Chinese University of Science and Technology makes progress in molecular quantum entanglement research

[ Instrument R & D of Instrumentation Network ] The Key Laboratory of Microscopic Magnetic Resonance of Chinese Academy of Sciences, led by Du Jiangfeng, a member of Chinese Academy of Sciences and Professor of Chinese University of Science and Technology, cooperated with the National Institute of Standards and Technology to realize the joint control of charged atoms and charged molecules in the ion trap system. The quantum entanglement state between single atom and single molecule is analyzed, and the quantitative entanglement method is used to determine that the generated quantum entanglement exceeds the critical threshold. This achievement has an important impetus for considering the use of molecules for quantum information processing in the future.

Physicist Dr. Qudsia Quraishi of the Bureau of Sensors and Electronic Equipment of the US Army Research Laboratory pointed out that the next-generation precision sensing system involves quantum sensors. Quantum sensors are based on laser-cooled atoms and are likely to greatly improve system performance. Laser-cooled atoms are small coherent gas atoms that can measure changes in gravitational or magnetic fields. They are not only very accurate, but also very sensitive.

There are currently a variety of systems that can be used to explore quantum sensing and quantum information processing. Among them, molecules as a system of multiple atoms, atomic groups can rotate and vibrate, which brings unique properties. For example, the rotation of an analog gyro and the vibration of a small ball connected by a spring. The molecules can have different rotational angular velocities and angles and vibration modes. These classic physical quantities can be quantized to form quantum states. Therefore, molecules can be in different quantum states with a large energy span, and the frequency corresponding to the energy difference between the states can reach from nearly zero to the optical frequency of hundreds of THz (million billion times per second), so the molecule can be used as a medium, It is used to match and communicate different quantum systems with very different frequencies to realize a composite quantum system and information processing platform.

In addition, polar molecules can produce long-range interactions, which is conducive to the realization of a new type of quantum information processing platform; polar molecules are very sensitive to electric fields and can interact with microwave photonic systems, cantilever beam oscillators and other systems. In order to connect a single molecule to other quantum carriers to transfer quantum information, demonstrating quantum entanglement is an important step. When the two particles are in an entangled state, it is no longer possible to describe the state of each particle individually. The two particles form a tight whole. Such correlation properties have important applications in quantum computing and some quantum precision measurements.

The positive and negative charge centers in the molecule do not coincide. From the perspective of the entire molecule, the distribution of charge is uneven and asymmetric. Such a molecule is a polar molecule, and a diatomic molecule bound by a polar bond must be a polar molecule. Polyatomic molecules bonded by polar bonds, such as CH4, are not polar molecules.

Recently, there has been considerable development of molecular research at home and abroad, and a series of breakthroughs have been made in information processing, including the binding and cooling of molecules, the high-quality reading of molecular quantum information, the exploration of quantum entanglement between a large number of molecules, and the high-precision Molecular spectrometry, etc. Research on the scale of single molecules bound in a vacuum has also developed rapidly, with progress in generating single molecules from two bound atoms and the interaction between single molecules and single atoms.

In this work, by binding charged calcium atoms and calcium hydride molecules in the ion trap system, the entangled state between them was prepared using laser control. This state is very peculiar: in simple terms, when the electron orbital state of calcium ions is in the ground state, the rotation of the molecule is also a whole state of the low-rotation energy state (described by the rotating quantum number); at the same time, it can "superimpose" very different Another kind of whole state-the former is in the excited state of the orbit, and the corresponding molecule is in a state of high rotational energy. The opposite excitation pairing can also be prepared. Here the ground state and excited state can store quantum information, similar to binary "0" and "1", also known as qubits. In order to show the frequency span of the molecular state, a pair of rotation dynamics close to the rotational energy was selected as bits in the experiment, with a frequency interval of 13.4 kHz (about 10,000 times per second) and an interval of 855 GHz (nearly trillion times per second). , Using laser pulses to quantitatively demonstrate entanglement with atoms.

Ion traps are roughly divided into three types: three-dimensional ion traps (3D Ion Trap), linear ion traps (Linear Ion Trap), and orbital ion traps (Orbitrap).

In addition to the orbital ion trap, the ion trap uses electromagnetic fields to confine the ions to a specific space. By changing the parameters of the electric field, the specific ions are brought into an unstable state, which eventually causes the ions to be ejected from the reserved holes or narrow slits to reach the detector . In addition to the orbital ion trap, the ion trap mass selector, due to its unique space-limiting characteristics, makes it occupy an important position in multi-stage mass spectrometry (MS-MS).

The laser control technology used here includes lasers of multiple wavelengths (colors), including ultraviolet and multiple infrared bands, which are used to match the corresponding atomic and molecular spectral lines to realize the processes of ion cooling, detection, and quantum state control. Several important technologies developed in recent years are combined here, including the use of electrical interactions between charged atoms and molecules to achieve information transfer, which can be used to indirectly read out information without losing molecules; using infrared lasers to achieve molecular transfer Dynamic high-precision control and other technologies. In the experiment, the researchers first initialized atoms and molecules to a certain low-energy state (ground state), and cooled their movements to near quantum limits.

Then the laser is used to prepare a single molecule rotation dimension, the superposition of the rotation high and low energy (can be understood as high and low speed), and then through a series of complex laser pulse sequences, such as the composition of the molecule with high rotation energy triggers the atom to be excited to high The energy state (excited state) produces the required quantum correlation-the entangled state. Finally, by observing the coordinated state association of atoms and molecules in different situations, all information can be integrated into a value ranging from 0 to 1, and a threshold exceeding 0.5 indicates the occurrence of entangled states. The value measured in the experiment is far above this threshold within the error range, indicating the generation of entangled states.

Source: Encyclopedia, University of Science and Technology of China