Scientists have recently managed to maintain quantum coherence in a molecular qubit for over one hundred nanoseconds at room temperature, hinting at potential breakthroughs in quantum computing.

Quantum computers could revolutionize information technology by changing the paradigm of computing. This is attributed to their basic units, called qubits, which can exist in any combination of states, unlike classical bits constrained to a definite value of 1 or 0. Due to this infinite variety of qubit states, a quantum computer should be able to easily handle computational problems that would take a conventional computer trillions of years to solve.

Scientists have successfully created qubits from particles such as photons, atoms, individual electrons, or even a superconducting loop. However, creating a qubit is one thing, building a working quantum computer out of thousands or even millions of qubits is an entirely different challenge, and attempts thus far have been fraught with substantial difficulties.

For a quantum computer to work, it is necessary to establish and manipulate subtle quantum interactions among multiple qubits — a state known as entanglement. However, for this to work, the qubits themselves need to remain stable or “coherent”, which means keeping it in a well-defined quantum state. The problem is, coherence is difficult to maintain as it easily crumbles when qubits interact with their surroundings — even radiation from space can throw them.

To solve this, a team of Japanese researchers led by Nobuhiro Yanai, associate professor at Kyushu University, has engineered a stable qubit using a special structure called a metal-organic framework. This structure involves combining pentacene molecules (made up of five connected benzene rings) with zirconium ions and organic dicarboxylate ligands. The pentacene molecules act like bridges, linking the ligands and ions together into a  framework made up of both organic molecules and metal ions–hence the name.

The role of the qubit was played by a pair of neighboring pentacene molecules, which were  coupled and exist within five different quantum states achieved by irradiating the metal-organic framework with various wavelengths of microwave radiation.

The metal-organic framework’s nanoscale voids offer the pentacene molecules a degree of freedom, but ultimately restricts their full movement under the radiation’s influence, ensuring they formed a desired quantum state and remained trapped in it for a significant amount of time.

“The metal-organic framework in this work is a unique system that can densely accumulate [pentacene molecules],” said Yanai in a press release. “Additionally, the nanopores inside the crystal enable [them] to rotate, but at a very restrained angle.”

The most important result of the study was that the team could maintain coherence for more than a hundred nanoseconds at room temperature, whereas previously this could only be achieved in similar systems at incredibly cold temperatures of about -200 degrees Celsius. At such temperatures, it was possible to maintain coherence only in photonic qubits, but in addition to needing such extreme conditions to operate, quantum computers using these photon qubits suffer from photon leakage.

Maintaining cryogenic temperatures is not only expensive but complicates the entire computing setup. Thus, creating a stable qubit that operates at room temperature is an impressive and practical achievement.

Looking ahead, the scientists are optimistic about extending coherence for even longer periods. They believe that by designing improved metal-organic frameworks and identifying more suitable molecules for qubits, they can push the boundaries further.

“It will be possible to generate quintet […] state qubits more efficiently in the future by searching for guest molecules that can induce more such suppressed motions and by developing suitable metal-organic framework structures,” concluded Yanai. “This can open doors to room-temperature molecular quantum computing.”

Reference: Akio Yamauchi et al, Room-temperature quantum coherence of entangled multiexcitons in a metal-organic framework, Science Advances (2024), DOI: 10.1126/sciadv.adi3147

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