Our work into quantum shells (QSs) has into the quantum optical regime. A recent breakthrough demonstrates that these unique nanostructures can serve as efficient sources of quantum light, specifically generating correlated photon pairs. This positions QSs as a promising new platform for quantum information science and technologies.
From Classical to Quantum Light: The Biexciton-Exciton Cascade
The ability of QSs to generate quantum light stems from their multiexciton properties. The core mechanism is the biexciton-exciton (BX-X) radiative cascade. In this process, a single QS absorbs a high-energy photon, creating a biexciton (two electron-hole pairs). The biexciton first recombines to emit one photon, leaving behind a single exciton. This exciton then recombines to emit a second photon. The two emitted photons are temporally correlated, forming a pair [1]. In most nanocrystals, this cascade is inefficient. The non-radiative Auger recombination process rapidly destroys the biexciton before it can radiatively decay, quenching the potential photon pair emission. As established in previous work, the QS geometry strongly suppresses Auger recombination. This suppression is the key enabling factor, allowing the BX-X cascade to proceed radiatively with high efficiency. The near-unity biexciton quantum yield measured in QSs directly predicts a high probability of generating correlated photon pairs [1].
(a) A diagram illustrating exciton−exciton repulsion in a quantum shell structure. (b) Low temperature (10 K), time dependent PL spectra of an individual QS showing energyseparated X and XX emission bands. The integration time of each frame is 1 s. The inset shows an energy diagram of a two-exciton state of a QS. (c) Room temperature emission and absorption profiles of a 6.0 nm core CdS-CdSe-CdS QS. (d) Characteristic TEM image of 6.0 nm core CdS-CdSe-CdS QSs. (e) High angle annular dark field (HAADF)-STEM images of 6.0 nm core quantum shells, illustrating the location of the CdSe shell layer.
Heralded Generation of Correlated Photon Pairs
Using a custom confocal microscope setup with time-correlated single-photon counting (TCSPC), we measured a second-order cross-correlation function between photons detected in two different spectral windows: one corresponding to the biexciton emission (“herald” photon) and the other to the exciton emission (“signal” photon). The correlation histogram revealed a strong, sharp peak at zero time delay. This “bunching” peak is the definitive signature that the detection of a biexciton photon (in the herald channel) is closely followed by the detection of an exciton photon (in the signal channel) from the same cascade event within the same nanocrystal [1]. This correlation allows for the heralded generation of single photons. By detecting the first photon of the pair (the biexciton photon), one can anticipate with high probability the imminent arrival of the second photon (the exciton photon). This is a useful capability for quantum cryptography and linear optical quantum computing, where knowing a single photon is on its way is essential.
(a) Schematics of radiative XX−X cascade and spectrally resolved correlation measurements.
Spectral Tunability and Material Properties
The use of colloidal QSs offers significant practical advantages over traditional epitaxial quantum dot sources of quantum light. The emission wavelengths of the photon pairs (both BX and X) can be easily tuned by adjusting the physical dimensions of the QS, particularly the thickness of the CdSe quantum well layer. This allows for generation of correlated photons at specific wavelengths desirable for integration with existing fiber-optic communication networks or other quantum systems. As colloidal nanomaterials, QSs can be synthesized in large quantities and integrated into various solid-state matrices or deposited onto photonic structures, opening pathways for developing scalable and integrated quantum photonic devices.
This work shows CdS/CdSe/CdS quantum shells as an efficient source of quantum light. The suppression of Auger recombination—a generic property of QSs that also enables their superior lasing performance—is directly harnessed to generate correlated photon pairs via the biexciton-exciton cascade. The observation of strong photon bunching and the potential for heralded single-photon generation demonstrates quantum shells as promising building blocks for future quantum technologies.
[1] Marder, A. A. et al. Heralded Generation of Correlated Photon Pairs from CdS/CdSe/CdS Quantum Shells. ACS Nano 2024, *18* (44), 30863–30870. DOI: 10.1021/acsnano.4c11723