One of the primary goals of quantum computing research is the development of a consistent "quantum speedup" -- a process that, in MIT Professor Scott Aaronson's words, means to "solve some actual computational problem faster using quantum coherence." In order to achieve such a speedup, it's necessary to take advantage of the ability of qubits (the basic unit of information in quantum computing) to exhibit quantum entanglement. Quantum entanglement allows qubits to exhibit multiple states -- enabling faster calculations than traditional bits, which can only exhibit one state at a time. Such entanglement has been demonstrated on a small scale in superconducting circuits by the Schoelkopf Lab at Yale, which last year published a paper demonstrating three qubit entanglement.
What's needed to build on this work is a much bigger scale of entangled qubits. And that scale may be possible soon, thanks to some important work by physicist Olivier Pfister and his team at the University of Virginia. Their research, which was published in Physical Review Letters describes the team's ability to entangle cluster states of Qmodes. Qmodes are part of a quantum computing architecture whereby the normal modes of light are actually used as qubits to perform quantum computing operations.
In this set of experiments, the Qmodes were generated as lasers emitted by a optical parametric oscillator. The qmodes were forced by the oscilaltor to create what's know as an optical frequency comb. This resulted in a series of Qmodes that were separated by known frequencies, and related to each other based on their phase. Using this method, Pfister and his team were able to entangle 15 cluster states of 4 Qmodes each, for a total of 60. The team ascertained that all 60 Qmodes were equally entangled.
This is an exciting step forward in quantum computing, but there are a couple of caveats. First of all, this is miles from the thousands of entangled qubits necessary to achieve quantum speedups. This seems like a pretty scalable solution, but that remains to be demonstrated. Moreover, although the authors state that "[t]here is no known fundamental impossibility to the implementation of quantum computing with Qmodes", there are some special challenges when it comes to entangling qubits optically as opposed to entangling them in a superconductor or other quantum computing method. So it may turn out that this is scalable, but not economical or practical. There's still a lot of work to do.
Quantum computing is an exciting field right now, but it's very much in its infancy, and while I'm excited about Pfister's work here and eager to see where it goes, we're still probably years away from being able to use this technique in a practical quantum computer.
h/t Next Big Future