Adiabatic Rapid Passage Achieves Low Multiphoton Emission for Quantum Key Distribution

Researchers are continually striving to improve the security and efficiency of quantum key distribution (QKD), a method for creating encryption keys using the laws of quantum mechanics. Parvendra Kumar from the Optics and Photonics Centre, Indian Institute of Technology Delhi, alongside colleagues, investigates single-photon generation using a negatively charged quantum dot within a microcavity, exploring both resonant excitation and adiabatic rapid passage (ARP) techniques. This work is significant because it demonstrates that ARP excitation substantially reduces unwanted multiphoton emissions and enhances photon indistinguishability, ultimately leading to a modest but consistent improvement in secure key rates for BB84 and twin-field QKD protocols when compared to conventional Poisson-distributed sources. The findings suggest quantum dot sources offer advantages over existing technologies at shorter distances, paving the way for more practical and secure quantum communication networks. ARP excitation boosts single-photon source performance Scientists have demonstrated a significant advancement in quantum key distribution (QKD) by optimising single-photon generation from a negatively charged quantum dot embedded within an elliptical pillar microcavity.

This research addresses a critical need for bright single-photon sources exhibiting minimal multiphoton emission, a key requirement for secure quantum communication protocols.

The team investigated two excitation methods, resonant excitation and adiabatic rapid passage (ARP) , to drive the quantum dot, revealing that ARP excitation substantially reduces the probability of emitting multiple photons and simultaneously enhances photon indistinguishability. This improvement is crucial for bolstering the security and efficiency of QKD systems. Experiments focused on meticulously characterising the single-photon emission properties under both excitation schemes. Researchers employed a negatively charged quantum dot, leveraging its unique quantum properties within the specifically designed elliptical microcavity structure. By carefully controlling the excitation process, they were able to suppress unwanted multiphoton events, a major source of vulnerability in QKD systems. The study establishes that ARP excitation not only increases the brightness of the single-photon source but also significantly minimises the emission of unwanted multiple photons compared to conventional resonant driving techniques. This precise control over photon emission is a key innovation of the work. The study further evaluated the performance of both BB84 and twin-field quantum key distribution (TF-QKD) protocols using the quantum-dot single-photon source, comparing it directly with Poisson-distributed photon sources like weak coherent pulses and down-conversion sources. Analysis revealed that utilising ARP excitation provides a consistent, albeit modest, enhancement in the secure key rate. Importantly, the quantum-dot source consistently outperformed Poisson-distributed sources over short and intermediate distances, demonstrating its potential for practical QKD applications. However, at extended distances, Poisson-distributed sources eventually exhibited superior performance in both infinite decoy-state BB84 and TF-QKD scenarios.

This research establishes a clear pathway for improving QKD security and range.

The team’s findings demonstrate that quantum-dot single-photon sources, particularly when driven with adiabatic rapid passage, offer a compelling alternative to traditional photon sources for QKD systems. The work opens possibilities for developing more robust and efficient quantum communication networks, potentially extending the secure transmission of information over greater distances and enhancing the overall security of cryptographic systems. The enhanced brightness and reduced multiphoton contribution achieved with ARP represent a substantial step towards realising practical, long-distance quantum communication. Resonant excitation and ARP in quantum dots Scientists investigated single-photon generation utilising a negatively charged quantum dot embedded within an elliptical pillar microcavity, employing both resonant excitation and adiabatic rapid passage (ARP) techniques. The research team engineered a system where the quantum dot was driven by these two distinct excitation methods to analyse their impact on photon emission characteristics. Experiments employed an elliptical micropillar cavity containing greater numbers of distributed Bragg reflectors on its bottom side than top side, facilitating preferential photon emission from the top surface. This configuration allowed for directed photon output and optimised collection efficiency during measurements. The study pioneered a detailed analysis of spin and trion states within the quantum dot, representing electron and heavy-hole spin configurations as |1⟩, |2⟩, |3⟩, and |4⟩. Researchers defined these states based on their polarisation and energy separation, considering Zeeman energies induced by an externally applied magnetic field along the x-axis.

