Quantum systems show fascinating counterintuitive properties of superposition, i.e. the ability to be simultaneously in multiple states, and entanglement, i.e. the ability to develop long-distance many-body correlations. These properties present opportunities to build quantum technology, such as sensing, communication and information processing, with capabilities not achievable in the classical domain. In recent years motivated by this promise, tremendous progress has been achieved to engineer and control a variety of individual quantum systems. Among these, spin defects in insulating materials, i.e. microscopic spin quantum bits (qubits) are particularly promising. Spin interacts with the environment through their magnetic dipole moments: a fact utilized for quantum sensing of magnetic fields with unprecedented spatial resolution and sensitivity. On the other hand, the environment in typical materials hosting spin qubits produces weak magnetic fields, consequently the quantum states encoded in the spin qubits survive for long times, a fact that makes spin qubits attractive candidates for quantum memory and information processing. The next frontier aims at scaling the functionality of spin-based quantum hardware, which includes developing the ability of spin qubits to:(i) sense signals beyond magnetic fields for developing novel quantum sensors, and (ii) controllably interact beyond few proximally-placed spin qubits while retaining individual addressability, for quantum information processing. This has however proved challenging due to the lack of a mediator which can controllably and strongly couple to spin qubits as well as to a wide range of external signals. In this project, the principle investigator will exploit magnons (i.e. the collective excitations in magnets), as a fundamentally novel mediator to address this challenge. In tight integration with research, the principal investigator will also develop a quantum engineering course for training undergraduate, graduate and industry professionals for enhancing the United States quantum-smart workforce.

The proposed research aims at unraveling novel magnon spin-qubit hybrid devices by integrating theory with proof-of-principle experiments. In particular, the principal investigator will pursue the following device types. (a) Sensing-type devices- the central aim of these devices will be to enhance sensing capability of spin qubits for signals, such as, electric fields, temperature. This will be achieved by using magnons as transducers of external signals to a magnetic signal. (b) Information processing-type devices- the central aim of these devices will be to address the challenge of designing a scalable information-processing architecture for spin qubits, where qubits can be coherently coupled across varied length scales while maintaining local addressability. For this purpose, theoretical schemes and proof-of-principle experiments will be demonstrated for coherently coupling classical signals to spin qubits locally via electrical pumping of designed magnon resonance modes. In addition, schemes will be developed for transferring information between spin-qubits and magnons in the quantum regime by designing the magnetic resonance modes. To achieve above goals, the principal investigator will translate the well-established design principles for photon/phonon-qubit hybrids (i.e. cavity and circuit quantum electrodynamics) to the proposed magnon-spin-qubit systems. Beyond these similarities, magnons also offer unique capabilities, such as condensation into superfluid-like, as well as, soliton-like modes, and inherent chiral propagation. The proposed research is thus expected to uncover fundamentally new device concepts unique to magnonic system, such as unidirectional chiral spin-spin coupling. Exploring such device concepts theoretically will also form an integral part of this proposal.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

National Science Foundation (NSF)
Division of Electrical, Communications and Cyber Systems (ECCS)
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Usha Varshney
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Purdue University
West Lafayette
United States
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