Researchers from MIT (19459007) and elsewhere have for the first time recorded the "temporal coherence" of a graphene qubit – meaning how long a special state can be maintained that allows it to simultaneously represent two logical states , The demonstration, which used a new graphene-based qubit, represents a significant advance in practical quantum computation.
Superconducting quantum bits (simply qubits) are artificial atoms that use different methods of producing bits. Quantum information, 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. But these qubits can also be a superposition of both states simultaneously, allowing quantum computers to solve complex problems for conventional computers practically impossible.
The time these qubits remain in this superposition 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, which, among other things, enable faster and more efficient computation. However, until now, no coherence has been recorded for these advanced qubits, so it is not known if they are practical for practical quantum computing.
In a paper published today in Nature Nanotechnology, researchers are showing for the first time: a coherent qubit of graphene and exotic materials. These materials allow the qubit to change state by voltage, much like transistors in today's conventional computer chips – and 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 postdoctoral fellow 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 time-coherent. This is an important prerequisite for the construction of more complex quantum circuits. Our device is the first device to have a measurable coherence time – a primary metric for a qubit – long enough for human control.
There are 14 other co-authors, including Daniel Rodan-Legrain, a graduate student in Jarillo-Herero's group, which has 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, in which an insulator (usually an oxide) is sandwiched between two superconducting materials (typically aluminum). In traditional, tunable qubit designs, a current loop creates a small magnetic field through which electrons jump back and forth between the superconducting materials, switching the qubit between 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 inserted a graphene sheet 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 excitation or superposition 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 represents the ideal "ballistic transport" for qubits, where most of the electrons move from one superconducting line to another without scattering with impurities, resulting in a fast and precise state change.
How Tension Helps 19659004] 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 requires a lot of space and energy loss," 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 a little of the magnetic field generated by the current disturbs a qubit it is not aiming at, which leads to computational problems.
For now the researchers – Qubit 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.
The researchers, however, already address some issues that cause this short lifespan most of which require structural changes. They use their new method of coherence testing to further investigate how electrons move ballistically around the qubits, with the goal of extending the coherence of qubits in general.
Publication: Joel I-Jan Wang et al., "Coherent control of a graphene-based hybrid superconducting circuit with van der Waals heterostructures", Nature Nanotechnology (2018)