Fri. Jan 21st, 2022

Shrinking qubits for quantum computation with atom-thin materials

Optical photomicrograph of the team’s superconducting qubit chip that is 1000 times smaller than others made using conventional manufacturing techniques. Credit: Abhinandan Antony / Columbia Engineering

In order for quantum computers to surpass their classic counterparts in speed and capacity, their qubits – which are superconducting circuits that can exist in an infinite combination of binary states – must be on the same wavelength. Achieving this, however, has happened at the expense of size. While the transistors used in classical computers have shrunk to nanometer scales, superconducting qubits these days are still measured in millimeters – one millimeter is one million nanometers.

Combine qubits into larger and larger circuit chips, and you end up with a relatively large physical footprint, which means that quantum computers take up a lot of physical space. These are not yet devices we can carry in our backpacks or carry around our wrists.

In order to reduce qubits while maintaining their performance, the field needs a new way of building the capacitors that store the energy that “supplies” qubits. In collaboration with Raytheon BBN Technologies, Wang Fong-Jen Professor James Hone’s laboratory at Columbia Engineering recently demonstrated a superconducting qubit capacitor built with 2D materials that are a fraction of previous sizes.

To build qubit chips in the past, engineers have had to use planar capacitors, which put the necessary charged plates side by side. Stacking these plates would save space, but the metals used in conventional parallel capacitors interfere with qubit information storage. In the current work, published on 18 November in Nano letters, Hone’s Ph.D. students Abhinandan Antony and Anjaly Rajendra laid an insulating layer of boron nitride in between two charged plates of superconducting niobium dieselene. These layers are each only a single atom thick and held together by van der Waals forces, the weak interaction between electrons. The team then combined their capacitors with aluminum circuits to create a chip that contained two qubits with an area of ​​109 square micrometers and only 35 nanometers thick – that’s 1,000 times smaller than chips produced under conventional methods.

As they cooled their qubit chip down to just above absolute zero, qubits found the same wavelength. The team also observed key characteristics that showed that the two qubits were entangled and functioned as a single unit, a phenomenon known as quantum coherence; That would mean the quantum state of the qubit could be manipulated and read out via electrical impulses, Hone said. The coherence time was short – just over 1 microsecond compared to about 10 microseconds for a conventionally built coplanar capacitor, but this is only a first step in exploring the use of 2D materials in this area, he said.

Separate work published on arXiv in August by researchers at MIT also utilized niobium dieselide and boron nitride to build parallel plate capacitors for qubits. The devices examined by the MIT team showed even longer cohesion times – up to 25 microseconds – indicating that there is still room to further improve performance.
From here, Hone and his team will continue to refine their manufacturing techniques and test other types of 2D materials to increase coherence times, which reflect how long the qubit stores information. New device designs should be able to screw down even more, Hone said, by combining the elements into a single van der Waals stack or by implementing 2D materials for other parts of the circuit.

“We now know that 2D materials can be the key to making quantum computers possible,” Hone said. “It is still very early days, but results like these will encourage researchers around the world to consider new uses of 2D materials. We hope to see much more work in this direction in the future.”

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More information:
Abhinandan Antony et al., Miniaturizing Transmon Qubits Using van der Waals Materials, Nano letters (2021). DOI: 10.1021 / acs.nanolett.1c04160

Provided by Columbia University School of Engineering and Applied Science

Citation: Shrinking qubits for quantum computation with atom-thin materials (2021, November 30) retrieved November 30, 2021 from

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