Isolated optically-active solid-state spins such as the Nitrogen-Vacancy (NV) center in diamond have demonstrated good properties as qubits for quantum information processing tasks. However, engineering a quantum register of spins around a central NV qubit enables more powerful applications. For example, given their weak coupling to the environment a register of nuclear spins has demonstrated enhanced quantum memory, quantum error correction, and many interesting quantum protocols. Still, thanks to their stronger coupling between spins and to external fields, a register of electronic spins would enable new complementary applications, especially in the areas of quantum sensing and quantum device characterization.
In this talk I will present two critical steps towards developing a small-scale quantum information processor based on electronic spins in diamond.
First, we demonstrate an approach to systematically scale up a system of electronic spins starting from a central NV qubit. Concretely, we develop a general method to characterize the Hamiltonian of an unknown interacting spin system via sweep of an external magnetic field, and applying this characterize 2 unknown optically-dark electron-nuclear spin defects around our NV. The knowledge of the Hamiltonian not only helps identify spin defects at the single-molecular level, but also enables coherent control over the multi-qubit system [1].
We then turn our attention to the environment surrounding our quantum register of spins, which causes decoherence (noise). More generally, as decoherence limits the power of quantum devices, a practical yet predictive model of decoherence is desired. Here, by combining two complementary techniques of quantum noise spectroscopy, we demonstrate one approach a practical yet predictive noise model, namely, to build a “self-consistent” classical noise model that is consistent with all observed decoherence under various qubit dynamics. As a crucial check of accurate characterization of the underlying bath, we verify the self-consistent noise model is predictive even under additional qubit dynamics. Finally, by characterizing the noise of our nanoscale system of electronic spins, we surprisingly find a non-uniform and complex quantum spin environment. Applied to multi-qubit devices, the self-consistent noise model enables quantum sensing of complex many-body environments with high spatial resolution, as well as characterization of correlated noise between qubits which has implications for practical realizability for quantum error correction [2].
Time permitting, I will also briefly introduce two past works: a more robust metric of entanglement detection [3], and achieving practical quantum advantage in sensing with quantum register of electronic spins [4].
[1] A. Cooper, W. K. C. Sun, J.C. Jaskula, and P. Cappellaro, Physical Review Letters 124, 083602 (2020).
[2] W. K. C. Sun, P. Cappellaro, Submitted (2022)
[3] W. K. C. Sun, A. Cooper, and P. Cappellaro, Physical Review A 101, 012319 (2020).
[4] A. Cooper, W. K. C. Sun, J.C. Jaskula, and P. Cappellaro, Physical Review Applied 12, 044047 (2019).
