Researchers from the John A. Paulson School of Engineering and Applied Sciences at Harvard University have developed a new method for protecting quantum information using mechanical vibrations, which are essentially microscopic sound waves.

A New Approach to Quantum Information Protection

In the laboratory, a technique was demonstrated that could help create compact quantum networks on chips and advance the development of hybrid quantum systems that combine different types of qubits. One promising method for building quantum networks relies on using the spin of an electron associated with an impurity in diamond to store quantum information. Mechanical vibrations, known as phonons, can transfer information between qubit nodes.

Advantages of Phonons

The laboratory developed a structure called a phononic resonator cavity, which traps mechanical vibrations and enhances their interaction with the electron spin inside the qubit. Phonons offer several advantages over light, which is typically used to transmit quantum information at the chip level. At the same frequency, phonons have a much shorter wavelength, allowing for more compact components and denser integration. Additionally, phonons can easily interact with both solid-state spins and electromagnetic fields, making them attractive for hybrid quantum technologies that unite different types of qubits within a single system.

The Challenge of Quantum Memory Protection

Using phonons introduces the challenge of protecting quantum memory, as qubits are highly sensitive to external disturbances. For effective storage and processing of information, qubits must maintain their quantum state for a sufficient period—this property is known as coherence. Typically, microwave pulses are used to shield quantum memory from external noise, but this method is not very effective for qubits placed in phononic resonator cavities. This limitation has made it difficult to achieve both strong interaction with phonons and long coherence times in a single device.

Mechanical Protection of Coherence

The research team proposed a solution by implementing what they call "fully mechanical coherence protection" for the spin with a silicon vacancy in diamond. Instead of traditional microwave pulses, they used a continuous mechanical control signal generated by phonons. This transformed the qubit into a special quantum state known as a "dressed" qubit, making it less susceptible to low-frequency environmental noise.

Because the protection is provided by a continuous mechanical field compatible with phononic resonator cavities, this method can be applied within the same structures that will eventually connect stationary nodes in quantum networks.

Results and Prospects

The new method allowed the coherence time of the spin with a silicon vacancy to be increased by about three times. These results show that suppressing noise with continuous mechanical waves can extend quantum coherence in real devices. Thus, microscopic sound waves could become an important tool for creating more reliable and compact quantum systems.