center allows it to exhibit stable, bright single-photon emission that can operate reliably even at room temperature. This room-temperature stability makes NV centers highly attractive for applications outside of specialized laboratory settings.
Semiconductor Quantum Dots (QDs) represent another major platform, created by nanostructures that confine electrons and holes in a small volume, leading to discrete, atom-like energy levels. QDs are appealing because they can be grown directly into existing semiconductor photonics circuits, promising ease of integration and scalability. However, to achieve the highest purity and indistinguishability, most high-performance QD emitters must be cooled to cryogenic temperatures, typically below 10 Kelvin, which adds complexity and cost to the overall system.
A more recent development involves utilizing Two-Dimensional (2D) materials, such as hexagonal Boron Nitride (hBN), which is atomically thin. Like diamond, hBN hosts single photon emitters via deep-level defect states within its wide bandgap, and some of these defects have demonstrated stable emission at room temperature. The two-dimensional nature of these materials offers an advantage for scalability and integration, as they can be easily stacked or combined with other electronic and photonic elements.
Driving Advances in Quantum Applications
The development of high-quality single photon emitters is now directly driving progress in two transformative fields: quantum cryptography and quantum computing. These devices provide the hardware necessary to implement protocols that were previously only theoretical. The resulting technologies promise a new era of secure communication and advanced computation.
Quantum Key Distribution (QKD)
In quantum cryptography, specifically Quantum Key Distribution (QKD), single photons are used to create inherently secure communication channels. The security relies on the no-cloning theorem, which states that an unknown quantum state cannot be perfectly copied. Information is encoded onto the polarization state of individual photons and sent across a link. If an eavesdropper attempts to measure the single photon, the act of measurement inevitably disturbs the photon’s quantum state, introducing detectable errors. The use of a true single-photon source is necessary, as a multi-photon pulse would allow a hacker to split off one photon and measure it without alerting the legitimate users (a photon number splitting attack). By guaranteeing only one photon is sent per pulse, SPEs ensure the security of the communication link remains intact.
Optical Quantum Computing and Networking
Single photon emitters are foundational components for optical quantum computing and quantum networking, where the photons act as “flying qubits.” Photons are used not only to perform calculations but also to transfer quantum information between stationary qubits, such as those made from trapped ions or superconducting circuits. This capability enables the construction of a future quantum internet, connecting distant quantum processors into a large, powerful network. Photons are particularly well-suited as flying qubits because they travel at the speed of light and exhibit long coherence times, meaning their quantum state remains intact over long distances. The ability to generate streams of identical, entangled photons is also used in quantum simulation experiments. This precise control over light at its most fundamental level is enabling the next generation of secure communication and faster computation.