Graphene just unlocked “impossible” quantum currents without magnets

Quantum physicist Talieh Ghiasi has demonstrated the quantum spin Hall (QSH) effect in graphene for the first time without any external magnetic fields. The QSH effect causes electrons to move along the edges of the graphene without any disruption, with all their spins pointing in the same direction. "Spin is a quantum mechanical property of electrons, which is like a tiny magnet carried by the electrons, pointing up or down," Ghiasi explains. "We can leverage the spin of electrons to transfer and process information in so-called spintronics devices. Such circuits hold promise for next-generation technologies, including faster and more energy-efficient electronics, quantum computing, and advanced memory devices."

On-chip integration

Realizing quantum transport in graphene typically requires applying large external magnetic fields that are not compatible with electronic circuitries. "In particular, the detection of quantum spin currents in graphene has always required large magnetic fields that are practically impossible to integrate on-chip. Thus, the fact that we are now achieving the quantum spin currents without the need for external magnetic fields opens the path for the future applications of these quantum spintronic devices," says Ghiasi.

Spin transport in graphene

The scientists from the Van der Zant lab were able to bypass the need for external fields by layering the graphene on top of a magnetic material: CrPS₄. This magnetic layer significantly altered the graphene's electronic properties, giving rise to the QSH effect in graphene. Ghiasi: "We observed that the spin transport in graphene gets modified by the neighboring CrPS₄ such that the flow of electrons in graphene becomes dependent on the electrons' spin direction."

Preserving spin information

The quantum spin currents that the scientists detect in the graphene-CrPS₄ stack are 'topologically' protected, implying that the spin signal travels stays intact over tens of micrometers long distances without losing the spin information in the circuit. "These topologically-protected spin currents are robust against disorders and defects, making them reliable even in imperfect conditions," Ghiasi says. Preserving spin signal without any loss of information is vital for building spintronic circuits.

This discovery paves the way toward ultrathin, graphene-based spintronic circuits, promising advancements in next-generation memory and computing technologies. The observed spin currents in graphene offer a powerful new route for efficient and coherent transfer of quantum information through electron spins. These robust spintronic devices could serve as essential building blocks in quantum computing, seamlessly linking qubits together within quantum circuits.