All-mechanical coherence protection and fast control of a spin qubit

In the quest to build a quantum internet, scientists are searching for a way to move delicate information between tiny, stationary computers without losing it along the way. These computers, known as qubits, often rely on the spin of an electron—a property that acts like a tiny, internal compass needle—to store data. The challenge is that these needles are incredibly sensitive; the slightest vibration or magnetic whisper from the environment can scramble the information, causing the computer to forget what it was doing. To solve this, researchers have long looked to light, or photons, to carry messages between nodes. However, light takes up a lot of space and can interfere with itself easily. A more compact alternative is the phonon, a particle of sound or vibration that travels through solid materials. Because sound waves are much smaller than light waves and interact strongly with the materials they travel through, they offer a promising path to building dense, efficient networks on a single chip. But to use sound as a messenger, scientists first had to prove they could protect the stationary qubit from noise while simultaneously using sound to control it, a task that seemed nearly impossible because the methods used to shield the qubit often clashed with the methods used to talk to it.

A team of researchers at Harvard University and the University of Chicago has now demonstrated a way to achieve this delicate balance using a specific type of defect in a diamond crystal called a silicon vacancy. In their experiment, they placed a single diamond chip inside a super-cold refrigerator to freeze out random thermal vibrations. On top of this diamond, they built tiny metal fingers, known as interdigital transducers, which act like speakers that convert electrical signals into mechanical waves. When they sent a specific microwave signal into these speakers, it generated a continuous stream of sound waves that traveled across the diamond surface. These waves did not just pass by the silicon vacancy; they shook the crystal lattice in a precise rhythm that matched the natural frequency of the electron's spin. This constant shaking created a new, stable state for the electron, effectively shielding it from the chaotic magnetic noise of the surrounding environment.

The researchers found that by keeping this mechanical field active, they could extend the time the electron held its information by more than three times compared to when the field was off. In the quiet, unshielded state, the electron's memory lasted for about 680 nanoseconds before fading. With the continuous mechanical protection, that time stretched to over 2 microseconds. This protection worked because the constant shaking made the electron less sensitive to the slow, drifting magnetic fluctuations that usually cause errors. Crucially, the team showed that they could still read and write information to this protected electron using the same mechanical waves. They used a second, weaker sound pulse to flip the electron's state and a laser to see what happened. This proved that the electron could be both shielded from noise and controlled by sound at the same time, a combination that had previously been a major hurdle for building phonon-based networks.

Beyond just protecting the memory, the team discovered that their method allowed for incredibly fast control. They were able to make the electron spin back and forth at a rate of 800 million times per second, a speed that sets a new record for this type of system. This rapid switching is essential for performing complex calculations quickly before the information has a chance to degrade. The researchers achieved this speed because their setup converted electrical signals into mechanical strain with high efficiency, allowing them to drive the electron much harder than previous methods that relied on magnetic fields. They also observed that at these extreme speeds, the electron's behavior became more complex, showing multiple frequencies at once, which suggests that the system is entering a new regime of physics where standard simple models no longer apply.

The significance of this work lies in its ability to unify two previously conflicting needs: the need to isolate a qubit from noise and the need to interact with it strongly. By using sound waves to create a protective shield that is also the tool for control, the researchers have cleared a path toward high-fidelity quantum gates, which are the basic operations required for a quantum computer to function. They demonstrated that the information could be initialized and read out directly using light, without needing complicated extra steps to transfer the state. This simplicity, combined with the speed and the extended memory time, suggests that silicon vacancies in diamond could serve as the stationary nodes for a future quantum network where information is routed by sound. While the researchers noted that the heating from the sound waves must be carefully managed to keep the diamond cold enough, their results show that the trade-off is manageable and that the benefits of this all-mechanical approach are substantial. The work represents a crucial step toward robust, on-chip quantum networks that could one day link quantum computers together using the vibrations of the solid world itself.