Newly realized nuclear-spin dark state promises reduced quantum decoherence

Quantum computers, which operate leveraging quantum mechanics phenomena, could eventually tackle some optimization and computational problems faster and more efficiently than their classical counterparts. Instead of bits, the fundamental units of information in classical computers, quantum computers rely on qubits (quantum bits), which can be in multiple states at once.

Silicon-based quantum dots, semiconductor-based structures that trap individual electrons, have been widely used as qubits, as the spin state of the electrons they confine can be leveraged to encode information. Despite their promise, many quantum computers developed so far are susceptible to decoherence, which entails the disruption of qubit states due to their interaction with the surrounding environment.

Researchers at the University of Rochester recently set out to experimentally realize a so-called nuclear-spin dark state, a condition that has been theorized to improve the performance of quantum computers, suppressing undesirable interactions and thus reducing decoherence. Their paper, published in Nature Physics, demonstrates the potential of this state for reducing decoherence in quantum systems and thus potentially improving control over quantum information processing.

“The motivation for this project was to explore a realization of the central spin problem in a new setting,” John Nichol, senior author of the paper, told Phys.org. “The central spin model is a widely studied system where one spin (the central spin) interacts with many other spins. This important model is used to understand phenomena including decoherence of qubits.”

The central spin model is a promising framework that predicts various phenomena, including the formation of so-called dark states. While various theoretical studies are built on this model, testing it in experimental settings has proved challenging due to the large number of spins required to demonstrate the effects it predicts.

“We were inspired to use the semiconductor silicon to explore this physics because while it still has a lot of nuclear spins, it doesn’t have as many as other semiconductors,” said Nichol. “We thought silicon would provide us with a more controllable platform in which to study central-spin physics.”

Typically, the nuclear spins in semiconductors interact with electron spins via the so-called hyperfine interaction. To realize the nuclear-spin dark state, researchers need to arrange nuclear spins following a specific configuration, to prevent their interactions with electron spins.

“We used a system of electrons in a silicon gate-defined double quantum dot,” explained Nichol. “Using voltage pulses, we manipulated the electron spins to generate dynamic nuclear polarization, which is the process of polarizing the nuclei with the electrons. During the process of dynamic nuclear polarization, the nuclei in the semiconductor became synchronized in such a way to cancel their interaction with the electrons.”

The findings gathered by Nichol and his colleagues appeared to confirm theoretical predictions about nuclear-spin dark states. The researchers realized a nuclear-spin dark state in a gate-defined silicon double quantum dot, demonstrating that such states can exist in real devices.

Notably, they found that the dark state they realized significantly suppressed interactions between nuclear and electron spins. In addition, these interactions remained suppressed for as long as the nuclei maintained their carefully engineered synchronization.

“The existence of the dark state had been predicted many years ago, but conclusive experimental evidence of its formation had to wait until now,” said Nichol. “Its existence is surprising in part because the dark state involves the synchronized evolution of thousands of nuclear spins, and the fact that they can undergo this collective evolution in a real system is surprising.”

Demonstrating that dark states can exist in silicon devices could have important implications for future research. Silicon is easily accessible and widely used to develop various electronic devices, including classical and quantum computing systems. Therefore, this recent study could soon pave the way for other demonstrations of nuclear-spin dark states in silicon quantum dots.

“We hope someday the dark state can be useful, especially for quantum applications,” added Nichol. “We have many unanswered questions about this dark state and are eager to explore them in future research. In our next studies, we plan to explore the properties of the dark state, including how robust it is to external factors. We will also focus on how it can be used. For instance, predictions suggest that the dark state should be an excellent quantum memory, and we are curious to see if this pans out.”

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