Harnessing quantum dots to produce low-energy single photons for applications in secure communications and quantum computing.
A collaborative team of physicists from Finland and Germany has made a groundbreaking discovery that could significantly advance quantum technologies, particularly in the realms of quantum cryptography and quantum computing.
In the world of quantum technologies, the ability to generate and manipulate individual elementary electromagnetic field components, known as photons, is critical.
For example, quantum cryptography is based on the fundamental principle that it’s impossible to observe the state of a photon without altering it, meaning the interception of a photon carrying information by an eavesdropper can be easily detected.
However, a challenge arises when two photons are generated which exist in the same quantum state. By intercepting one of them, the eavesdropper could obtain the information carried by the other photon, compromising the security of the communication.
The problem that the field currently faces is in creating single, unique photons as the energy they carry is so small. Overcoming this challenge and producing solitary photons is a complex and demanding scientific and engineering task.
Generating single photons
To date, scientists have succeeded in generating single photons, for example, by irradiating defects in the crystal lattice of certain materials with a laser beam, but these particles are too energetic for practical use. Researchers need to generate lower-energy photons, which correspond to longer wavelengths in the electromagnetic spectrum.
In a recent study published in Advanced Quantum Technologies, the team proposed a method of generating single photons in an energy range more suited to telecommunications, which would also allow existing communications infrastructure to be used for quantum cryptography.
“Emission wavelengths in the telecommunication spectral window are of particular interest since they offer the least absorption and […] dispersion of photons in optical fibers,” wrote the scientists in their study. “The experimental implementation of [quantum cryptography] protocols has succeeded in over several hundred kilometers of optical fiber using [particular] laser pulses.”
However, the use of weak laser pulses introduces the possibility of producing multiple identical photons, which can be exploited for eavesdropping, necessitating the development of more intricate security protocols.
The scientists therefore took an outside-the-box approach, generating low-energy single photons from gallium antimonide quantum dots.
Quantum dots, which recently won the Nobel Prize in Chemistry, are semiconductor crystals of nanometre dimensions with distinctive conductive properties determined by their size. The gallium-based quantum dots used in the study had radii of 12 nanometers, lending them important optical and physical properties due to subtle quantum mechanical effects.
Based on the specific properties of electrons in gallium antimonide, which are responsible for emitting electromagnetic waves, the physicists hypothesized that quantum dots made from it could be an excellent source of single, low-energy photons.
Their experiments, in which they irradiated the quantum dots with an infrared laser, proved successful and led to the generation of single photons in the same quantum state and with wavelengths that correspond to the telecommunications range.
“Non-classical light sources are a major building block in quantum communication applications as well as for photonic quantum computing,” the authors wrote. “Compared to several other physical systems, like vacancy centers in diamond and trapped atoms, which can provide single photons, […] quantum dots offer superior optical properties, like low multi-photon contribution and high indistinguishability.”
Not only could quantum dots allow scientists to harness and optimize existing fiber-optic networks and satellites for secure quantum communications, but they may drive advancement in quantum computing, which uses single photons as qubits, elementary units of information.
Reference: Johannes Michl, et al., Strain-Free GaSb Quantum Dots as Single-Photon Sources in the Telecom S-Band, Advanced Quantum Technologies (2023). DOI: 10.1002/qute.202300180
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