Scalable entanglement in an optical frequency comb

Quantum computing’s appeal comes from using systems in highly entangled states to solve problems that would take too long on even the most powerful classical computers. Still, useful quantum computation would require a register of some hundreds or thousands of quantum entities, such as two-level qubits or continuously valued qumodes. With an eye on the needed scalability, Olivier Pfister and colleagues at the University of Virginia have now shown that in a single step, they can simultaneously manipulate at least 60 modes in an optical frequency comb, entangling them in clusters of four. Their experiment is a step toward creating a highly entangled state known as a cluster state, the universal starting point for so-called one-way quantum computing. Most efforts to entangle optical modes into a cluster state build up the state bit by bit; the larger the desired cluster state, the more equipment and effort is required. In contrast, Pfister and colleagues’ experiment shows that many of the necessary entanglements can be produced in parallel using minimal equipment—just one nonlinear optical element and one input laser frequency. Connecting the four-qumode clusters into larger cluster states is mostly a matter of adding more input frequencies; in recent theoretical work , Pfister, Steven Flammia , and Nicolas Menicucci found that 15 input frequencies should suffice to build a cluster state of arbitrary size. (M. Pysher et al., Phys. Rev. Lett. 107, 030505, 2011 .)—Johanna Miller