An optical micrograph of the new superconducting qubit chip that’s 1000 times smaller than others made with conventional fabrication techniques. Image: Abhinandan Antony/Columbia Engineering.
An optical micrograph of the new superconducting qubit chip that’s 1000 times smaller than others made with conventional fabrication techniques. Image: Abhinandan Antony/Columbia Engineering.

For quantum computers to surpass their classical counterparts in speed and capacity, their qubits – superconducting circuits that can exist in an infinite combination of binary states – need to be on the same wavelength. But achieving this has come at the cost of size. Whereas the transistors in classical computers have been shrunk down to nanometer scales, today's superconducting qubits are measured in millimeters – one millimeter is one million nanometers.

Combining qubits together into larger and larger circuit chips produces, relatively speaking, a big physical footprint, which means quantum computers take up a lot of physical space. They are not yet devices we can carry around in our backpacks or wear on our wrists.

To shrink qubits down while maintaining their performance, the field needs a new way to build the capacitors that store the energy that 'powers' the qubits. In collaboration with Raytheon BBN Technologies, James Hone’s lab at Columbia Engineering recently demonstrated a superconducting qubit capacitor built with two-dimensional (2D) materials that’s a fraction of previous sizes.

To build qubit chips, engineers have up to now used planar capacitors, which arrange the necessary charged plates side-by-side. Stacking those plates would save space, but the metals used in conventional parallel capacitors interfere with qubit information storage.

In this study, reported in a paper in Nano Letters, Hone’s PhD students Abhinandan Antony and Anjaly Rajendra sandwiched an insulating layer of boron nitride between two charged plates of superconducting niobium diselenide. These layers are each just a single atom thick and held together by van der Waals forces, the weak interactions between electrons.

The team then combined their capacitors with aluminum circuits to create a chip containing two qubits. This chip has an area of 109µm2 and is just 35nm thick—that’s 1000 times smaller than chips produced with conventional approaches.

When the team cooled this qubit chip down to just above absolute zero, the qubits found the same wavelength. The team also observed key characteristics that showed that the two qubits were becoming entangled and acting as a single unit, a phenomenon known as quantum coherence. This means the qubit’s quantum state could be manipulated and read out via electrical pulses, said Hone. The coherence time was short – a little 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.

Separate work published on arXiv in August 2021 from researchers at MIT also took advantage of niobium diselenide and boron nitride to build parallel-plate capacitors for qubits. The devices studied by the MIT team showed even longer coherence times – up to 25 microseconds – indicating that there is still room to further improve performance.

From here, Hone and his team will continue refining their fabrication techniques and test other types of 2D materials to increase coherence times, which reflect how long a qubit can store information. According to Hone, new device designs should be able to shrink things down even further, by combining the elements into a single van der Waals stack or by deploying 2D materials for other parts of the circuit.

“We now know that 2D materials may hold the key to making quantum computers possible,” Hone said. “It is still very early days, but findings like these will spur researchers worldwide to consider novel applications of 2D materials. We hope to see a lot more work in this direction going forward.”

This story is adapted from material from Columbia Engineering, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.