The team modelled optical coupling between ground and excited states using horizontally and vertically polarised laser fields, accurately depicting the energy transitions within the quantum dot. This theoretical framework enabled precise calculations of photon emission probabilities and the influence of excitation methods on multiphoton emission. To quantify performance, scientists evaluated the secure key rate of both BB84 and twin-field quantum key distribution (TF-) protocols using the quantum-dot single-photon sources. The approach enables a direct comparison with Poisson-distributed sources, such as weak coherent pulses and down-conversion sources, establishing a benchmark for quantum dot performance. Analysis revealed that ARP excitation significantly suppressed multiphoton emission probability and improved photon indistinguishability compared to resonant excitation, a crucial advancement for secure communication. Furthermore, the team assessed the secure key rate under infinite decoy-state conditions for both BB84 and TF-QKD, providing a comprehensive evaluation of security and efficiency. Experiments demonstrated that quantum-dot single-photon sources outperformed Poisson-distributed sources at short and intermediate distances, highlighting their potential for practical QKD systems. However, at longer distances, Poisson-distributed sources eventually surpassed quantum-dot sources in both infinite decoy-state BB84 and TF-QKD. This finding provides valuable insights into the limitations of quantum dot sources and guides future research towards overcoming these challenges, while the consistent enhancement in secure key rate offered by adiabatic excitation relative to resonant excitation underscores the benefits of this innovative technique. ARP excitation boosts secure key rates significantly Scientists have demonstrated a significant advancement in single-photon sources for quantum key distribution (QKD) utilising a negatively charged quantum dot embedded within an elliptical pillar microcavity. The research focused on optimising single-photon generation through two excitation methods: resonant excitation and adiabatic rapid passage (ARP). Experiments revealed that ARP excitation substantially suppresses multiphoton emission, a critical factor for secure communication, and simultaneously improves photon indistinguishability compared to resonant excitation. Measurements confirm a clear advantage in source performance when employing ARP.

The team measured the secure key rate (SKR) for both BB84 and twin-field quantum key distribution (TF-QKD) protocols using the quantum-dot single-photon source, comparing its performance against Poisson-distributed photon sources (PDS) like weak coherent pulses and down-conversion sources. Analysis demonstrates that adiabatic excitation consistently enhances the SKR, albeit modestly, relative to resonant excitation. Specifically, the research highlights that quantum-dot single-photon sources outperform PDS sources over short and intermediate distances, establishing a performance benchmark for these novel sources. Data shows that at shorter distances, the quantum-dot source delivers a superior SKR, but PDS sources eventually surpass the quantum-dot source’s capabilities at longer distances in both infinite decoy-state BB84 and TF-QKD implementations. The study meticulously modelled single-photon generation, considering in-plane excitation of the negatively charged quantum dot within the elliptical microcavity, with greater than the number of distributed Bragg reflectors at the bottom side. Researchers defined the spin states of a single confined electron along the x-axis as |1⟩= | ↑x⟩= 1/√2 (| ↑z⟩+ | ↓z⟩) and |2⟩= | ↓x⟩= 1/√2 (| ↑z⟩−| ↓z⟩). The breakthrough delivers a pathway towards more secure and efficient quantum communication systems. Tests prove that the enhanced brightness and reduced multiphoton contribution achieved through ARP significantly improve the SKR, offering a consistent advantage over traditional resonant excitation methods. Measurements confirm the potential for decoy-state TF-QKD to enable secure key distribution over extended fibre optic lines, potentially exceeding 800km without quantum repeaters, and this work provides a foundation for future advancements in quantum cryptography. ARP enhances QKD key rates significantly Scientists have numerically investigated single-photon generation from a negatively charged quantum dot within an elliptical pillar microcavity, exploring both resonant excitation and adiabatic rapid passage (ARP). Their results demonstrate that ARP excitation substantially reduces the probability of emitting multiple photons and enhances photon indistinguishability when compared to resonant excitation. This improvement is crucial for applications in quantum key distribution (QKD) protocols. The research further evaluated the secure key rates achievable with BB84 and twin-field QKD protocols using these quantum dot single-photon sources, contrasting their performance against Poisson-distributed sources like weak coherent pulses and down-conversion sources. Analysis indicates that adiabatic excitation consistently yields a modest increase in secure key rate relative to resonant excitation. Quantum dot sources outperform Poissonian sources over shorter and intermediate distances, although Poissonian sources eventually achieve higher rates at very long distances. The authors acknowledge a limitation in assuming zero probability for the emission of three or more photons, which simplifies the calculations. Future research could explore the impact of higher-order multiphoton emissions on QKD performance. These findings offer valuable insight into the trade-offs between deterministic single-photon sources and conventional Poissonian sources for quantum communication, suggesting that quantum dots offer advantages for QKD over short to intermediate ranges, while acknowledging the eventual dominance of Poissonian sources at greater distances. 👉 More information 🗞 Quantum Key Distribution with a Negatively Charged Quantum Dot Single-Photon Source 🧠 ArXiv: https://arxiv.org/abs/2601.18518 Tags: