Scientists integrate solid-state spin qubits into nanomechanical resonators

In a new Physical Exam Letters In a study, scientists propose a new method to combine solid-state spin qubits with nanomechanical resonators for scalable and programmable quantum systems.

Quantum information processing requires qubits to have long coherence times, be stable, and be scalable. Solid-state spin qubits are sought-after candidates for these applications because they have long coherence times. However, they are not scalable.

THE PRL A study led by Frankie Fung, a graduate student in Professor Mikhail Lukin’s group at Harvard University, addressed this challenge in an interview with Phys.org.

“Although small quantum registers using solid-state spin qubits have been demonstrated, they rely on magnetic dipole interactions, which limit the interaction range to tens of nanometers,” he said. “The short interaction distance and the difficulty of fabricating spin qubits coherently at such close spacings make it difficult to control systems containing large arrays of qubits.”

In the PRL In this study, the researchers proposed an architecture that mediates the interaction between spin qubits using a nanomechanical resonator, a mechanical oscillator.

Diamonds as qubits

The team’s approach relied on nitrogen vacancy centers in diamonds acting as qubits.

Typically, diamond structures are made up of carbon atoms in a tetrahedral structure, meaning they are bonded to four other carbon atoms.

However, using methods such as chemical vapor deposition, one of the carbon atoms can be replaced with a nitrogen atom. This results in a missing carbon atom adjacent to the nitrogen, creating a vacancy.

The nitrogen atom adjacent to a vacancy forms the NV center, which has an unpaired electron with spin states used as qubits.

NV centers offer many advantages due to their unique optical properties. They have long coherence times, which means that their interaction with the environment is low, making them very stable.

Additionally, they are optically compatible, meaning it is easy to pass information in and out using light. Since they have unpaired electrons, they are also very sensitive to magnetic fields.

These properties make them ideal for use as qubits, especially when integrated into semiconductor devices.

The problem comes from the short-range interaction between the qubits themselves. Indeed, solid-state spin qubits interact with each other via magnetic dipole interactions, which are short-range.

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

Mechanical resonators as mediators

To address the long-range interaction of qubits, the researchers propose to couple the NV centers of diamonds with mechanical resonators.

“Our research aims to use nanomechanical resonators to mediate the interactions between these spin qubits. Specifically, we propose a novel architecture, in which the spin qubits inside individual scanning probe tips can be moved on a nanomechanical resonator that mediates the spin-spin interactions,” Fung explained.

Nanomechanical resonators are tiny structures capable of oscillating at high frequencies (typically in the nanometer range). They are sensitive to external fields and forces.

By combining the qubits with a nanomechanical resonator, the researchers create a means for nonlocal interactions between qubits. This could enable the creation of large-scale quantum processors, which would address the scalability problem of solid-state quantum systems.

Refine the architecture

The research team’s architecture therefore consists of a spin qubit inside individual scanning probe tips, which are precise scanning devices capable of collecting information.

“The scanning probe tips can be moved on a mechanical resonator that mediates spin-spin interactions. Because we can choose which qubits to move on this mechanical resonator, we can create programmable connectivity between spin qubits,” Fung explained.

The individual qubits are NV centers inside a diamond nanopillar. This structure allows the NV center to be close to a micromagnet, which creates the magnetic field used to manipulate the spin state of the electrons.

“The fact that the nanopillar acts as a waveguide that reduces the laser power needed to excite the NV center is also an advantage,” Fang added. This happens because the nanopillar guides the laser to exactly where it needs to go, the NV center.

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

In theory, the device works as follows. The micromagnet creates a magnetic field around the qubit and the resonator. This magnetic field changes the electron spin state of the qubit.

The change in spin state causes a different interaction between the qubit and the nanomechanical resonator, causing it to oscillate at a different frequency. This oscillation affects other qubits, thus affecting their spin state.

The architecture allows for non-local qubit interactions.

Architecture feasibility and hybrid quantum systems

To show that their architecture is feasible, the researchers demonstrated the coherence of the qubit on the mechanical transport of the micromagnet.

Fung said: “As a proof-of-principle measure, we stored coherent information in the NV center, moved it through a large field gradient, and showed that the information was preserved afterwards.”

Coherence was also demonstrated via the quality factor, indicating the efficiency of a resonant system.

For the architecture, the quality factor was about one million at low temperature, suggesting that the nanobeam resonator can maintain highly coherent mechanical motion despite being functionalized with a micromagnet. However, the highest quality factor recorded for mechanical resonators is 10 billion.

“While this coupling is not yet strong enough to make this architecture a reality, we believe there are several realistic improvements that could get us there,” Fung said.

The researchers are working on introducing an optical cavity with a nanomechanical resonator.

Fung explained: “The cavity would not only allow us to measure mechanical motion more precisely, but also potentially prepare the mechanical resonator in its ground state. This greatly expands the experiments we can perform, such as transferring a single quanta of information from spin to mechanics and vice versa.”

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

“A hybrid quantum system can exploit the advantages of different types of qubits while mitigating their drawbacks. Because they can be fabricated on-chip, nanomechanical resonators can be integrated with other components, such as an electrical circuit or an optical cavity, opening up possibilities for long-range connectivity,” Fung concludes.

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