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.
Highlights
Quantum devices rely on the readout of a quantum system after a period of controlled evolution or interaction with other systems
Weak signals are typical for quantum systems and limit the information that can be gained about the state of the system in one readout step
We present an error correction protocol, which can improve repetitive readout fidelity by correcting errors caused by the γ± transitions
Summary
Quantum devices rely on the readout of a quantum system after a period of controlled evolution or interaction with other systems. High readout fidelity is critical to such devices, because it enables heralded state initialization,[1,2,3] quantum error correction,[4] and improved sensitivity of quantum sensors.[5,6] Weak signals are typical for quantum systems and limit the information that can be gained about the state of the system in one readout step. All quantum measurements exhibit backaction, which changes the device’s state during readout. In the absence of backaction, namely for an ideal quantum non-demolition measurement, the problem of a weak signal could be overcome by multiple repeated readout steps. We describe and demonstrate an error correction protocol that protects spin memories from measurement backaction in the form of depolarization due to spin–spin interactions
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