A superconducting quantum computing circuit package (1). The package contains a substrate (2) on which a circuit is formed, the circuit including a plurality of circuit elements. The substrate (2) includes holes (8) arranged between the circuit elements which extend through a thickness of the substrate (2). The package also contains a holder (3) with a surface (9) on which the substrate (2) is received, and a cover (4) arranged on an opposite side of the substrate (2). The holder (3) and the cover (4) are formed from a metal and/or a superconductor. The holder (3) also contains projections (12) arranged on and projecting from the surface (9). The projections (12) protrude through the holes (8) in the substrate (2) and contact the cover (4) so to suppress electromagnetic modes in the frequency range of operation of the quantum computing circuit.