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arXiv:2609.17009

Indistinguishability of single Raman photons from single atoms

Pascal Baumgart, Max Bergerhoff, Jürgen Eschner

quant-phSubmitted Sep 15, 2026• Announced Sep 16, 2026• 0 citations
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Abstract

We theoretically investigate the indistinguishability of single photons generated from single trapped $^{40}$Ca$^+$ ions in a Raman scattering process driven by few-nanosecond excitation pulses. Of particular interest is how spontaneous decay back to the initial state affects Hong-Ou-Mandel (HOM) photon interference. Numerical simulations identify the mean number of back-decays as a measurable quantity that correlates with achievable HOM visibility. Optimization of the excitation pulse with respect to a trade-off between photon yield and indistinguishability is analyzed. Finally, we compare the performance of different trapped-ion species for long-range dual-rail entanglement swapping via HOM photon interference.

Comments
14 pages, 7 figures

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We theoretically investigate the indistinguishability of single photons generated from single trapped 40^{40}40Ca+^++ ions in a Raman scattering process driven by few-nanosecond excitation pulses. Of particular interest is how spontaneous decay back to the initial state affects Hong-Ou-Mandel (HOM) photon interference. Numerical simulations identify the mean number of back-decays as a measurable quantity that correlates with achievable HOM visibility. Optimization of the excitation pulse with respect to a trade-off between photon yield and indistinguishability is analyzed. Finally, we compare the performance of different trapped-ion species for long-range dual-rail entanglement swapping via HOM photon interference.
Indistinguishability of photonic qubits emitted from trapped 40^{40}40Ca+^++ ions via pulsed excitation
We investigate the indistinguishability of Raman photons generated from two trapped 40^{40}40Ca+^++ ions using few-nanosecond excitation pulses. We elucidate how spontaneous scattering back to the initial state affects Hong-Ou-Mandel interference. We identify the mean number of back-decays as a measurable single-emitter quantity that correlates with achievable interference visibility of photons from two identical emitters.
We investigate two schemes for generating indistinguishable single photons, a key feature of quantum networks, from a trapped ion coupled to an optical cavity. Through selection of the initial state in a cavity-assisted Raman transition, we suppress the detrimental effects of spontaneous emission on the photon's coherence length, measuring a visibility of 81(2)81(2)%81(2) without subtraction of background counts in a Hong-Ou-Mandel interference measurement, the highest reported for an ion-cavity system. In comparison, a visibility of 50(2)% was measured using a more conventional single-photon scheme. We demonstrate through numerical analysis of the single-photon generation process that our scheme produces photons of a given indistinguishability with a greater efficiency than the conventional one. Single-photon schemes such as the one demonstrated here have applications in distributed quantum computing and communications, which rely on high-fidelity entanglement swapping and state transfer through indistinguishable single photons.
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Interfacing different physical systems is important in the context of building a practical quantum information network. As a first step towards interfacing different physical systems, we aim to demonstrate the indistinguishability between single photons produced via two different processes by observing the Hong-Ou-Mandel interference effect. The single photons are produced via a four-wave mixing process in a cold atomic ensemble of Rb and spontaneous emission from a single Rb atom in an optical dipole trap. This thesis reports the progress of the single Rb atom setup towards achieving this goal. We measure the temporal profile of the spontaneously emitted single photon and show that the characteristic decay time is in accordance with the values in the literature. Finally, we demonstrate that we can excite the atom with an excitation probability of 78% using a π pulse.