Nanodevice Shifts Light’s Color at Single-Photon Level

False-color scanning electron micrograph of a nanophotonic frequency converter, consisting of a ring-shaped resonator (shaded blue) into which light is injected using a waveguide (shaded red). The input signal, depicted as a purple arrow, is converted to a new frequency (blue arrow) through the application of two pump lasers (Source: K. Srinivasan, NIST)

False-color scanning electron micrograph of a nanophotonic frequency converter, consisting of a ring-shaped resonator (shaded blue) into which light is injected using a waveguide (shaded red). The input signal, depicted as a purple arrow, is converted to a new frequency (blue arrow) through the application of two pump lasers (Source: K. Srinivasan, NIST)

Converting a single photon from one color, or frequency, to another is an essential tool in quantum communication, which harnesses the subtle corre­lations between the subatomic properties of photons to securely store and transmit information. Scientists at the National Institute of Standards and Techno­logy NIST have now developed a minia­turized version of a frequency converter, using techno­logy similar to that used to make computer chips.

The tiny device, which promises to help improve the security and increase the distance over which next-generation quantum communication systems operate, can be tailored for a wide variety of uses, enables easy integration with other information-processing elements and can be mass produced. The new nanoscale optical frequency converter efficiently converts photons from one frequency to the other while consuming only a small amount of power and adding a very low level of noise, namely background light not associated with the incoming signal.

Frequency converters are essential for addressing two problems. The frequen­cies at which quantum systems optimally generate and store information are typically much higher than the frequencies required to transmit that information over kilometer-scale distances in optical fibers. Converting the photons between these frequencies requires a shift of hundreds of terahertz.

A much smaller, but still critical, frequency mismatch arises when two quantum systems that are intended to be identical have small variations in shape and composition. These variations cause the systems to generate photons that differ slightly in frequency instead of being exact replicas, which the quantum communi­cation network may require.

The new photon frequency converter, an example of nano­photonic engineering, addresses both issues. The key component of the chip-integrated device is a tiny ring-shaped resonator, about 80 micrometers in diameter and a few tenths of a micrometer in thickness. The shape and dimensions of the ring, which is made of silicon nitride, are chosen to enhance the inherent properties of the material in converting light from one frequency to another. The ring resonator is driven by two pump lasers, each operating at a separate frequency. In a scheme known as four-wave-mixing Bragg scattering, a photon entering the ring is shifted in frequency by an amount equal to the difference in frequencies of the two pump lasers.

Like cycling around a racetrack, incoming light circulates around the resonator hundreds of times before exiting, greatly enhancing the device’s ability to shift the photon’s frequency at low power and with low background noise. Rather than using a few watts of power, as typical in previous experiments, the system consumes only about a hundredth of that amount. Importantly, the added amount of noise is low enough for future experiments using single-photon sources.

While other techno­logies have been applied to frequency conversion, “nano photonics has the benefit of potentially enabling the devices to be much smaller, easier to customize, lower power, and compatible with batch fabri­cation techno­logy,” said Srinivasan. “Our work is a first demon­stration of a nano­photonic technology suitable for this demanding task of quantum frequency conversion.” (Source: NIST)

Reference: Q. Li et al.: Efficient and low-noise single-photon-level frequency conversion interfaces using silicon nano photonics, Nat. Phot., online 18 April 2016, DOI: 10.1038/nphoton.2016.64

Link: Center for Nanoscale Science and Technology (K. Srinivasan), National Institute of Standards and Technology NIST, Boulder, Colorado, USA

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