Like the transistors in a classical computer, superconducting qubits are the
building blocks of a quantum computer. While engineers have been able to
shrink transistors to nanometer scales, however, superconducting qubits are
still measured in millimeters. This is one reason a practical quantum
computing device couldn't be miniaturized to the size of a smartphone, for
instance.
MIT researchers have now used ultrathin materials to build superconducting
qubits that are at least one-hundredth the size of conventional designs and
suffer from less interference between neighboring qubits. This advance could
improve the performance of quantum computers and enable the development of
smaller quantum devices.
The researchers have demonstrated that hexagonal boron nitride, a material
consisting of only a few monolayers of atoms, can be stacked to form the
insulator in the capacitors on a superconducting qubit. This defect-free
material enables capacitors that are much smaller than those typically used
in a qubit, which shrinks its footprint without significantly sacrificing
performance.
In addition, the researchers show that the structure of these smaller
capacitors should greatly reduce cross-talk, which occurs when one qubit
unintentionally affects surrounding qubits.
"Right now, we can have maybe 50 or 100 qubits in a device, but for
practical use in the future, we will need thousands or millions of qubits in
a device. So, it will be very important to miniaturize the size of each
individual qubit and at the same time avoid the unwanted cross-talk between
these hundreds of thousands of qubits. This is one of the very few materials
we found that can be used in this kind of construction," says co-lead author
Joel Wang, a research scientist in the Engineering Quantum Systems group of
the MIT Research Laboratory for Electronics.
Wang's co-lead author is Megan Yamoah '20, a former student in the
Engineering Quantum Systems group who is currently studying at Oxford
University on a Rhodes Scholarship. Pablo Jarillo-Herrero, the Cecil and Ida
Green Professor of Physics, is a corresponding author, and the senior author
is William D. Oliver, a professor of electrical engineering and computer
science and of physics, an MIT Lincoln Laboratory Fellow, director of the
Center for Quantum Engineering, and associate director of the Research
Laboratory of Electronics. The research is published today in Nature
Materials.
Qubit quandaries
Superconducting qubits, a particular kind of quantum computing platform that
uses superconducting circuits, contain inductors and capacitors. Just like
in a radio or other electronic device, these capacitors store the electric
field energy. A capacitor is often built like a sandwich, with metal plates
on either side of an insulating, or dielectric, material.
But unlike a radio, superconducting quantum computers operate at super-cold
temperatures—less than 0.02 degrees above absolute zero (-273.15 degrees
Celsius)—and have very high-frequency electric fields, similar to today's
cellphones. Most insulating materials that work in this regime have defects.
While not detrimental to most classical applications, when quantum-coherent
information passes through the dielectric layer, it may get lost or absorbed
in some random way.
"Most common dielectrics used for integrated circuits, such as silicon
oxides or silicon nitrides, have many defects, resulting in quality factors
around 500 to 1,000. This is simply too lossy for quantum computing
applications," Oliver says.
To get around this, conventional qubit capacitors are more like open-faced
sandwiches, with no top plate and a vacuum sitting above the bottom plate to
act as the insulating layer.
"The price one pays is that the plates are much bigger because you dilute
the electric field and use a much larger layer for the vacuum," Wang says.
"The size of each individual qubit will be much larger than if you can
contain everything in a small device. And the other problem is, when you
have two qubits next to each other, and each qubit has its own electric
field open to the free space, there might be some unwanted talk between
them, which can make it difficult to control just one qubit. One would love
to go back to the very original idea of a capacitor, which is just two
electric plates with a very clean insulator sandwiched in between."
So, that's what these researchers did.
They thought hexagonal boron nitride, which is from a family known as van
der Waals materials (also called 2D materials), would be a good candidate to
build a capacitor. This unique material can be thinned down to one layer of
atoms that is crystalline in structure and does not contain defects.
Researchers can then stack those thin layers in desired configurations.
To test hexagonal boron nitride, they ran experiments to characterize how
clean the material is when interacting with a high-frequency electric field
at ultracold temperatures, and found that very little energy is lost when it
passes through the material.
"Much of the previous work characterizing hBN (hexagonal boron nitride) was
performed at or near zero frequency using DC transport measurements.
However, qubits operate in the gigahertz regime. It's great to see that hBN
capacitors have quality factors exceeding 100,000 at these frequencies,
amongst the highest Qs I have seen for lithographically defined, integrated
parallel-plate capacitors," Oliver says.
Capacitor construction
They used hexagonal boron nitride to build a parallel-plate capacitor for a
qubit. To fabricate the capacitor, they sandwiched hexagonal boron nitride
between very thin layers of another van der Waals material, niobium
diselenide.
The intricate fabrication process involved preparing one-atom-thick layers
of the materials under a microscope and then using a sticky polymer to grab
each layer and stack it on top of the other. They placed the sticky polymer,
with the stack of 2D materials, onto the qubit circuit, then melted the
polymer and washed it away.
Then they connected the capacitor to the existing structure and cooled the
qubit to 20 millikelvins (-273.13 C).
"One of the biggest challenges of the fabrication process is working with
niobium diselenide, which will oxidize in seconds if it is exposed to the
air. To avoid that, the whole assembly of this structure has to be done in
what we call the glove box, which is a big box filled with argon, which is
an inert gas that contains a very low level of oxygen. We have to do
everything inside this box," Wang says.
The resulting qubit is about 100 times smaller than what they made with
traditional techniques on the same chip. The coherence time, or lifetime, of
the qubit is only a few microseconds shorter with their new design. And
capacitors built with hexagonal boron nitride contain more than 90 percent
of the electric field between the upper and lower plates, which suggests
they will significantly suppress cross-talk among neighboring qubits, Wang
says. This work is complementary to recent research by a team at Columbia
University and Raytheon.
In the future, the researchers want to use this method to build many qubits
on a chip to verify that their technique reduces cross-talk. They also want
to improve the performance of the qubit by finetuning the fabrication
process, or even building the entire qubit out of 2D materials.
"Now we have cleared a path to show that you can safely use as much
hexagonal boron nitride as you want without worrying too much about defects.
This opens up a lot of opportunity where you can make all kinds of different
heterostructures and combine it with a microwave circuit, and there is a lot
more room that you can explore. In a way, we are giving people the green
light—you can use this material in any way you want without worrying too
much about the loss that is associated with the dielectric," Wang says.
Reference:
Joel Wang, Hexagonal boron nitride as a low-loss dielectric for
superconducting quantum circuits and qubits, Nature Materials (2022).
DOI: 10.1038/s41563-021-01187-w.
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Physics