Physicists record lifetime of graphene qubits

First measurement of its kind could provide stepping stone to practical quantum computing.

Researchers from MIT and elsewhere have recorded the “temporal coherence” of a graphene qubit — how long it maintains a special state that lets it represent two logical states simultaneously — marking a critical step forward for practical quantum computing
Researchers from MIT and elsewhere have recorded the “temporal coherence” of a graphene qubit — how long it maintains a special state that lets it represent two logical states simultaneously — marking a critical step forward for practical quantum computing

For the first ever time, MIT scientists have quantified the temporal coherence (lifetime) of graphene qubits- meaning to what extent it can keep up a special state that enables it to speak to two coherent states at the same time.

As of late, specialists have been incorporating graphene-based materials into superconducting quantum computing gadgets, which guarantee quicker, progressively proficient computing, among different advantages. Up to this point, be that as it may, there’s been no recorded coherence for these advanced qubits, so there’s no knowing whether they’re feasible for practical quantum computing.

In a new study, scientists demonstrated a coherent qubit made from graphene and exotic materials. These materials empower the qubit to change states through voltage, much like transistors in today’s traditional computer chips — and not at all like most different kinds of superconducting qubits. Also, the specialists put a number to that coherence, timing it at 55 nanoseconds, before the qubit comes back to its ground state.

First author Joel I-Jan Wang, a postdoc in Oliver’s group in the Research Laboratory of Electronics (RLE) at MIT said, “Our motivation is to use the unique properties of graphene to improve the performance of superconducting qubits. 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.”

Superconducting qubits rely on a structure known as a ‘Josephson junction’, 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 hop back and forth between the superconducting materials, causing the qubit to switch states.

But this flowing current consumes a lot of energy and causes other issues. Thus, scientists replaced the insulator with graphene, an atom-thick layer of carbon that’s inexpensive to mass produce and has unique properties that might enable faster more efficient computation.

For the fabrication of qubits, scientists turned to van der Waals materials that can be stacked like Legos on top of one another, with little to no resistance or damage.

For their Josephson junction, specialists sandwiched a sheet of graphene in the middle of the two layers of a van der Waals encasing called hexagonal boron nitride (hBN). Essentially, graphene takes on the superconductivity of the superconducting materials it contacts.

The chose van der Waals materials can be made to usher electrons around utilizing voltage, rather than the traditional current-based magnetic field. In this manner, so can the graphene — thus can the whole qubit.

When voltage gets applied to the qubit, electrons bounce back and forth between two superconducting leads connected by graphene, changing the qubit from the ground (0) to excited or superposition state (1). The bottom hBN layer serves as a substrate to host the graphene.

The top hBN layer encapsulates the graphene, protecting it from any contamination. Because the materials are so pristine, the traveling electrons never interact with defects. This represents the ideal “ballistic transport” for qubits, where a majority of electrons move from one superconducting lead to another without scattering with impurities, making a quick, precise change of states.

Wang said, “The work can help tackle the qubit scaling problem. Currently, only about 1,000 qubits can fit on a single chip. Having qubits controlled by voltage will be especially important as 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.”

“Additionally, voltage control means greater efficiency and a more localized, precise targeting of individual qubits on a chip.”

For now, the researchers’ qubit has a brief lifetime. For reference, conventional superconducting qubits that hold promise for a practical application have documented coherence times of a few tens of microseconds, a few hundred times greater than the researchers’ qubit.

The work 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.

Other co-authors of the study include Daniel Rodan-Legrain, a graduate student in Jarillo-Herrero’s group who contributed equally to the work with Wang; MIT researchers from RLE, the Department of Physics, the Department of Electrical Engineering and Computer Science, and Lincoln Laboratory; and researchers from the Laboratory of Irradiated Solids at the École Polytechnique and the Advanced Materials Laboratory of the National Institute for Materials Science.

The paper describing the study published in the journal Nature Nanotechnology.