Non-invasive method to measure atomic density opens new window into quantum world

New Delhi, January 8: Scientists have developed a new non-invasive technique that enables real-time, local density measurements of cold atoms without significantly disturbing their quantum state — a breakthrough that could accelerate advances in quantum computing and quantum sensing.

The technique has been demonstrated by researchers at the Raman Research Institute (RRI), an autonomous institute under the Department of Science and Technology. The method allows scientists to probe how densely atoms are packed at microscopic scales, a critical requirement for emerging quantum technologies.

Cold atom experiments rely on laser cooling and trapping techniques to reduce atomic motion to near absolute zero, where quantum properties become prominent. Such atoms serve as key resources for quantum computers, precision sensors and simulators. However, existing detection techniques — such as absorption and fluorescence imaging — have major limitations. Absorption imaging struggles with dense atomic clouds, while fluorescence imaging requires long exposure times and often disrupts or destroys the atomic state being measured.

To overcome these challenges, the RRI team developed Raman Driven Spin Noise Spectroscopy (RDSNS), a technique that combines spin noise spectroscopy with coherently driven Raman transitions between atomic spin states. Spin noise spectroscopy detects the natural fluctuations of atomic spins by measuring tiny polarisation changes in laser light passing through the atomic sample, while the Raman beams dramatically amplify the signal.

According to the researchers, the Raman beams enhance the detection signal by nearly a million times. By tightly focusing the probe laser to just 38 micrometres, the technique achieves a probing volume of only 0.01 cubic millimetres, enabling measurements on a tiny region containing around 10,000 atoms. Crucially, the signal directly reveals the local atomic density, rather than just the total number of atoms.

The team demonstrated the method on potassium atoms held in a magneto-optical trap. Using RDSNS, they observed that the central density of the atomic cloud saturated within one second, while the total atom number measured through fluorescence imaging took nearly twice as long to stabilise. This contrast highlights a key advantage of RDSNS — while fluorescence provides a global count, the new technique reveals how tightly atoms are packed locally.

“The technique is non-invasive, as the probe is far-detuned and operates at low power, allowing even microsecond-scale measurements with accuracy within a few percent,” said Bernadette Varsha FJ and Bhagyashri Deepak Bidwai, research assistants at the QuMIX laboratory at RRI.

Sayari, a doctoral researcher at RRI and lead author of the study, said real-time, non-destructive imaging methods such as RDSNS are strong candidates for quantum sensing and computing. “It uncovers many-body dynamics by capturing transient microscopic density fluctuations and can help benchmark theoretical models using spatially resolved data,” she said.

To validate the technique, researchers compared RDSNS-derived density profiles with results obtained using the inverse Abel transform applied to fluorescence images. The results showed strong agreement. Unlike the Abel transform, which assumes axial symmetry, RDSNS works reliably even for asymmetric or dynamically evolving atomic clouds.

The researchers noted that fast, precise and non-invasive density measurements are crucial for quantum technologies such as gravimeters, magnetometers and other atom-based sensors, where performance depends sensitively on accurate knowledge of atomic density. By enabling micron-scale probing without disturbing the system, RDSNS opens new avenues to study phenomena such as density wave propagation and quantum transport.

“We expect this technique to find wide applications in real-time diagnostics of cold atom experiments, particularly for neutral-atom quantum computing, quantum simulations and studies of non-equilibrium dynamics,” said Saptarishi Chaudhuri, who leads the Quantum Mixtures (QuMIX) laboratory at RRI.

Supported under India’s National Quantum Mission, the work positions RRI at the forefront of precision measurement in quantum research, demonstrating that progress in quantum science can come from gentler, smarter ways of observing the quantum world.