Observing quantum systems can alter their state. Moreover, the more particles are combined into a system, the harder it becomes to measure them without distorting the results. A team of physicists has developed a method that allows for tracking a large quantum system for over a day without interfering with it.

Features of Quantum Systems

Quantum effects are easily disrupted by external noise generated by the surrounding environment. To minimize interference, researchers typically use small, well-controlled systems, such as pairs of atoms, which are carefully isolated—cooled and placed in dark, quiet conditions. However, larger systems are more promising for practical applications, even though it is more challenging to preserve quantum properties in them due to significant noise.

A New Approach to Observation

Researchers from Johns Hopkins University have developed a method that significantly simplifies the study of quantum phenomena. Their system uses macroscopic ensembles of spins and enables tracking their evolution over time, as well as directly observing spin fluctuations. Importantly, the quantum effects in the system are not destroyed. The sensitivity of the method is close to the fundamental limit set by the laws of quantum mechanics. The results have been published in the journal Nature Physics.

Description of the Experimental Setup

The setup is based on a superconducting circuit, with data read out using a SQUID—a highly sensitive magnetic field detector. Fluorine-19 nuclei in Teflon and hydrogen nuclei in nylon were placed inside the receiving coil, which was connected to the detector. The entire system was cooled to temperatures below one kelvin. This arrangement allowed researchers to obtain information about the spins through magnetic resonance data without external excitation.

Operating Principle and Results

Natural thermal fluctuations in the superconducting microcircuit served as the only source of motion. The magnetic field was tuned so that the spin resonance frequency matched the circuit’s resonant frequency. This made it possible to monitor changes in the properties of the superconducting circuit and, consequently, the behavior of the spins.

The measured fluctuations in spin angles matched theoretical predictions and scaled according to the number of spins and their polarization. This confirms that the observed noise was indeed due to quantum spin projection, not external interference. Continuous observation lasted for 26 hours.

Significance and Prospects

The achieved measurement accuracy is comparable to the quantum limit for solid samples containing nearly five sextillion (5×10²¹) spins. The smallest measured fluctuation angle was nine nanoradians, which is extremely precise.

The developed system can be used for non-invasive magnetic resonance spectroscopy, allowing researchers to study materials without altering their properties during measurement. This is especially important when working with explosive or highly sensitive substances.