Scientists have integrated solid-state spin qubits with nanomechanical resonators

Spin qubits (orange) within diamond nanopillars move (black arrows) on magnetically functionalized mechanical resonators (blue), enabling mechanically mediated spin-spin interactions. Image courtesy of Frankie Fung.

In a new study in Physical Review Letters, scientists propose a new approach to combine solid-state spin qubits with nanomechanical resonators to enable scalable and programmable quantum systems.

Quantum information processing requires qubits with long coherence times, stability, and scalability. Solid-state spin qubits are candidates for these applications because of their long coherence times. However, they are not scalable.

The PRL study, led by Frankie Fung, a graduate student in the group of Professor Mikhail Lukin at Harvard University, addressed this challenge in a conversation with Phys.org.

"While small quantum registers using solid-state spin qubits have been demonstrated, they rely on magnetic dipole interactions, which limits the interaction range to tens of nanometers," he said. The short interaction distances and the difficulty of consistently fabricating spin qubits at such close spacing make it challenging to control systems containing large arrays of qubits.

In the PRL study, the researchers proposed an architecture that uses nanomechanical resonators (mechanical oscillators) to mediate interactions between spin qubits.

As a diamond for qubits

The team's approach relies on nitrogen vacancy centers in diamonds as qubits.

Usually, a diamond structure consists of carbon atoms in a tetrahedral structure, which means they are bonded with four other carbon atoms.

However, using methods such as chemical vapor deposition, one of the carbon atoms can be replaced by nitrogen atoms. This results in the absence of carbon atoms adjacent to nitrogen, resulting in vacancy.

The nitrogen atom adjacent to the vacancy forms an NV center with unpaired electrons whose spin state is used as a qubit.

NV centers have a number of advantages due to their unique optical properties. They have a long coherence time, which means they have a low interaction with their environment, making them very stable.

In addition, they are optically compatible, which means that optical input and output information is easy to use. Since they have unpaired electrons, they are also highly sensitive to magnetic fields.

These properties make them ideal for use as qubits, especially when integrating them with solid-state devices.

This problem arises due to the short-range interaction between the qubits themselves. This is because solid-state spin qubits interact with each other through magnetic dipole interactions, which are short-range.

The interaction between qubits is necessary to create entangled states, which are the basis of quantum information processing.

Mechanical resonator as a medium

To account for the long-range interaction of qubits, the researchers propose to couple the NV center in the diamond with a mechanical resonator.

"Our research aims to use nanomechanical resonators to mediate the interaction between these spin qubits. More specifically, we propose a new architecture in which spin qubits within the tip of a single scanning probe can move across a nanomechanical resonator that mediates spin-spin interactions," Fung explained.

Nanomechanical resonators are tiny structures that can oscillate at high frequencies, usually in the nanometer range. They are sensitive to external fields and forces.

By coupling qubits with nanomechanical resonators, researchers are creating a way for non-local qubits to interact. This has the potential to make it possible to create large-scale quantum processors, thus addressing the shortcomings of the scalability of solid-state quantum systems.

Optimize the architecture

As a result, the research team's architecture consists of spin qubits within the tips of individual scanning probes, which are precise scanning devices that can collect information.

"The scanning probe tip can be moved over a mechanical resonator that mediates spin-spin interactions. Since we can choose which qubits to move on this mechanical resonator, we can create programmable connections between spin qubits," Fung explained.

A single qubit is the NV center within a diamond nanopillar. This structure brings the center of the NV closer to the micromagnet, which creates a magnetic field for manipulating the spin state of the electron.

"The nanopillars act as waveguides, reducing the laser power required to excite the NV center, which also helps," Fang added. This happens because the nanopillar directs the laser light to the exact location where it needs to go, which is the NV center.

The micromagnet is located on the silicon nitride nanobeam, completing the nanomechanical resonator.

Theoretically, the setup works as follows. Micromagnets create magnetic fields around qubits and resonators. This magnetic field changes the electron spin state of the qubit.

The change in spin state causes the qubits to interact with the nanomechanical resonator differently than before, causing it to oscillate at different frequencies. This oscillation affects other qubits, which affects their spin state.

The architecture allows non-native qubit interactions.

Architectural feasibility and hybrid quantum systems

To prove that their architecture is achievable, the researchers demonstrated the coherence of qubits for the mechanical transport of micromagnets.

"As a proof-of-principle measurement, we stored some coherent information in the NV center, moved it in a large field gradient, and showed that the information was later retained," Fung said.

Coherence is also demonstrated by the mass factor, which indicates the efficiency of the resonant system.

For this structure, the mass factor at low temperatures is about one million, which indicates that the nanobeam resonator can maintain a highly coherent mechanical motion despite being functionalized with micromagnets. However, the highest recorded quality factor for mechanical resonators is 10 billion.

"While this coupling isn't strong enough to make this architecture a reality, we believe there are several realistic improvements that will allow us to achieve our goals," Fung said.

Researchers are working to introduce an optical cavity with a nanomechanical resonator.

"This cavity not only allows us to measure mechanical motion more precisely, but also has the potential to put the mechanical resonator in the ground state," Fung explains. This greatly expands the experiments we can do, such as transferring information from a single quantum from spin to mechanics and vice versa.

Researchers also believe that nanomechanical resonators are ideal intermediaries between different qubits, as they can interact with various forces, such as Coulomb repulsion and radiant pressure.

"Hybrid quantum systems can take advantage of the advantages of different kinds of qubits while mitigating their disadvantages. Since nanomechanical resonators can be fabricated on a chip, they can be integrated with other components, such as circuits or optical cavities, which opens up possibilities for long-distance connections," Fung concluded.

More information: F. Fung et al., Programmable Quantum Processors Based on Spin Qubits with Mechanically Mediated Interactions and Transport, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.263602

期刊信息： Physical Review Letters ， arXiv