An optical cavity raised the first cascade photon’s measured interference visibility to 90%, compared with a typical value near 60%, according to findings published in Physical Review Letters by Stefan Schumacher and colleagues. Accelerating the first emission improved coherence for both photons in the experiment, while accelerating the second transition had the opposite effect.
Quantum dots release stored energy in two steps
A quantum dot confines electronic excitations inside a semiconductor and functions similarly to an artificial atom. Exciting it twice creates a biexciton, which is a bound complex containing two electrons and two holes, where each hole represents a missing electron in the semiconductor.
The complex releases its stored energy in two steps. Lead author Timon Baltisberger from the University of Basel explains that the biexciton decays into an exciton and emits one photon, after which the remaining exciton decays to release a second photon and return the dot to its ground state. That sequence, known as a biexciton cascade, provides a path toward generating entangled photon pairs.
Microcavity mirrors use Purcell effect to accelerate transitions
The research team placed an indium gallium arsenide quantum dot inside an open optical microcavity. One mirror formed part of the underlying semiconductor structure, while a curved upper mirror completed the assembly and let light escape toward the collection system.
Adjusting the distance between the mirrors tuned the cavity resonance and selectively accelerated either the first or second transition inside the dot. This acceleration relies on the Purcell effect, which alters an emitter’s radiative decay rate based on its optical environment. The experiment varied the ratio between the biexciton lifetime and the exciton lifetime across two orders of magnitude.
Enhanced transitions increase photon interference visibility
When the open cavity enhanced the first transition, the biexciton-to-exciton photon reached a raw interference visibility of 90% with a two-percentage-point uncertainty. The second photon reached 80% with an uncertainty of six percentage points. An unmodified cascade typically produces a theoretical visibility near 60% based on standard lifetime ratios.
Testing interference between successive photons of the same transition separated by 13.1 nanoseconds required spectral filters to select either the first or second emission. After correcting for imperfect single-photon purity, visibility estimates rose to 94% for the first photon and 82% for the second.
Crystal Vibrations and Photon Purity
Indistinguishability and single-photon purity depend differently on cavity settings. When the cavity enhanced the biexciton transition, the measured zero-delay photon correlation reached 2.3%, indicating a purity near 98%. Resonating the exciton transition instead dropped that correlation below 0.1% due to cavity feeding.
In the first configuration, the remaining exciton can emit into the cavity while simultaneously creating a phonon, which is a vibration of the crystal lattice. That phonon supplies the energy difference required for an otherwise mismatched emission, causing both cascade photons to enter the cavity output.
What causes timing jitter in a biexciton cascade?
What causes timing jitter in a biexciton cascade?
The second photon cannot emerge before the first emission finishes, and uncertainty in the first emission introduces timing jitter that limits overall photon indistinguishability.
How did the open microcavity alter the transitions?
Changing the distance between the semiconductor mirror and the curved upper mirror tuned the cavity resonance, using the Purcell effect to selectively accelerate either the first or second transition within the quantum dot.
What obstacles remain for creating a practical entangled photon source?
Crystal vibrations, or phonons, cause unwanted cavity feeding during the cascade, and the open cavity currently collects the second photon inefficiently through other optical modes.
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