Quantum cryptography is a topic of considerable interest. A simple and robust experiment and theory for a senior level undergraduate investigation of quantum key distribution are described. In the proposed experiment, key principles from the BB84 protocol, used in quantum cryptography, are emulated using an optical apparatus and computational scripts independently.

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.