The achievement represents a major step towards a "quantum internet," in which future computers will be able to rapidly and securely send and receive quantum information.
Quantum computers are seen as providing a route towards computers that will be able to outperform current systems because they work in a fundamentally different way. Instead of processing discrete ones and zeros, quantum information, whether stored in electron spins or transmitted by laser photons, can be in a superposition of multiple states simultaneously. Moreover, the states of two or more objects can become entangled, so that the status of one cannot be completely described without this other. By handling entangled states, quantum computers will be able to evaluate many possibilities simultaneously, as well as transmit securely.
These entangled states, however, can be very fragile and can last only a few microseconds before losing coherence. To achieve the goal of a quantum internet, over which coherent light signals can relay quantum information, these signals will need to be able to interact with electron spins inside distant computers.
Researchers led by Osaka University used laser light to send quantum information to a quantum dot by altering the spin state of a single electron trapped there. While electrons don't spin in the usual sense, they do have angular momentum, which can be flipped when absorbing circularly polarized laser light.
"Importantly, this action allowed us to read the state of the electron after applying the laser light to confirm that it was in the correct spin state," explained first author Takafumi Fujita. "Our readout method used the Pauli exclusion principle, which prohibits two electrons from occupying the exact same state. On the tiny quantum dot, there is only enough space for the electron to pass the so-called Pauli spin blockade if it has the correct spin."
Quantum information transfer has already been used for cryptographic purposes. "The transfer of superposition states or entangled states allows for completely secure quantum key distribution," senior author Akira Oiwa said. "This is because any attempt to intercept the signal automatically destroys the superposition, making it impossible to listen in without being detected."
The rapid optical manipulation of individual spins is a promising method for producing a quantum nano-scale general computing platform, according to the research team. An exciting possibility is that future computers may be able to leverage this method for many other applications, including optimisation and chemical simulations.
