For the first time, scientists at MIT and from other institutions have been able to record the “temporal coherence” of a graphene qubit—that is, the time frame for which it can maintain a special state that enables it to represent two logical states at the same time.
According to the researchers, a new type of graphene-based qubit was used for the demonstration, which represents a crucial step ahead for practical quantum computing.
Superconducting quantum bits, or qubits, are artificial atoms that synthesize bits of quantum information—the basic component of quantum computers—by making use of different methods. Analogous to conventional binary circuits in computers, qubits have the ability to maintain one of two states in accordance with the classic binary bits, a 0 or 1. However, these qubits can also turn out to be a superposition of both states at the same time, which could enable quantum computers to solve complex problems that cannot be practically solved by conventional computers.
The time period for which the qubits remain in this superposition state is called as their “coherence time.” The length of the coherence time is directly proportional to the ability of the qubit to compute complex problems.
In the recent past, graphene-based materials have been incorporated into superconducting quantum computing devices by researchers. Such devices promise more efficient and faster computing, apart from other advantages. However, to date, there has been no recorded coherence for these advanced qubits; hence, it is not known whether they can be used for practical quantum computing.
In a paper published in Nature Nanotechnology on December 31st, 2018, for the first time, the researchers have exhibited a coherent qubit developed from graphene and exotic materials. These materials enable the states of the qubit to be changed through voltage, quite similar to transistors in the present-day conventional computer chips—and in contrast to a majority of the other types of superconducting qubits. Furthermore, the scientists clocked the coherence at 55 ns, before the qubit returned to its ground state.
The study combined expertise from co-authors William D. Oliver, a physics professor of the 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 MIT who researches innovations in graphene.
“Our motivation is to use the unique properties of graphene to improve the performance of superconducting qubits,” stated Joel I-Jan Wang, first author of the study, who is a postdoc in Oliver’s group in 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 temporally quantum coherent, a key requisite for building more sophisticated quantum circuits. Ours is the first device to show a measurable coherence time—a primary metric of a qubit—that’s long enough for humans to control.”
The study involves 14 other co-authors, including Daniel Rodan-Legrain, a graduate student in Jarillo-Herrero’s group who contributed equally to the study with Wang; MIT researchers from RLE, the Department of Physics, the Department of Electrical Engineering and Computer Science, and Lincoln Laboratory; and scientists from the Laboratory of Irradiated Solids at the École Polytechnique and the Advanced Materials Laboratory of the National Institute for Materials Science.
A pristine graphene sandwich
Superconducting qubits are dependent on a structure called a “Josephson junction,” in which an insulator (normally an oxide) is sandwiched between two superconducting materials (normally aluminum). In conventional tunable qubit designs, a small magnetic field created by a current loop causes electrons to hop back and forth between the superconducting materials, making the qubit to switch states.
However, a lot of energy is consumed by this flowing current, thereby causing other issues. In the recent past, some research teams have used graphene as an alternative to the insulator. Graphene is an atom-thick layer of carbon that can be mass-produced at a lower cost and has distinctive properties that might allow more efficient and faster computation.
The researchers created their qubit from a category of materials known as van der Waals materials—atomic-thin materials that can be stacked similar to Legos on top of one another, with little to no resistance or damage. It is possible to stack these materials in particular ways to develop different electronic systems. In spite of their near-flawless surface quality, very few research teams have ever used van der Waals materials for quantum circuits, and none of them have earlier been shown to have temporal coherence.
To achieve their Josephson junction, the scientists sandwiched a sheet of graphene between two layers made of a van der Waals insulator known as hexagonal boron nitride (hBN). Most significantly, graphene adopts the superconductivity of the superconducting materials that it comes into contact with. It is possible to make chosen van der Waals materials to steer electrons around using voltage, rather than the conventional current-based magnetic field. Hence, the graphene can do the same—and so can the entire qubit.
Upon applying voltage to the qubit, electrons bounce back and forth between two superconducting leads that are connected by graphene, thereby changing the qubit from ground (0) to superposition or excited state (1). The bottom hBN layer acts as a substrate for hosting the graphene. The top hBN layer encloses the graphene, thus preventing contamination of the graphene layer. Since the materials are highly pristine, the traveling electrons do not interact with flaws at all. This is the perfect “ballistic transport” for qubits, where a major portion of the electrons moves from one of the superconducting leads to the other without scattering with impurities, thus enabling a rapid, precise change of states.
How voltage helps
According to Wang, the study can be helpful in tackling the qubit “scaling problem.” At present, it is possible to fit only around 1000 qubits on a single chip. It would be specifically important to be able to control the qubits by voltage since millions of qubits start being crammed on a single chip.
Without voltage control, you’ll also need thousands or millions of current loops too, and that takes up a lot of space and leads to energy dissipation.
Joel I-Jan Wang, Postdoc, Research Laboratory of Electronics (RLE), MIT.
Moreover, voltage control relates to improved efficiency and a highly localized, precise targeting of individual qubits on a chip, preventing “cross talk.” This occurs when a tiny bit of the magnetic field developed by the current interferes with a qubit that it has not targeted, leading to computation problems.
For the time being, the lifetime of the qubit created by the researchers is very short. For reference, the coherence times recorded for conventional superconducting qubits that look promising for practical application are a few tens of microseconds, a few hundred times greater than the qubit developed by the researchers.
However, the research team has already been addressing various problems that lead to such a short lifetime. A majority of these problems mandate structural modifications. The team has also been using its new coherence-probing technique to further analyze the way in which electrons move ballistically around the qubits, with the goal of extending the coherence of qubits by and large.