Chiral emission from quantum emitters in waveguides in three experiments

Short answer: In chiral quantum optics, the direction in which an emitter sends a photon into a waveguide depends on the emitter's spin or helicity. Three experiments report this with quantum emitters: a 2015 photonic-crystal waveguide with directionality above 90 percent, a 2016 nanobeam waveguide with spin-path readout up to 95 plus or minus 5 percent, and a 2020 quantum dot waveguide with about 54 percent anisotropy. All are laboratory devices.

What is chiral light-matter interaction?

The 2015 Nature Nanotechnology abstract says nanophotonic waveguides confine photons to one dimension and so increase light-matter interaction. In a regular waveguide, an emitter interacts equally with photons travelling in either direction. The symmetry is broken in structures with non-transversal local electric-field components, so that photon emission and scattering may become directional.

What did the 2015 experiment show?

The authors show that the helicity of the optical transition of a quantum emitter determines the direction of single-photon emission in a specially engineered photonic-crystal waveguide. They observe single-photon emission into the waveguide with a directionality exceeding 90 percent, under conditions where practically all emitted photons couple to the waveguide. They say this chiral interaction enables deterministic and highly directional emission for achievable on-chip non-reciprocal photonic elements, which may serve as building blocks for single-photon optical diodes, transistors and deterministic quantum gates. That is their outlook, and no such device is reported.

What did the nanobeam experiment show?

The 2016 Nature Communications paper starts from the aim of converting faithfully between matter qubits and photonic qubits in integrated circuits. Quantum dots have well-defined spin states that couple efficiently to photons, and the authors embed them in nanophotonic waveguides. They demonstrate that the naturally occurring electromagnetic field chirality in nanobeam waveguides leads to unidirectional emission from quantum dot spin states, with in-plane transfer of matter-qubit information. The chiral behaviour occurs despite the non-chiral geometry and material. With dot registration techniques they place an emitter deterministically at a chiral point and realise spin-path conversion by design. They say the chiral effects are much more tolerant to dot position than in standard photonic crystal waveguides, reach spin-path readout up to 95 plus or minus 5 percent, and have potential as the basis of spin-logic and network implementations. The last point is an expectation.

What did the 2020 quantum dot paper show?

The 2020 paper in Physica Status Solidi RRL focuses on tailoring single quantum dot waveguide systems to emit single photons with high directionality. The authors use low-temperature in situ electron-beam lithography, enabled by cathodoluminescence mapping, to select suitable dots and integrate them deterministically into linear waveguides at chiral points determined by numerical calculations. They observe excitonic and biexcitonic emission and characterise directional emission of circularly polarized photons. The results show an anisotropy of 54 percent for directional emission, and antibunching in an autocorrelation experiment confirms single-photon emission. They describe this as a prerequisite for use in integrated quantum circuits.

How to read this

Directionality, spin-path readout and anisotropy are related but different measures, so the three percentages cannot be ranked. The 54 percent value is lower than the others, and the 2020 abstract presents it as a step in a fabrication method. This post is based on abstracts, and full texts were not checked. It makes no claim about products.

FAQ

What decides the emission direction?

The helicity or spin of the emitter's optical transition, per the 2015 abstract.

What directionality was reported?

Above 90 percent in 2015, and a 54 percent anisotropy in 2020.

Why place the dot at a chiral point?

The 2016 and 2020 authors position emitters where the field chirality gives directional emission.

Is this an optical diode?

The 2015 authors say such elements are possible building blocks. None is demonstrated in the abstracts.