ZJU researchers pioneer in constructing Schrödinger's cat state in artificial quantum system via chiral states

Recently, Dr. WANG Dawei and Prof. WANG Haohua from the Interdisciplinary Center for Quantum Information, in collaboration with several domestic and overseas teams, have successfully synthesized antisymmetric spin exchange interactions in an artificial quantum system and explored a novel approach to chiral spin clusters in superconducting circuits. Relevant findings are published in the January 21 issue of Nature Physics.

When it comes to quantum mechanics, Schrödinger’s cat will, without any slightest doubt, be touched upon. A cat, a flask of poison, and a radioactive source are placed in a sealed box. If an internal monitor (e.g. Geiger counter) detects radioactivity (i.e. a single atom decaying), the flask is shattered, releasing the poison, which kills the cat. The interpretation of quantum mechanics implies that after a while, the cat is simultaneously alive and dead. Yet, when one looks in the box, one sees the cat either alive or dead not both alive and dead. This poses the question of when exactly quantum superposition ends and reality collapses into one possibility or the other.

This revolution has something to do with the concept of “chirality”, signaling that an object and its image cannot be superposed. French scientist Pasteur discovered the chirality of the molecule when measuring variations in polarization after light passed through a solution. However, after the discovery of quantum mechanics, German theoretical physicist Hund proposed that since the interaction between the atoms of the constituent molecules did not break the parity, the stationary state of a molecule should be the quantum superposition of left-handed and right-handed molecules. This contradicts the existence of a large number of stable chiral molecules. On the contrary, the quantum superposition state of left-handed and right-handed molecules is extremely volatile, and it can be perceived as a Schrödinger’s cat state that is susceptible to environmental noises. This contradiction is also known as the Hund Paradox.

Here, “parity” can be simply construed as “left-right symmetry”. LI Zhengdao and YANG Zhenning discovered basic parity-breaking interactions—weak interactions in the nucleus. Some physicists have surmised that the presence of chiral molecules may be attributable to the effect of weak interactions on the ground state energy of the molecule. However, no experiment has been conducted to support this speculation. Synthesizing antisymmetric spin exchange interactions in an artificial quantum system can facilitate our understanding of the formation of chiral molecules and the decoherence principle of the chiral molecular superposition state.

In this study, WANG Dawei proposed the idea of synthesizing antisymmetric spin exchange interactions in superconducting qubits in an effort to delve into quantum superposition and quantum entanglement. Spin is one of the rudimentary properties of microscopic particles. There are two spin states of electrons. For a synthetic superconducting qubit, its lowest two states can be labeled as the two states of spin, corresponding to energy values of 0 and 1. These two values are also regarded as the binary digits of bits in quantum computing. There are two kinds of interactions between spins—symmetrical interactions after the exchange of spin positions, and antisymmetric interactions of the signs after exchanging spins. Symmetric spin exchange interactions have been implemented in artificial quantum systems while antisymmetric spin exchange, though playing a crucial role in topological magnetic excitation, inverse constant sub-Hall effect and quantum spin liquid, are difficult to synthesize in artificial systems.

To make progress in addressing these questions, it would be helpful to construct an artificial quantum system that breaks the parity symmetry and that can be prepared in a superposition of two chiral states. Researchers report the synthesis of the parity-breaking antisymmetric spin exchange interactions in all-to-all connected superconducting circuits, which allows them to show various chiral spin dynamics in up to five-spin clusters. They also demonstrate the entanglement of up to five qubits in Greenberger–Horne–Zeilinger states based on a three-spin chiral logic gate. 

These results are a step towards quantum simulation of magnetism with antisymmetric spin exchange interactions and quantum computation with chiral spin states.