Researchers from MIT and elsewhere they have for the first time recorded the "temporal coherence" of a graphene qubit – this means how long it can maintain a special state that allows it to represent two logical states at the same time. The demonstration, which used a new type of graphene-based qubit, represents a crucial step forward for practical quantum computing, say the researchers.
Superconducting quantum bits (simple, qubits) are artificial atoms that use different methods to produce pieces of quantum information, the fundamental component of quantum computers. As with traditional binary circuits in computers, qubits can maintain one of the two states corresponding to the classic binary bits, a 0 or 1. But these qubits can also be a superposition of both states simultaneously, allowing quantum computers to solve complex problems. practically impossible for traditional computers.
The amount of time these qubits remain in this superposition state will be their & # 39; coherence time & # 39; called. The longer the coherence time, the greater the possibility for the qubit to calculate complex problems.
Recently, researchers have incorporated graphene-based materials into superconducting quantum computing devices that promise faster, more efficient computing alongside other benefits. So far, however, there has been no consistency for these advanced qubits, so there is no idea if they are feasible for practical quantum computing.
In a paper published today in Nature Nanotechnology, the researchers show for the first time a coherent qubit made from graphene and exotic materials. With these materials, the qubit can change by state of tension, just like transistors in traditional computer chips today – and unlike most other types of superconducting qubits. In addition, the researchers put a number on that coherence, by clocking it at 55 nanoseconds, before the qubit returns to its ground state.
The work combines expertise from co-authors William D. Oliver, a professor of physics of practice and Lincoln Laboratory Fellow whose work focuses on quantum computing systems, and Pablo Jarillo-Herrero, the Cecil and Ida Green professor of physics at the MIT that investigates innovations in graphene.
"Our motivation is to use the unique properties of graphene to improve the performance of superconducting qubits," said 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 from graphene is temporarily quantum coherent, an important requirement for building more advanced quantum circuits, ours is the first device to show a measurable coherence time – a primary metric of a qubit – which is long enough to be controlled by humans. "
There are 14 other co-authors, including Daniel Rodan-Legrain, a graduate student in the Jarillo-Herrero group who also contributed to the work with Wang; MIT researchers from RLE, the Department of Physics, the Department of Electrical Engineering and Computer Science, and the Lincoln Laboratory; and researchers from the Laboratory for Irradiated Solids at the École Polytechnique and the Advanced Materials Laboratory of the National Institute for Materials Science.
An immaculate sandwich of graphene
Superconducting qubits rely on a structure known as a "Josephson transition" where an insulator (usually an oxide) is sandwiched between two superconducting materials (usually aluminum). In traditional tunable qubit designs, a current loop creates a small magnetic field that causes electrons to jump back and forth between superconducting materials, changing the qubit state.
But this flowing stream consumes a lot of energy and causes other problems. Recently, some research groups have replaced the insulator with graphene, an atomic layer of carbon that is not expensive to produce and has unique properties that allow faster and more efficient calculation.
To fabricate their qubit, the researchers turned to a class of materials called van der Waals materials: atom-thin materials that can be stacked as Legos, with little to no resistance or damage. These materials can be stacked in specific ways to create different electronic systems. Despite their almost impeccable surface quality, only a few research groups have ever applied van der Waals materials to quantum circuits and none of them have previously shown that they have temporary cohesion.
For their Josephson junction, the researchers placed a layer of graphene between the two layers of a van der Waals insulator called hexagonal boron nitride (hBN). It is important that graphene assumes the superconductivity of the superconducting materials it touches. The selected van der Waals materials can be made to introduce electrons around the use of voltage, instead of the traditional current-based magnetic field. That is why the graphene – and therefore the whole qubit.
When voltage is applied to the qubit, electrons bounce back and forth between two superconducting leads connected by graphene, changing the qubit from ground (0) to excited or superposition (1). The lower hBN layer serves as a substrate for hosting the graphene. The upper hBN layer envelopes the graphene and protects it from any contamination. Because the materials are so pristine, the moving electrons never interact with defects. This represents the ideal "ballistic transport" for qubits, where a majority of electrons pass from one superconducting conduit to the other without scattering with impurities, thus providing a rapid, precise change of states.
How tension helps
The work can help to reduce the & # 39; scale problem & # 39; of qubit, Wang says. At the moment only about 1,000 qubits fit on a single chip. Having qubits that are controlled by voltage will be especially important, because millions of qubits are crammed on one chip. "Without voltage regulation you also need thousands of millions of power loops, and that takes a lot of space and leads to energy dissipation," he says.
In addition, voltage regulation means greater efficiency and a more localized, accurate targeting of individual qubits on a chip, without cross talk & # 39; This happens when a small magnetic field created by the current interferes with a qubit that is not targeted, causing calculation problems.
For now, the qubit of the researchers has a short life span. By comparison, conventional superconducting qubits that are promising for practical application have documented coherence times of several tens of microseconds, a few hundred times greater than the qubit of the researchers.
But the researchers are already tackling various problems that cause this short life span, most of which require structural adjustments. They also use their new method of coherence determination to further investigate how electrons move ballistically around the qubits, with the aim of increasing the coherence of qubits in general.
Publication: Joel I-Jan Wang, et al., "Coherent control of a hybrid superconducting circuit made with graphene-based van der Waals heterostructures," Nature Nanotechnology (2018)