Short answer: Microwave-to-optical quantum transduction converts quantum information between microwave photons, which superconducting qubits use, and optical photons, which can travel through optical fibre. It is a missing link for connecting superconducting quantum processors over distance. A 2023 Nature Nanotechnology paper reports a chip-based transducer with 0.9% efficiency, so the technology is at a research stage and efficiency is a central challenge.
Why it is needed
A 2025 tutorial on arXiv (2505.02057) explains that different qubit platforms have complementary strengths, which makes any quantum network heterogeneous. Superconducting technology is described as one of the most promising platforms for scalable, fast quantum computing at the node scale, and it operates at microwave frequencies. For links beyond a few metres, the paper says quantum links should operate at optical frequencies. A quantum transducer that converts one type of qubit to the other is therefore required. The tutorial frames transduction within a communications engineering framework, categorizes transduction modalities, and describes one in which the transducer itself can act as an entanglement source.
A 2020 perspective in Quantum Science and Technology adds that individual infrared photons are excellent carriers of quantum information in optical fibre and have been used to transmit it over distances of up to hundreds of kilometres, while many solid-state qubits, such as superconducting circuits, electron spins in quantum dots or NV centres, typically operate at microwave frequencies. The perspective reviews the leading approaches, with emphasis on atomic ensembles, opto-electro-mechanics and electro-optics, and discusses metrics and challenges.
What a useful transducer must do
The 2023 paper in Nature Nanotechnology states that useful entanglement links between processing units require several conditions to be met at once. The transducer must add less than a single quantum of input-referred noise and work with high efficiency, large bandwidth and a high repetition rate. The authors present a design for an integrated transducer in which a planar superconducting resonator is coupled to a silicon photonic cavity through a mechanical oscillator made of lithium niobate on silicon.
What the device achieved
They report a transduction efficiency of 0.9% with 1 microwatt of continuous optical power and a spectral bandwidth of 14.8 MHz. With short optical pulses the added noise is limited to a few photons, at repetition rates up to 100 kHz. The device couples directly to a 50 ohm transmission line, and the authors say it can be scaled to a large number of transducers on one chip. They describe this as laying foundations for distributed quantum computing. The efficiency and added-noise figures show how far this device is from the conditions listed above.
What this does not tell you
These sources do not report a working network of superconducting processors linked by transducers. The figures are from one laboratory device, and other approaches reported elsewhere may differ. The papers describe transduction as an active research area with challenges remaining.
FAQ
What is quantum transduction?
Converting quantum signals from one form of energy to another, here between microwave and optical photons.
Why can superconducting qubits not use optical fibre directly?
They operate at microwave frequencies, while links over distance should be optical, per the 2025 tutorial.
What does a good transducer need?
Added noise below one quantum, high efficiency, large bandwidth and a high repetition rate.
How efficient is the 2023 device?
The authors report 0.9% efficiency with 1 microwatt of continuous optical power.
Is this a working quantum network?
No. The sources describe research devices and open challenges.
All field notes