Multimode Sources Impact DLCZ Quantum Repeaters, Enabling Long Distance Generation for Secure Computing

Long distance quantum communication represents a significant technological challenge, yet holds immense potential for applications ranging from secure computing to advanced cryptography. Emil R. Hellebek and Anders S. Sørensen, from the Niels Bohr Institute at the University of Copenhagen, investigate a critical component in realising this technology, namely the impact of multimode light sources on quantum repeaters based on the DLCZ protocol. Their work addresses a key practical consideration, demonstrating how the characteristics of the light source used to create entangled states affect the overall performance of the quantum repeater. By optimising parameters for both pulsed and continuous light source operation, the researchers provide valuable insights into achieving high fidelity quantum communication over extended distances and offer guidance for future experimental implementations of this promising technology.

The generation of long-distance entanglement at a high rate represents a major quantum technological goal, promising applications such as secure quantum computing on remote servers and quantum cryptography. This work investigates how multimode sources impact DLCZ-type quantum repeaters, a crucial component in achieving this goal, analysing how source characteristics affect entanglement generation rates and fidelity. As these technologies mature, understanding the details of underlying components becomes crucial. This paper considers the impact of multimode emission from the SPDC source on quantum repeaters based on the DLCZ scheme, exploring both pulsed and continuous driving of the SPDC source and finding that using very narrow laser pulses is crucial for obtaining high-fidelity entangled states, although this places demands on experimental precision.

Entanglement Swapping, Pulsed and Continuous Wave Analysis

This research details the theoretical framework for quantum entanglement swapping in both pulsed and continuous wave (CW) scenarios. Entanglement swapping creates entanglement between two particles that never directly interacted, achieved by entangling each particle with a third, then performing a measurement on those third particles. The analysis covers two methods for generating and manipulating entangled photons: pulsed, using short bursts of light, and continuous wave, using a continuous stream of light. The document provides the mathematical framework to describe the density matrix and coefficients for the entangled states in both scenarios, crucial for simulating, predicting, and optimising entanglement swapping experiments.

The research defines the density matrix, a mathematical object describing the quantum state of a system, essential for dealing with mixed states and calculating measurement probabilities. The equations define various coefficients that characterize the density matrix elements, depending on experimental parameters such as pulse duration and light intensity. Recursive equations allow calculation of these coefficients at successive steps of the entanglement swapping process, modelling the system’s dynamics, and functions describe the density matrix elements as functions of time and other parameters. The research explores diagonal elements, representing probabilities of finding photons in different polarization states, and off-diagonal elements, representing coherence between quantum states, both crucial for understanding the entanglement swapping process. Key parameters include a decay rate representing photon loss, a time duration representing pulse length, and an efficiency factor representing detector performance. The team demonstrates that multimode emission from spontaneous parametric downconversion (SPDC) sources affects the fidelity of entangled states, particularly as the number of entanglement swaps increases. They explored both pulsed and continuous driving of the SPDC source, identifying optimal parameters for pulse width and temporal acceptance windows to mitigate these effects. The findings reveal that careful control of laser intensity and pulse characteristics is crucial for maintaining high fidelity in quantum repeaters, allowing for greater swap depths and approaching the ideal performance of single-mode sources.

Researchers highlight the importance of balancing the probability of successful entanglement swaps with the purity of the resulting quantum state, demonstrating how to optimize acceptance windows for both pulsed and continuous driving schemes. By considering realistic limitations on laser power and incorporating multiplexing techniques within atomic ensemble memories, the team shows that high-performance quantum communication is achievable despite source imperfections. The authors acknowledge that the analysis focuses on specific parameters and approximations, and further research is needed to explore the impact of additional noise sources and more complex repeater architectures, suggesting that future work could investigate the interplay between different optimization parameters and explore the potential for adaptive control strategies to compensate for source fluctuations. Nevertheless, this study provides valuable insights into the practical challenges of building long-distance quantum communication networks and offers a pathway towards realizing high-fidelity quantum repeaters.