A
Microwave Power Standard Based on the
Measurement of an Atomic Resonnance
Dave Paulusse, Nelson Rowell
and Alain Michaud
Institute
for National
Measurement Standards,
National Research Council, Ottawa, Ont., Canada K1A 0R6
(e-mail:
Alain.Michaud@nrc-cnrc.gc.ca)
Presented at:
Congrès ACP-2002-CAP Congress, Québec City, June
2-5 2002
Abstract— We demonstrate the feasibility of a novel microwave power standard
based on the electromagnetic interaction with cold atoms.
Under
the effect of the radiation, the internal state population will undergo
a Rabi flopping oscillation. The measurement of this
frequency
will allow the determination of the electromagnetic field strength.
Our experiment uses Rubidium atoms which
are
captured in a standard magneto-optical trap (MOT). The use of cold
atoms has several advantages, namely the interaction time and atomic
density are increased by many orders of magnitude and the Doppler
effect becomes negligible. Simply shutting off the lasers and
letting the atoms fall into the interaction region does the
measurement. Before the atoms fall, the magnetic field is ramped to
zero, the cooling laser is detuned and the light intensity reduced to
further reduce the temperature. Finally a short pulse of resonant light
is applied to prepare the internal state of all the atoms. These steps
are done in about 20 ms.
When the atoms are in the interaction
zone, a pulse
of microwave radiation is applied. When the frequency of the field is
resonant with the transition (6.8 GHz), only the field
amplitude
determines the Rabi frequency. After the atoms have crossed the
interaction region, a laser beam probes the population distribution. As
this will destroy the atomic coherence, the experiment has to be
repeated many times for varying pulse duration times. The population
inversion can then be plotted as a function of the time. The
microwave field comes from a radiating structure of which the
radiation pattern should be known accurately. For that reason we used a
simple geometry i.e. the open end of a waveguide terminated by an
‘infinite’ metallic flange. The waveguide (standard
R-70 type) attaches
directly to the output of a directional coupler that has a power sensor
attached to its side arm. Once the experiment is completed
the
coupler and power sensor can be used directly as a calibrated transfer
standard. The use of a second coupler would allow the system to
stabilize the output power of a source. In this paper we report on the
progress of this work. We show some schematic describing the experiment
and we present our latest experimental results.