Researchers have found a missing piece in the puzzle of optical quantum computing
Jung-Tsung Shen, associate professor in the Preston M. Green Department of Electrical & Systems Engineering, Washington University in St. Louis has developed a deterministic, high-fidelity two-bit quantum logic gate that takes advantage of a new form of light. This new logic gate is orders of magnitude more efficient than the current technology. His research was published in May 2021 in the journal Physical Review A.
Where voltage determines the value of a bit (a 1 or a 0) in a classical computer, researchers often use individual electrons as “qubits,” the quantum equivalent. Electrons have several traits that suit them well to the task: they are easily manipulated by an electric or magnetic field, and they interact with each other. For the past two decades scientists have been trying to use photons as qubits instead of electrons. Photons have no charge, which can lead to the opposite problems: they do not interact with the environment like electrons, but they also do not interact with each other. It has also been challenging to engineer and to create ad hoc (effective) inter-photon interactions. Less than a decade ago, scientists working on this problem discovered that, even if they weren’t entangled as they entered a logic gate, the act of measuring the two photons when they exited led them to behave as if they had been. The unique features of measurement are another wild manifestation of quantum mechanics.
Shen was able to build a two-bit quantum logic gate with such efficiency because of the discovery of a new class of quantum photonic states — photonic dimers, photons entangled in both space and frequency. His prediction of their existence was experimentally validated in 2013, and he has since been finding applications for this new form of light. When a single photon enters a logic gate, nothing notable happens — it goes in and comes out. But when there are two photons, “That’s when we predicted the two can make a new state, photonic dimers. It turns out this new state is crucial.” When two independent photons (representing two optical qubits) enter the logic gate, “The design of the logic gate is such that the two photons can form a photonic dimer,” Shen said. “It turns out the new quantum photonic state is crucial as it enables the output state to have the correct sign that is essential to the optical logic operations.”
Source: Washington University in St. Louis news release