The authors introduce quantum databases, a new database abstraction that allows to defer the making of choices in transactions until an application or user forces the choices by observation. Conceptually, a transaction is in a quantum state - in one of many possible worlds, which one is unknown - until fixed by observation. Practically, their abstraction enables late binding of values read from the database. This allows more transactions to succeed in environments with high contention. This is particularly important for applications in which transactions compete for scarce physical resources represented by data items in the database, such as seats in airline reservation systems or meeting slots in calendaring systems.