Progress towards an error correction scheme in quantum computing: monitoring single photon loss in real-time

A quantum computer promises to solve certain problems significantly faster than a classical computer, but is much more susceptible to errors. These errors are one of the greatest impediments to realizing a successful quantum computer, therefore making a robust quantum error correction (QEC) scheme imperative. One candidate for such a scheme relies on encoding a quantum bit of information onto a state of light in a cavity, and then proceeding to measure the photon number parity of that cavity state repeatedly in real-time. This measurement-based QEC, where the error syndrome is a change of photon number parity induced by the decay of single photons, requires highly quantum non-demolition measurements that are fast compared to the average lifetime of the cavity. Here we show the first realization of a fast and repeated monitoring of photon number parity in real-time. Our results indicate that we are very sensitive to the decay of single photons and that our measurements do not induce extra decay channels for the cavity state. This successful monitoring of an error syndrome for quantum error correction presents an important step towards extending the lifetime of a quantum bit.