In the Harvard–MIT setup, an array of individually controllable optical tweezers created by an acousto-optical deflector (AOD) is used to trap individual atoms, whose positions are captured by fluorescence imaging using a CCD camera. The position information is then used to selectively switch off unoccupied tweezers, and move the remaining, occupied sites tweezers to create a defect-free 1-D array of atoms. A separate team, from the University of Paris-Saclay, used a similar setup to create 2-D arrays of atoms. [Image: Courtesy of the researchers/MIT News Office] [Enlarge image]
In recent years, scientists have taken immense strides in manipulating individual atoms and photons. But deterministically building large ensembles of atoms—to serve, for example, as multi-qubit registers in quantum computing—remains a challenging proposition. Now, two separate research teams, one based in the United States and one in France, have used lasers to trap and manipulate more than 50 atoms, with individual control, into 1-D and 2-D patterns, creating what amount to defect-free crystals of atoms on the fly (Science, doi: 10.1126/science.aah3752, 10.1126/science.aah3778).
The teams achieved this feat, in essence, by using arrays of separate, individually controllable optical tweezers to capture scores of atoms from an ultracold cloud—and then taking pictures of those atoms, and using the resulting data on their positions to move and manipulate them further. OSA Member Vladan Vuletic of the Massachusetts Institute of Technology (MIT), one of the senior authors on the U.S. study, characterized the scheme as “like Legos of atoms that you build up, and you can decide where you want each block to be.”
Single-atom control, real-time feedback
While scaling up ion-trap arrangements has attracted a great deal of effort, and some success in small quantum-computing experiments, neutral atoms, because of their lack of electrostatic repulsion, have some advantages for building up ensembles of qubits. But that same lack of interaction means that it’s tougher to trap and manipulate atoms than ions.
To herd such neutral particles into an orderly ensemble, the U.S. research team—led by Vuletic of MIT and by Manuel Endres, Markus Greiner, and OSA Fellow Mikhail Lukin of Harvard University—began with an array of 100 tightly focused optical microtweezers, created by passing an 809-nm external-cavity diode laser through an acoustic optical deflector (AOD). Crucially, a multi-tone radio-frequency (RF) signal, with a specific RF tone for each microtweezer, drives the AOD and allows individual control of the microtweezers.
The researchers trained this controllable tweezer array onto a cloud of rubidium-87 atoms that had been laser cooled to near absolute zero, picking off and trapping individual ultracold atoms in a large share of the microtweezers. The team then used high-resolution fluorescence imaging to capture, with a CCD camera, information on which traps were occupied by a single atom and which were empty. The empty traps were turned off in real time, and the remaining traps were arranged into a regular, vacancy-free array, which the team documented with a second high-res fluorescence image. (The image above shows a view of the experimental setup.)
Using this combination of single-atom detection and real-time feedback and position control, the team found that it could trap and manipulate ensembles upwards of 50 atoms into defect-free 1-D arrays in less than 400 ms, and hold those arrays in place for several seconds. That’s long enough, Vuletic points out, to do hundreds of thousands of quantum operations.
Taking it to 2-D
The other team, from the Institut d’Optique Graduate School of the University of Paris-Saclay, France, used a very similar setup to trap an ensemble of atoms in a 2-D rather than a 1-D array. Using a spatial light modulator (SLM), the researchers, led by Thierry Lahaye and Antoine Browaeys, created 2-D squares of up to 100 optical traps, and controlled them by superimposing a second, AOD-controlled laser beam to allow deterministic rearrangement of the trap positions. As with the U.S. team, the French scientists used a CCD image to determine which traps were actually occupied by atoms. They were then able to use the “control knob” afforded by the AOD to rearrange those atoms into an orderly 2-D array.
The team found that it could swap the atoms into a 2-D pseudo-crystal in less than 50 ms after the initial image was captured. The scientists were even able to assemble a “gallery” of 2-D arrays built with the technique, including chains of atoms and square, triangular and kagome structures. And, according to the researchers, improving the algorithms used for real-time feedback and control could allow “scaling up our approach to hundreds of atoms.”
Maxwell’s demon lives!
Interestingly, both teams drew analogies between their atom-sorting setups and “Maxwell’s demon,” the microscopic imp in James Clerk Maxwell’s celebrated thought experiment who locally conquers entropy by selectively opening a tiny door between two chambers of gas, allowing only certain molecules to flow through. Through real-time feedback and rearrangement, the two groups were likewise able to overcome the entropy associated with the position of individual atoms within each array—creating, in essence, defect-free artificial crystals of atoms.
Both teams believe that the ability to create such arrays deterministically could open new horizons in quantum information processing using cold neutral atoms, as well as in basic quantum physics experiments and quantum engineering. There is even the possibility of leveraging the scheme to do “cold” chemistry—manipulating individually controlled atoms, one at a time, to create complex atomic species or molecules.