Decoherence Suppression in Transmon Qubits by Encapsulation in Hexagonal Boron Nitride

Magnetic surface adsorbates are a dominant contributor to qubit dephasing from 1/f flux noise. At the same time, surface adsorbates can act as strongly coupled two-level state (TLS) defects, leading to strong enhancement of the qubit energy relaxation rate at discrete operating frequencies. Here we describe an approach to reduce decoherence that involves encapsulation of transmon qubits in few-layer hexagonal boron nitride (hBN). The inert hBN membrane is expected to inhibit nucleation of microscopic adsorbates that induce both dephasing and relaxation, without contributing additional loss. We describe two series of experiments to probe the effectiveness of hBN encapsulation at reducing decoherence. First, we perform T1 swap spectroscopy to map out the spectrum of strongly-coupled TLS defects for devices with and without hBN encapsulation. Next, we use single-shot Ramsey tomography to extract the power spectral density of magnetic flux noise for both encapsulated and reference devices. These results shed light on the microscopic sources of qubit decoherence. We discuss prospects for extending this technique to larger multiqubit arrays.