Novel single-photon source technology will significantly advance quantum communications and computing

In the field of quantum technology, the ability to generate and manipulate individual elementary electromagnetic field components (i.e., photons) is critical. For example, quantum cryptography is based on the fundamental principle that it is impossible to observe the state of a photon without altering it, which means that an eavesdropper's interception of a message-carrying photon is easily detected.

However, the conundrum arises when two photons with the same quantum state are generated. By intercepting one of the photons, the eavesdropper can gain access to the information carried by the other photon, thereby jeopardizing the security of the communication.

The problem the field is currently facing is how to generate unique single photons, as they carry very little energy, and overcoming this challenge and generating single photons is a complex and difficult scientific and engineering task.

So far, scientists have succeeded in generating single photons, for example by irradiating defects in the lattice of certain materials with laser beams, but these particles are too energetic to be of practical use: the researchers need to generate photons with lower energies, which correspond to longer wavelengths in the electromagnetic spectrum.

In a recent study published in Advanced Quantum Technologies, the team proposed a way to generate single photons in an energy range more suitable for telecommunications, which would also allow existing communications infrastructure to be used for quantum encryption.

The emission wavelengths in the telecom spectral window are of particular interest because they minimize absorption and dispersion of photons in optical fibers," the scientists wrote in the study. Using (specific) laser pulses, experimental implementations of (quantum cryptography) protocols have been successful in hundreds of kilometers of optical fiber."

However, the use of weak laser pulses may produce multiple identical photons, which could be used for eavesdropping, necessitating more complex security protocols.

As a result, scientists have taken an out-of-the-box approach, using gallium antimonide quantum dots to generate low-energy single photons.

Quantum dots, which recently won the Nobel Prize in Chemistry, are nano-sized semiconductor crystals whose unique conductive properties depend on their size. The gallium-based quantum dots used in the study have a radius of 12 nanometers, and they have important optical and physical properties due to subtle quantum mechanical effects.

The electrons in gallium antimonide have the special property of emitting electromagnetic waves, and based on this property, the physicists hypothesized that quantum dots made from gallium antimonide could be an excellent source of individual low-energy photons.

Their experiments irradiating the quantum dots with an infrared laser proved successful and produced single photons in the same quantum state with wavelengths consistent with the telecommunication range. "Non-classical light sources are a major component of quantum communication applications as well as photonic quantum computing. In contrast to some other physical systems, such as vacancy centers and trapped atoms in diamond, which provide single photons, quantum dots have superior optical properties, such as low multiphoton contributions and high indistinguishability."