Microwave–optical quantum transduction

Superconducting quantum devices use microwaves, while long-distance links work better with light. An interface must bridge them. Can it transfer a quantum signal from the processor to an optical fibre without losing too much of it or adding unwanted noise?[1]

A quantum signal leaving a superconducting processor must enter a converter, change from microwave to optical frequency and emerge at an optical port. Can the complete interface preserve useful quantum information while meeting its loss, noise, usable frequency range and heating requirements?

Conversion efficiency compares converted optical photons with microwave input photons. Internal conversion excludes losses associated with coupling through the ports. As an illustration using assumed values, take 90% internal conversion and port-coupling fractions of 80% and 50%. In the review’s two-port efficiency model,

$$\eta_{\mathrm{external}}=0.80\times0.90\times0.50=0.36.$$

The external conversion efficiency is 36%; these are invented inputs, not measured device results. This calculation alone says nothing about added noise or how faithfully a quantum state is preserved. An August 2026 atomic-memory experiment adds controllable retrieval timing, with efficiency normalised to the microwave input incident on the active medium. This omits spatial-overlap losses that matter at a complete hardware interface. September pump-free Bell-pair work proposes a heralded protocol, where a measurement signals success, for teleportation in a theoretical model.[1] [2] [3]

It must transfer the quantum signal without too much loss or extra noise.[1]

Positive unassisted one-way quantum capacity in the pure-loss model requires transmissivity above one half; added noise also matters. Heralded or teleportation protocols have different requirements. Area-normalised conversion does not remove mode-matching losses at a real hardware interface.[1] [2] [3]

Optical links could let separated microwave-based quantum processors exchange quantum information. This requires more than a bright converted signal: loss and extra noise must leave the information usable at the receiving device.[1]

Networking depends on the delivered quantum information and the protocol that uses it. Controllable timing could coordinate distant operations, while low loss and noise constrain whether the delivered state is useful. Comparing efficiencies at the same interface boundary prevents an impressive internal conversion number from hiding port losses.[1] [2]