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Take a trip near absolute zero… where atoms behave like optical waves!

Several decades have passed since Young’s famous interference experiment, proving the wave theory of light: one photon could be at two different places at the same time… hard to believe, and even more when talking about atoms.

Dr Andrea Bertoldi is an atom cooling and interferometry specialist. He uses extremely cold rubidium atoms to measure the universal gravitational constant G (MAGIA experiment, Florence - Italy), to develop atomic clocks using weak measurements (BIARO experiment, Palaiseau - France) and, in the near future, to develop a gravitational wave detector based on atom interferometry (MIGA, Bordeaux - France).
Several decades have passed since Young’s famous interference experiment, proving the wave theory of light: one photon could be at two different places at the same time… hard to believe, and even more when talking about atoms.
Dr Andrea Bertoldi is an atom cooling and interferometry specialist. He uses extremely cold rubidium atoms to measure the universal gravitational constant G (MAGIA experiment, Florence - Italy), to develop atomic clocks using weak measurements (BIARO experiment, Palaiseau - France) and, in the near future, to develop a gravitational wave detector based on atom interferometry (MIGA, Bordeaux - France).
Perhaps the passion for cold matter comes from Andrea’s origins: the center of the Alps. He started working in atomic physics during his PhD at the University of Trento working with the group of Leonardo Ricci. There they developed a new way of trapping neutral species using combined static potentials [1,2].
Dr Andrea Bertoldi
Dr Andrea Bertoldi
After his PhD, Dr Bertoldi moved to Florence and began work on the MAGIA experiment with Professor Tino’s group.
The goal of the experiment was measuring the gravitation constant G by atom interferometry.

The method adopted was innovative: instead of measuring the torsion induced by heavy sources on macroscopic suspended masses, they used microscopic probes (cold rubidium atoms) in free fall to measure gravity simultaneously in two places (via Raman Atom Interferometry) to obtain a gravity-gradient value.
Repeating the measurement for different configurations of heavy source masses (500 kg of tungsten), they canceled differentially the Earth's influence and obtained a value of G in the 10-3 precision level [3,4].
BIARO Cooled Atom Vacuum Chamber
In 2008, Andrea transferred to the Institut d’Optique in Palaiseau to work on the BIARO experiment in Alain Aspect and Philippe Bouyer’s group. Here, they trapped rubidium atoms in an optical dipole trap.
These atoms are trapped by injecting telecom radiation at 1.5 µm in a macroscopic optical cavity. The atoms are further cooled by evaporation below 200 nK to obtain a Bose-Einstein Condensate (BEC).

This macroscopic quantum state was experimentally demonstrated for the first time in 1995. Using cold atoms at a few µK, they recently demonstrated the feedback control of internal states of atomic clouds and their protection against collective noise using weak, non-demolition measurements [5].

Now, they are trying to implement an atomic clock with extended interrogation time, exploiting repeated coherence preserving measurements and feedback. The aim is to increase measurement sensitivity with in time-keeping devices.
Atoms, at an initial temperature of 300 K, are cooled to few hundred µK using a magneto-optic trap (MOT). This technique combines laser cooling and an inhomogeneous magnetic field to turn the radiative pressure1 of the red detuned light into a dissipative and central force.
The transition line commonly used to cool rubidium is the D2 line between the 52S1/2 and  52P3/2 levels (at about 780 nm). To further lower the atomic temperature, atoms are trapped in a conservative potential, which can be magnetic or optical, and the effective potential depth is progressively reduced.
The hottest atoms escape from the well, and the remaining ensemble thermalizes at a lower temperature. The temperature is thus reduced by evaporation, and the phase space density increased. In our case, we use an optical dipole trap obtained with telecom light at 1.5 µm.

The most common way to measure the temperature of cold atomic cloud is the time-of-flight (TOF) method, pioneered by W. Phillips at MIT in the 80’s. It consists of releasing the atomic cloud from the trap and characterizing its expansion versus time. A cold sample increases more slowly in size with respect to a hot one.

"Apart from cooling the atoms in the MOT, we use radiation at 780 nm to probe the rubidium atoms on the D2 line", explains Andrea. "We implemented a weak non-destructive heterodyne detection [6,7] to count the population difference between the two hyperfine levels of rubidium. Using this tool in a feedback scheme we could control a coherent spin state. Thus, correcting the effect of a noise source. The optical dipole trap uses light at 1560 nm (fiber laser amplified with an EDFA), which causes a strong differential light shift on the D2 line due to the upper transition at about 1529 nm. To probe the atoms in the trap we cancel the effect using another fiber laser slightly shifted on the blue wavelength of that transition".
The EYLSA by Quantel has been plugged in the BIARO experiment. "It has been very fast: the installation took some hours", says Andrea.

"We use optical beating between the EYLSA laser and a reference laser (locked on rubidium cell) to produce the error signal required for the frequency lock. Soon, we obtained cold atoms in the 2D-MOT and 3D-MOT. Turning on a system that immediately delivers an intense beam (1 W) at the right wavelength saves a lot of time.
During my PhD, I spent several months building extended cavity diode lasers, injecting tapered amplifiers, and aligning optics!
And the advantage is not just during the assembly of the experiment, but also later when the setup must perform reliably on the long term. The system is all fibered so presumably very robust and stable".
Andrea Bertoldi will leave the Institut d’Optique in Palaiseau to work in Bordeaux on the MIGA project, which aims to develop the first combined atom-laser antenna for gravitational wave detection.

We hope that in the southwest of France he will also have the time to enjoy the beautiful nature and the fine wines!

1 Radiative pressure (photon momentum) radiative pressure is a force applied on objects by light sources. When an atom absorbs a photon (with a defined precise wavelength), it is submitted to a small force with same direction than photon momentum.  In case of emission, the direction of emitted photon is random and the force is opposite to the direction of emitted photon.

Publications of Dr. Andrea Bertoldi

[1] Combined static potentials for confinement of neutral species, L. Ricci, D. Bassi and A. Bertoldi, Phys. Rev. A 76, 023428 (2007).
[2] Dynamics of a cold atom cloud in an anharmonic trap, A. Bertoldi and L. Ricci, Phys. Rev. A 81, 063415 (2010).
[3] Atom interferometry gravity-gradiometer for the determination of the Newtonian gravitational constant G, A. Bertoldi, G. Lamporesi, L. Cacciapuoti, M. de Angelis, M. Fattori, T. Petelski, A. Peters, M. Prevedelli, J. Stuhler and G. M. Tino, Eur. Phys. J. D 40, 271 (2006). 
[4] Determination of the Newtonian Gravitational Constant Using Atom Interferometry, G. Lamporesi, A. Bertoldi, L. Cacciapuoti, M. Prevedelli, and G. M. Tino, Phys. Rev. Lett. 100, 050801 (2008). 
[5] Feedback control of atomic coherent spin state, T. Vanderbruggen, R. Kohlhaas, A. Bertoldi, S. Bernon, A. Aspect, A. Landragin, P. Bouyer, arXiv:1207.3203.
[6] Heterodyne non-demolition measurements on cold atomic samples: towards the preparation of non-classical states for atom interferometry, S. Bernon, T. Vanderbruggen, R. Kohlhaas, A. Bertoldi, A. Landragin and P. Bouyer, New J. Phys. 13, 065021 (2011).
[7] Spin-squeezing and Dicke-state preparation by heterodyne measurement, T. Vanderbruggen, S. Bernon, A. Bertoldi, A. Landragin and P. Bouyer, Phys. Rev. A 83, 013821 (2011).