Long-Distance Entanglement of Spin Qubits via Ferromagnet

We propose a mechanism of coherent coupling between distant spin qubits interacting dipolarly with a ferromagnet. We derive an effective two-spin interaction Hamiltonian and find a regime where the dynamics is coherent. Finally, we present a sequence for the implementation of the entangling controlled-not gate and estimate the corresponding operation time to be a few tens of nanoseconds. A particularly promising application of our proposal is to atomistic spin qubits such as silicon-based qubits and nitrogen-vacancy centers in diamond to which existing coupling schemes do not apply.

Popular Summary

The general idea of a quantum computer is to use the amazing and often counterintuitive laws of quantum mechanics to process information. As quantum phenomena are mostly observable at very low temperatures, quantum computers operating at room temperature would seem like an unattainable holy grail. Recently, however, atomic-scale nitrogen-vacancy centers (NV centers) in diamond, whose quantum spin states are long lived and can be manipulated with a high degree of control, have emerged as a promising candidate for qubits at room temperature. The next step on the road map to quantum computers based on such qubits is to make the qubits interact with each other in a controlled and scalable way even when they are separated over long distances. Until now, no theoretical proposals for doing so with these atomistic spin-based qubits existed. In this paper, we present a simple mechanism and design that fill this gap.

NV-center-based spin qubits can be generated below the surface of a diamond. The key idea in our proposal is to make two spatially separated spin qubits interact through a third party, to which both are coupled. The third party is a “dogbone-shaped” ferromagnet placed on the surface and close to the two qubits and it interacts with each of them through the ubiquitous magnetic dipole-dipole interaction that is only effective at short distances. The qubit-qubit interaction is then effectively mediated by the exchange of short-lived traveling magnons—collective excitations of the spins in the ferromagnet. By tuning the energy gap between the binary quantum states of the qubits to closely match the excitation energies of the magnons, fast and sizable interactions even at distances as large as micrometers can be achieved. Indeed, experimentally realistic regimes of parameters exist for coherent dynamical interactions between the qubits.

Given the generality of the underlying mechanism, our proposal can also be applied to other solid-state spin-based qubits, such as silicon-based qubits and electron-spin qubits localized in quantum dots.

Figure 1

The schematics of the ferromagnetic coupler setup. The orange dogbone shape denotes the ferromagnet that is coupled via magnetic dipole interaction to spins of nearby quantum dots (red spheres with green arrows). The ferromagnet is assumed to be a monodomain, and its magnetization is denoted by blue arrows ($M$) that can take arbitrary orientation. $L$ is the length of the quasi-1D ferromagnetic channel that is approximately equal to the distance between the qubits. The shape of the ferromagnetic coupler is chosen such that it enables strong coupling to the spin qubits while maintaining the spatially slowly decaying 1D susceptibility between the two disks.