The S = 1/2 Heisenberg bilayer antiferromagnet with randomly removed inter-layer dimers is studied using quantum Monte Carlo simulations. A zero-temperature multi-critical point (p * , g * ) at the classical percolation density p = p * and inter-layer coupling g * ≈ 0.16 is demonstrated. The quantum critical exponents of the percolating cluster are determined using finite-size scaling. It is argued that the associated finite-temperature quantum critical regime extends to zero inter-layer coupling and could be relevant for antiferromagnetic cuprates doped with non-magnetic impurities.PACS numbers: 75.10. Jm, 75.10.Nr, 75.40.Mg, 75.40.Cx Randomly diluted quantum spin systems combine aspects of the percolation problem [1] with the physics of thermal and quantum fluctuations. In systems that can be tuned through a T = 0 phase transition as a function of some parameter one can hence study divergent quantum fluctuations coexisting with classical fluctuations due to percolation. A multi-critical point, where the two types of fluctuations diverge simultaneously, is realized in the transverse Ising model with dimensionality D > 1 [2,3]. In models with O(N ) symmetry and N > 2 such a point was believed not to exist, because quantum fluctuations were argued to always destroy the long-range order on the percolating cluster [3]. Several studies of diluted 2D Heisenberg antiferromagnets were consistent with this scenario [4,5,6]. However, recent quantum Monte Carlo simulations have shown that longrange order in the 2D Heisenberg model persists until the percolation point [7,8,9] and that the percolating cluster is ordered as well [8,9]. This implies that the phase transition is a classical percolation transition. It also suggests that a multi-critical point, at which the percolating cluster is quantum critical, could be reached by including other interactions. In this Letter it will be shown that the O(3) multi-critical point can be realized in the Heisenberg bilayer with dimer dilution, i.e., where adjacent spins on opposite layers are removed together. This system is illustrated in Fig. 1, and a schematic T = 0 phase diagram is shown in Fig. 2. In analogy with quantum critical points in clean 2D Heisenberg antiferromagnets [10,11], one can expect a finite-T universal quantum critical scaling regime to extend to couplings well beyond the T = 0 critical coupling g * , possibly all the way to decoupled layers (g = 0). This quantum criticality could then be realized in layered antiferromagnets doped with non-magnetic impurities. It may already have been observed in La 2 Cu 1−x (Zn,Mg) x O 4 , for which recent neutron scattering experiments [12] show a correlation length divergence roughly consistent with the dynamic exponent z ≈ 1.3 extracted here.The clean S = 1/2 bilayer Heisenberg model has been extensively studied in the past [13,14]. It undergoes a quantum phase transition between an antiferromagnetic and a quantum disordered state as a function of the ratio g = J 2 /J 1 of the inter-and intra-plane couplings. The criti...