  The effect of disorder is studied on the field-induced quantum phase
transition in the frustrated spin-ladder compound H8C4SO2Cu2(Cl[1-x]Brx)4 using
bulk magnetic and thermodynamic measurements. The parent material (x=0) is a
quantum spin liquid, which in applied fields is known to form a magnon
condensate with long-range helimagnetic order. We show that bond randomness
introduced by a chemical substitution on the non-magnetic halogene site
destroys this phase transition at very low concentrations, already for x=0.01.
The extreme fragility of the magnon condensate is attributed to random
frustration in the incommensurate state.
