Inside a quantum computer, the temperature is colder than outer space. At just a few thousandths of a degree above absolute zero, fragile quantum bits (qubits) perform calculations that could revolutionize everything from drug discovery to cryptography. But there's a problem: these delicate quantum processors are drowning in cables.

Every qubit in today's superconducting quantum computers needs its own dedicated coaxial cable snaking through the refrigerator, carefully conditioned at each temperature stage to prevent interference and heat. The most powerful quantum processors currently use more than 100 qubits, which means hundreds of cables competing for limited space and cooling power. Experts predict this wiring bottleneck could limit machines to just thousands of qubits, far short of the millions needed for truly useful quantum computers.

Now, researchers at the Institute of Science and Technology Austria have demonstrated a radical solution: replace those microwave cables with beams of light.

The Wiring Crisis

Picture a dilution refrigerator, the ultra cold chamber where quantum computers live. It looks like a golden chandelier hanging from the ceiling, with layer upon layer of shields protecting the quantum processor at the bottom. Running through this structure are thick bundles of coaxial cables, each about the diameter of a pencil, carrying microwave signals to control and read out individual qubits.

These aren't ordinary cables. Each one must be carefully attenuated (weakened) on the way down to prevent noise from corrupting the quantum states. Then, on the way back up, the faint signals must be amplified without adding too much noise, isolated to prevent interference, and thermalized at each temperature stage to avoid heating the system.

The result is an engineering nightmare. The cables take up precious space, add significant heat load to a system with extremely limited cooling power, and cost thousands of dollars each. More fundamentally, they create a hard physical limit: you can only fit so many cables into a refrigerator before running out of space or exceeding the cooling capacity.

If quantum computers are to scale from hundreds to millions of qubits, the conventional wiring approach simply won't work.

Enter the Photons

The breakthrough came from recognizing that classical data centers solved a similar problem years ago. When electrical wires couldn't keep up with increasing data transfer demands, engineers switched to fiber optic cables carrying light instead of electricity. Optical fibers can carry far more information, generate less heat, and take up much less space than their electrical counterparts.

Could the same approach work for quantum computers?

The challenge is that qubits operate in the microwave frequency range, around 6 billion cycles per second. Optical fibers, by contrast, carry light at frequencies nearly 200 trillion cycles per second. Converting between these vastly different frequencies while maintaining quantum information and avoiding added noise is extraordinarily difficult.

Previous attempts at optical quantum control had significant limitations. Some approaches were inefficient, wasting most of the light and generating excessive heat. Others had limited bandwidth or introduced too much noise. Some could only work in one direction, either sending control signals down or reading measurement signals back up, but not both.

The Austrian team solved all these problems at once using a single remarkable device called an electro-optic transducer.

A Crystal Clear Solution

At the heart of their system sits a small disc made of lithium niobate, a special crystal that can convert between microwave and optical frequencies. The disc is about the size of a coin and features a whispering gallery mode resonator, where light travels around the edge like sound waves hugging the curved wall of a cathedral dome.

The magic happens when three different frequencies meet inside this crystal. An optical carrier wave at telecom wavelengths travels through a fiber to the device. A microwave cavity couples to the same crystal. And a pump laser provides the energy to enable conversion between the two.

When the pump laser is on, microwave signals can become optical signals and vice versa, all within the same device. It's like having a two way translator that works in both directions simultaneously.

The researchers call this simultaneous conversion approach "radio over fiber" readout. Instead of sending a microwave pulse down a coaxial cable to probe the qubit and then amplifying the weak microwave response coming back, they send modulated laser light down an optical fiber. The light gets converted to microwaves, probes the qubit, and the reflected microwave signal gets converted back to light before traveling back up through the same optical system.

Crucially, this approach eliminates the need for a microwave circulator, the bulky component that normally separates incoming and outgoing microwave signals. In the all optical scheme, the conversion happens so efficiently that the readout works without any cryogenic microwave components at all.

Putting Light to the Test

To prove their concept, the team built a superconducting transmon qubit, the workhorse of current quantum computers, enclosed in a three dimensional superconducting cavity. They connected this qubit system directly to the electro-optic transducer with a short coaxial cable.

Then they ran a comprehensive set of tests, comparing three different readout methods side by side: conventional microwave readout, hybrid microwave-in optical-out readout, and the new all optical approach.

The results were striking. For the all optical readout, they sent a 140 milliwatt laser pulse to the transducer for just 2 microseconds. This optical pulse got converted to microwaves, probed the qubit state, and the reflection got converted back to light. Using optical detectors at room temperature, they could distinguish whether the qubit was in its ground state or excited state with 82% accuracy in a single measurement.

