Researchers Engineer Correlations in Quantum Photon Sources

Jiun-Shiuan Shiu of National Cheng Kung University and colleagues from National Tsing Hua University have, for the first time, unified quantum theory to predict correlations in DLCZ Raman photon sources. The propagation-inclusive open-system theory accounts for changes in atomic population during the write process and realistic conditions affecting how light travels. Experiments confirmed the theory’s predictions regarding time-dependent Stokes generation and enhanced correlations achieved with shorter write pulses; these correlations scale with the number of spin-wave excitations.

A new theoretical model accurately predicts the behaviour of photon sources used in developing quantum networks. This framework considers both how photons travel and how the source’s internal state evolves over time, enabling greater control over the characteristics of the light emitted. By combining these previously separate aspects, the team has provided a strong basis for engineering the connections between photons, a vital step in constructing more efficient quantum memories and networks.

A new quantum theory predicts the behaviour of light emitted from atomic ensembles, key components in emerging quantum networks. These networks rely on correlated pairs of photons to transmit information; the team’s work focuses on the Duan-Lukin-Cirac-Zoller (DLCZ) protocol, which aims to create single photons on demand.

Existing models often simplify the process, but this unified theory accounts for changes within the atoms during light creation and how light travels through the material, offering a more realistic simulation. By accurately predicting photon characteristics, the researchers hope to engineer more efficient quantum memories and networks; the full details of this propagation-inclusive open-system quantum theory, including how it models random fluctuations, are presented below.

Spin-wave scaling governs ultrafast photon correlations in atomic ensembles

A theoretical framework and experimental validation demonstrate enhanced correlations with shorter write pulses, improving upon previous models that struggled to accurately predict behaviour. Specifically, correlations scaled with the mean spin-wave excitation number, a phenomenon previously unquantified. Earlier approaches could not simultaneously account for both write-induced population redistribution and realistic light propagation within the atomic ensemble, hindering precise control over photon characteristics.

By combining Heisenberg-Langevin dynamics with Maxwell-Schrödinger propagation, scientists created a propagation-inclusive open-system quantum theory capable of modelling the entire DLCZ process, from initial spin-wave creation to emitted photon correlations. Experiments confirmed that the scaling of correlated photon pairs differed from that of accidental background noise, directly linked to the number of spin-wave excitations created within the atomic material. A clear relationship between correlation strength and the mean excitation number was observed, providing valuable insight into the process.

Temporal slicing was successfully implemented, selectively extracting anti-Stokes photons emitted at specific times after the initial write pulse, achieving precise control over the emitted light’s characteristics. This gating did not require real-time feedback from detecting the initial Stokes photons, simplifying the experimental setup and reducing complexity.

Modelling light-atom interactions clarifies control parameters for enhanced photon emission

Quantum memories are essential building blocks for future networks, demanding precise control over the properties of emitted photons. The new theory successfully models the complex interaction between light propagation through the atomic material and the resulting changes in atomic properties. While simulations may not fully capture all real-world complexities, this theoretical advancement represents a step forward in understanding and optimising quantum memory devices.

The model successfully links control settings to the quality of emitted photons, a vital factor for building practical quantum networks, and allows for better engineering of correlations, the linked behaviour of photons, essential for secure communication and distributed quantum computing. Further investigation will focus on the extent to which ‘realistic propagation and noise conditions’ were incorporated into the simulations, alongside a clear comparison to the performance of less thorough models.

The researchers National Tsing Hua University have established a predictive model for generating correlated photons using the Duan-Lukin-Cirac-Zoller (DLCZ) protocol, a technique employing atomic ensembles to create single photons on demand. This approach accurately simulates the entire process, from the initial creation of collective excitations within the atoms to the emission of linked photon pairs, and provides insights into optimising the DLCZ protocol for improved performance.

The research demonstrated a new theoretical model that accurately predicts how control parameters affect the emission of correlated photons using the Duan-Lukin-Cirac-Zoller protocol. Researchers intend to further refine the model by incorporating more realistic conditions and comparing its performance to simpler approaches.