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Decoy-State and Purification Protocols for Superior Quantum Key Distribution with Imperfect Quantum-Dot-Based Single-Photon Sources: Theory and Experiment

Yuval Bloom1,*, Yoad Ordan1,*, Tamar Levin1, Kfir Sulimany1, Eric G. Bowes2, Jennifer A. Hollingsworth2, and Ronen Rapaport1,†

  • *These authors contributed equally to this work.
  • †Contact author: ronen.rapaport@mail.huji.ac.il
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PRX Quantum 6, 030332 – Published 21 August, 2025

DOI: https://doi.org/10.1103/7fdd-m92n

Abstract

The original proposal of quantum key distribution (QKD) was based on ideal single-photon sources, which, 40 years later, are still challenging to develop. Therefore, the development of decoy-state protocols using weak coherent states (WCS) from lasers set the frontier in terms of secure-key rates and distances. Here, we propose and experimentally demonstrate two simple-to-implement QKD protocols that allow practical far from ideal sub-Poissonian photon sources to outperform state-of-the-art WCS. By engineering the photon statistics of a biexciton-exciton cascade in room-temperature single-photon sources based on giant colloidal quantum dots coupled to nanoantennas, we show that either a truncated decoy-state protocol or a heralded-purification protocol can be employed to achieve a significantly increased performance in terms of the maximal allowed channel loss for secure-key creation, which can exceed even that of ideal WCS by more than 3 dB. We then experimentally emulate a BB84 QKD using such a quantum dot source, verifying the superiority of our protocols over the best possible BB84 WCS performance. These protocols can be utilized efficiently on a host of various quantum emitters having controllable photon statistics with a finite photon-number basis, offering a practical approach to QKD without the hindering requirements on the single-photon purity of the photon source.

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Popular Summary

Secure quantum communication (QC) allows two parties to share secret information using the principles of quantum mechanics, an approach that remains secure even in the quantum computation era. But the ideal version of QC requires perfect single-photon sources—devices that emit exactly one photon on demand. However, despite many years of research, creating and operating such ideal sources are still outstanding challenges. As a result, many QC systems today rely on weak laser pulses. While not perfect, these can be made more secure using various techniques such as decoy-state protocols. These laser-based QC systems set the current standard for security performance, secret-key-rate generation, and maximal secure transmission distance.

Here, we push the field of quantum communication toward a new approach by addressing the limitations of existing far-from-perfect single-photon sources. Instead of suppressing multiphoton events entirely, which is hard to do, we show how two-photon emissions can instead be used in new QC protocols to improve the secure communication performance. To do this, we use efficient and high-rate sub-Poissonian photon sources, based on semiconductor nanocrystal quantum dots, that randomly emit either zero, one, or two photons per excitation pulse. We then introduce two simple-to-implement techniques that rely on engineering the probability of the emitted photon-number distribution from the source at each pulse, thus either introducing “decoy” or “photon purification” into the transmission line. Such methods enable the generation of secure-key sharing at distances significantly longer than the current state-of-the-art solutions, beating even the theoretical limits set by ideal protocols based on weak lasers. Finally, we demonstrate experimentally, using our quantum-dot sources and implementing our protocols, that high performance and secure quantum communication are possible without the requirement of nearly perfect single-photon purity.

Our methods are broadly applicable to various other quantum light platforms with controllable photon emission statistics, offering a practical route for applicable superior quantum communication platforms.

Article Text

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PRX Quantum 6, 030332– Published 21 August, 2025
  • Received 1 October 2024
  • Revised 26 May 2025
  • Accepted 14 July 2025
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