Many of us enjoyed donning our construction hats and building little houses out of Lego blocks when we were young. Now, researchers at the University of Calgary are playing a similar game, but rather than using Legos, they are stacking light particles, which could be an important step on the road to quantum computing. The problem is that working with light particles is notoriously difficult. But the Calgary team has managed to manipulate the quantum properties of light so as to stack two-story quantum houses of any style and architecture. Researchers have demonstrated stacking light particles like Lego blocks, which could be an important step on the road to quantum computing. Here, the primary author of the Nature Photonics paper, Erwan Bimbard, aligns the experimental setup. Photo by Ron Switzer, courtesy of the Institute for Quantum Information Science at the University of Calgary. “Constructing complex quantum entangled systems while maintaining control over their individual components offers unprecedented opportunities for fundamental studies of nature,” said Dr. Alexander Lvovsky, leader of the team. “It also leads to measurement instruments of extraordinary sensitivity, exponentially faster computers, unconditionally secure communication systems and quantum-controlled chemistry.” Among various physical systems that may be suitable for quantum technological applications, light stands alone as the only one that can serve as a communication agent. The vision of Lvovsky’s research is implementing light as the principal physical medium for quantum information processing. In his experiments, which are detailed in the February 2010 issue of Nature Photonics, arbitrary superpositions of zero-, one- and two-photon states of light are produced. This means that the team has taken a step toward being able to construct any arbitrary quantum state – one of the “holy grails” of quantum information technology. But this was no easy task. “The degree of control reported in our work has so far been achieved only with other quantum systems – trapped ions and microwave resonators, but not with traveling photons,” Lvovsky said. The previous “industry best” in stacking photons was manufacturing superpositions of zero- and one-photon states. What led to the group’s success was a combination of high-intensity parametric down-conversion – which allows multiple photon pairs to be produced from a single laser pulse with a reasonable probability – plus high-bandwidth balanced detection – to measure the states that are produced. In the setup, mirrors and lenses are used to focus a blue laser beam into a specialized crystal. The crystal takes high-energy blue photons and converts them into a quantum superposition of lower-energy red photons, which emerge in two directions, or “channels.” By measuring one of the channels using ultrasensitive single-photon detectors, the team prepared the desired quantum state in the other. “Such an operation is possible because the photons in the two channels are entangled. This means that a measurement made in one channel would result in an immediate change in the other, regardless of whether the particles were an arm’s length apart or light-years away,” Lvovsky said. “Albert Einstein called this quantum weirdness ‘spooky action at a distance.’ ” The researchers’ next step is to work on characterizing two basic optical processes: photon creation and annihilation operators. Once this is completed, Lvovsky and his colleagues will experiment with entanglement distillation and also will continue to improve on their technique.