Sending Quantum Information Securely from Laser Light to a Quantum Dot

Scientists from Osaka University have demonstrated how information encoded in the circular polarization of a laser beam can be translated into the spin state of an electron in a quantum dot (QD). They used laser light to send quantum information to a QD by altering the spin state of a single electron trapped on the QD.

“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,” professor Takafumi Fujita said. Although electrons don’t spin in the traditional sense, they do have angular momentum, which can be flipped when absorbing circularly polarized laser light.

The researchers used a Pauli spin blockade in a double QD to verify that a circularly polarized single photon can excite a single electron spin via the transfer of angular momentum. According to Fujita, the Pauli exclusion principle 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,” he said. The photon polarization dependence of the excited spin state was finally confirmed for the heavy-hole exciton excitation.

This is a schematic image of the spin detection of a circularly polarized photon exciting an electron spin. The yellow nanofabricated metal electrodes form the pockets required to trap the electrons, move them, and sense them. Courtesy of Osaka University.

The angular momentum transfer observed by the researchers is considered by them to be a fundamental step toward providing a route to instant injection of spins, distributing single spin information, and possibly toward extending quantum communication.

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. The way that quantum computers handle entangled states allows them to evaluate many possibilities simultaneously, as well as transmit information from place to place securely. However, these entangled states can be fragile, lasting only microseconds before losing coherence. To realize the goal of a quantum internet, over which coherent light signals can relay quantum information, these signals must be able to interact with electron spins inside distant computers.

The work of the Osaka team could be a step toward realizing hacker-proof, interconnected quantum computers. “The transfer of superposition states or entangled states allows for completely secure quantum key distribution,” professor 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 research was published in Nature Communications (https://doi.org/10.1038/s41467-019-10939-x).