As engineered quantum systems become more complex, they inevitably interact more closely with the environment surrounding them.  Each resonator, flux-tuning line, and ground plane contains a multitude of fluctuating quantum modes that couple to the experimental system, causing it to evolve in inherently unpredictable ways.  From the perspective of the macroscopic world, this “bath”, or “reservoir”, of extraneous modes leads to decay and dephasing in the experiment, erasing phase coherence in a quantum superposition and inexorably driving the system toward the ground state.

Figure caption: Controlling a system requires removing entropy, but measurement is not the only way to accomplish this. Counterintuitively, decoherence, or the interaction of a system with a large reservoir, can be engineered to drive a system to a non-trivial ground state, and even produce the kinds of states that it usually destroys. We have used this technique, called bath engineering, to prepare and stabilize superposition states of a qubit, entangled states between separated qubits and many body eigenstates of an Bose-Hubbard chain.