Topological photonic crystal

[Image: JQI]

Quantum optical devices, such as those used in quantum simulation and sensing, depend on the reliable transit of single photons. Each photon is important, so minimizing the number that get deflected is critical. Researchers from the Joint Quantum Institute (JQI), University of Maryland, USA, have recently demonstrated a photonic chip that both generates and steers single photons making sure they don’t get lost en route—even when that route goes around corners (Science, doi:  10.1126/science.aaq0327).

Leveraging topological order

The JQI team’s chip is a GaAs photonic crystal with embedded InAs quantum dots that act as emitters. Photonic crystals can be used to guide and control light by changing the size, pattern, and distribution of holes within their structure to create conduits. Any flaws in the structure, however, can alter the light’s intended path.

The JQI team has overcome this, not by creating a flawless structure, but by changing the topology, or shape and pattern of holes. The researchers constructed two lattices composed of triangular holes with different spacing and, therefore, different band gaps.

The boundary between the two structures where the band gaps overlap traps the photons emitted by the quantum dots in helical edge modes that propagate along the interface. When the researchers shone a pump laser on the center of the device to prompt emission from the quantum dots, they confirmed that the emitted light was collected at the left and right edges of the, initially straight, interface.

One-way street

Next, by applying a magnetic field, the researchers forced a quantum dot to emit single photons in two states with opposite circular polarization. Photons with one polarization coupled with the topological helical edge mode and were collected at the right edge of the interface, while photons with the opposite polarization travelled in the other direction. The interface thus can act as a sort of one-way street for polarized light.

After introducing a 60-degree bend in the structure, the team performed the same measurement, with the same result, proving that the structure minimizes deflection of the photons. If the photons had been scattered or reflected at the bend, as might be expected, the researchers would have measured fewer photons of one polarization at the edge after the bend, and a mixture of both transmitted and reflected polarizations at the other edge. Instead, the photons remained coupled with the helical edge modes, a sign that they continued along the same path as they had before the bend was introduced.

Protective environment

"This design incorporates well-known ideas that protect the flow of current in certain electrical devices," said Mohammad Hafezi, JQI Fellow and one of the study’s lead authors. "Here, we create an analogous environment for photons, one that protects the integrity of quantum light, even in the presence of certain defects."

Because the arrangement of holes is flexible, the researchers say their result could enable systematic assembly of photon pathways and new types of optical devices that take advantage of tailored interactions between quantum emitters and other kinds of matter.