Ultrafast quantum simulation of large-scale quantum entanglement

【Main points】
•    Magnetic materials were mimicked with a quantum simulator working on an ultrafast timescale. The system is based on an innovative technique that uses an ultrafast laser to manipulate artificial crystals of 30,000 atoms aligned in a cubic array with a spacing of 0.5 microns.
•    The research group applied their novel "ultrafast quantum computer" scheme, which was demonstrated last year, to quantum simulation.
•    This novel scheme improves the simulation speed by three orders of magnitude, thus solving the issue of external noise, which has been one of the biggest concerns for quantum simulation.
•    The research group succeeded in simulating the formation dynamics of "quantum entanglement", which is difficult to measure in actual magnetic materials, on a timescale of several hundred picoseconds.
 
【Outline】
   A research group led by Professor Kenji Ohmori at the Institute for Molecular Science, National Institutes of Natural Sciences are using an artificial crystal of 30,000 atoms aligned in a cubic array with a spacing of 0.5 micron, cooled to near absolute zero temperature (Fig.1). By manipulating the atoms with a special laser light that blinks for 10 picoseconds (pico = one trillionth of a second), they succeeded in executing quantum simulation of a model of magnetic materials (Fig.1). Their novel "ultrafast quantum computer" scheme demonstrated last year was applied to quantum simulation. Their achievement shows that their novel "ultrafast quantum simulator" is an epoch-making platform, as it can avoid the issue of external noise, one of the biggest concerns for quantum simulators^*1. The "ultrafast quantum simulator" is expected to contribute to the design of functional materials and the resolution of social problems.
 
The result was published online in Physical Review Letters of the American Physical Society on September 22, 2023.

1. Research background:
1-1. Quantum simulator ^*1:
   Quantum technology, which has seen intensified competition in development in recent years, such as quantum computers^*5, quantum simulators^*1, and quantum sensors^*6, is a qualitatively new technology that takes advantage of the "wave nature^*11" of electrons and atoms. Since quantum technology has the potential to revolutionize functional materials, pharmaceuticals, information security, artificial intelligence, etc., huge investments are being made around the world. A quantum simulator^*1 is a device that simulates the complex behavior of electrons and other microscopic particles in a solid by mapping them onto a highly controllable model material. It is expected to solve problems that would take an infinite amount of time even with the fastest supercomputer, thus bringing about disruptive innovation to solve social problems such as logistics and traffic congestion, and in developing superconductive and magnetic materials.
   On the other hand, quantum states created by quantum mechanical particles, such as electrons and atoms, are easily degraded by noise from the external environment and lasers, which makes it difficult to develop quantum computers. In 2022, a research group led by Professor Kenji Ohmori at the National Institutes of Natural Sciences realized an ultrafast two-qubit gate^*4 that operates in only 6.5 nanoseconds (nano = one billionth of a second) using cold atoms, improving the speed of the two-qubit gate^*4 by two orders of magnitude as compared to conventional cold-atom approach, thus paving the way for the realization of an ultrafast quantum computer that can ignore the effects of noise. If their ultrafast approach can be applied to quantum simulations, it is also expected to solve the issue of noise and to realize a highly reliable and innovative quantum simulator^*1.

2. Research results:
2-1. Summary of results:
   The research group performed ultrafast quantum simulations of a model of magnetic materials by preparing an atomic array of 30,000 atoms, cooled to near absolute zero^*2 (Figure 1) and manipulating them at high precision using a laser pulse that blinks for only 10 picoseconds (pico = one trillionth of a second). The ultrafast quantum simulator succeeded in simulating the formation of quantum entanglement^*9 (the topic of the Nobel Prize in Physics last year), which is a correlation unique to quantum mechanical particles, in 600 picoseconds, the fastest in the world. The ultrafast quantum simulator applies the novel “ultrafast quantum computer” scheme to a quantum simulator: it circumvents the Rydberg blockade effect with an ultrafast laser. Overcoming the noise issue and achieving high speed and accurate controls are the keys to reliable quantum simulation. The world's fastest quantum simulation realized by the group is three orders of magnitude faster than conventional simulators and is more than 1,000 times faster than noise, allowing the noise effects to be ignored.
   Quantum entanglement^*9, a peculiar correlation that appears in quantum mechanical particles such as atoms and electrons that constitute matter, is a concept essential for understanding the "quantum" world, while it is considered extremely difficult to measure in large-scale systems and real materials. This achievement, which simulates the formation of large-scale "quantum entanglement^*9" at an ultrafast timescale, is expected to contribute to the development of quantum technology by understanding "quantum entanglement," an essential resource for quantum computers and quantum networks, in future large-scale systems close to the practical level.
   In addition, quantum simulations of magnetic materials are expected to advance our understanding of the origin of physical properties of materials such as magnetism. It will also provide guidance for the design of next-generation devices and functional materials that exhibit dramatic functionality through the use of quantum mechanical effects.

