Periodic Reporting for period 1 - QUESCA (Quantum Enhanced Sensing with Cold Atoms)
Periodo di rendicontazione: 2017-08-01 al 2019-07-31
The overarching goal of the project is to develop a robust technique for measuring the number of atoms in a minimally disturbing way, without increasing the temperature of the atomic cloud and without destroying quantum correlations. This measurement will be used to prepare an ultra-cold cloud with atom number uncertainty significantly reduced compared to the typical precision that can be achieved so far. Achieving a resolution better than the atom shot noise with such a minimally-perturbing measurement will lead to the creation of non-classical atomic states, with properties that offer quantum-enhanced sensitivity.
A wide range of instruments that play an important role in the modern civilization can profit from developments in the field of cold atoms. Time-keeping atomic clocks, inertial sensors for navigation or seismography and gravitometers for geophysics exploration are applications that will benefit from sensitivity enhancement of cold-atom sensors. In addition, similar improvements may lead to more precise experiments for the study of quantum systems, advancing our understanding of the universe.
The research was performed on Rb atoms, confined in a magnetic trap with a rotating magnetic field. Standard cooling and trapping techniques were used to load at a few microKelvin temperature approximately 2x10^5 atoms in the trap, with shot to shot atom number fluctuations on the order of 15%. Linearly polarized light, far detuned from the optical resonance, was used to probe the atom number in the ensemble. The setup was developed with care to diminish fluctuations and drifts in the probe beam path. The shot to shot fluctuation of the trap position with respect to the light beam was found to be on the order of a few micrometers. To compensate for this, the probe light beam diameter was expanded to be significant larger than the ensemble size so that all the atoms experienced effectively homogeneous probe illumination. A pinhole, introduced in the plane where the atoms were imaged, collected the part of the beam that had interacted with the atoms. The pinhole diameter was chosen according to the size of the atomic ensemble and the shot to shot variation in its position, so that fluctuations in position result in measurement imprecision below the shot noise of atoms. The polarization orientation of probe light was measured with balanced polarimetry, which is robust against technical probe noise. Additional technical noise mitigation was provided by the signal following synchronously the rotating magnetic field of the trap, and thus appearing at a specified frequency away from the low frequency spectrum.
The imprecision of the atom number estimator was limited by the photon shot noise of probe light and the stochastic loss of atoms due to the unavoidable absorption of light that exists in every light-matter interaction. The efficiency of the technique was characterized by evaluating the shot to shot atom number variation after performing a measurement that estimated the atomic population using the Faraday paramagnetic effect. With this scheme, the atom number imprecision was reduced by a factor of 40 to approximately 0.8% for ensembles of 5x10^4 atoms, without any measurable heating. This equates to an x40 improvement in the knowledge of the atomic population for the prepared cold ensemble, which was directly mapped to the precision of the final measurement.
In addition, upgrades in the setup realized during the course of the project led to the development of a waveguide and accelerator ring for neutral atoms. We demonstrated the first hypersonic transport of coherent matter-waves over macroscopic distances without measurable effect on the quantum state. This work was published in the scientific journal “Nature”.
The method developed in the project can be an improvement over the state of the art due to the simplicity of the system and the robustness against technical noise. The proposed technique can be implemented with readily available components, while the detection scheme of balanced polarimetry and sinusoidally modulated signal offer an advantage for suppressing experimental noise. The AC signal also allows for the introduction of stroboscopic probing, which can further improve the precision.
The research performed aspires to popularize the use of the Faraday paramagnetic interaction as a precise technology for sample preparation that can be integrated in practical devices with cold atoms. Establishing a technique with the ability to prepare an ensemble with small uncertainty in the atom number will enhance the sensitivity of cold atom sensors. Furthermore, the minimally-disruptive, correlation preserving character of the measurement approach can be used to realize non-classical states and paves the way for quantum technologies based on cold atoms. By contributing advances in the field of cold-atoms, the research performed in the project can ultimately find applications in both practical life and in fundamental science.