Solid-state quantum memory using the 31P nuclear spin

Abstract

The transfer of information between the entities that do the processing and memory is crucial — and problematic — for quantum computation. In classical systems the information transfer can include a copying step, where errors can be spotted and corrected, but in quantum systems copying is fundamentally precluded. Morton et al. demonstrate a technology that could solve the problem: the coherent storage and readout of information between electron-spin processing elements and memory elements based on a nuclear spin. The system utilizes phosphorus-31 spin donors in a silicon-28 crystal. The nuclear spin acts as a memory element that can faithfully store the full state of the electron spin for more than a second, then transfer it back to the electron spin with about 90% efficiency. The transfer of information between processing entities and memory is crucial for quantum computation; it is challenging because the process must remain coherent at all times to preserve the quantum nature of the information. This paper demonstrates coherent storage and readout of information between electron-spin processing elements and memory elements based on a nuclear spin. The transfer of information between different physical forms—for example processing entities and memory—is a central theme in communication and computation. This is crucial in quantum computation1, where great effort2 must be taken to protect the integrity of a fragile quantum bit (qubit). However, transfer of quantum information is particularly challenging, as the process must remain coherent at all times to preserve the quantum nature of the information3. Here we demonstrate the coherent transfer of a superposition state in an electron-spin ‘processing’ qubit to a nuclear-spin ‘memory’ qubit, using a combination of microwave and radio-frequency pulses applied to 31P donors in an isotopically pure 28Si crystal4,5. The state is left in the nuclear spin on a timescale that is long compared with the electron decoherence time, and is then coherently transferred back to the electron spin, thus demonstrating the 31P nuclear spin as a solid-state quantum memory. The overall store–readout fidelity is about 90 per cent, with the loss attributed to imperfect rotations, and can be improved through the use of composite pulses6. The coherence lifetime of the quantum memory element at 5.5 K exceeds 1 s.