2-2. Experimental method (Fig. 1,2): 
   The experiment was conducted using rubidium atoms^*12. First, 30,000 gaseous rubidium atoms were cooled to an ultralow temperature of fewer than 10 millionths of one Kelvin using laser cooling^*13. Then, an artificial crystal was prepared by arranging the atoms at a 0.5-micron spacing in a cubic array using an optical lattice*3. They then irradiated ultrashort laser pulses that blink for only 10 billionths of a second to excite electrons trapped in the 5S orbitals of atoms into giant 35D electron orbitals (Rydberg orbitals^*7) and observed what happens to the artificial crystal. The researchers observed the formation of “quantum entanglement^*9,” a correlation unique to quantum mechanical particles, on a timescale of a few hundred picoseconds (pico = one trillionth of a second) due to the strong interaction between the distant atoms.

3. Future development and social significance of this research:
   The ultrafast quantum simulation of magnetic materials achieved with the cold-atom platform was realized using the unique scheme developed by the same research group to manipulate an array of 30,000 atoms with an ultrafast laser. The research group has demonstrated that the ultrafast quantum simulator is a revolutionary platform.
   The innovative ultrafast quantum simulator developed by the research group is expected to be further upgraded in the future to elucidate the origin of physical properties of materials such as magnetism, to provide guidelines for designing quantum materials that exhibit dramatic functions (next-generation devices and functional materials that utilize quantum mechanical effects), and thus to bring innovation to material research. It is also expected to contribute to the development of quantum technology by understanding quantum entanglement, an indispensable resource for quantum computers and quantum networks, in a large-scale system close to the future practical level. Furthermore, it is expected to develop as a tool for solving social issues such as logistics, traffic congestion, and electric power transportation, which are difficult to solve even with supercomputers, by using quantum mechanical effects.

4. Terminology:
*1 Quantum simulator
A kind of quantum computer^*5 dedicated to the simulation of quantum many-body systems is referred to as a “quantum simulator”. Quantum mechanical particles such as atoms are assembled into an artificial quantum many-body system. It is then used to simulate and understand the properties of, for instance, an ensemble of many electrons interacting with each other in a solid, instead of having the properties calculated with a classical computer such as a supercomputer. 

*2 Absolute zero
“Absolute temperature” is a temperature scale in which zero degree is defined as the temperature at which all atoms and molecules stop moving. The unit is Kelvin. Zero Kelvin is called “absolute zero temperature” and is -273.15 degree Celsius, and zero degree Celsius is +273.15 Kelvin.
*3 Optical lattice
A periodic array of optical traps that use standing waves of light created by the interference of opposing laser beams to trap atoms at ultra-low temperatures. In this study, opposing laser beams from six directions are used to arrange 30,000 atoms into a three-dimensional square lattice with a spacing of 0.5 microns..

*4 Two-qubit gate
The two-qubit gate is the source of high performance of quantum computers. It is a logical operation on the quantum state of two qubits, so that they are entangled.

*5 Quantum computer
A computer that applies the properties of quantum superposition^*10 and entanglement^*9 to information processing. It performs information processing on a group of quantum particles, such as atoms, by manipulating their state (superposition of logical 0 and 1) and performing logical operations among multiple particles. By using the superposition and entanglement property of quantum systems, it is expected that calculations that would take an ordinary computer a very long time can be performed much faster.

