Events

When: Monday, March 2, 2026 at 4:30pm

Where: Davis Auditorium, Schapiro CEPSR

Title: "New platforms for quantum sensing and quantum computing"

Abstract: Correlated phenomena play a central role in condensed matter physics, but in many cases there are no tools available that allow for measurements of correlations at the relevant length scales (nanometers -microns). We have recently demonstrated that nitrogen vacancy (NV) centers in diamond can be used as point sensors for measuring two-point magnetic field correlators [1]. NV centers are atom-scale defects that can be used to sense magnetic fields with high sensitivity and spatial resolution. Typically, the magnetic field is measured by averaging sequential measurements of single NV centers, or by spatial averaging over ensembles of many NV centers, which provides mean values that contain no nonlocal information about the relationship between two points separated in space or time. We recently proposed and implemented a sensing modality whereby two or more NV centers are measured simultaneously, from which we extract temporal and spatial correlations in their signals that would otherwise be inaccessible. We demonstrate measurements of correlated applied noise using spin-to-charge readout of two NV centers and implement a spectral reconstruction protocol for disentangling local and nonlocal noise sources. We also demonstrate massively multiplexed magnetometry with high fidelity spin readout of hundreds of NV centers simultaneously [2]. Finally, we develop protocols to use optically unresolved NV center pairs, entangled states of NV center pairs, and nuclear spins as multi-qubit sensors for measuring correlated noise, enabling covariance magnetometry at nanometer length scales [3]. This novel quantum sensing platform will allow us to measure new physical quantities that are otherwise inaccessible with current tools, particularly in condensed matter systems where two-point correlators can be used to characterize charge transport, magnetism, and non- equilibrium dynamics.

Separately, I will describe our recent efforts to tackle noise and microwave losses in superconducting qubits. Building large, useful quantum systems based on transmon qubits will require significant improvements in qubit relaxation and coherence times, which are orders of magnitude shorter than limits
imposed by bulk properties of the constituent materials. This indicates that loss likely originates from uncontrolled surfaces, interfaces, and contaminants. Most efforts aimed at achieving millisecond coherence times have focused on avoiding loss and decoherence through novel qubit designs, such as 3D transmons, fluxonium qubits, Kerr-cat qubits, and zero-pi qubits, which require radically new processor architectures and gate schemes. By contrast, materials improvements are a powerful approach to reducing loss and decoherence in superconducting qubits because such improvements can be readily translated to large scale processors. We recently demonstrated 2D transmon qubits that have both lifetimes and coherence times exceeding 0.3
milliseconds by using tantalum as the material in the capacitor [4]. Following this discovery, we have parametrized the remaining sources of loss in state-of-the-art devices using systematic measurements of the dependence of loss on temperature, power, and geometry [5]. This parametrization, complemented by direct materials characterization, allows for rational, directed improvement of superconducting qubits. We have used this playbook to tackle the dominant sources of loss and noise to realize further improvement, and we have demonstrated 2D transmons with coherence times exceeding 1 millisecond and lifetimes up to 1.68 milliseconds [6]. Because we have improved the underlying material system without alteration to the qubit architecture, these qubits are readily translated to existing control schemes, and we demonstrate single qubit gates with 99.994% fidelity.

[1] "Nanoscale covariance magnetometry with diamond quantum sensors," J. Rovny, Z. Yuan, M. Fitzpatrick, A. I.
Abdalla, L. Futamura, C. Fox, M. C. Cambria, S. Kolkowitz, N. P. de Leon, Science 378, 6626 1301-1305 (2022).
[2] "Massively multiplexed nanoscale magnetometry with diamond quantum sensors," K.H. Cheng, Z. Kazi, J. Rovny,
B. Zhang, L. Nassar, J. D. Thompson, N. P. de Leon, Phys. Rev. X 15, 031014 (2025).

[3] "Multi-qubit nanoscale sensing with entanglement as a resource," J. Rovny, S. Kolkowitz, N. P. de Leon,
arXiv:2504.12533, in press at Nature.
[4] "New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds," A.
P. M. Place, L. V. H. Rodgers, P. Mundada, B. M. Smitham, M. Fitzpatrick, Z. Leng, A. Premkumar, J. Bryon, A.
Vrajitoarea, S. Sussman, G. Cheng, T. Madhavan, H. K. Babla, X. H. Le, Y. Gang, B. Jaeck, A. Gyenis, N. Yao, R. J.
Cava, N. P. de Leon, A. A. Houck, Nature Communications 12, 1779 (2021).
[5] "Disentangling Losses in Tantalum Superconducting Circuits," K. D. Crowley, R. A. McLellan, A. Dutta, N.
Shumiya, A. P. M. Place, X. H. Le, Y. Gang, T. Madhavan, M. P. Bland, R. Chang, N. Khedkar, Y. C. Feng, E. A.
Umbarkar, X. Gui, L. V. H. Rodgers, Y. Jia, M. M. Feldman, S. A. Lyon, M. Liu, R. J. Cava, A. A. Houck, N. P. de
Leon, Phys. Rev. X 13, 041005 (2023).
[6] "2D transmons with lifetimes and coherence times exceeding 1 millisecond," M. P. Bland, F. Bahrami, J. G. C.
Martinez, P. H. Prestegaard, B. M. Smitham, A. Joshi, E. Hedrick, A. Pakpour-Tabrizi, S. Kumar, A. Jindal, R. D.
Chang, A. Yang, G. Cheng, N. Yao, R. J. Cava, N. P. de Leon, A. A. Houck, arXiv:2503.14798, Nature (2025).