The electron’s spin is truly a perfect candidate for a quantum bit (qubit) – a basic unit of information in quantum computing. Many researchers are trying to find suitable qubits for specific applications. One of them is a research group led by Josep Orenstein at the Lawrence Berkeley National Laboratory (Berkeley Lab) has published a paper in Nature Physics, and their finding stressed utilising magnon wave packets to transport quantum information over a considerable amount of distance in antiferromagnetic materials. 

WHAT IS A MAGNON WAVE PACKET?

Magnon wave packets are collective excitations of electron spins in magnetic material. In other words, they are quantised spin waves that carry information about the spin state of electrons while propagating through the material. 

The nature of the spin waves arises from perturbations in electron spin that cause a ripple-like effect, influencing the spins of neighbouring electrons since electron spins are coupled to each other in magnetic materials. According to quantum mechanics, light or sound, spin waves can be quantized. Thus, a magnon is generally a quantized spin wave. Also, a magnon can be thought of as a quasi-particle representing the collective excitation of electron spins in a magnetic material.

The superposition of different magnon modes with the same frequencies and wave vectors form a magnon wave packet – a localised excitation of magnons that propagate through materials. This kind of wave packet can be influenced by several factors, such as laser pulses and microwave excitation.

HOW DOES A MAGNON WAVE PACKET CARRY QUANTUM INFORMATION?

As mentioned above, a magnon wave packet can reveal any information about the spin state of electrons. Electron spins and their orientation is the source of the magnetism in material. For example, antiferromagnetic materials show no net magnetisation and this results from the fact that the electron spins are oriented in alternating directions and cancel the magnetic fields. The type of magnetism arising from the material defines the speed and distance of the propagation of a magnon wave packer. In antiferromagnetic materials, those wave packets travel faster, and over longer distances than previously studied conventional approaches. By manipulating and detecting magnon wave packets, scientists can encode and process or transmit quantum information. 

INSIGHTS FROM THE STUDY

Orenstein's group utilised pairs of laser pulses to perturb the order of antiferromagnetism in one location of material while probing at another. They observed that magnon wave packets propagate in all directions in a similar way to how the ripples on the pond are created by dropped pebbles. The antiferromagnetic material is made of CrSBr (chromium sulfide bromide) and the observed speed of the magnon wave packet traveling through this material was faster than the predicted results. 

It was explained that the discrepancy between the propagation speed and the existing models can be related to the long-range interaction between the spins. Orenstein recalls that each spinning electron is like a tiny bar magnet and notes “If we imagine replacing the spheres by tiny bar magnets representing the spinning electrons, the picture changes completely. Furthermore, he concluded that instead of local interactions, each bar magnet couples to every other one throughout the entire system, through the long-range interaction that pulls a refrigerator magnet to the fridge door”. 

SUMMARY

The research work conducted at Berkeley lab has proven that the potential of magnon wave packets in antiferromagnetic materials is a potential candidate for quantum information transmission and storage. The capability of transmitting quantum information specifically in longer distances with faster speed is crucial for the development of quantum technology. 

Scientists are revealing breakthrough discoveries in the quantum world one by one, speeding up innovation and development; and we can expect to see more and more findings that will revolutionize today’s technology.

The full published article: 

Sun, Y., Meng, F., Lee, C. et al. Dipolar spin wave packet transport in a van der Waals antiferromagnet. Nat. Phys. (2024). https://doi.org/10.1038/s41567-024-02387-2

Source: 

https://www.earth.com/news/transmitting-quantum-information-using-electron-spin/