16 February 2018
Researchers from the University of Maryland’s Joint Quantum Institute, led by associate professor Mohammad Hafezi and Professor Edo Waks, have created a photonic chip that not only generates single photons, but can also steer them.
“This design incorporates well known ideas that protect the flow of current in certain electrical devices,” said Hafezi. “Here, we create an analogous environment for photons; one that protects the integrity of quantum light – even in the presence of certain defects.”
The chip features a photonic crystal, made by punching holes through a sheet of semiconductor. According to the research team, the repeated hole pattern appears to a photon to be much like a real crystal made from a grid of atoms. Different hole patterns change the way that light passes through the crystal; for instance, modifying hole size and separation allows certain light colours to pass, while blocking others.
However, even carefully fabricated devices have flaws that alter the light’s intended route. Looking to mitigate this issue, the team etched thousands of triangular holes in an array. Hole spacing was shifted along the device’s centre, opening a different light path.
Light is generated by stimulating quantum emitters embedded into the photonic crystal with lasers. Photons coming from the two most energetic states of a single emitter are of different colour and rotate in opposite directions.
The team tested the chip by first changing a quantum emitter from its lowest energy state to one of its two higher energy states. Upon relaxing back down, the emitter produces a photon, which enters a nearby travel lane. Continuing this process many times, using photons from the two higher energy states, the researchers saw these photons preferred to travel in opposite directions, said to be evidence of the underlying crystal topology.
To confirm the design could offer ‘protected lanes’ for single photons, the team created a 60° turn in the hole pattern. The topology protected the photons and allowed them to continue on their way unhindered.
“On the internet, information moves around in packets of light containing many photons and losing a few doesn’t hurt you too much,” said researcher Sabyasachi Barik. “In quantum information processing, we need to protect each photon and make sure it doesn’t get lost along the way. Our work can alleviate some forms of loss, even when the device is not completely perfect.”
The design, which is said to be flexible, could allow researchers to systematically assemble pathways for single photons, said Prof Waks. “Such a modular approach may lead to new types of optical devices and enable tailored interactions between quantum light emitters or other kinds of matter.”
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