Error corrected spin-state readout in a nanodiamond

Abstract

Quantum state readout is a key component of quantum technologies, including applications in sensing, computation, and secure communication. Readout fidelity can be enhanced by repeating readouts. However, the number of repeated readouts is limited by measurement backaction, which changes the quantum state that is measured. This detrimental effect can be overcome by storing the quantum state in an ancilla qubit, chosen to be robust against measurement backaction and to allow error correction. Here, we protect the electronic-spin state of a diamond nitrogen-vacancy center from measurement backaction using a robust multilevel 14N nuclear-spin memory and perform repetitive readout, as demonstrated in previous work on bulk diamond devices. We achieve additional protection using error correction based on the quantum logic of coherent feedback to reverse measurement backaction. The repetitive spin readout scheme provides a 13-fold enhancement of readout fidelity over conventional readout and the error correction a 2-fold improvement in the signal. These experiments demonstrate full quantum control of a nitrogen-vacancy center electronic-spin coupled to its host 14N nuclear spin inside a ~25 nm nanodiamond, creating a sensitive and biologically compatible platform for nanoscale quantum sensing. Our error-corrected repetitive readout scheme is particularly useful for quadrupolar nuclear magnetic resonance imaging in the low magnetic field regime where conventional repetitive readout suffers from strong measurement backaction. More broadly, methods for correcting longitudinal (bit-flip) errors described here could be used to improve quantum algorithms that require non-volatile local memory, such as correlation spectroscopy measurements for high resolution sensing.Error correction: Protecting spins in nanodiamondA scheme for protecting the quantum states in a nanodiamond is demonstrated that improves readout fidelity by enabling repeated measurements. The ability to readout a quantum state is key to quantum technologies, such as quantum sensors, computation and secure communication. Readout fidelity can be enhanced by making multiple measurements, but this often has the unfortunate effect of changing the quantum state being measured. A team of researchers from the University of Cambridge led by Mete Atature now demonstrate a simple error-correction code to protect the electronic-spin state of a nitrogen-vacancy center in diamond from measurement backaction, enabling them to significantly enhance the readout fidelity. Aside from showing the potential of this platform, this approach for correcting errors could also be used to improve quantum algorithms that require a non-volatile local memory.