Researchers from the Massachusetts Institute of Technology and other countries for the first time recorded the "temporal coherence" of graphene qubit – that is, how long it can maintain a particular state, allowing it to simultaneously represent two logical states. The researchers say that a demonstration in which a new kind of graphene-based qubit was used represents an important step forward for practical quantum computing.
Superconducting quantum bits (just qubits) are artificial atoms that use various methods to get bits of quantum information, a fundamental component of quantum computers. Like traditional binary circuits in computers, qubits can support one of two states corresponding to classical binary bits, 0 or 1. But these qubits can also be a superposition of both states simultaneously, which can allow quantum computers to solve complex problems that are almost impossible for traditional computers. .
The time during which these qubits remain in this state of superposition is called their “coherence time”. The longer the coherence time, the greater the qubit's ability to calculate complex tasks.
Recently, researchers have introduced graphene-based materials into superconducting quantum computing devices that promise faster and more efficient calculations, among other benefits. Until now, however, there was no registered consistency for these advanced qubits, so it is not known whether they are feasible for practical quantum computing.
An article published today in The nature of nanotechnologyResearchers demonstrate for the first time a coherent qubit of graphene and exotic materials. These materials allow a qubit to change states through voltage, like transistors in modern traditional computer chips, and unlike most other types of superconducting qubits. Moreover, the researchers determined the number for this coherence, increasing it to 55 nanoseconds before the qubit returns to its ground state.
The work brought together the experience of co-authors William D. Oliver, a professor of physics and a laboratory at Lincoln, whose work focuses on quantum computing systems, and Pablo Harillo-Herrero, a professor of physics, Cecil and Ida Green from the Massachusetts Institute of Technology, who is engaged in innovation research. in graphene.
“Our motivation is to use the unique properties of graphene to improve the characteristics of superconducting qubits,” says first author Joel I-Yang Wang, a postdoc from the Oliver group at the Electronics Research Laboratory (RLE) at the Massachusetts Institute of Technology. “In this paper, for the first time, we show that a superconducting qubit made of graphene is quantum-coherent in time, which is a key condition for building more complex quantum chains. We are the first device to show a measurable coherence time — the primary metric of a qubit — enough for people to control. "
There are 14 other co-authors, including Daniel Rodan-Legrain, a graduate student in the group of Jarillo-Herrero, who made the same contribution to working with Van; Researchers at the Massachusetts Institute of Technology from RLE, the Faculty of Physics, the Faculty of Electrical Engineering and Computer Science, and the Lincoln Laboratory; and researchers from the laboratory of irradiated solids at the Polytechnic School and the Laboratory of Advanced Materials of the National Institute of Materials Science.
Untouched graphene sandwich
Superconducting qubits are based on a structure known as the “Josephson junction”, where an insulator (usually an oxide) is located between two superconducting materials (usually aluminum). In traditional tuned qubit designs, the current loop creates a small magnetic field that causes electrons to jump back and forth between superconducting materials, causing the qubit to switch states.
But this current draws a lot of energy and causes other problems. Recently, several research groups have replaced the insulator with graphene, a carbon layer atomic thickness that is inexpensive in mass production and has unique properties that can provide faster and more efficient calculations.
To fabricate their qubit, the researchers turned to a class of materials called van der Waals materials – atomic-thin materials that can be stacked on top of each other, like Legos, on each other, with little or no resistance or damage. These materials can be stacked in a certain way to create various electronic systems. Despite their almost flawless surface quality, only a few research teams have ever used van der Waals materials for quantum chains, and none of them have previously demonstrated transient coherence.
For their Josephson junction, researchers placed a layer of graphene between two layers of a van der Waals insulator called hexagonal boron nitride (hBN). It is important to note that graphene acquires superconductivity of superconducting materials to which it relates. Selected van der Waals materials can be used to excite electrons using voltage instead of traditional current-based magnetic field. Thus, the same can be said about graphene, like the whole qubit.
When voltage is applied to the qubit, the electrons bounce back and forth between two superconducting leads connected by graphene, changing the qubit from the ground (0) to an excited state or a superposition state (1). The bottom layer hBN serves as a substrate for graphene. The top layer hBN seals graphene, protecting it from any contamination. Because the materials are very clean, traveling electrons never interact with defects. This represents an ideal “ballistic transport” for qubits, where most electrons move from one superconducting lead to another without scattering on impurities, which leads to a fast and accurate change of states.
How stress helps
The work can help solve the problem of qubit scaling, says Wang. Currently, only about 1000 qubits can fit on a single chip. Control of voltage controlled qubits will be especially important as millions of qubits begin to fit on a single chip. “Without voltage regulation, you will also need thousands or millions of current loops, and this will take up a lot of space and lead to energy dissipation,” he says.
In addition, voltage management means greater efficiency and more localized, accurate targeting to individual qubits on a chip without crosstalk. This happens when a small part of the magnetic field created by the current collides with a qubit, which it does not aim at, causing problems with calculations.
At the moment, qubit researchers have a short lifetime. For reference, the traditional superconducting qubits, which promise practical application, have documented coherence times of several tens of microseconds, several hundred times larger than the qubit of researchers.
But researchers are already addressing several problems that cause this short lifespan, most of which require structural changes. They also use their new coherent sounding method to further explore how electrons move ballistically around qubits in order to expand the coherence of qubits as a whole.
Graphene's ballistic Josephson junctions enter microwave circuits.
Coherent control of a hybrid superconducting chain based on graphene-based van der Waals heterostructures, The nature of nanotechnology (2018). DOI: 10.1038 / s41565-018-0329-2, https://www.nature.com/articles/s41565-018-0329-2