Researchers at MIT and elsewhere have for the first time recorded the "temporal coherence" of a graphene qubit. This means how long a special state can be maintained, by which two logical states can be represented simultaneously. The demonstration, which used a new graphene-based qubitite, is a crucial step in practical quantum computing, the researchers say.
Superconducting quantum bits (simply qubits) are artificial atoms that use various methods to generate quantum information bits, the fundamental component of quantum computers. Similar to conventional binary circuits in computers, qubits can maintain one of two states that correspond to the classical binary bits, a 0 or 1
The length of time these qubits remain in this overlay state is called the "coherence time". The longer the coherence time, the greater the ability of the qubit to calculate complex problems.
Recently, researchers have incorporated graphene-based materials into superconducting quantum computing devices that promise, among other things, faster and more efficient computing. However, until now, no coherence has been recorded for these advanced qubits, so it is not known if they are feasible for practical quantum computing.
In an article published today in Nature Nanotechnology researchers are demonstrating a coherent qubit of graphene and exotic materials for the first time. These materials allow the qubit to change state by voltage, similar to today's computer chip transistors – unlike most other types of superconducting qubits. In addition, the researchers set a figure for this coherence and made it 55 nanoseconds before the qubit returned to ground state.
The work combined the expertise of co-authors William D. Oliver, physics professor of practice, and Lincoln Laboratory Fellow, whose work focuses on quantum computer systems, and Pablo Jarillo-Herrero, Cecil, and Ida Green, professor of physics at MIT, researching innovations in graphene.
"Our motivation is to use the unique properties of graphene to improve the performance of superconducting qubits," says lead author Joel I-Jan Wang, a postdoc in Oliver's group at the Research Laboratory of Electronics (RLE) at MIT , "In this work, we show for the first time that a superconducting qubit made of graphene is time-coherent, a key prerequisite for building more complex quantum circuits." Our first device shows a measurable coherence time – a primary metric of a Qbit – that is long enough controlling humans. "
There are 14 other co-authors, including Daniel Rodan-Legrain, a graduate student in the group of Jarillo-Herrero, who contributed equally to work with Wang. MIT researcher from RLE, the Department of Physics, the Department of Electrical and Computer Engineering, and the Lincoln Laboratory; and Researchers of the Irradiated Solids Laboratory at the École Polytechnique and the Advanced Materials Laboratory of the National Institute of Materials Science.
An original graphene sandwich
Superconducting qubits are based on a Josephson junction structure known as a, where an insulator (usually an oxide) is sandwiched between two superconducting materials (typically aluminum). In conventional tunable qubit designs, a current loop creates a small magnetic field through which electrons jump back and forth between the superconducting materials, causing the qubit to change states.
However, this flowing current consumes a lot of energy and causes other problems. Recently, some research groups have replaced the insulator with graphene, an atomic carbon layer that can be inexpensively mass-produced and that has unique properties that could enable faster and more efficient computation.
Researchers turned to producing their qubit A class of materials called Van der Waals materials – atomic thin materials that can be stacked like Legos, with little to no resistance or damage. These materials can be stacked in a specific way to create various electronic systems. Despite their almost flawless surface quality, only a few research groups have used Van der Waals materials for quantum switching and so far no temporal coherence has been demonstrated.
For their Josephson junction, the researchers placed a graphene plate in the interstices between the two layers of a van der Waals insulator called hexagonal boron nitride (hBN). It is important that graphene takes over the superconductivity of the superconducting materials it touches. The selected van der Waals materials may be made to traverse electrons using voltage instead of the conventional current-based magnetic field. Hence the graphene – and with it the entire qubit.
When voltage is applied to the qubit, electrons bounce back and forth between two superconducting lines connected by graphene, thereby changing the qubit from ground (0) to excited or superimposed state (1). The lower hBN layer serves as a substrate to accommodate the graphene. The upper hBN layer encapsulates the graphene and protects it from any contamination. Because the materials are so flawless, the moving electrons never interact with defects. This is the ideal "ballistic transport" for qubits, where most of the electrons move from one superconductor to another without scattering with impurities, resulting in a fast and precise state change.
How Tension Helps 19659005] The work can help solve the "scaling problem" of qubits, says Wang. Currently, only about 1,000 qubits fit on a single chip. It is especially important that the qubits are controlled by the voltage as millions of qubits are jammed on a single chip. "Without voltage regulation, you also need thousands or millions of current loops, and that takes up a lot of space and leads to energy dissipation," he says.
In addition, voltage control means higher efficiency and a localized effect, precise targeting of individual qubits on a chip without "crosstalk". This happens when part of the magnetic field generated by the current disturbs a qubit that it is not aiming at, which leads to computational problems.
The researchers' qubit currently has a short lifespan. For reference, conventional superconducting qubits, which are promising for practical application, have documented coherence times of several tens of microseconds, several hundred times more than the researchers' qubit.
However, the researchers already address some issues that cause this short lifespan most of which require structural changes. With their new method of coherence testing, they also study how electrons move ballistically around the qubits, with the goal of extending the coherence of qubits in general.
Graphene ballistic Josephson contacts enter into microwave circuits
Coherent Control of a Graphene-based Hybrid Superconducting Circuit with Van der Waals Heterostructures, Nature Nanotechnology (2018). DOI: 10.1038 / s41565-018-0329-2, https://www.nature.com/articles/s41565-018-0329-2