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arXiv:2512.16617 · Physical Review Letters

Indistinguishable photons from a two-photon cascade

Timon L. Baltisberger, Francesco Salusti, Mark R. Hogg, Malwina A. Marczak, Nils Heinisch, Sascha R. Valentin, Stefan Schumacher, Arne Ludwig, Klaus D. Jöns, Richard J. Warburton

quant-phSubmitted Dec 18, 2025• Announced Dec 19, 2025• 1 citation
arXivHTMLPDF

Abstract

Decay of a four-level diamond scheme via a cascade is a potential source of entangled photon pairs. A solid-state implementation is the biexciton cascade in a semiconductor quantum dot. While high entanglement fidelities have been demonstrated, the two photons, XX and X, are temporally correlated, typically resulting in poor photon coherence. Here, we demonstrate a high two-photon interference visibility (a measure of the photon coherence) for both XX (V=94$\pm$2%) and X (V=82$\pm$6%) photons. This is achieved by Purcell-enhancing the biexciton transition in a low-noise device. We find that the photon coherence follows the well-known quantum optics result upon tuning the XX:X lifetime ratio over two orders of magnitude.

Journal reference
Phys. Rev. Lett. 137 (2026), 073603
DOI
10.1103/t8sk-b2w4
Comments
10 pages, 4 figures in main paper, 5 figures in supplementary information

