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NIMS Quantum Dot Research Realizing Innovative Quantum Light Sources

In recent years, the realization of quantum communication and quantum computers has been rapidly moving closer to reality. Against this backdrop, “quantum light sources” are attracting attention as a key enabling technology. If we can generate single photon (individual quanta of light) and/or pairs of entangled photons on demand, then ultimately secure communication, as well as dramatic advances in computational power long thought impossible, come into view. At NIMS, researchers are pushing the limits of quantum light sources through a materials-science-driven approach. By leveraging unique techniques for fabricating high-quality quantum dots, they have succeeded in developing both single-photon sources and entangled-photon sources, opening the door to a new era of quantum technologies.

The Power of Light in Quantum Communication and Quantum Computing

In classical optical communication over standard optical fiber, information is carried in laser pulses. Each bit of information is encoded in a pulse containing on the order of 100,000 to 1,000,000 photons. Because so many photons are involved, it is extremely difficult to detect any attempt to tap even a small fraction of them during transmission. By contrast, in quantum cryptographic communication, a single photon carries one bit of information. If an eavesdropper intercepts a photon in transit, the resulting disturbance can be detected immediately. As a result, quantum cryptography—for example, quantum key distribution—can in principle provide communication whose secrecy is guaranteed by the laws of physics.

In the realm of computation, classical computers represent information using digital bits that take the value 0 or 1. Quantum computers, on the other hand, use quantum objects such as electrons or photons, which can be prepared in a superposition^*1 state, allowing a single quantum bit (qubit) to represent 0 and 1 at the same time. Furthermore, by exploiting entanglement^*2 between multiple qubits and interference effects arising from the wave nature of quantum states, it is, at least in theory, possible to perform massively parallel computations.

*1 Superposition

A phenomenon in which particles that obey the laws of quantum mechanics (such as electrons or photons) can simultaneously occupy multiple states—for example, different spin directions, momenta, positions, or energies. A particle in a superposition does not have a definite state until it is measured; upon measurement, its state collapses to a single outcome.

*2 Entanglement

A quantum-mechanical state in which two or more particles share strong correlations. When one particle in an entangled pair is measured, the state of the other particle is determined in a correlated way, even if the two are far apart.

Several physical platforms are being pursued for quantum computers, including semiconductor-based and superconducting implementations. Among these, optical quantum computers perform computation using photons as qubits. In one scheme, known as the single-photon approach, the logical states 0 and 1 are encoded in the polarization^*3 of a single photon.

*3 Polarization

Light consists of oscillating electric and magnetic fields that propagate through space like a wave. When the direction of oscillation of the electric field is aligned, the light is said to be polarized.

Optical quantum computers offer several advantages. They operate at the speed of light and are relatively robust against environmental noise, making it easier to maintain quantum coherence than in semiconductor-based schemes that use electron spins as qubits. Moreover, they can in principle operate at room temperature,^*4 eliminating the need for special cooling systems such as those required for superconducting approaches, which must be kept below 1 kelvin (K).
However, moving toward practical implementation of optical quantum computers presents major challenges. It is essential to develop quantum light sources that generate high-purity single photons and entangled photon pairs, as well as highly sensitive single-photon detectors. Meeting these requirements poses substantial technical hurdles. To address them, NIMS has long pursued the research and development of quantum dots as materials for quantum light sources.

*4 Operation at room temperature

While individual components such as light sources and detectors may need to be cooled, the overall system itself can operate at room temperature.