MIT and Waterloo Team Increase Quantum Register Size

Researchers from MIT and the University of Waterloo have developed a method to control an environmental spin defect beyond the coherence limit of a central spin, a significant breakthrough in quantum computing. This could enhance the size and performance of solid-state quantum registers, which are vital for quantum metrology and communication. The team’s approach uses weakly coupled probe spin and double resonance control sequences to mediate the transfer of spin polarization. This development could advance nanoscale sensing, enable correlated noise spectroscopy for error correction, and facilitate spin-chain quantum wires for quantum communication.

What is the Significance of Controlling an Environmental Spin Defect Beyond the Coherence Limit of a Central Spin?

A team of researchers from the Massachusetts Institute of Technology (MIT) and the University of Waterloo have made a breakthrough in the field of quantum computing. The team, consisting of Alexander Ungar, Paola Cappellaro, Alexandre Cooper, and Won Kyu Calvin Sun, have developed a method to control an environmental spin defect beyond the coherence limit of a central spin. This development could potentially increase the size and performance of solid-state quantum registers, which are crucial for applications in quantum metrology and quantum communication.

Previously, works on multi-qubit electronic spin registers in the environment of a nitrogen-vacancy (NV) center in diamond only included spins directly coupled to the NV. This direct coupling is limited by the central spin coherence time, which significantly restricts the maximum attainable size of the register. The team’s approach addresses this problem by using a weakly coupled probe spin together with double resonance control sequences to mediate the transfer of spin polarization between the central NV spin and an environmental spin that is not directly coupled to it.

The team’s work paves the way for engineering larger quantum spin registers, which could advance nanoscale sensing, enable correlated noise spectroscopy for error correction, and facilitate the realization of spin-chain quantum wires for quantum communication.

How Does This Development Impact Quantum Information Processing?

Optically active solid-state spin defects with individual control are promising building blocks for quantum information processing. Notably, defects in diamond, the nitrogen-vacancy (NV) center, are a leading candidate for applications in quantum sensing. Meanwhile, the silicon vacancy center is a key prospect for realizing efficient quantum networks.

Engineering a quantum register consisting of individually controllable environmental spins surrounding a central optically active spin enables more powerful and interesting applications. For instance, a quantum register of nuclear spins is ideal for storing and processing quantum information given its weak coupling to the environment. Such a nuclear-spin register has been used to demonstrate enhanced quantum memory with record lifetimes, quantum error correction, and quantum simulation.

A register consisting of electronic spins, which feature stronger coupling to external fields and other spins, can enable new and complementary applications. In the areas of quantum sensing and quantum device characterization, such an electronic spin register can be used for correlated noise spectroscopy and error characterization, as well as high-resolution sensing in spatial and frequency domains.

What are the Limitations of Current Electronic Spin Registers?

Until now, electronic spin registers comprising an NV center electronic spin and optically inactive dark spins in its environment have been limited in size. They only include spins that are directly coupled to the central NV via the magnetic dipolar interaction. Alternatively, registers consisting of multiple NVs have been limited in size to pairs of NVs that are directly coupled.

In the case of dark electronic spin registers, the coherence volume, which is defined as the volume encompassing all of the spins that can be coherently controlled, has been limited by the coherence time of the central NV. If spins beyond this first layer could be accessed, the coherence volume of electronic spin registers could be scaled up beyond the NV coherence limit.

How Does the Team’s Approach Overcome These Limitations?

The team’s approach to surpass the coherence limit is to utilize a coherent first-layer spin to detect and subsequently control a second-layer spin not directly coupled to the central spin. Although control of nuclear spins beyond this first-layer limit has been achieved in other works, achieving the same control for electronic spins is a more significant challenge, limiting the scalability of such registers.

The electron-electron coupling strength, several orders of magnitude larger compared to nuclear-nuclear coupling, leads to more complex interaction graphs, requiring more complicated control schemes over a given coherence volume. To overcome this problem, one must decrease the spin density and expand the control to higher layers of spins. Thus, achieving control of an electronic spin beyond the first layer is a key first step toward building a practical electronic spin register.

What are the Implications of This Research?

The team’s research extends the coherence volume of a network of dark electronic spins beyond the coherence limit of the optically active central NV. This breakthrough could potentially increase the size and performance of solid-state quantum registers, which are crucial for applications in quantum metrology and quantum communication.

The team’s work paves the way for engineering larger quantum spin registers, which could advance nanoscale sensing, enable correlated noise spectroscopy for error correction, and facilitate the realization of spin-chain quantum wires for quantum communication. This development is a significant step forward in the field of quantum computing and could have far-reaching implications for the future of technology.