512 qubits! Significant breakthroughs have been made in quantum computing of neutral atoms
In the field of quantum computing, a core challenge faced by all quantum architectures is to maintain high fidelity control and low crosstalk of a single qubit while expanding the scale of the system. At present, the neutral atom array has become a promising quantum architecture, which can break through the current limitations on system scale, coherence, high fidelity state preparation and control.
In the neutral atom system, a single neutral atom is captured in the optical tweezers array, and the coherent interaction between atoms is generated by exciting them to the Rydberg state. The atomic array experiment has reached the system scale of hundreds of atoms, including the programmable quantum simulator of 256 atoms (qubits) by Mikhail Lukin’s team of Harvard University, which proves the potential of this platform.
However, these demonstrations are limited to arrays of single atomic elements, where the same properties of atoms make crosstalk-free control and non-destructive readout of a large number of atomic qubits challenging.
In a recent study, Hannes Bernien's team at the University of Chicago implemented a two-element atomic array that can independently control a single rubidium atom and cesium atom. The researchers used 512 optical tweezers to capture 256 rubidium and 256 cesium atoms, and observed that the crosstalk between the two elements was negligible.
A record 512 atomic system is achieved. The paper has been published in Physical Review x[1].
Dielemental atomic array
The researchers used a dual-wavelength optical tweezers array to load and capture individual atoms from laser-cooled rubidium and cesium atomic clouds. The experimental device is shown in Figure 1. They used an acoustooptic deflector (AOD) and a spatial light modulator (SLM) to produce optical tweezers at laser wavelengths of 811 nm (red) and 910 nm (dark red), respectively. The optional spatial filter in the AOD path can selectively shield the trap and generate the required geometry.
Then, AOD and SLM acquisition arrays are combined by a polarization beam splitter (PBS). These combined traps propagate along the shared beam path and are focused into the vacuum chamber by the high NA microscope objective, resulting in arbitrary array geometry. The same objective lens images the trap onto a charge-coupled device (CCD) camera to achieve feedback-based intensity homogenization. 780 nm (blue) atomic fluorescence of rubidium and 852 nm (yellow) atomic fluorescence of cesium are collected using the first objective lens, and reflected the electron multiplier CCD (EMCCD) camera along the shared beam path using a custom dichroic mirror. By separating the fluorescence wavelength before EMCCD and performing separate spatial filtering, the signal background ratio is improved.
Fig. 1 experimental device
As the first demonstration of the dielemental atomic array, the researchers interleaved the rubidium optical tweezers array with the cesium optical tweezers array to form a double crystal lattice with 512 positions, in which each rubidium atom is located at the center of four cesium atoms on the two-dimensional lattice. After loading the optical tweezers array from the two-element magneto-optical trap (MOT), the subsequent fluorescence images of rubidium and cesium atoms in the optical tweezers were taken respectively.
The average and single fluorescence images of the double lattice are shown in Fig. 2. Figures 2a and 2b show the average and single fluorescence images of rubidium (blue) and cesium (gold) atoms loaded at the same time. Figures 2C and 2D show average and single fluorescence images containing only cesium and rubidium atoms, respectively. Each atomic position has a spatial resolution, so a single atomic detection can be carried out for two elements.
Fig. 2 dual element 512 atomic array
In order to further prove the independent loading and control of rubidium and cesium atoms, various two-element arbitrary arrays were also constructed, as shown in Figure 3, including rubidium atom modified cesium hexagonal array, binary cellular lattice and two famous Chicago landmarks: hills building and bean (Cloud Gate).
Fig. 3 arbitrary geometry with two-element array
Lay the foundation for a larger scale quantum computer
Hannes bernien, the principal researcher of the project and assistant professor of Pritzker School of molecular engineering at the University of Chicago, has studied with Professor Ronald Hanson of Delft University of Technology in the Netherlands and Professor Mikhail Lukin of Harvard University. Bernien won the Young Scientist Award at the quantum 2020 conference jointly organized by the Chinese Physical Society and the University of science and technology of China.
Bernien said that in a hybrid array composed of atoms of two different elements, the nearest neighbor of any atom can be the atom of another element with completely different frequencies. This makes it easier for researchers to measure and manipulate individual atoms without any interference from surrounding atoms. It also allows researchers to avoid the standard complexity of atomic arrays: it is difficult to fix an atom in one place for a long time.
Bernien said: "When you do these experiments with a single atom, at some point, you will lose atoms, and then you must reinitialize your system. First, create a new cold atom cloud and wait for the single atom to be captured by the laser again. Our hybrid design can experiment with these elements separately. We can experiment with the atoms of one element and refresh other atoms at the same time, we always have qubits available, and so on. "
In summary, the hybrid array created by Bernien's team contains 512 atoms. As far as quantum computers are concerned, 512 qubits are enough: IBM's superconducting quantum computer has only 127 qubits at most. Although Bernien's device is not a quantum computer yet, quantum computers made of atomic arrays are easier to expand, which will bring some important new insights.
"In fact, we don't know what happens when you expand a very coherent system. This method of capturing atoms can be an excellent tool for exploring the quantum effects of large systems in unknown states," Bernien said
The hybrid nature of this atomic array also opens the door to many applications that are impossible for a single kind of atom. Because the two substances are independently controllable, the atoms of one element can be used as quantum memory, while the other element can be used for quantum computing, playing the roles of ram and CPU on the computer respectively.
Link：
[1]https://journals.aps.org/prx/abstract/10.1103/PhysRevX.12.011040
[2]https://phys.org/news/2022-03-elements-possibilities-hybrid-atomic-quantum.html
