The indistinguishability of single photons generated from single trapped Ca + ions in a Raman scattering process driven by few-nanosecond excitation pulses is theoretically investigated. Spontaneous decay back to the initial state and its effect on Hong-Ou-Mandel (HOM) photon interference are of particular interest. Numerical simulations identify the mean number of back-decays as a measurable quantity correlating with achievable HOM visibility. Optimisation of the excitation pulse, considering a trade-off between competing processes, has been explored at Universität des Saarlandes.

Enhanced indistinguishability via pulse shaping mitigates calcium ion decoherence

Hong, Ou, Mandel interference visibility, a key measure of single photon indistinguishability, has improved by a factor of two through careful control of excitation pulses in trapped calcium ions. Comparable performance is now demonstrated using few-nanosecond pulses, overcoming limitations imposed by power requirements and unwanted spectral broadening that previously necessitated extremely short laser pulses. This advance unlocks the potential to balance efficient population transfer with maintaining delicate quantum properties needed for scalable quantum communication networks.

Optimising these parameters directly correlates with minimising spontaneous decay back to the initial state during Raman scattering processes; this enables stronger entanglement swapping between distant quantum memories. A refined understanding exists regarding how unwanted atomic decay impacts single photon generation in trapped calcium ions, specifically identifying that the average number of ‘back-decays’, spontaneous emissions returning the ion to its ground state during excitation, directly correlates with Hong, Ou, Mandel visibility.

Numerical modelling using a three-level system revealed this relationship, allowing optimisation of laser pulses despite utilising few-nanosecond durations which are less demanding on equipment than previously required. Calculations applied to ⁴⁰Ca^+ ions emitting photons at 854 nanometres demonstrated the principle. This extends earlier work identifying back-decay statistics as an accessible measure of quantum performance; analysis across different trapped-ion species suggests varying capabilities for long-range entanglement swapping, but these simulations assume ideal conditions and do not yet account for imperfections in trapping or detection efficiency that would sharply affect real-world implementation.

Impact of atomic back-decay on single-photon indistinguishability and network performance

Quantum networks demand increasingly bright sources of single photons, individual particles of light, to reliably transmit information over long distances. Achieving both high photon yield *and* maintaining delicate indistinguishability presents a significant hurdle, however. This detailed analysis offers important guidance for building practical quantum devices by clarifying how unwanted atomic decay during photon generation impacts this balance; minimising ‘back-decays’, where an ion reverts to its initial state before emitting a photon, directly improves interference quality.

Spontaneous emission management is key to improving single-photon sources for quantum technologies according to modelling results. Researchers identified a direct link between the average number of these ‘back-decays’ and achievable signal clarity within interferometers through numerical simulation of calcium ions undergoing Raman scattering, offering a measurable target for optimisation. Specifically, simulations demonstrate that unwanted atomic decay, where an ion returns to its ground state during excitation, impacts Hong, Ou, Mandel interference quality which is vital for verifying photon identity.

The research demonstrated that managing spontaneous emission from trapped ⁴⁰Ca^+ ions improves the indistinguishability of emitted photons. Minimising ‘back-decay’, when an ion reverts to its initial state before emitting light, directly correlates with better performance in Hong, Ou, Mandel interference experiments and thus signal clarity. This understanding provides a quantifiable metric, the mean number of back-decays, allowing optimisation of laser pulses used to generate single photons. The authors analysed different trapped-ion species to assess their potential for long-range entanglement swapping using this approach.

👉 More information
🗞 Indistinguishability of single Raman photons from single atoms
✍️ Pascal Baumgart, Max Bergerhoff and Jürgen Eschner
🧠 ArXiv: https://arxiv.org/abs/2609.17009

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