Quantum Feedback Control of Non-classical Microwave Fields in Cavity Quantum Electrodynamics Experiments
by
Serge Haroche(Collège de France, Paris)
→
Europe/Stockholm
Oskar Klein Auditorium (FR4)
Oskar Klein Auditorium (FR4)
Description
In our Cavity Quantum Electrodynamics experiments, we use a beam of
Rydberg atoms to manipulate and probe non-destructively microwave photons
trapped in a very high Q superconducting cavity. In recent years, we have
realized ideal quantum non-demolition (QND) measurements of photon numbers,
observed the radiation quantum jumps due to cavity relaxation and prepared
non-classical fields such as Fock and Schrödinger cat states. Combining QND
photon counting with a homodyne mixing method, we have reconstructed the
Wigner functions of these non-classical states and, by taking snapshots of
these functions at increasing times, obtained movies of the decoherence
process .
The next step in this research program was to demonstrate ways to protect
these non-classical states against decoherence. One natural method is to
implement a procedure analogous to the feedback strategies used to
stabilize complex systems in classical physics. A probe measures the system
state, a controller determines the action leading it towards the chosen
operating point and an actuator realizes this action. Juggling, for
instance, relies on feedback loops from the eye to the hands through the
brain. Transferring the feedback concept to the quantum world faces a
fundamental difficulty: quantum measurement changes randomly the state of
the system and this change must be taken into account in the feedback
procedure. In our first demonstration experiment, we have chosen as an
operating point a fixed photon number in our cavity. The "eye" in this
"photon juggling game" is the beam of Rydberg atoms which performs a weak
QND measurement extracting a partial information about the photon number.
The "brain" is a computer using this information to estimate in real time
the state of the field in the cavity and the "hand" is a classical
microwave source feeding the cavity. These operations are performed in
successive loops bringing the field towards the desired photon number.
Subsequent quantum jumps of the field are detected by the computer and
their effect is reversed by the feedback procedure. In this way, a
pre-determined photon number is maintained in a steady state. We hope to be
able to extend soon these experiments to the protection of other
non-classical states such as Schrödinger cats of radiation.
