Symmetry-protected privacy: beating the rate-distance linear bound over a noisy channel

Understand

There are two main factors limiting the performance of quantum key distribution --- channel transmission loss and noise.

• Previously, a linear bound was believed to put an upper limit on the rate-transmittance performance.
• Remarkably, the recently proposed twin-field and phase-matching quantum key distribution schemes have been proven to overcome the linear bound.
• In practice, due to the intractable phase fluctuation of optical signals in transmission, these schemes suffer from large error rates, which renders the experimental realization extremely challenging.