That might not sound impressive compared to the 89% accuracy they achieved with conventional microwave readout, but the key is what they eliminated: all the cryogenic microwave amplifiers, circulators, isolators, and carefully thermalized cables that normally sit between the qubit and room temperature. The optical approach needs only the transducer itself and optical fibers.

Even more importantly, when they measured fundamental properties of the qubit like its lifetime and coherence, they found no significant difference between optical and microwave readout methods. The qubit's excited state lasted about 34 microseconds before decaying, and its quantum superposition survived for about 1.3 microseconds, regardless of which readout method they used.

The Heat Question

One major concern with optical readout is heat. Laser light carries far more energy per photon than microwaves. Even if only a small fraction of that light gets absorbed, could it heat up the system and destroy the fragile quantum states?

To answer this question definitively, the researchers performed an exhaustive series of measurements. They used a near quantum limited amplifier to make extremely sensitive measurements of the qubit while varying the intensity and repetition rate of the optical pulses.

The results were reassuring. At moderate repetition rates (up to about 1,000 pulses per second), they observed minimal impact on the qubit. The device showed no evidence of direct radiation damage from the high energy optical photons, despite the absence of any shielding.

What they did observe was indirect heating through optical absorption in the transducer. When the laser was on continuously at high repetition rates, it acted as a localized heat source that warmed up the mixing chamber of the refrigerator. This thermal effect eventually degraded qubit performance, but only at repetition rates far higher than needed for practical operation.

By carefully measuring the temperature of different components, they traced exactly how heat flowed through the system. The lithium niobate transducer heated up first, then gradually warmed the refrigerator's base plate, which eventually affected the qubit cavity. But at duty cycles appropriate for quantum computing, the impact remained negligible.

The key insight is that this heating depends entirely on the efficiency of the optical coupling. In their proof of principle device, only 22% of the light coupled into the transducer efficiently, meaning most of it got absorbed as waste heat. But this is an engineering problem, not a fundamental limitation. With better optical coupling and more efficient devices, the heat load could drop dramatically.

The Path Forward

The demonstrated all optical readout eliminates several major obstacles to scaling quantum computers, but important challenges remain.

First, the current device requires relatively high optical power, about 140 milliwatts during readout pulses. While this is manageable for a single qubit with low duty cycle, scaling to thousands of simultaneous readouts would overwhelm the refrigerator's cooling capacity.

However, recent work with different types of electro-optic devices shows a clear path to improvement. Some electromechanical transducers have achieved similar performance with a billion times less optical power, though with narrower bandwidth. Integrated photonic devices could offer even better efficiency while maintaining high speed.

Second, the current system requires more photons to achieve the same readout accuracy as conventional approaches. The measured quantum efficiency was about 0.015%, meaning only a tiny fraction of the readout photons contribute useful information. But again, this stems mainly from losses in the current prototype, not fundamental physics. With optimized optical coupling and lower loss components, the efficiency could improve by orders of magnitude.

Third, while the demonstration used a single qubit, real quantum computers need to read out many qubits simultaneously. The beauty of the optical approach is that wavelength division multiplexing, a mature technology from telecommunications, could enable many independent channels on a single fiber. Different qubits could be read out at different optical wavelengths, all traveling through the same optical infrastructure.

Looking ahead, the same electro-optic device that performed readout could also generate the microwave pulses needed to control the qubit. The researchers calculated that their transducer operating at the qubit frequency could produce a complete qubit operation in about 110 nanoseconds, fast enough for high fidelity quantum gates.

A Modular Quantum Future

Beyond simply reducing cable clutter, optical interconnects could enable entirely new quantum computer architectures. Current machines pack all their qubits into a single dilution refrigerator, limited by space and cooling constraints. With optical links, you could potentially connect multiple quantum processors in different refrigerators, creating a modular quantum network.

This matters because quantum error correction, the technique needed to build reliable quantum computers from imperfect qubits, requires vast numbers of physical qubits for each useful logical qubit. Estimates suggest millions of physical qubits to run meaningful algorithms. Fitting that many qubits and their control systems into a single refrigerator seems nearly impossible. But a distributed architecture connected by optical links could overcome this barrier.

The technology also has applications beyond quantum computing. Superconducting photon detectors used in astronomy and communications could benefit from optical readout. Classical superconducting logic circuits being developed for ultra low power computing could use similar optical interconnects. Even classical data centers are moving processors into cryogenic environments to reduce power consumption, and they too need efficient ways to communicate with room temperature systems.

Quantum Meets Telecom

Perhaps most intriguingly, the work demonstrates that superconducting quantum circuits and telecom wavelength light can peacefully coexist. Many researchers worried that shining bright lasers anywhere near sensitive superconducting qubits would destroy their quantum properties. The new results show that with proper engineering, this isn't the case.

