Scientists have developed a technique to measure the local density of ultra-cold atoms in real time without significantly disturbing them, a step that could aid work on quantum computers and quantum sensors, a government statement said on Thursday.
Researchers at the Raman Research Institute, an autonomous institute under the Department of Science and Technology, demonstrated a method called Raman Driven Spin Noise Spectroscopy, or RDSNS, that is designed to overcome limits in commonly used imaging techniques for cold atom experiments.
Cold atoms, created by cooling and trapping atoms with lasers to temperatures close to absolute zero, are widely used in research because quantum behaviour becomes easier to observe. However, the tools used to detect their quantum state can alter the atoms during measurement, the statement said.
Absorption imaging can struggle with dense atomic clouds because the probe beam may not penetrate far enough to give an accurate density profile. Fluorescence imaging often needs longer exposure times to collect scattered photons and can also be destructive, the statement added.
RDSNS combines spin noise spectroscopy, which detects natural fluctuations in atomic spins by measuring polarisation changes in a laser passing through an atomic sample, with two additional laser beams that drive atoms between neighbouring spin states. The statement said the approach boosts the measured signal by nearly a million times.
The researchers said the probe can be focused to 38 micrometres, allowing a small measurement volume of 0.01 cubic millimetres that targets about 10,000 atoms. They said the resulting signal provides a direct measure of local density rather than only a total atom count.
Using potassium atoms held in a magneto-optical trap, the team observed that the central density of the atomic cloud saturated within one second, while the total number of atoms, measured using fluorescence, took almost twice as long.
The researchers said the result highlights a difference between the methods, with fluorescence reflecting global atom counts while RDSNS indicates how tightly atoms are packed in a specific region.
“The technique is non-invasive, as the probe is far-detuned and operates at low power, allowing even microsecond-scale measurements to achieve accuracy within a few percent,” research assistants Bernadette Varsha FJ and Bhagyashri Deepak Bidwai said, according to the statement.
The team validated the technique by comparing local density profiles with results from fluorescence images processed using an inverse Abel transform, which typically requires axial symmetry. The statement said RDSNS remained effective even when atomic clouds were asymmetric or changing over time.
The researchers said fast and non-invasive density measurements could be useful for quantum technologies such as gravimeters and magnetometers, where knowing atom density precisely can affect performance. They added that the method could help study effects including density wave propagation and quantum transport.
The work was supported under India’s National Quantum Mission, the statement said.
