Scientists integrate solid-state spin qubits with nanomechanical resonators

new Physics Review Letter studyScientists propose a new way to combine solid-state spin qubits and nanomechanical resonators for scalable, programmable quantum systems.

Quantum information processing requires qubits with long coherence times, stability and scalability. Solid-state spin qubits have been considered as candidates for these applications because they have long coherence times, but they are not scalable.

of PRL The research, led by Frankie Huang, a graduate student in Professor Mikhail Lukin’s group at Harvard University, addressed this challenge in an interview with Phys.org.

“Miniature quantum registers using solid-state spin qubits have been demonstrated, but they rely on magnetic dipole interactions that limit the interaction range to tens of nanometers. Systems containing large arrays of qubits are difficult to control due to the short interaction distance and the difficulty of consistently fabricating spin qubits with such close spacing,” he said.

In PRL In their study, the researchers proposed an architecture that uses nanomechanical techniques to mediate the interactions between spin qubits. Resonatormechanical oscillator.

Diamonds as qubits

The team’s approach relied on nitrogen-vacancy centres in diamond to act as quantum bits.

Typically, the diamond structure is Carbon atom It has a tetrahedral structure, meaning it is bonded to four other carbon atoms.

But if you use a method like Chemical Vapor DepositionOne of the carbon atoms can be replaced by a nitrogen atom, resulting in the loss of the carbon atom adjacent to the nitrogen and the formation of a vacancy.

The nitrogen atom adjacent to the vacancy forms an NV centre, which has an unpaired electron with a spin state that can be used as a quantum bit.

NV centers offer many advantages due to their unique optical properties: they have long coherence times, which means they interact less with the environment and are extremely stable.

Additionally, they are optically compatible, making it easy to transfer information in and out using light, and their unpaired electrons give them excellent sensitivity to magnetic fields.

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

The problem arises due to short-range interactions between the qubits themselves, because solid-state spin qubits interact with each other via short-range magnetic dipole interactions.

The interaction between qubits is necessary to create the quantum entangled state, which Quantum Information Processing.

Mechanical resonators as intermediaries

To address the long-range interactions of qubits, the researchers propose coupling the NV centres in diamond with mechanical resonators.

“Our research aims to use nanomechanical resonators to mediate the interactions between these spin qubits. More specifically, we propose a new architecture in which spin qubits in individual scanning probe tips can be moved on nanomechanical resonators that mediate the spin-spin interactions,” Fang explained.

Nanomechanical resonators are tiny structures that can vibrate at high frequencies (typically on the nanoscale) and are sensitive to external fields or forces.

By coupling the qubits to nanomechanical resonators, the researchers were able to achieve nonlocal Quantum Bits interactions, which could enable the creation of large-scale quantum processors and address the scalability shortcomings of solid-state quantum systems.

Architectural improvements

The team’s architecture therefore consists of spin qubits inside individual scanning probe tips, which are precise scanning devices that can gather information.

“The tip of the scanning probe can be moved on a mechanical resonator that mediates the spin-spin interaction. We can choose which qubits move on this mechanical resonator, thus creating programmable connections between spin qubits,” Fang explained.

The individual qubits are NV centers inside diamond nanopillars. This structure allows the NV centers to be brought close to micro-magnets, generating the magnetic field used to manipulate the electron spin state.

“It also helps that the nanopillars act as waveguides, reducing the laser power needed to excite the NV centres,” Fang added. This happens because the nanopillars guide the laser exactly where it needs to be: to the NV centres.

The micromagnets are placed on the silicon nitride nanobeams to complete the nanomechanical resonators.

In theory, this setup works like this: Micromagnets generate a magnetic field around the qubit and resonator, which changes the electron spin state of the qubit.

The change in spin state causes the qubit to interact with the nanomechanical resonator in a different way than before, vibrating at a different frequency, which in turn affects the other qubit, affecting its spin state.

This architecture allows for nonlocal qubit interactions.

Architectural feasibility and hybrid quantum systems

To show that this architecture is feasible, the researchers demonstrated the coherence of qubits in the mechanical transport of micromagnets.

“As a proof-of-principle measurement, we stored coherent information in an NV centre, moved it through a large magnetic field gradient, and showed that the information was still preserved afterwards,” Fang said.

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

With this architecture, the quality factor at low temperatures is about one million, suggesting that the nanobeam resonator can maintain highly consistent mechanical behavior despite being functionalized with micromagnets, although the highest recorded quality factor for a mechanical resonator is 10 billion.

“The coupling is not yet strong enough to make this architecture a reality, but we think there are some real improvements that can get us there,” Fang said.

Researchers are working on introducing optical cavities with nanomechanical resonators.

Fang explained: “This cavity not only allows us to measure mechanical motion more precisely, but also makes it possible to prepare mechanical resonators in their ground state. This greatly expands the experiments we can perform, such as transferring single quantum information from spin to machine and vice versa.”

The researchers also believe that nanomechanical resonators are ideal intermediaries between different quantum bits because they can interact with a variety of forces, including Coulomb repulsion and radiation pressure.

“Hybrid quantum systems can harness the advantages of different types of qubits while mitigating their drawbacks. Because nanomechanical resonators can be fabricated on-chip, they can be integrated with other components such as electrical circuits and optical cavities, opening up the possibility of long-distance connections,” Fan concluded.

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