  Quantum computers can potentially achieve an exponential speedup versus
classical computers on certain computational tasks, as recently demonstrated in
systems of superconducting qubits. However, these qubits have large footprints
due to their large capacitor electrodes needed to suppress losses by avoiding
dielectric materials. This tactic hinders scaling by increasing parasitic
coupling among circuit components, degrading individual qubit addressability,
and limiting the spatial density of qubits. Here, we take advantage of the
unique properties of the van der Waals (vdW) materials to reduce the qubit area
by a factor of $>1000$ while preserving the required capacitance without
increasing substantial loss. Our qubits combine conventional aluminum-based
Josephson junctions with parallel-plate capacitors composed of crystalline
layers of superconducting niobium diselenide (NbSe$_2$) and insulating
hexagonal-boron nitride (hBN). We measure a vdW transmon $T_1$ relaxation time
of 1.06 $\mu$s, which demonstrates a path to achieve high-qubit-density quantum
processors with long coherence times, and illustrates the broad utility of
layered heterostructures in low-loss, high-coherence quantum devices.
