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| Detecting Magnetic Fields Using Single Atom Arrays |
A previously unheard-of magnetometer with unprecedented spatial resolution has been developed using a sequence of atoms initially designed for quantum memory by researchers.
In addition to being suitable options for quantum memories and quantum computers, two-dimensional arrays of ultra-cold atoms can be used as sensors to identify other phenomena such as magnetic fields. Demonstrating better performance in terms of spatial resolution than classical devices, an array of atoms magnetometer was recently showcased for the first time by a team from the Technical University of Darmstadt in Germany. Team leader Gerhard Birkl announced the findings at the Atomtronics Conference held last month in Spain.
The aim of the emerging discipline of atomtronics in physics is to utilize atoms akin to how electrons are used in traditional electronics. While the term is often used to describe circuits where atoms move along paths determined by lasers, it frequently denotes various technologies involving the manipulation of atoms using light.
This new study actually employs an experimental setup based on laser cooling and laser trapping. Birkl states, "We can use light to cool single rubidium atoms and arrange them into two-dimensional arrays." Unlike other quantum technologies, these methods, honored with the 1997 Nobel Prize in Physics and widely used, do not require sophisticated cryogenic infrastructure.
Birkl and his colleagues trap rubidium atoms into a square-shaped array in their laboratory, with a width of 0.2 mm. They can manipulate each atom using an additional laser serving as an optical tweezer. "We can create any atom pattern we desire," says Birkl. So far, they have been able to arrange approximately 1300 atoms in their arrays with a spacing of a few microns. Birkl claims the concept could be scaled up to a million or more atoms.
Since each atom in this system can be in one of two internal states corresponding to the two potential states of a qubit, it can be used as a quantum memory.
By exciting atoms to Rydberg states that allow interaction between them, the system can also perform quantum computations. However, Birkl and his team explored a different avenue in their showcased work in Spain: quantum sensing. The plan is to subject the system to a test magnetic field that fluctuates spatially but remains constant over time. Since each atom in the array acts as an independent sensor, the system can investigate these variations. "It's like having a CCD camera for magnetic fields," says Birkl. The pixel size of this camera is 7 µm, corresponding to the separation between atoms in the array.
The researchers began operating the device with all atoms in the ground state. Then, a magnetic field was applied. The two energy levels of the atoms' ground state split into 12 different levels due to the presence of this external field. The degree of energy splitting depends on the intensity of the magnetic field and can be explored with high-resolution spectroscopy akin to atomic clocks.
With this method, the group could measure field changes with a sensitivity of 100 nanotesla and a resolution of micrometers. According to their estimates, the smallest observable field is approximately equal to the Earth's magnetic field or 25 microtesla per second of measurement time.
Although Birkl notes that the plan is still in its early stages of development, this sensitivity is relatively low. Researchers are optimistic that by fine-tuning the settings of the experiment, they can increase the sensitivity to 1 picotesla per second of measurement time. A device with such optimization could be used to scan magnetic fields surrounding some high-temperature superconductors with local asymmetries.
However, there are other potential applications besides magnetometry. Birkl continues, "We can expand our sensor-grid platform to investigate radio frequency waves, electric fields, and possibly gravitational fields." Applications requiring high precision and spatial resolution, including biology and materials science, could benefit from this. Cold atom expert Donatella Cassettari from the University of St Andrews in the UK, not involved in the study, says, "this experiment represents a significant advancement in quantum sensors." "While atomic state manipulation relies on highly complex methods, it has the potential to become a reliable and widely applicable sensor."