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.

Two teams of the quantum nanoelectronics laboratory are involved in the simulation of naturally occurring quantum system. Two different approaches are employed. On one hand, a circuit composed of a cavity coupled to a transmon qubit hosts the digital quantum simulation of topological insulators, exotic materials exhibiting protected edge state. This digital approach, based on quantum walk algorithm, efficiently simulates spin-orbit coupled particles running on a lattice. The powerful toolbox offered by cavity quantum electrodynamics architectures enables the measurement of the topological invariant of the simulated system using Schrödinger cat superposition state. The direct measurement of such quantity in solid-state materials remains an outstanding challenge, owing to the non-local nature of the topological order. This protocol paves the way for the simulation and characterization of complex quantum materials.

On the other hand, a circuit composed of a chain of coupled transmon qubits can perform quantum simulations of molecule orbitals in the context of quantum chemistry. Here, a hybrid algorithm leverages best of the classical and quantum world. It takes advantage of the large set of degrees of freedom available in the quantum system in combination with the well-developed classical optimization capabilities in order to simulate complex chemical processes. As a first step, the electronic orbitals of a di-hydrogen molecule is simulated using two coupled transmons. The rising discipline of quantum chemistry simulations open the door to a better understanding of biochemical complexes sustaining biomedical innovation.

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.