” Right now, we can have perhaps 50 or 100 qubits in a device, however for practical use in the future, we will require thousands or millions of qubits in a device. It will be really crucial to miniaturize the size of each private qubit and at the very same time prevent the undesirable cross-talk in between these hundreds of thousands of qubits. Superconducting qubits, a specific kind of quantum computing platform that uses superconducting circuits, contain inductors and capacitors. And the other problem is, when you have 2 qubits next to each other, and each qubit has its own electric field open to the complimentary area, there may be some unwanted talk between them, which can make it challenging to control simply one qubit. And capacitors built with hexagonal boron nitride consist of more than 90 percent of the electric field between the upper and lower plates, which recommends they will considerably suppress cross-talk amongst surrounding qubits, Wang states.
Using ultrathin materials to lower the size of superconducting qubits may pave the way for personal-sized quantum devices.
Like the transistors in a classical computer system, superconducting qubits are the building blocks of a quantum computer system. While engineers have been able to diminish transistors to nanometer scales, nevertheless, superconducting qubits are still determined in millimeters. This is one factor a practical quantum computing gadget couldnt be miniaturized to the size of a smartphone, for example.
MIT scientists have actually now used ultrathin products to construct superconducting qubits that are at least one-hundredth the size of standard designs and experience less interference in between neighboring qubits. This advance might improve the performance of quantum computer systems and make it possible for the development of smaller sized quantum devices.
The scientists have shown that hexagonal boron nitride, a material consisting of just a few monolayers of atoms, can be stacked to form the insulator in the capacitors on a superconducting qubit. This defect-free material makes it possible for capacitors that are much smaller sized than those normally used in a qubit, which shrinks its footprint without substantially compromising efficiency.
In addition, the researchers reveal that the structure of these smaller capacitors need to considerably decrease cross-talk, which takes place when one qubit unintentionally impacts surrounding qubits.
MIT researchers used the 2D product hexagonal boron nitride to develop much smaller capacitors for superconducting qubits, allowing them to diminish the footprint of a qubit by two orders of magnitude without compromising efficiency. Credit: Figure thanks to the researcher; edited by Christine Daniloff and Jose-Luis Olivares, MIT
” Right now, we can have possibly 50 or 100 qubits in a device, but for practical use in the future, we will need thousands or millions of qubits in a gadget. It will be really important to miniaturize the size of each individual qubit and at the same time avoid the undesirable cross-talk between these hundreds of thousands of qubits. This is among the very few products we found that can be used in this sort of building and construction,” says co-lead author Joel Wang, a research scientist in the Engineering Quantum Systems group of the MIT Research Laboratory for Electronics.
Wangs co-lead author is Megan Yamoah 20, a previous trainee in the Engineering Quantum Systems group who is presently 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 teacher of electrical engineering and computer technology 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 was released on January 27, 2022, in Nature Materials.
Qubit predicaments
Superconducting qubits, a specific type of quantum computing platform that utilizes superconducting circuits, include inductors and capacitors. Simply like in a radio or other electronic device, these capacitors keep the electric field energy. A capacitor is frequently constructed like a sandwich, with metal plates on either side of an insulating, or dielectric, product.
Unlike a radio, superconducting quantum computers run at super-cold temperature levels– less than 0.02 degrees above outright zero (-273.15 degrees Celsius)– and have really high-frequency electric fields, similar to todays cellular phones. The majority of insulating materials that operate in this program have flaws. While not detrimental to most classical applications, when quantum-coherent details passes through the dielectric layer, it may get lost or soaked up in some random method.
” Most typical dielectrics utilized for integrated circuits, such as silicon oxides or silicon nitrides, have many flaws, leading to quality aspects around 500 to 1,000. This is merely too lossy for quantum computing applications,” Oliver states.
To navigate this, traditional 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 size of each private qubit will be much larger than if you can include whatever in a little gadget. And the other issue is, when you have two qubits next to each other, and each qubit has its own electric field open to the totally free space, there may be some unwanted talk between them, which can make it challenging to control just one qubit.
So, thats what these scientists did.
They thought hexagonal boron nitride, which is from a household called van der Waals products (also called 2D materials), would be an excellent candidate to build a capacitor. This special material can be thinned down to one layer of atoms that is crystalline in structure and does not contain flaws. Scientists can then stack those thin layers in wanted setups.
To evaluate hexagonal boron nitride, they ran experiments to define how clean the product is when connecting with a high-frequency electric field at ultracold temperature levels, and discovered that extremely little energy is lost when it travels through the material.
” Much of the previous work identifying hBN (hexagonal boron nitride) was performed at or near zero frequency utilizing DC transportation measurements. Nevertheless, qubits operate in the gigahertz regime. Its great to see that hBN capacitors have quality factors surpassing 100,000 at these frequencies, among the greatest Qs I have actually seen for lithographically defined, integrated parallel-plate capacitors,” Oliver says.
Capacitor construction
They used hexagonal boron nitride to construct a parallel-plate capacitor for a qubit. To produce the capacitor, they sandwiched hexagonal boron nitride in between really thin layers of another van der Waals product, niobium diselenide.
The detailed fabrication procedure included preparing one-atom-thick layers of the materials under a microscope and after that using a sticky polymer to get each layer and stack it on top of the other. They put the sticky polymer, with the stack of 2D products, 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 most significant challenges of the fabrication procedure is working with niobium diselenide, which will oxidize in seconds if it is exposed to the air. To prevent that, the entire assembly of this structure has to be performed in what we call the glove box, which is a huge box filled with argon, which is an inert gas which contains an extremely low level of oxygen. We need to do everything inside this box,” Wang states.
The resulting qubit is about 100 times smaller sized than what they made with traditional strategies on the exact same chip. The coherence time, or lifetime, of the qubit is just a couple of microseconds much shorter with their new design. And capacitors constructed with hexagonal boron nitride consist of more than 90 percent of the electric field between the upper and lower plates, which suggests they will considerably reduce cross-talk amongst neighboring qubits, Wang states. This work is complementary to current research by a team at Columbia University and Raytheon.
In the future, the scientists wish to use this method to develop many qubits on a chip to validate that their technique minimizes cross-talk. They likewise want to enhance the performance of the qubit by finetuning the fabrication process, and even building the whole qubit out of 2D materials.
” Now we have actually cleared a path to show that you can securely use as much hexagonal boron nitride as you want without stressing too much about defects. This opens up a great deal of opportunity where you can make all sort 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 providing people the green light– you can utilize this material in any method you want without worrying too much about the loss that is related to the dielectric,” Wang says.
Reference: “Hexagonal boron nitride as a low-loss dielectric for superconducting quantum circuits and qubits” by Joel I-J. Wang, Megan A. Yamoah, Qing Li, Amir H. Karamlou, Thao Dinh, Bharath Kannan, Jochen Braumüller, David Kim, Alexander J. Melville, Sarah E. Muschinske, Bethany M. Niedzielski, Kyle Serniak, Youngkyu Sung, Roni Winik, Jonilyn L. Yoder, Mollie E. Schwartz, Kenji Watanabe, Takashi Taniguchi, Terry P. Orlando, Simon Gustavsson, Pablo Jarillo-Herrero and William D. Oliver, 27 January 2022, Nature Materials.DOI: 10.1038/ s41563-021-01187-w.
This research study was funded, in part, by the U.S. Army Research Office, the National Science Foundation, and the Assistant Secretary of Defense for Research and Engineering by means of MIT Lincoln Laboratory.