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April 11, 2014

Research Advances on Key Quantum Computing Elements

Tiffany Trader
quantum action

Equal parts fascinating and confounding, the field of quantum computing keeps making headway. Two exciting developments are described in the current issue of Nature, one from a collaboration between Harvard University and MIT researchers and the other from the Max Planck Institute of Quantum Optics in Germany. Their work concerns the fundamental building blocks that make quantum computing possible.

As summarized in Popular Mechanics, the scientists figured out a way to combine atoms and particles of light – photons – to create a quantum versions of the switch and logic-gate – two essential elements of classic computing systems.

Quantum computing has long been considered the holy grail of computing. This bizzare world of particle superposition and spooky action at a distance promises to unlock the door to unprecedented kinds of computing tasks. Beyond the killer app of encryption, all sorts of seemingly uncanny things become possible, such as simulations of the universe itself.

At their core, all modern computers involve data and rules. In classical computing, the smallest unit of data is a bit, represented as a 0 or a 1. In quantum computing, the bit becomes a q-bit and instead of just being able to represent two states, it can exist in multiple states. “Superposition,” as this phenomenon is called, allows a lot of information to be acted on in a very small space, setting the stage for incredibly fast supercomputers.

Superposition states are fragile, though, and must be coaxed into being. “At this point, very small-scale quantum computers already exist,” says Mikhail Lukin, the head of the Harvard research team. “We’re able to link, roughly, up to a dozen qubits together. But a major challenge facing this community is scaling these systems up to include more and more qubits.”

The new quantum logic gate and switch introduce a new method of connecting particles, using trapped rubidium atoms and photons. The Harvard and MIT scientists created the switch by coupling one rubidum atom with a single photon, enabling both the atom and photon to switch the quantum state of the other particle. Being able to go from a ground state to an excited state, the atom-photon coupling can transmit information like a transistor in a classical computing system.

The German research group used mirror-like sheets and lasers to trap the atom, forming quantum gates, which change the direction of motion or polarization of photons. When the rubidium atom is in superposition, the photon both does and does not enter the mirror, and both does and does not get a polarization change. Via an attribute of quantum physics called entanglement swapping, multiple photons can share superposition information. These engtangled photons are made to bounce repeatedly off the mirror-trapped rubidium atom, acting as the input for the logic gate.

“The Harvard/MIT experiment is a masterpiece of quantum nonlinear optics, demonstrating impressively the preponderance of single atoms over many atoms for the control of quantum light fields,” says Gerhard Rempe, a professor at the Max Planck Institute of Quantum Optics who was part of the research team upon reading the paper from his US counterparts. “The coherent manipulation of an atom coupled to a photonic crystal resonator constitutes a breakthrough and complements our own work … with an atom in a dielectric mirror resonator.”

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