"Real-Time Quantum Feedback With Cold Atoms"
J. M. Geremia
California Institute of Technology
Wednesday, February 11, 2004 11 AM -12:00 Noon in 375 LeConte Hall
Abstract
Free from observation, a quantum mechanical system evolves deterministically
- its entire evolution can be predicted (at least in principle) from its state at one
point in time. However, when a measurement is performed, quantum mechanics
postulates that the observer will obtain a random post-measurement outcome.
Conveniently, quantum measurement can produce states that are difficult to
otherwise obtain. Of course, quantum mechanics does not predict the outcomes
of individual experiments, only their likelihood. Therefore using measurement
to prepare sought-after states is hindered by non-determinism, and the most desired
measurement outcomes are often the least likely. Fortunately, the probabilistic
nature of quantum observation does not preclude external intervention. I will
describe our recent experiment to steer a quantum nondemolition (QND) measurement
of spin angular momentum in a cloud of cold atoms toward a fiducial outcome.
We have employed real-time feedback to prepare unconditional spin-squeezing and
have used the resulting quantum entanglement to perform atomic magnetometry below
the standard quantum limit (i.e., the shotnoise limit).
