Two-dimensional (2D) materials are not expected to be metals at low temperature due to electron localization [1]. Consistent with this, pioneering studies on thin films reported only superconducting and insulating ground states, with a direct transition between the two as a function of disorder or magnetic field [2][3][4][5][6]. However, more recent works have revealed the presence of an intermediate quantum metallic state occupying a substantial region of the phase diagram [7-10] whose nature is intensely debated [11][12][13][14][15][16][17]. Here, we observe such a state in the disorder-free limit of a crystalline 2D superconductor, produced by mechanical co-lamination of NbSe 2 in inert atmosphere. Under a small perpendicular magnetic field, we induce a transition from superconductor to the quantum metal. We find a unique power law scaling with field in this phase, which is consistent with the Bose metal model where metallic behavior arises from strong phase fluctuations caused by the magnetic field [11][12][13][14].Global superconductivity emerges in a sample when conduction electrons form Cooper pairs and condense into a macroscopic, phase-coherent quantum state. In two dimensions, the phase coherence can be disrupted even at zero temperature by increasing disorder, either by degrading crystal quality or applying magnetic fields to create vortices [2]. Granular or amorphous superconducting thin films, for which disorder levels can be controlled during growth, have thus provided an established platform for the study of quantum phase transitions in 2D superconductors. Within the conventional theoretical framework, increasing disorder or magnetic field perpendicular to a strongly disordered film at T = 0 induces a direct transition to an insulating state as the normal state sheet resistance approaches the pair quantum resistance h/(2e) 2 = 6.4 kΩ [2,4]. As film quality has improved over time, however, an intervening metallic phase with resistance much lower than the normal state resistance has been observed in several systems with generally less disorder [7][8][9][10]. Its origin is not well understood, and the various theoretical treatments can be generally divided between purely bosonic-based models, in which Cooper pairing persists in the metallic phase but phase coherence is lost [11][12][13][14], and models that also incorporate other fermionic degrees of freedom [15][16][17].Recently, mechanical exfoliation has emerged as a technique to produce ultra-clean, crystalline 2D materials, with graphene being a well-known example [18]. Like amorphous films, the thickness of these samples can be easily controlled down to the level of individual atomic layers. In contrast to amorphous films, a 2D superconductor exfoliated from a lay- Figure 1. Environmentally controlled device fabrication. a) Schematic of heterostructure assembly process. Boron nitride (BN)/graphite (G) on a polymer stamp (PDMS) is used to electrically contact and encapsulate NbSe2 in inert atmosphere. The heterostructure is lithographically patte...