This is one of the necessary steps in creating a domestic quantum computer.
Single atoms can act as qubits, i.e. elements for storing and transmitting information in quantum computers. These computers are believed to employ accelerated machine learning methods, calculate the behavior of multicomponent systems thus enabling new materials creation, test medications at the molecular level and quickly decode modern data encryption systems.
Trapping a single atom in an optical tweezers or, as it is also called, a dipole trap is the first step to creating an array of qubits and conducting quantum calculations. The array contains many atoms, with each one being held by "its own" optical tweezers. Accordingly, the task is not only to capture atoms, but also to register them correctly.
The electronic states of cold atoms can exist for a few seconds, which is quite a long time in the context of quantum computing; therefore, such atoms are convenient to be used as qubits. About 20 research groups in the world are working with single cold atoms, with two of them in Russia: at Rzhanov Institute of Semiconductor Physics SB RAS and at Lomonosov Moscow State University.
"We solved a complex problem consisting of several tasks: first, we needed to cool the atoms (that is, reduce their speed), this is done using laser beams: the flow of photons from the laser is absorbed by the atoms and slows them down. Secondly, a single atom must be trapped. This is also performed by laser beam with a very precise focus of several microns: it is the typical size of the spot for holding an atom. And thirdly, we needed to photograph an atom: in a short time of a hundred milliseconds we "registered" infrared photons which the atom emits while trapped, about 1000 photons per second (this is a very small amount and a household video camera will not register them). The conditions of our experiment require that the captured atoms are registered in a short time. Thus they can be used as qubits," explains Ilya Beterov, Senior Researcher at Rzhanov Institute of Semiconductor Physics SB RAS, Associate Professor of the Optical Information Technologies Department, NSTU NETI.
Foreign research groups use highly sensitive scientific EMCCD video cameras with electronic multiplication for registering, but they are expensive.They cost about five million rubles and have not been delivered to Russia since 2015. Novosibirsk physicists worked with a scientific sCMOS video camera of the previous generation which is significantly cheaper (it cost about six hundred thousand rubles).
The scientists were able to achieve impressive results: they reliably registered an atom with a minimum exposure time of 50 milliseconds. This is typical of experiments conducted by researchers in France, Germany, Korea, and other countries using more advanced EMCCD cameras. In recent experiments, the longest time during which Novosibirsk scientists observed a single atom was 40 seconds.
"We had to place the lens of the optical tweezers as far away from the cloud of cold atoms as possible, so that they did not interact with the glass, the dielectric surface. This process can negatively influence the further two-qubit quantum operations experiments. This is why we used a long-focus lens, but as a result, it was more difficult to register the photons emitted by the atom as a smaller number of them gets into the lens when it is far from the atom. In addition, a single atom's emission is rather low, so all of it had to be focused on a single pixel of the camera's matrix. However, it later turned out that if we just try to register a single atom, we see almost nothing against the background of the video camera noise, because the tweezers laser removes the atoms from resonance with the illuminating radiation. In order to solve this problem, we turned off the dipole trap for a very short time, no more than one millionth of a second. During this time a single atom does not have time to leave the trap. We repeated this for several thousand cycles, accumulating the signal during the time when the dipole laser is turned off," adds Ilya Beterov.
According to the researcher's observations, the work of the Novosibirsk team is the first to implement the simultaneous use of a long-focus lens and a sCMOS video camera, and the result may be of interest not only to Russian physicists.
"Foreign research groups can also be underfinanced, and this is important for everyone that a significantly cheaper video camera shows an acceptable result for the experiment," says Ilya Beterov.
The next step of Novosibirsk scientists is to learn how to perform high precision one-cubit operations and turn to two-cubit operations. That is, to put it simply, to "collect" the logical elements of a quantum computer of cold atoms changing the electronic states of the atom and controlling them.
The research is supported by the Russian Science Foundation (Project No. 18-12-00313), as well as by Russian Foundation for Advanced Research Projects.
For the reference: since 2018, Rzhanov Institute of Semiconductor Physics SB RAS has been a member of the scientific consortium whose work is aimed at the development of quantum technologies and in particular at the creation of a domestic quantum computer. The consortium was based at Lomonosov Moscow State University and includes leading universities and research institutes, such as Saint Petersburg State University, Bauman Moscow State Technical University, National Research Nuclear University "MEPhI", the Institute of Solid State Physics RAS, Valiev Institute of Physics and Technology RAS, Prokhorov General Physics Institute RAS, and others.
Based on the materials of Rzhanov Institute of Semiconductor Physics SB RAS public affairs service
Illustration:
1. The experimental setup for trapping and registering a single rubidium atoms
2. Photo of a single atom trapped in optical tweezers