Atomically Thin Materials Significantly Shrink Qubits

Quantum computing – This is a damn complex technology, the development of which faces many technical obstacles. Among these problems, two critical problems stand out: miniaturization and quality of the qubit.

IBM accepted superconducting qubit roadmap achieving a 1121 qubit processor by 2023which leads to the expectation that 1000 qubits with today's qubit form factor it is quite feasible. However, current approaches would require very large chips (50 millimeters per side or larger) at the scale of small wafers or use chiplets on multi-chip modules. While this approach will work, the goal is to find a better path to scalability.

Now researchers from MIT managed to reduce the size of qubits and this is done in such a way as to reduce the interference that occurs between neighboring qubits. WITH researchers have increased the number superconducting qubits which can be added to the device 100 times.

“We are engaged in both miniaturization and quality of qubits,” said William Oliverdirector of Center for Quantum Engineering at the Massachusetts Institute of Technology. “Unlike traditional transistor scaling, where only the quantity really matters, for qubits large numbers are not enough; they also need to be high-performance. Sacrificing performance for the number of qubits is a dead giveaway in quantum computing. They have to go hand in hand.”

The key to this significant increase in qubit density and reduction in interference is the use of two-dimensional materials, in particular the 2D insulator hexagonal boron nitride (hBN). MIT researchers have demonstrated that multiple atomic monolayers of hBN can be stacked on top of each other to form an insulator in capacitors superconducting qubit.

Like other capacitors, the capacitors in these superconducting circuits are shaped like a sandwich, with insulating material sandwiched between two metal plates. The big difference between these capacitors is that the superconducting circuits can only operate at extremely low temperatures—less than 0.02 degrees above absolute zero (-273.15°C).

Superconducting qubits are measured at temperatures as low as 20 millikelvins in a dilution refrigerator.Nathan Fiske/MIT

In this environment, insulating materials such as PE-CVD, silicon oxide or silicon nitride, have quite a few defects that are too large for quantum computing applications. To get around these material disadvantages, most superconducting circuits use what are called coplanar capacitors. In these capacitors, the plates are located sideways to each other rather than on top of each other.

As a result, the dielectric of the capacitor is its own silicon substrate under the plates and, to a lesser extent, the vacuum above the plates. Proprietary silicon is chemically pure and therefore has few defects, and the large size dilutes electric field at the interfaces of the plates, all this leads to the creation of a low-loss capacitor. The lateral dimension of each wafer in this open design ends up being quite large (typically 100 by 100 micrometers) to achieve the required capacitance.

In an effort to move away from the large side-by-side configuration, MIT researchers began searching for an insulator that has very few defects and is compatible with the plates of superconducting capacitors.

“We decided to study hBN because it is the most widely used insulator in 2D materials research due to its purity and chemical inertness,” said one of the lead authors. Joel Wangis a research scientist in the Quantum Systems Engineering Group at the Electronics Research Laboratory at the Massachusetts Institute of Technology.

On either side of the hBN, the MIT researchers used a two-dimensional superconducting material. niobium diselenide. According to Wang, one of the most challenging aspects of making the capacitors was working with niobium diselenide, which oxidizes in air in a matter of seconds. This requires capacitor assembly to occur in a glove box filled with argon.

While this would seem to make it difficult to expand production of these capacitors, Wang does not see this as a limiting factor.

“What determines the quality factor of a capacitor is whether there are two interfaces between the two materials,” Wang said. “Once the sandwich is made, the two interfaces are ‘sealed’ and we do not see any noticeable degradation over time when exposed to the atmosphere.”

The lack of degradation is due to the fact that about 90 percent of the electric field is contained within the sandwich structure, so oxidation the outer surface of niobium diselenide no longer plays a significant role. Ultimately, this significantly reduces the capacitor footprint and reduces crosstalk between adjacent qubits.

“The main challenge to scaling up production will be the growth of HBN and 2D at wafer scale. superconductors like [niobium diselenide]and how these films can be stacked at wafer scale,” Wang added.

Wang believes this study shows that 2D hBN is a good candidate insulator for superconducting qubits. He says the work done by the MIT team will serve as a roadmap for other hybrid technologies. 2D materials to create superconducting circuits.