In a significant advance toward practical quantum computing, researchers have successfully trapped and controlled individual electrons on the surface of superfluid helium at temperatures above 1 Kelvin. The achievement, detailed in the journal Physical Review X, overcomes a major engineering hurdle for scaling up quantum computers by drastically reducing the extreme cooling typically required for such systems.
This new method, developed by the Chicago-based company EeroQ, could substantially simplify the refrigeration infrastructure needed to build and operate large, fault-tolerant quantum processors. Most leading quantum platforms, such as those using superconducting or spin-based qubits, must operate in dilution refrigerators at temperatures near absolute zero, often around 10 millikelvin. By demonstrating stable control of electrons at a temperature more than two orders of magnitude higher, this work validates a promising pathway to building more scalable and commercially viable quantum machines.
Overcoming Cryogenic Bottlenecks
A primary obstacle in the race to build large-scale quantum computers is managing heat. Qubits, the fundamental units of quantum information, are incredibly sensitive to thermal noise, which can destroy the fragile quantum states they need to perform calculations. To protect them, scientists place quantum processors inside complex dilution refrigerators, which cool the hardware to just a fraction of a degree above absolute zero.
While effective, this approach presents significant scaling challenges. The cooling power of these refrigerators is extremely limited, typically around 1 milliwatt at 100 millikelvin. As engineers try to add more qubits and control lines into a processor, the heat generated quickly outstrips the cooling capacity. This limitation is a critical bottleneck, making it physically difficult and prohibitively expensive to scale current architectures to the thousands or millions of qubits needed for revolutionary applications.
An Ultra-Pure Qubit Environment
The EeroQ team’s platform is based on a unique architecture that uses single electrons trapped above the surface of superfluid helium. This system is considered one of the cleanest and most pristine environments in physics, offering a naturally isolated platform for a qubit. The electrons levitate about 10 nanometers above the helium surface, held in place by a balance of forces. Because the helium is exceptionally pure, there are minimal defects or vibrations to disturb the electron’s quantum state, which is essential for maintaining long coherence times—the duration a qubit can hold its information.
This approach has long been theoretically promising but has been difficult to realize in practice. The recent experiments mark a critical step by proving that these electron-on-helium qubits can be precisely managed under less demanding thermal conditions. This confirms that the platform is not just a theoretical curiosity but a practically feasible option for building advanced quantum computers.
The Experimental Method
Detecting a Single Electron
To achieve this breakthrough, the researchers developed a device that integrates the electron trap with a superconducting coplanar waveguide (CPW) resonator on a chip. This resonator acts as a highly sensitive detector. The team defined an electron trap using gate electrodes placed beneath the helium surface. By monitoring microwave signals passing through the resonator, they could detect tiny changes in its resonant frequency.
As electrons were loaded one by one into the trap from a nearby reservoir, each additional electron caused a distinct “dispersive frequency shift.” The team was able to resolve these shifts clearly, allowing them to confirm the presence of a single trapped electron. These experimental observations were in strong agreement with the team’s theoretical models, which treated each electron as a classical oscillator coupled to the cavity’s field.
Operating in a Warmer Environment
Conducting these measurements above 1 K introduced substantial new challenges. At these temperatures, the thermal energy is several times greater than the electron’s motional frequency, creating a much noisier environment than in previous millikelvin experiments. Furthermore, the non-negligible vapor pressure of the helium presented another potential source of interference. Despite these conditions, the researchers successfully demonstrated reproducible control and readout, establishing the viability of the technique at these higher temperatures.
A Path to Scalable Quantum Processors
The ability to operate at temperatures above 1 K opens the door to using different, more powerful cryogenic technologies. Instead of relying on dilution refrigerators, systems could be built using pumped helium-4 cryostats, which offer cooling powers greater than 100 milliwatts. This increased cooling capacity is crucial for managing the heat loads from the thousands of control lines and components necessary for a large-scale quantum processor. By relaxing the cooling constraints, the technology significantly reduces a key barrier to scaling.
Founded in 2017, EeroQ has focused on this electron-on-helium approach with the long-term goal of combining its inherent purity and stability with standard superconducting circuits. This latest result is a major validation of that roadmap, proving that single-electron control is feasible in conditions compatible with larger and more practical quantum hardware. This breakthrough could accelerate the development of quantum computers capable of tackling problems in fields like drug discovery and materials science that are intractable for even the most powerful classical supercomputers.
