Single photons get cleaner as a terahertz transmitter shrinks to one chip
A narrow filter sharply reduced multi-photon errors, a nonrepeating pattern produced room-temperature lasing, and an integrated lithium-niobate transmitter carried sub-terahertz links; each result removes a familiar photonics constraint while remaining a laboratory demonstration.
Science··Night
Filtering unwanted photons in the spectrum
A supposed fundamental error limit in single-photon sources can be beaten by looking at where photons fall in the spectrum, not only when they emerge. When the driving pulse is shorter than the emitter lifetime, photons released during that pulse spread across a markedly broader spectrum than the emitter’s natural linewidth. The researchers used a narrowband spectral filter to remove most of that broad component. That distinction determines which photons the filter rejects. Measured second-order coherence at zero time delay fell by almost an order of magnitude without damaging the desired single-photon emission. That measurement tracks the chance that more than one photon appears in the same event; reducing it is directly useful because multi-photon events create errors in quantum communication and linear optical quantum computing. Rather than redesigning all of the source physics, the result uses the unwanted events’ distinct spectral signature for a narrower intervention. The Nature Communications study presents this laboratory result against the assumption that spontaneous emission and re-excitation place a firm ceiling on single-photon purity.[2]
Room-temperature lasing from a nonrepeating pattern
The Illinois work loosens a different entrenched constraint in photonic-crystal surface-emitting lasers. The usual design relies on a regularly repeating lattice of holes etched vertically into semiconductor. Graduate student Erin Raftery first etched a silicon-dioxide layer and then grew semiconductor over it. Because the buried dielectric layer supplies the required refractive-index contrast, the design no longer needs vertical holes through the upper material. The structure could therefore be drawn rather than confined to a tiled pattern, allowing a quasi-periodic design that never repeats. The device produced laser light at room temperature. That geometric freedom could in principle open the emission wavelength and beam shape to tuning, but the team has not yet demonstrated electrical injection. Kent Choquette’s group describes the result as a physics demonstration. The advance expands the pattern language available on the chip, while work toward an electrically driven and manufacturable device continues.[1]
Bringing a sub-terahertz transmitter onto one chip
The third study integrated a transmitter to reduce coupling losses caused by separate optical components. Researchers bonded an indium-phosphide uni-travelling-carrier photodiode onto a thin-film lithium-niobate modulator chip, bringing the discrete optical assembly normally required in photonic terahertz systems onto one chip. The integrated photodiode reached a 3-decibel bandwidth of 240 gigahertz and an internal responsivity of 0.67 amperes per watt. In sub-terahertz wireless links, the transmitter carried 40 gigabits per second at 150 gigahertz and 32 gigabits per second at 200 gigahertz, using NRZ and PAM4 formats. These values come from a laboratory demonstration; the device has not yet been used in a deployed communications network. The shared direction of the three advances is clearest here: spectral filtering, new patterning and device integration each target a loss or limit that had been built into an established design. Each solution works at a different layer of photonics and demonstrates its gain through a measurable laboratory output.[3], [2], [1]
