A team of scientists in Innsbruck, Austria, made an important step toward distributed quantum computing with cavities linking remote atom-based registers. They demonstrated precise control of the coupling of each of two trapped ions to the mode of an optical resonator.
A key goal in quantum computing is the demonstration of a quantum network, that is, a framework for distribution and remote processing of quantum information. One promising model for such a network consists of local processors linked to one another over long distances via optical fiber. Each local processor, consisting of several quantum bits, would be confined between two highly reflective mirrors. These mirrors form an optical resonator, which functions as a coherent interface between light (in the fiber) and matter (the quantum bits).
Significant experimental progress has been made in recent years with single atoms as quantum bits in optical resonators. However, the necessary control of multiple quantum bits within a resonator has not yet been demonstrated. A team led by Tracy Northup and Rainer Blatt at the Institute for Experimental Physics at the University of Innsbruck and the Institute of Quantum Optics and Quantum Information at the Austrian Academy of Sciences has now shown that they can trap two calcium ions within a resonator. "By precisely positioning each ion, we choose whether one or both ions interacts with photons, that is, with light quanta in the resonator. When both ions interact with the resonator, this interaction can be used to generate quantum-mechanical entanglement the two ions," explains Tracy Northup. "In fact, entanglement is created via the measurement process: at the moment when we detect two photons, one from each ion, at a pair of detectors, the ions become entangled," says Northup. Afterwards, the entanglement is analyzed by observing correlations in the ions’ fluorescence on a camera.
If the two ions were located in spatially separated cavities, the same protocol could be used to generate efficient remote entanglement between the ions. Thus, this work represents an important step towards a cavity-based interface with an ion-trap quantum computer. Furthermore, the scheme is applicable to other quantum-computing platforms, such as arrays of neutral atoms or quantum dots.
Heralded entanglement of two ions in an optical cavity. B. Casabone, A. Stute, K. Friebe, B. Brandstätter, K. Schüppert, R. Blatt, and T. E. Northup. Phys. Rev. Lett. 111, 100505 (2013)