MAGIA

Collaboration:

- Italy: Firenze University and INFN and LENS
- Germany: Peters Universität Konstanz
- USA: Yale University

Laboratory: European Lab. for Non-Linear Spectroscopy - LENS (Firenze)

National Responsible: G. Tino (Firenze)

1. Goal of the experiment

Measurement of the Gravitational Constant G with a new method based on atomic interferometry .

More on the experiment...

2.

Activities during 2002

General

Laser system: n ew laser-diodes were tested, new and better frequency locks were mounted and the beam paths were aligned. We could get a Rubidium MOT with 3*107 atoms to demonstrate the function of the laser-system. A time of flight measurement allowed us to determine the temperature of the cloud to 30 microK.

One new laser was setup together with a tapered amplifier to enhance its power up to 500mW. This laser will be used to perform Raman-cooling in an optical lattice on the atoms.

Phase-lock system: t wo new lasers were mounted. They are superposed and their beat signal is detected. This signal is mixed down electronically and fed into a phase-lock loop to stabilize the phase of one laser relative to the other. The lasers are stabilized with a difference frequency equal to the hyperfine splitting of Rb87 or Rb85 which is 6.8 GHz and 3.0 GHz respectively.

We reach a locking-bandwidth of 4.5 MHz. The phase noise of the two lasers (from 1kHz to 6MHz) is 0.07 radian. Although quite good, it is not enough for our experiment, and further improvements are on the way. The phase-lock is of crucial importance for the experiment, as the phase-front of the beat of these lasers will be the reference frame for the entire experiment.

A new locking scheme: b orn out of the necessity to find a suitable locking system for our MOT-laser without modulating the light, we developed a new locking method, which suits well for our purpose.

The method can be designated as Doppler-free DAVLL (dichroic atomic vapor laser lock). The setup has in common with the classical saturation spectroscopy the pump probe configuration, but differs in the following points: a magnetic field along the propagation axis of the light changes the absorption behaviour of the s+ and s- component of the (linearly polarised) probe beam. The s+ and s- components of the probe beam are shifted in opposite directions and are detected independently on two different photodiodes. The resulting difference signal between those two signals is of dispersive character and has a very low offset and therefore noise. The laser can be directly locked on this signal, which in addition is insensitive to stray magnetic fields. A publication is in preparation.

New apparatus: t he vacuum apparatus was designed, ordered and mounted. The apparatus consists of a cell for trapping the atoms and launching them, of a second cell to perform Raman cooling in a comoving frame and of a 1m high tube in which the interferometry will be performed. Thanks to its low thermal expansion, its high resistance, its low magnetic permeability, high stiffness and low weight, we use Titanium as material of choice for the apparatus.

The following achievements have been obtained:

- Construction of the vacuum system and of laser source setup (100 %)

- Construction of the system with two lasers phase locked and amplified (100 %)

- Preliminary characterization of the atomic interferometer (50 % (*))

(*) The realization of the laser system and vacuum apparatus required more time than expected because of unexpected technical problems. Preliminary characterization of parts of the final interferometer were performed: 1) cooling and trapping of the atoms in a magneto-optical trap, 2) measurement of the temperature by time of flight scheme. This work will be completed in the first half of 2003.

Milestones 2003

Optimization of the atomic fountain | 31-05-2003
Evaluation and design of the source mass | 31-05-2003
Optimization of the atomic interferometer parameters | 30-09-2003
Analysis and reduction of systematic errors | 30-09-2003
Design of the support and motion structure for the source mass | 30-09-2003
Measurement of small g with the atomic interferometer | 31-12-2003

3. INFN contribution in terms of manpower and financial support.

Manpower: 7 researchers (4.9 FTE), 2 technicians (0.7 FTE)

Budget for the Year 2003: 0.7 % of the CSN2 budget for 2003

4. Number of publications in refereed journals (year 2002): 4

5. Conference talks (year 2002): 0

6. Number of undergraduate and doctoral thesis.

- 1 laurea thesis in progress
- 1 doctoral thesis in progress

7. Leadership role and major responsibilities in the experiment.

- G. Tino, spokesperson

8. Innovative instruments

9. Competing experiments

10. International Committee which has reviewed the experiment