Squeezed light boosts quantum key distribution over noisy fibre links

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Researchers have shown that squeezed light can make quantum key distribution work better over noisy optical fibre. Quantum Zeitgeist reports on a Science Advances paper describing an experimental continuous variable system. The results suggest squeezing helps most where ordinary methods struggle

Continuous variable QKD encodes information in properties of light such as amplitude and phase. Most systems use ordinary coherent states. Squeezed states reduce noise in one property at the expense of another, which can help when the channel itself is noisy

Over a 30 kilometre fibre link, squeezed states beat coherent states once the excess noise rose to about 60 mSNU. The squeezed system kept producing secret keys above 175 mSNU, while the coherent state version failed beyond about 80. On a 20 kilometre channel at around 117 mSNU, the squeezed protocol reached a secret key fraction of 0.0167 bits per symbol

The key enabler is digital signal processing that rebuilds the squeezed states, replacing optical phase locked loops that are hard to run. That keeps the hardware close to standard coherent state systems, which makes it more practical. The security analysis included finite size effects, and those tests ran on a GPU

Coherent states still perform better at very low noise, so squeezing is not a universal upgrade. But real fibre networks are rarely perfect, and tolerance to noise matters a lot. This looks like a useful step towards QKD that works in messier conditions

Could squeezed light make QKD practical over existing networks? And how does CV QKD compare with other approaches?

Quantum Key Rates Improve With Squeezed Light Over Fiberquantumzeitgeist.com