*6 Quantum sensors
A device that measures physical quantities using the quantum mechanical properties of microscopic particles, such as atoms, electrons, and light. It is expected to enable measurement with higher sensitivity than conventional measuring devices.

*7 Rydberg orbitals 
An electron orbital extending far from the atomic nucleus. The distance from the nucleus to the Rydberg orbital reaches from nanometers to micrometers. Electrons moving in Rydberg orbitals are called Rydberg electrons, and atoms with Rydberg electrons are called Rydberg atoms.

*8 Rydberg blockade
A phenomenon in which, when electrons in two neighboring atoms are laser excited into Rydberg orbitals^*7, simultaneous excitation is strongly suppressed and only electrons in one of the atoms are excited into a Rydberg orbital^*7. This is due to the long-range interaction between Rydberg atoms.

*9 Quantum entanglement
A special correlation that can occur between two quantum systems when there is a quantum superposition^*10. The state of one qubit depends on the state ("0" or "1") of the other qubit, and it is no longer possible to determine the state of only one qubit without considering the states of the other qubit. In 2022, the Nobel Prize in Physics was awarded to three researchers for their work on the existence of "quantum entanglement" using photons.

*10 Quantum superposition
A property unique to quantum mechanics that several different states can be taken simultaneously. In a classical computer, a bit (the unit of information) is either in state 0 or in state 1 at a certain moment. The situation is much different in a quantum computer where a quantum object, such as an atom, can be in a superposition of two states: the atom being at the same time “in state 0 and in state 1”. Furthermore, there are many ways of superposing two states. Thinking of a quantum state as a wave, it becomes apparent that two waves can be superposed with their crest aligned (“state 0 plus state 1”) or with the crest of wave 1 aligned with the trough of wave 2 (“state 0 minus state 1”).

*11 Wave nature
Microscopic particles such as electrons and atoms have a wave nature that is not present in visible particles around us such as soccer balls. Unlike particles, waves can overlap and exist simultaneously over a large spatial area. Therefore, microscopic particles such as electrons and atoms can be in different states at the same time and exist in different places at the same time, which is a mysterious property not found in visible particles.

*12 Rubidium atom 
An alkali metal atom with atomic number 37. It has one electron in the 5th orbital (5s) around the nucleus.

*13 Laser cooling
Laser cooling is a technique that uses laser light to remove energy from atoms and thus decrease their temperature. When an atom absorbs the laser light, it receives the momentum of the laser photon and is subjected to a force in the direction of the laser light. If the atoms are traveling against the direction of the laser beam, the force gradually slows them down and lowers the energy of the atoms. This makes it possible to cool an atom down to about 1/100,000 of a Kelvin. 

5. Publication information:
JOURNAL: Physical Review Letters
ARTICLE TITLE: Picosecond-scale ultrafast many-body dynamics in an ultracold Rydberg-excited atomic Mott insulator
AUTHORS: V. Bharti, S. Sugawa, M. Mizoguchi, M. Kunimi, Y. Zhang, S. de Léséleuc, T. Tomita, T. Franz, M. Weidemüller, and K. Ohmori
ARTICLE PUBLICATION DATE: 22-Sep-2023 (online）
DOI：https://doi.org/10.1103/PhysRevLett.131.123201

6. Research Group:
Institute for Molecular Science, NATIONAL INSTITUTES OF NATURAL SCIENCES 
SOKENDAI (The Graduate University for Advanced Studies)
Universität Heidelberg, Germany
Shanxi University, China

7. Financial Supports:
This work was supported by the following fundings.

Moonshot R&D Program, JST (JPMJMS2269)
Quantum Technology Flagship Program Q-LEAP, MEXT of Japan (JPMXS0118069021)
Grant-in-Aid for Specially Promoted Research, JSPS　(16H06289)
Humboldt Research Award, Alexander von Humboldt Foundation and Heidelberg University
Grant-in-Aid for Scientific Research(B), JSPS (21H01021)
Grant-in-Aid for Early-Career Scientists, JSPS (20K14389)

8. Expert Contact:
Kenji Ohmori
Professor / Director
ohmori@ims.ac.jp

9. Media Contact:
Public Relations Manager
Institute for Molecular Science
press@ims.ac.jp