Researchers Use Quantum Effects to Achieve Ultrabroadband Bandwidth

Researchers at the University of Rochester have used a thin-film nanophotonic device and the phenomenon of quantum entanglement to generate very large bandwidth — a record for ultrabroadband, the researchers said. The researchers entangled photons and, specifically, their frequencies in the work.

The breakthrough could lead to enhanced sensitivity and resolution for experiments in metrology and sensing, including in spectroscopy, nonlinear microscopy, and quantum OCT.

Quantum entanglement occurs when two quantum particles are connected, even when they are apart. Any observation of one particle affects the other as if the particles were in communication. When this entanglement behavior involves photons, entangling the photons’ frequencies — the bandwidth of which can be controlled — is a possibility.

To date, most devices used to generate broadband entanglement of light divide a bulk crystal into small sections, each with slightly varying optical properties, and each generating different frequencies of the photon pairs. The frequencies are then added together to give a larger bandwidth.

The process is inefficient, however, and reduces brightness and the purity of the photons, said Usman Javid, a Ph.D. student in the lab of Qiang Lin, a professor of electrical and computer engineering who led the research. In these devices, there will always be a trade-off between the bandwidth and the brightness of the generated photon pairs.

“We have completely circumvented this trade-off with out dispersion engineering technique to get both: a record-high bandwidth at a record-high brightness,” Javid said.

Researchers in the lab of Qiang Lin at the University of Rochester have generated record ‘ultrabroadband’ bandwidth of entangled photons using the thin-film nanophotonic device illustrated here. At top left, a laser beam enters a periodically poled thin-film lithium niobate waveguide (banded green and gray). Entangled photons (purple and red dots) are generated with a bandwidth exceeding 800 nm. Illustration by Usman Javid and Michael Osadciw, courtesy of University of Rochester.

The nanophotonic device consists of a thin film of lithium niobate. It uses a single waveguide with electrodes on both sides. Whereas a bulk device can be millimeters across, the Rochester team’s device has a thickness of 600 nm. This makes it more than a million times smaller in its cross-sectional area than a bulk crystal, and, as a result, the propagation of light extremely sensitive to the dimensions of the waveguide.

Even a variation of a few nanometers can change the phase and group velocity of the light through which it is propagating. As a result, the device allows precise control over the bandwidth in which the pair generation process is momentum matched.

“We can then solve a parameter optimization problem to find the geometry that maximizes this bandwidth,” Javid said.

The device could additionally lead to the higher-dimensional encoding of information in quantum networks for information processing and communications. However, the device is ready only to be deployed in experiments in a laboratory setting, according to Javid. For it to be used commercially, a more efficient and cost-effective fabrication process is needed. Lithium niobate fabrication will take some time to mature enough to make financial sense, he said.

“This work represents a major leap forward in producing ultrabroadband quantum entanglement on a nanophotonic chip,” Qiang Lin said.

The research was published in Physical Review Letters (www.doi.org/10.1103/PhysRevLett.127.183601).