Electrons and holes recombine one by one

For certain quantum computing applications, such as quantum cryptography, data stored in qubits will have to travel between systems in different locations. But transporting the state of an electron-spin qubit is difficult. Recent experiments employing mechanisms such as nearest-neighbor state exchange or long-range coupling have ferried spin states over distances from tens of microns to millimeters. That may be long on the qubit scale, but it’s many orders of magnitude too short for practical information transfer.

One way to approach long-distance information transfer is by encoding an electron-spin qubit’s state into a photon’s polarization. Then the data could easily travel through either free space or fiber optics. Although that controlled encoding is not yet possible, Tzu-Kan Hsiao and his coworkers at Cambridge University have now realized one component necessary to make it a reality: Their new LED semiconductor device can combine electron–hole pairs one at a time to produce photons. Because the photons are produced sequentially by single electrons, rather than in bulk as in normal LEDs, information encoded in the light for transport could, in principle, be reconstructed when it reaches its destination.

In the experimental setup shown in the upper figure, reservoirs of electrons (left) and holes (right) are induced by applying positive and negative voltages to the surface gates. The reservoirs are separated by a channel with a potential slope along it. A transverse voltage squeezes the channel so electrons must pass through single file. Surface acoustic waves (SAWs) produced by a transducer create an oscillating potential that acts as a conveyer belt: Electrons from the reservoir get trapped in the wave minima and carried up the channel’s potential slope. They then combine with holes and emit photons (lower figure). This animation illustrates the process.

The resulting electroluminescence showed peaks of equal height every 860 ps, a time that corresponds to the wave frequency of 1.163 GHz. That periodicity confirmed that the electrons were traveling in the wave minima. Hsiao and his coworkers chose the wave frequency and 9 dBm power so that only one electron was trapped in each minimum, which they verified by measuring the current across the channel.

Now that the researchers have demonstrated control over single electron–hole recombination, their next goal is to show spin-to-polarization transfer from the electrons to the emitted photons. Full quantum information transfer would also require an inverse polarization-to-spin transfer to rewrite transmitted information onto qubits. In the meantime, the device can be used as a single-photon source for on-chip photonics. (T.-K. Hsiao et al., Nat. Commun. 11, 917, 2020 .)