Can a strain-tuned quantum dot make an ideal single-photon source?
A quantum dot built into a tiny curved-mirror cavity on a piezoelectric base can be squeezed into resonance and then emits bright, nearly perfect single photons.
Source
Tunable quantum dots in monolithic Fabry-Perot microcavities for high-performance single-photon sources
Study at a glance
- Design
- Other — Fabrication and optical characterisation of a single InAs quantum dot deterministically placed in a Fabry-Perot microcavity on a piezoelectric actuator, with FDTD design simulations.
- N
- No sample N; photon statistics reported for one device, with Q-factor trends averaged over several cavities per defect size.
- Population
- InAs/GaAs quantum dots in monolithic dielectric/semiconductor Fabry-Perot microcavities on PMN-PT
- Outcome
- Cavity Q factor, strain tuning range, Purcell factor, extraction efficiency, single-photon purity g2(0), and HOM indistinguishability
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What they did
The team grew quantum dots inside a semiconductor mirror structure, located individual dots by fluorescence imaging, and built a small lens-shaped silica bump above each one topped by a dielectric mirror, forming a Fabry-Perot microcavity. The thin membrane was transferred onto a piezoelectric crystal, so a voltage could strain the dot and shift its emission colour into resonance with the cavity. They then measured cavity quality, tuning, emission lifetime, brightness, photon antibunching and two-photon interference.
What they found
Cavity quality rose with defect width to a plateau near a Q of about 15,000, and strain tuned the dot by about 1.3 nm with the cavity mode staying put, giving roughly 50-fold brighter emission on resonance. The coupled dot's lifetime shortened to about 100 ps, a Purcell factor of about 9, with an extraction efficiency of 0.58. Single-photon purity was high, with g2(0) of 0.044, and corrected indistinguishability was 0.922.
The limits
What it doesn't show
The headline photon statistics come from a single device, so yield and device-to-device variation are not established. Measured Q factors fell short of simulations, which the authors attribute to imperfect evaporated dielectric layers and surface losses; the simulated extraction efficiency of 94.9% was not reached. The indistinguishability figure is corrected for setup imperfections (raw visibility was lower), and the tuning range is only about a nanometre.
Key terms
- Quantum dot
- A nanoscale semiconductor island that confines electrons and holes, so it emits light at discrete energies like an artificial atom.
- Purcell effect
- Speeding up spontaneous emission by placing an emitter in a resonant cavity with a small mode volume.
- Fabry-Perot microcavity
- A resonator formed by two mirrors facing each other; here the mirrors are stacked Bragg reflectors.
- g2(0)
- The second-order correlation at zero delay; values near zero mean the source rarely emits two photons at once.
- Hong-Ou-Mandel interference
- Two identical photons meeting on a beam splitter leave together; the visibility of this effect measures indistinguishability.
- Strain tuning
- Changing an emitter's energy by mechanically deforming its crystal lattice, here using a piezoelectric actuator.
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Quiz yourself
What tuned the quantum dot into resonance?
Common questions
Why not just tune the dot with temperature?
Heating shifts both the dot and the cavity and degrades photon coherence; strain shifted the dot while leaving the cavity mode nearly unchanged.
Why is a monolithic cavity better than an open cavity?
Open cavities move one mirror with nano-positioners, making them bulky and sensitive to vibration, whereas the monolithic device is compact and mechanically robust.
What does an indistinguishability of about 0.92 mean in practice?
Successive photons are nearly identical quantum states, which is needed for them to interfere in photonic quantum computing and repeaters.
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