PORTLAND, Ore. The world's first entanglement of quantum bits (qubits) in a solid-state superconducting Josephson Junction circuit has been reported by University of Maryland researchers. Though far from a quantum logic circuit and even farther from a quantum computer, the demonstration holds hope that engineering improvements could someday produce such a computer.
"Our findings indicate that you could use Josephson Junctions to build a quantum computer," said professor Fred Wellstood, leader of the project and director of the University's Center for Superconductivity Research. Others contributing to the project include Andrew Berkley, Mark Gubrud, Joseph Foley, Matt Kenyon, Jan Olaf Gaudestad, Huizhong Xu and Roberto Ramos.
Unlike the usual physical states such as voltage or current used to encode information in electronic circuits, quantum states have only a probable existence until they are measured. That counters intuition, but it also offers a high degree of parallelism since all possible states of an elementary particle have a probable existence simultaneously.
There are many different quantum statesfrom electron spin direction to photon polarization anglesbut all share the common ability to exist in a nebulous state that is not resolved until an observation of them is made. By encoding "1s" and "0s" in these unresolved nebulous states, quantum computers promise to perform parallel operations on both values in a single step. Quantum entanglement is important because it enables the final result to be read out without disturbing the sequence of parallel operations.
Wellstood said entanglement blurs the distinction between individual particles so that it is impossible to describe the particles separately no matter how far away they are physically separated.
"We view entanglement as essential to quantum computing because it packs more information into quantum bits than is possible with classical computing bits. Six quantum bits, for instance, can represent 64 pieces of information," said Berkley.
Wellstood claims his approach "scales up" more easily than competing methods because it is based on solid-state electronic devices, not free-floating subatomic particles. By demonstrating entanglement between two Josephson junctions, Wellstood said he has provided important evidence that quantum computers are possible.
The Josephson junction device used by the team is composed of two superconductors separated by an insulating layer so thin that electrons can tunnel through it. Otherwise he uses the same techniques used to make conventional ICs, concluding that his approach is well suited for scaling up.
"We can go to the thousands of devices you need to build a real working quantum computer," said Wellstood.
Wellstood's work builds on that of AT&T's Albert Chan, whose nearest-neighbor qubits were shown to be capable of interbit coupling that potentially performs calculations without resolving the qubits nebulous quantum state.