The optical wavelength used in the experiment, around 1550 nanometers, is the same wavelength used in fiber optic networks worldwide. This means quantum computers could potentially tap into the vast existing infrastructure and component ecosystem of the telecommunications industry. Off the shelf fiber optic components, multiplexers, amplifiers, and detectors could be adapted for quantum computing use.

The demonstrated circulator free readout is particularly elegant. In conventional microwave systems, circulators are necessary to separate the outgoing readout pulse from the incoming measurement signal. But these ferrite based components are bulky, lossy, and require careful magnetic shielding. Eliminating them simplifies the cryogenic setup considerably.

The Austrian team's approach uses the same optical fiber and the same transducer for both downconversion (generating the microwave readout pulse) and upconversion (detecting the reflected signal). The separation happens naturally through the frequency conversion process, without needing any additional components.

Not Just for Qubits

While the immediate application is quantum computing, the underlying technology addresses a broader challenge: communicating with any cryogenic system that operates at microwave frequencies.

Superconducting electronics, which operate with essentially zero resistance, offer enormous advantages for both classical and quantum information processing. But they require cryogenic temperatures, creating the same wiring bottleneck. Classical superconducting processors being developed for energy efficient computing face exactly the same input/output limitations.

Photonic readout could enable new applications that were previously impractical. For example, arrays of thousands of superconducting nanowire single photon detectors could be read out optically with wavelength division multiplexing, eliminating the need for thousands of individual amplifier chains.

Radio frequency sensors operating at cryogenic temperatures for improved sensitivity could use electro-optic upconversion to interface with room temperature electronics. The high bandwidth of optical links would preserve the full temporal resolution of fast signals while avoiding cable losses.

The Big Picture

Stepping back, this research represents a convergence of several cutting edge fields: superconducting quantum computing, integrated photonics, cryogenic engineering, and optical communications. It shows how solutions from one domain (fiber optic telecommunications) can solve problems in another (quantum computing scalability).

The work also highlights an important principle in quantum technology development: quantum systems don't need to be completely isolated from the classical world. With careful engineering, you can have intense laser pulses, room temperature detectors, and fragile quantum superpositions all working together in the same system.

For quantum computing specifically, the all optical readout opens new architectural possibilities. Instead of viewing the dilution refrigerator as a monolithic system with fixed capacity, we can imagine quantum processors as modular units connected by optical networks. Scale out rather than scale up.

The researchers note that their current device represents just a first step. The whispering gallery mode resonator, while achieving impressive performance, is relatively large and challenging to integrate. Future versions using chip scale integrated photonics could be much more compact and efficient.

Advances in integrated lithium niobate photonics over the past few years have been dramatic, with orders of magnitude improvements in optical mode confinement and electro-optic efficiency. Applying these techniques to cryogenic transducers should yield substantial performance gains.

Alternative platforms like aluminum nitride and silicon photonics with electromechanical coupling also show promise for even more efficient frequency conversion at potentially lower cost and with better integration with superconducting circuits.

A Light at the End of the Cable

The quantum computing community has long recognized that input/output bottlenecks could limit scalability as severely as qubit quality or error rates. This work demonstrates that optical interconnects provide a viable path forward.

The key achievements are clear: true all optical qubit readout with no cryogenic microwave components, single shot state discrimination comparable to conventional methods, and most importantly, no observable degradation of qubit coherence despite relatively intense optical pulses.

The remaining challenges are equally clear: improving optical coupling efficiency, reducing the required optical power, and scaling to many qubits with wavelength multiplexing. But these are engineering challenges, not fundamental physics limitations.

As quantum processors grow from hundreds to thousands and eventually millions of qubits, the wiring inside dilution refrigerators will need to fundamentally change. Optical links offer more than just a way to reduce cable count. They enable new architectures, tap into mature telecommunications technology, and potentially allow modular, distributed quantum systems that can scale far beyond what's possible today.

The future of quantum computing might just be optical.

Publication Details

Published: February 11, 2025 (online)
Journal: Nature Physics
Publisher: Springer Nature
DOI: https://doi.org/10.1038/s41567-024-02741-4

Credit and Disclaimer

This article is based on original research published in Nature Physics by researchers from the Institute of Science and Technology Austria. The content has been adapted for a general audience while maintaining scientific accuracy. For complete technical details, comprehensive data, full methodology, and in depth analysis, readers are strongly encouraged to access the original peer reviewed research article through the DOI link provided above. All factual information, data interpretations, and scientific conclusions presented here are derived from the original publication, and full credit goes to the research team and their institution.