Scientists integrate semiconductor spin qubits with nanomechanical resonators

In New Physical Review Letters researchers propose a new method for combining solid-state spin qubits with nanomechanical resonators for scalable and programmable quantum systems.

Quantum information processing requires qubits to have long coherence times, stability and be scalable. 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 Professor Mikhail Lukin’s group at Harvard University, addressed this challenge in an interview with Phys.org.

He said: “While small quantum registers using solid-state spin qubits have been demonstrated, they rely on magnetic dipolar interactions that limit the interaction range to tens of nanometers. The short interaction distance and the difficulty of consistently creating spin qubits at such small spacings make it challenging to control systems containing large array of qubits.”

IN PRL researchers have designed 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 consist 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 by a nitrogen atom. This results in a missing carbon atom adjacent to the nitrogen, creating a vacancy.

The nitrogen atom adjacent to the 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 a long coherence time, which means that their interaction with the environment is low, making them very stable.

Plus, they’re optically compatible, meaning it’s easy to input and output information using light. Because 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 semiconductor devices.

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

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

Mechanical resonators as mediators

To solve the long-distance interaction of qubits, scientists propose to connect NV centers in diamonds with mechanical resonators.

“Our research focuses on using nanomechanical resonators to mediate interactions between these spin qubits. More specifically, we propose a new architecture where the spin qubits inside individual scanning probe tips can be moved via a nanomechanical resonator that mediates spin-spin interactions,” explained Fung. .

Nanomechanical resonators are tiny structures that can oscillate at high frequencies (typically nanoscale). They are sensitive to external fields and forces.

By coupling the qubits to a nanomechanical resonator, the researchers create a pathway for nonlocal qubit interactions. This potentially enables the creation of large-scale quantum processors that address the scalability disadvantage of solid-state quantum systems.

Improving architecture

The research team’s architecture therefore consists of a rotating qubit inside individual scanning probe tips, which are precise scanning devices that can collect information.

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

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

“It also helps that the nanopillar acts as a waveguide, reducing the laser power needed to excite the NV center,” Fang added. This happens because the nanopillar guides the laser exactly where it needs to go, the center of the NV.

The micromagnet is placed on a silicon nitride nanobeam that complements the nanomechanical resonator.

In theory, the setup 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.

Changing the spin state causes the qubit to interact with the nanomechanical resonator differently than before, so it oscillates at a different frequency. This oscillation affects the other qubits and affects their spin state.

The architecture allows for non-local qubit interactions.

Architecture Feasibility and Hybrid Quantum Systems

To show that their architecture is achievable, the researchers demonstrated qubit coherence with the mechanical transport of a micromagnet.

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

Coherence was also demonstrated through a quality factor indicating the effectiveness of the resonance system.

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

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

Scientists 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 do, such as transferring a single quantum of information from spin to mechanics and vice versa.”

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

“A hybrid quantum system can take advantage of different kinds of qubits while mitigating their disadvantages. Because they can be fabricated on a chip, nanomechanical resonators can be integrated with other components such as an electrical circuit or an optical cavity, opening up the possibility of long-distance connectivity.” Fung concluded.

© 2024 Science X Network