Physicists create "high temperature" silicon spin qubits

The biggest challenge in quantum computing is achieving scalability. Classical computing, which has faced this problem before, now relies on silicon chips housing billions of fin field-effect transistors (FinFETs). These devices can also be used for quantum applications: At low temperatures, electrons or holes trapped under the gate can act as spin qubits. This approach could potentially enable quantum hardware and its classical control electronics to be integrated on the same chip. However, this requires qubit operation at temperatures around 1K (-272.15 degrees Celsius), where cooling overcomes heat dissipation.

Therefore, controlling each qubit requires additional measurement wires to connect the control electronics at room temperature to the qubits in the cryostat. The number of these measurement wires is limited because each wire generates heat. This inevitably creates a bottleneck in the routing, which in turn limits scaling.

To improve scalability, the University of Basel in Switzerland and the IBM Research Lab in Zurich demonstrated that FinFETs can carry spin qubits operating at around 4K. The researchers achieved fast electrical control of hole spins with drive frequencies up to 150 MHz, single-qubit gate fidelity at the fault-tolerant threshold, and Rabi oscillation quality factor greater than 87.

The researchers say that their device is characterized by industry compatibility and high quality, and is fabricated in a flexible manner, which will accelerate further development. The paper has been published in the journal Nature Electronics [1].

Quantum Coherence Laboratory, University of Basel

Building  on classic silicon technology

In classical computers, the solution to the scalability problem lies in silicon chips, which today include billions of "fin field-effect transistors" (FinFETs). These FinFETs are small enough for quantum applications; at extremely low temperatures near absolute zero (-273.15 degrees Celsius, or 0 K), negatively charged single electrons or positively charged "holes" can act as spin qubits.

Spin qubits store quantum information in two states, spin-up (intrinsic angular momentum up) and spin-down (intrinsic angular momentum down).

In this work, the qubit developed by Kuhlmann's group at the University of Basel is based on the FinFET architecture, using holes as spin qubits. In contrast to electron spins, hole spins in silicon nanostructures can be directly manipulated with fast electrical signals.

The newly developed qubits are based on so-called holes (red), whose spins (arrows) in one direction or the other store information. They are arranged in a silicon transistor-based architecture.

Possibility of higher operating temperatures

Another major hurdle to scalability is temperature; previous qubit systems typically had to operate in the extremely low range of 0.1K. Controlling each qubit requires additional measurement wires to connect the control electronics at room temperature to the qubits in the cryostat. The number of these measurement wires is limited because each wire generates heat. This inevitably creates a bottleneck in the routing, which in turn limits scaling.

Bypassing this "cabling bottleneck" was one of the main goals of Kuhlmann's research team and required the measurement and control electronics to be built directly into the cooling unit. "However, integrating these electronics requires qubits to operate at temperatures around 1K, and the cooling capacity of the cryostat increases dramatically to compensate for the heat dissipation of the control electronics," explains Dr. Leon Camenzind from the Department of Physics at the University of Basel. PhD student Simon Geyer, co-lead author of the study with Camenzind, added: "We have overcome the 4K temperature limit with our qubits, reaching the boiling point of liquid helium. Here, we can achieve much larger cooling capability, thereby integrating state-of-the-art cryogenic control technology.”

Close to industry standard

Building quantum computers using proven technologies such as the FinFET architecture has the potential to scale to very large numbers of qubits.

Kuhlmann deploys: "Our approach to existing silicon technology brings us closer to industry practice. The samples were created at the BiNIG and RoReR Nanotechnology Center at the IBM Research Laboratory in Zurich, headquartered at the University of Basel, the The research team is one of its members."

Link:

[1] https://www.nature.com/articles/s41928-022-00722-0

[2]https://www.unibas.ch/en/News-Events/News/Uni-Research/Hot-spin-quantum-bits-in-silicon-transistors.html