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Decay of a four-level diamond scheme via a cascade is a potential source of entangled photon pairs. A solid-state implementation is the biexciton cascade in a semiconductor quantum dot. While high entanglement fidelities have been demonstrated, the two photons, XX and X, are temporally correlated, typically resulting in poor photon coherence. Here, we demonstrate a high two-photon interference visibility (a measure of the photon coherence) for both XX (V=94±\pm±2%) and X (V=82±\pm±6%) photons. This is achieved by Purcell-enhancing the biexciton transition in a low-noise device. We find that the photon coherence follows the well-known quantum optics result upon tuning the XX:X lifetime ratio over two orders of magnitude.
A well controlled, triggered source of entangled photons is desirable for many applications in quantum information processing. The two-photon cascade from a biexciton state, two electrons and holes, confined in a quantum dot can be such a source of polarisation-entangled photons provided the two decay paths from the biexciton carry no “which-path” information. In recent years many techniques have been employed to make the two optical decay paths from the biexciton state indistinguishable. A number of these techniques will be discussed and entangled-photon emission from a single quantum dot with a high fidelity in the expected Bell-state from the cascade will be demonstrated. This source of entangled photons allows optical interferometry beyond the limits imposed by the photon wavelength. Interference fringes of the entangled biphoton state reveals a periodicity half of that obtained with the single photon, and much less than that of the pump laser. High fringe visibility indicates that biphoton interference is less sensitive to decoherence than interference of two sequential single photons. The effect of the exciton fine-structure splitting on our entangled photon source will be shown. Surprisingly the entanglement is found to persist despite relatively large separations between the intermediate energy levels of up to 4µeV. Measurements demonstrate that entanglement of the photon pair is robust to the dephasing of the intermediate exciton state responsible for the first order coherence time of either single photon. We distinguish between the first-order coherence time, and a parameter defined as the cross-coherence time, this is illustrated in the figure.
In this work we present a cavity-enhanced InAs/In0.53Al0.23Ga0.24As\mathrm{In_{0.53}Al_{0.23}Ga_{0.24}As}In0.53​Al0.23​Ga0.24​As quantum dot (QD) single-photon source in the telecom C-band with a record-low biexciton emitter decay time of \SI{67.4(2)}{ps} under resonant two-photon excitation (TPE). We observe strong multiphoton suppression associated with gX(2)(0)=0.006(1)g^{(2)}_\mathrm{X}(0) = 0.006(1)gX(2)​(0)=0.006(1) and gXX(2)(0)=0.007(1)g^{(2)}_\mathrm{XX}(0) = 0.007(1)gXX(2)​(0)=0.007(1) for the exciton (X) and biexciton (XX) emission, respectively. Due to a asymmetric Purcell enhancement of the XX-X cascade, the two-photon interference (TPI) visibility of XX photons under πππ-pulse excitation of VTPI=90(3)%V_{\rm{TPI}} = 90(3)\%VTPI​=90(3)% reaches the theoretical limit and clearly exceeds the ∼60%\sim60\%∼60% expected for standard XX-X cascades without photonic engineering. Furthermore, adding a second timed laser pulse coinciding with XX emission energy, we demonstrate stimulated TPE in the telecom C-Band. The result is an improved TPI visibility of the X photons of VTPI=0.69(3)V_{\rm{TPI}}=0.69(3)VTPI​=0.69(3) compared to TPE with VTPI=0.61(4)V_{\rm{TPI}}=0.61(4)VTPI​=0.61(4), with both being reduced compared to the theoretical values due to present dephasing effects. The advances presented in this work hold important promises for the implementation of advanced schemes of quantum communication using deterministic quantum light sources.
We investigate the degree of indistinguishability of cascaded photons emitted from a three-level quantum ladder system; in our case the biexciton-exciton cascade of semiconductor quantum dots. For the three-level quantum ladder system we theoretically demonstrate that the indistinguishability is inherently limited for both emitted photons and determined by the ratio of the lifetimes of the excited and intermediate states. We experimentally confirm this finding by comparing the quantum interference visibility of noncascaded emission and cascaded emission from the same semiconductor quantum dot. Quantum optical simulations produce very good agreement with the measurements and allow us to explore a large parameter space. Based on our model, we propose photonic structures to optimize the lifetime ratio and overcome the limited indistinguishability of cascaded photon emission from a three-level quantum ladder system.
A well controlled, triggered source of entangled photons is desirable for many applications in quantum information processing. The two-photon cascade from a biexciton state, two electrons and holes, confined in a quantum dot can be such a source of polarisation-entangled photons provided the two decay paths from the biexciton carry no “which-path” information. In recent years many techniques have been employed to make the two optical decay paths from the biexciton state indistinguishable. We will demonstrate the effect of the exciton fine-structure splitting on a single-dot based entangled photon source. Surprisingly the entanglement is found to persist despite relatively large separations between the intermediate energy levels of up to 4µeV. Measurements demonstrate that entanglement of the photon pair is robust to the dephasing of the intermediate exciton state responsible for the first order coherence time of either of the single photons. We distinguish between the first-order coherence time (2*), and a parameter defined as the cross-coherence time (HV) which is illustrated in the figure. The cross-coherence time is found to be at least an order of magnitude greater than the first-order coherence time. We will also report on recent progress towards an ideal source of triggered polarisation-entangled photons with a single quantum dot.
We demonstrate the fabrication of highly efficient sources of entangled photon pairs by inserting a semiconductor quantum dot in an optical cavity. Two electron-hole pairs trapped in a quantum dot (QD) radiatively recombine emitting a cascade of two polarization entangled photons. To extract both photons, we use a photonic molecule consisting of two identical micropillars, one empty, the other embedding a chosen QD. By adjusting the diameter of the pillars and their relative distance, we ensure that both optical transitions of the QD are simultaneously resonant to cavity modes. The emitted photon pairs are efficiently extracted thanks to Purcell effect. Doing so, we obtain the brightest sources of entangled photon pairs to date. We further show that the implementation of Purcell effect allows increasing the fidelity of the two photon state by reducing spin induced phase shift during the radiative cascade.
Photon indistinguishability, entanglement, and antibunching are key ingredients in quantum optics and photonics. Decay cascades in quantum emitters offer a simple method to create entangled-photon-pairs with negligible multipair generation probability. However, the degree of indistinguishability of the photons emitted in a cascade is intrinsically limited by the lifetime ratio of the involved transitions. Here we show that, for the biexciton-exciton cascade in a quantum dot, this ratio can be widely tuned by an applied electric field. Hong-Ou-Mandel interference measurements of two subsequently emitted biexciton photons show that their indistinguishability increases with increasing field, following the theoretically predicted behavior. At the same time, the emission line width stays close to the transform-limit, favoring applications relying on the interference among photons emitted by different sources.