Quantum cryptography with highly entangled photons from semiconductor quantum dots

Most modern quantum key distribution (QKD) systems utilize the BB84 protocol and single photons that are generated using lasers through the process called spontaneous parametric down-conversion. This technique has a range of limitations, mainly due to low extraction efficiency of polarized photons (approximately 0.01). Higher numbers usually lead to increased number of noise photons, which can cause bit errors and potential security loopholes in QKD schemes. Addressing these issues correctly requires expensive instrumental setups.

Semiconductor quantum dots can emit single pairs of polarization-entangled photons deterministically, but their drawback lies in the extremely low-extraction efficiency. However, the use of polarization entangled photon pairs substantially reduces the security threats due to the fact that quantum dots can be used to achieve exceptionally high entanglement fidelity values, or the”closeness” of quantum states.

In a new research paper, a team of scientists demonstrate the first implementation of the BBM92 protocol using a quantum dot source with an entanglement fidelity as high as 0.97. BBM92 is essentially a two-photon variation of the BB84 protocol.

To prove this principle, the team performed the quantum key generation between two buildings, connected by 350 metre long fiber, and managed to achieve an average key rate of 135 bits/s and a qubit error rate (QBER) of 1.91% over a time span of 13 hours, without using additional signal processing. The authors also conclude that using quantum dots as light sources for entanglement-based quantum networks could already provide key generation rates in the range of Gbit/s.

In conclusion, we have presented an implementation of the BBM92 quantum key distribution protocol using semiconductor quantum emitters as sources of highly entangled photon pairs with an ultra-low multi-photon emission probability.

With polarization entangled photons from droplet etched GaAs quantum dots (QDs) embedded in a simple planar cavity, a raw key rate of 135 bits/s and a QBER of 1:91% have been achieved through a 350m long optical fiber without time filtering, resulting in a net secure key rate of 86 bits/s. These values allow for implementing protocols with a higher demand on the QBER, like the device independent scheme.

Our work demonstrates the viability of quantum dots as light sources for entanglement-based quantum key distribution and quantum networks. By embedding them in state-of-the-art photonic structures, key generation rates in the Gbit/s range are at reach.

Link to research paper: https://arxiv.org/abs/2007.12726