Nondestructive microwave detection of a coherent quantum dynamics in cold atoms

In this study, published in the journal Communications Physics, Commun. Phys. 4, 35 (2021), we demonstrate a new technology for detecting cold atoms. By measuring the reflected power of a microwave signal sent to the atoms using a horn, we first determined the presence of a cloud of cold rubidium 87 atoms by measuring the atomic spectrum (medium response) around the hyperfine clock transition. Then, using two antennas, a horn and a monopole, we observed Rabi oscillations in stroboscopic mode. In this configuration, the reflected signal at the horn was used to detect the Rabi oscillations, and thus the collective spin state of the atoms, while the monopole antenna was used to drive the spin dynamics. With this measurement, we demonstrated the non-destructive nature of this method, as well as a detection bandwidth (around 30 kHz) far superior to that typically possible in cold atom experiments.

Spectrum of the microwave reflection coefficient in the absence (pink) and presence (orange) of a cold atom cloud.

Compact chip-scale guided cold atom gyrometers for inertial navigation : Enabling technologies and design study

A possible motivation for the development of compact cold atom gyroscopes on atom chips is the research in geophysics and the realization of General Relativity tests in space, such as the observation of the geodetic effect predicted by Leonard Schiff in 1960. Thus, in the paper AVS Quantum Sci. 1, 014702 (2019), published in the journal AVS Quantum Science, we gave a preliminary answer to the question: is it possible to design a compact cold guided-atoms gyroscope on a chip to reliably measure the geodetic effect using a satellite in a low earth orbit of 642 km altitude?

Diagram v - 2T (launch speed-interrogation time) showing the 5% accuracy frontier in the measurement of the geodetic effect with an on-chip cold atom gyrometer, for a fixed integration time of 4 months.

Pumping dynamics of single vacuum chamber cold atom experiments

This work focuses on the investigation of the different physical processes leading to the pressure dynamics in an ultra–high vacuum (UHV) system. The considered system was designed for a cold atom experiment that uses an atom chip as a platform for a compact quantum inertial sensor. To address the compactness requirement of a practical cold–atom–based sensor, the system is composed of a single vacuum chamber containing the atom source (⁸⁷Rb in this case) and a sputtered ion pump. Consequently, the pressure in the chamber has to vary from ∼10^-8 mbar (magneto-optical trap loading phase) down to ∼10^-11 mbar (sensor operation phase). The obtained results were published in the article Phys. Rev. Appl. 12, 014033 (2019).

Stability analysis of a magnetic waveguide with self-generated offset field

In this work we demonstrate that a stable magnetic guide can be realized in an atom chip without applying an external offset field. Such a magnetic guide can be realized using three parallel microwires, fed with modulated currents having a well defined phase difference between them. To demonstrate stable guiding, we found a steady periodic solution that allows us to linearize the nonlinear dynamics of both, the atomic motion and the atom magnetic moment. As detailed in our published article, Phys. Rev. A 97, 033405 (2018), the periodic nature of the system dynamics tells us that here we are concerned with orbital stability. To circumvent the difficulty in finding an analytical expression for the eigenvalues of the monodromy matrix of the system dynamics, we introduced a Lyapunov transformation of the system variables, from which we derived an equation of state for the system parameters of the modulated guide.

Assembled physics package

Atom chip fabrication

We manufacture our atom chips in our clean room facilities at Paris Observatory.

After defining the wire pattern for trapping and cooling, and the interferometry functions, we use metal evaporation deposition to make the micro wires.

By processing the images obtained with an electron microscope, we can determine the roughness of the fabricated micro-wires. Furthermore, we can also calculate, using the sample wall profile, the variation of the magnetic field generated by the wire wall defects as a function of the distance to the chip. In the case of a long spatial variation, i.e. a low spatial wave vector k, the fluctuations are too small to be observed which diminishes the effects of the rough magnetic field. Similarly, in the case of short spatial variations, i.e. a high wave vector, the effects of the field are compensated.

Experiment prototyping

The experience in itself and the atom chip will be designed taking into account the power consumption mobility constraints typical of embedded applications in inertial navigation and in situ geophysical studies. In this way, we can envision the realization of an atom interferometer capable of measuring simultaneously rotations/accelerations along the three inertial axes.

Interferometer magnetic guiding potential simulation

Sketch of the experimental setup

During the interferometer operation, a cloud of ultra-cold atoms ⁸⁷Rb will be coherently split in two, and these new ensembles will be constrained to propagate along a circular trajectory defined by a magnetic guide of a fews millimeters radius. At the output of the magnetic guide an atomic interference signal, sensitive to rotations via the Sagnac effect, will be measured. By virtue of its design, this experience offers the possibility of exploring quantum states engineering protocols (quantum non demolition measurements). We expect that such a strategy will allow, from one side, the realization of an inertial sensor for continuous measurements on the same guided atom cloud (working principle similar to that of optical fiber interferometers) and, from another side, the implementation of Quantum Metrology tasks.