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Physicists record the life of graphene qubitites



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This view shows the graphene layers used for the membranes. Credit: University of Manchester

Researchers from MIT and other sites have, for the first time, registered the "temporal coherence" of a qubit of graphene, which means how long it can maintain a special state that allows it to represent two logical states simultaneously. The demonstration, which used a new type of qubit based on graphene, represents a decisive step for practical quantum computing, researchers say.


The superconducting quantum bits (just qubits) are artificial atoms that use various methods to produce quantum information bits, the fundamental component of quantum computers. Similar to traditional binaries on computers, qubits can hold one of the two states corresponding to classic binary bits, 0 or 1. But these qubits can also be a superposition of both states simultaneously, which could allow computers Quantum solving complex problems that are virtually impossible for traditional computers.

The amount of time that these qubits remain in this superposition state is known as their "consistency time". The longer the time of coherence, the greater the capacity of qubit to calculate complex problems.

Recently, researchers have been incorporating graphene-based materials in superconducting quantum computing devices, which promise a faster and more efficient computing, among other advantages. So far, however, there has not been a consistent record for these advanced qubits, so we do not know if they are feasible for practical quantum computing.

In an article published today in Nanotechnology of nature, the researchers demonstrate, for the first time, a coherent qubit based on graphene and exotic materials. These materials allow qubit to change states through voltage, like transistors in traditional computer chips today and unlike most other qubits superconductors On the other hand, the researchers put a number on this consistency, setting it to 55 nanoseconds, before the qubit returns to its fundamental state.

The work combined the experience of co-authors William D. Oliver, a professor of practice physics and the Lincoln Laboratory Fellow, whose work focuses on quantum computing systems, and Pablo Jarillo-Herrero, Cecil i Ida Green Professor of Physics at MIT researching graphene innovations.

"Our motivation is to use the unique properties of graphene to improve the performance of superconducting qubits," says first author Joel I-Jan Wang, a postdoctor from the Oliver group at the Research Laboratory of # 39; Electronic (RLE) of MIT. "In this work, we show for the first time that a superconductor qubit from graphene is temporarily a coherent quantum, a key requirement for the construction of more sophisticated quantum circuits. Our is the first device that shows a measurable consistency time – a metric main quintile: this is long enough for humans to control. "

There are 14 other authors, including Daniel Rodan-Legrain, a graduate student from the Jarillo-Herrero group who has also contributed to work with Wang; The RIT MIT researchers, the Department of Physics, the Department of Electrical and Computer Engineering, and the Lincoln Laboratory; and researchers from the Irradiated Solids Laboratory at the École Polytechnique and the Advanced Materials Laboratory of the National Institute of Materials Science.

A pure graphene sandwich

The superconducting qubits are based on a structure called "Josephson union", where an insulator (usually an oxide) is interspersed between two superconducting materials (usually aluminum). In traditional qubit designs, a current loop creates a small magnetic field that causes electrons to go between superconducting materials, causing qubit to change states.

But this flow that flows consumes a lot of energy and causes other problems. Recently, some research groups have replaced the insulator with graphene, a layer of carbon atom that is economical to produce in mass and has unique properties that can allow faster and more efficient computing.

To produce their qubit, researchers went to a class of materials, called van der Waals materials, thin atomic materials that can be stacked like Legos, with little or no resistance or damage. These materials can be stacked specifically to create various electronic systems. Despite its almost impeccable surface quality, only some research groups have applied van der Waals materials to quantum circuits, and none of them have shown that it exhibits a temporary coherence.

For Josephson's union, the researchers wrapped a graphene sheet between the two layers of a van der Waals insulator called boron hexagonal nitride (hBN). It is important to note that the graphene assumes the superconductivity of the superconducting materials it touches. Selected van der Waals materials can be made to use electrons around the use of voltage, instead of the magnetic field based on traditional current. Therefore, it can also graphene, and it can also be the full qubit.

When the voltage is applied to the qubit, the electrons bounce sideways to another between two superconductive conductors connected by a graphene, changing the qubit from the ground (0) to the excited state or superimposed (1). The lower layer of hBN serves as a substrate to accommodate graphene. The top layer of hBN encapsulates graphene, protecting it from any contamination. Because the materials are so diverse, the electrons that travel never interact with defects. This represents the ideal "ballistic transport" for qubits, where most electrons move from one superconductor to another without dispersing with impurities, making a rapid and precise change of the states.

How the tension helps

Work can help face qubit's "climbing problem", says Wang. Currently, only about 1,000 qubits can fit in a single chip. Having qubits controlled by voltage will be especially important since millions of qubits begin to be piled up in a single chip. "Without voltage control, you will also need thousands or millions of current circuits, which occupy a lot of space and causes energy dissipation," he says.

In addition, voltage control means more efficiency and a more localized and precise orientation of the individual qubits on a chip, without "cross-talk". This happens when a bit of the current generated magnetic field interferes with a qubit that is not oriented, causing computing problems.

For now, the researchers' qubit has a short life. As a reference, conventional superconductive qubits that have a practical application promise have documented consistency times of a few tens of microseconds, a few hundred times larger than the qubit of the researchers.

But the researchers are already addressing several problems that cause this short life, most of which require structural modifications. They are also using their new coherence test method to investigate more about how the electrons move ballistically around the qubits, with the goal of increasing the consistency of qubits in general.


Explore more:
Ballistic Graphics Josephson's unions enter microwave circuits

More information:
Coherent control of a hybrid superconductor circuit made with van der Waals heterostructures based on graphene, Nanotechnology of nature (2018). DOI: 10.1038 / s41565-018-0329-2, https://www.nature.com/articles/s41565-018-0329-2

Journal Reference:
Nanotechnology of nature

Provided by:
Massachusetts Institute of Technology

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