The authors acknowledge discussions with E. Berg, S. Das Sarma, Y.-Z. Chou, A. Ghazaryan, L. Levitov, A. Macdonald, M. Serbyn, C. Varma, and M. Zaletel. Funding: This project was primarily funded by the Department of Energy under DE-SC0020043. Support to purchase the cryogen-free dilution refrigerator was provided by Army Research Office under award W911NF-17-1-0323. AFY acknowledges the support of the Gordon and Betty Moore Foundation under award GBMF9471 and the Packard Foundation under award 2016-65145 for general group activities. KW and TT acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan (Grant Number JPMXP0112101001) and JSPS KAKENHI (Grant Numbers 19H05790, 20H00354 and 21H05233). Author contributions: HZ and LH fabricated the devices. HZ, LH, YS, WH, CLP, and FY performed the measurements. HZ, LH, YS, WH, CLP, FY and AFY analyzed the data. LC assembled the cryogenic instrumentation. TT and KW grew the hexagonal boron nitride crystals. HZ and AFY conceived the experiment and wrote the paper. Competing interests: The authors declare no competing interests. Data and materials availability: All data shown in the main text and the supplementary materials are available in the Dryad data repository (49).