Epsilon Near Zero Materials Boost Quantum Dot Performance
Researchers Increase Quantum Dots with “Epsilon Near Zero” Materials, Breaking Quantum Communication Speed.
A global team of scientists has improved quantum emitter efficiency at telecommunications frequencies, a critical step toward the "quantum internet." The scientists used colloidal quantum dots (QDs) and a customized coating of indium tin oxide (ITO) to boost beam directionality and brightness while shortening emission lifespan by 54 times. In their recent study, Purdue and Heriot-Watt researchers describe how to build high-speed, on-chip quantum devices that interact with fiber-optic infrastructures.
Limitations of Quantum Speed
Quantum technologies like secure communication and quantum computation require single-photon generators with high repetition rates for “photons on demand”. PbS/CdS (core/shell) quantum dots are popular due to their size-tunable near-infrared emission and room temperature operation, but their extended decay periods have proven a drawback.
Native lifetimes of these emitters are one to three microseconds. This long recovery period limits a quantum network's data transmission speed. Researchers tried to modify emitters' environments to release photons faster using the Purcell effect.
“Epsilon-Near-Zero” Method
The researchers used Epsilon-near-zero (ENZ) or near-zero-index (NZI) materials. The fraction of dielectric permittivity in these materials drops at a certain frequency, creating a distinctive optical environment that can suppress electric fields or modify light emission.
The researchers employed transparent conducting oxide indium tin oxide (ITO) for touchscreens, which is widely used in electronics. ITO is attractive because its Epsilon Near Zero features can be changed during fabrication and it is CMOS-compatible, making it easy to incorporate into semiconductor manufacturing.
Findings from experiments
The researchers compared PbS/CdS quantum dots on 240 nm ITO thin films to those on glass slides to measure the enhancement. With a confocal microscope and superconducting nanowire single-photon detectors (SNSPD), they used a custom time-correlated single-photon counting (TCSPC) device to measure results with sub-nanometer accuracy.
The results were revolutionary for 1350 nm quantum dots, which are inside the ITO's Epsilon Near Zero bandwidth:
The ITO substrate's photoluminescence lifespan reduced from 544 nanoseconds on glass to 10 nanoseconds, increasing speed 54-fold.
When saturation intensity raised from 400 to 3000 kcps, brightness rose 7.5-fold.
Laser-like Directionality: The light's “emission cone” was lowered from 17.6° to 10.3° to increase light collecting into optical fibers or on-chip components.
Reduce Epsilon Near Zero Effect
The scientists performed a control experiment to show that the Epsilon Near Zero condition caused these advances, not the ITO material. They used a second batch of quantum dots that glow at 1450 nm, beyond the ENZ zone, to make ITO act like a metal.
In this “outside” case, benefits were less obvious. About 10% of the lifetime was lost, and the emission cone dropped to 12.8°. This confirmed that the emitter-ENZ spectrum overlap is the key cause of performance gains.
Future of Quantum Networking
Epsilon Near Zero settings manage light-matter interactions reliably and scalablely, according to this study. Creating integrated on-chip devices requires correct emission engineering at telecom wavelengths.
The researchers say this study opens the door to more advanced effects like fully optical quantum networks and super-radiance, where several emitters synchronize to create a large light pulse. The study uses CMOS-compatible materials like ITO to examine mass-manufacturable, high-performance quantum light sources.
The researchers noted that “our results directly address a critical drawback” of quantum dots, suggesting that this method may enable the production of high-repetition-rate photon sources for secure communications in the future. As quantum optics moves from lab to integrated circuits, ITO and other materials like it may help bridge the gap between quantum theory and technology.