Quantum mechanics is an impressively successful theory that tells us how very small (atomic and subatomic) objects behave. Despite the success so far, quantum mechanics has important aspects that remain counterintuitive and require deeper understanding. One such aspect of current interest involves measurements on quantum systems. Fundamentally, when measuring some aspect of a quantum system, the system is disturbed by the measurement, such that other aspects of the system change in response to the measurement. The most famous example is "Heisenberg's uncertainty principle," which, for example, states that when one has better knowledge of the position of an atom, one must necessarily have worse information about the atom's velocity. More generally, this disturbance effect goes by the name of "quantum back-action." The goal of this project is use ultracold atoms to observe and study several manifestations of quantum back-action that have been predicted theoretically. Broadly speaking, this research addresses the following questions: Under what conditions does back-action significantly influence the future of the system? Can precisely engineered back-action be useful as a tool to control the system? These questions are nontrivial, as back-action generally refers to a random disturbance to a quantum system. However, under carefully arranged conditions, the randomness of the back-action can control quantum systems in well-defined ways. This research will advance fundamental understanding of quantum mechanics, measurement, and information, and it will provide new tools that may be useful in future technologies such as quantum computers and precision-measurement devices for such quantities as acceleration and magnetic field, whose performance will ultimately be limited by quantum effects.
More specifically, the effects of quantum back-action on the dynamics of ultracold atoms will be studied in three separate scenarios. In the first scenario, a single trapped atom undergoes coherent transitions between two states when driven by a laser field. Spontaneous emission not only causes the atom to jump from the excited state to the ground state at random times, but also provides information about the present state of the atom. By selecting only the (random) cases where an atom does not spontaneously emit at all, the experiment is predicted to show that spontaneous emission nevertheless has an effect, via the relative probabilities for the atom to be in each state (i.e., the measurements should not be explainable by a theory that does not include spontaneous emission). In the second scenario, atoms confined to an optical lattice undergo spontaneous emission, which produces heating (momentum diffusion). Although momentum diffusion should increase the rate at which the atoms spread through the lattice, an interesting prediction is that for small spontaneous-emission rates, the atoms' spreading should be suppressed due to the inhibition of quantum tunneling. For larger spontaneous-emission rates, the spreading of the atoms should then again increase as expected. In the final scenario, a realization of the predicted "blowtorch" effect is to be realized with atoms in an optical lattice: by making a space-dependent "temperature" (spontaneous-emission rate) for the atoms, a pumping effect for the atoms (realized as a steady current of atoms) should be observable, demonstrating a fundamental effect in nonequilibrium thermodynamics.
