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Massive Colloidal Quantum Dots

Best Weak Coherent State Systems Lose to Imperfect Photon Sources in Quantum Key Distribution

Two new methods enable easily accessible, imperfect SPS to outperform weak coherent state (WCS) systems in quantum key distribution (QKD). This discovery potentially revolutionise QKD without flawless single-photon emitters by tackling a long-standing secure quantum communication problem.

QKD is crucial for safe optical communication in the quantum computing era. It uses quantum physics to create cryptographic keys between parties. Information should be encoded onto pure single-photon states for optimal security.

However, the lack of perfect SPS, which are challenging to create and maintain, has historically impeded the practical implementation of Quantum Key Distribution QKD. Perfect sources prevent photon number splitting (PNS) assaults and other advanced eavesdropping methods. Since multi-photon emissions reduce the efficacy of simple QKD approaches, realistic SPS purity and emission rate must meet tight criteria.

QKD systems often use attenuated lasers creating weak coherent state (WCS) due to the difficulty of developing stable, optimum SPS systems for practical application. WCS is more accessible yet has limitations. A pulse's possibility of emitting multiple photons is never zero due to its Poissonian photon distribution.

This renders them vulnerable to PNS attacks since eavesdroppers can collect multi-photon pulses and extract data unnoticed. WCS's secure key rate (SKR) is also limited by a high vacuum state probability and photon number emission probability dependence.

WCS systems have advanced decoy-state procedures to address these issues. These methods improve PNS attack detection and communication channel characterisation by changing photon distributions. Despite enhanced decoy-state techniques, WCS effectiveness is limited by Poissonian statistics and the fundamental relationship between photon number probabilities. It established the transmission distance "frontier" and secure key rates.

Yuval Bloom, Yoad Ordan, Tamar Levin, Kfir Sulimany, Eric G. Bowes, Jennifer A. Hollingsworth, and Ronen Rapaport define the new research as “radically different, more realistic approach” in PRX Quantum. Instead of perfect SPS, the team uses defective sources' regulated, sub-Poissonian photon statistics. They source single photons at ambient temperature using nanoantennas and gigantic colloidal quantum dots (gCQDs).

Due to their shortened photon-number Fock space, which can only include zero, one, or two photons (N=0, 1, or 2) and has a negligible probability of more than two photons, these gCQDs emit a biexciton-exciton (BX-X) cascade The optical excitation power may be simply modified to control the odds of obtaining these Fock states.

Two new QKD techniques have been put out and shown to work experimentally:

Decoy State on Truncated Fock Basis (DTB): Alice, the sender, smoothly controls SPS excitation level, alternating intensities for diverse photon statistics. Instead than being used for the secure key itself, these “decoy states” are employed to describe the channel and identify eavesdropping. Due to the reduced photon-number basis of the giant colloidal quantum dot gCQD, a precise analysis of the channel characteristics may be performed using only two decoy states, which is a major benefit over WCS techniques. WCS protocols, on the other hand, are either limited to two decoy states or require an infinite number for an exact answer. The maximal channel loss (MCL) was clearly improved by about 2 dB over WCS with unlimited decoy states when BB84 QKD was experimentally simulated with a basic gCQD employing DTB. With optimized gCQD devices (SPS1), theoretical studies indicate an even higher MCL increase of over 3 dB. Because of this, the DTB protocol is on par with or better than optimum WCS protocols and even cutting-edge cryogenic SPS. The method outperforms WCS even when using SPS with single photon purities as low as ~65% and g(2)(0) values as high as ~0.6. Heralded Purification (HP): In order to maximize the likelihood of two-photon emission (P2), Alice runs the SPS in the saturation regime at a single, high excitation power. A beam-splitter (BS) and a single photon detector (SPD) are employed as a purification step before to the BB84 encoding unit. A detection event at Alice’s detector essentially removes multi-photon events since emissions with N>2 photons are minimal. This results in an effectively pure single-photon source (where the effective two-photon probability P̃2 is close to zero) for that particular pulse. A lower signal rate is the price paid for this. Because of the extremely low effective two-photon probability (P̃2), realistic calculations utilizing an SPS2 device (gCQD on a hybrid nanocone-antenna) showed that BB84+HP allows for a greater MCL (~1 dB) compared to WCS with unlimited decoy states. Although the low P2 of the bare giant colloidal quantum dot gCQD prevented testing results with a bare gCQD from outperforming WCS with decoy, the potential for optimized devices (SPS2) is substantial.

Third-order correlation measurements demonstrated that three-photon emission probabilities are extremely small supporting the truncated Fock basis assumption. Experimental validation using a bare giant colloidal quantum dot gCQD verified the viability of controlling photon emission by varying laser intensity.

By avoiding the strict single-photon purity criteria that have long prevented the broad deployment of SPS, these protocols offer a realistic and feasible solution to create innovative photon sources with higher QKD performance. Their utility is further increased by the use of small, room-temperature, and readily integrated SPS devices, such as giant colloidal quantum dots gCQDs.