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‘Magnetic Josephson effect’ seen for the first time

19 Apr 2012
A magnetic Josephson junction

A fundamental prediction of superconductivity theory has been demonstrated in the lab for the first time. An international team of physicists has observed coherent quantum phase slip, a phenomenon similar to the well-known Josephson effect in which magnetic flux takes the place of electric charge. Its discovery has fundamental implications for our understanding of macroscopic quantum systems and could also lead to intriguing applications, including a possible way to produce a qubit in a quantum computer.

In 1962 the British physicist Brian Josephson developed a theory of how superconducting electrons tunnel across a thin insulating layer between two superconductors – a structure now called a Josephson junction. This was quickly verified in the lab and Josephson was awarded the 1973 Nobel Prize for Physics. The Josephson junction has become an important technology in its own right. For example, superconducting quantum interference devices (SQUIDs) that, depending on their design, use either one or two Josephson junctions are among the most sensitive magnetometers to have been invented. The devices have also shown promise as possible quantum bits (qubits) in quantum computers.

Phase slips through a superconductor

In 2006 Hans Mooji and Yuli Nazarov at Delft University in the Netherlands did theoretical work on the quantum tunnelling of magnetic flux between two areas of free space through a thin layer of superconductor. This effect is called coherent quantum phase slip, and Mooji and Nazarov argued that it is an exact analogue of the Josephson effect. This is because, while free space shows no resistance to the flow of magnetic flux, one of the fundamental properties of a superconductor is the Meissner effect, whereby it expels any magnetic field from its interior. It therefore behaves as the magnetic equivalent of an insulator. However, in the subsequent six years, no-one successfully showed whether or not coherent quantum phase slip across a superconductor could actually occur.

Now, Oleg Astafiev and colleagues at the NEC Green Innovation Research Laboratories and the Institute for Physical and Chemical Research in Ibaraki, Japan, are claiming the first experimental observations of coherent quantum phase slip.

The experiment was done on a quantum-mechanical circuit called a Mooji–Harmans qubit – a ring of superconductor that narrows at one point into a very thin nanowire. If coherent quantum phase slip did not occur, magnetic flux inside the ring would be unable to get out, and magnetic flux outside would be unable to get in because of the impermeability of a superconductor to magnetic flux. However, Astafiev’s group observed clear evidence of magnetic interaction between the inside and the outside of the ring while the ring remained in the superconducting state – clear evidence that flux was crossing the nanowire by quantum tunnelling.

‘Two significant features’

Alexey Bezryadin, at the University of Illinois at Urbana-Champaign, believes that the work marks a significant achievement, both in terms of its progress in fundamental physics and its potential for application. “I would say there are two significant features to this work,” he says. “One is that the observation of these coherent quantum phase slips extends the applicability of quantum mechanics to more complex macroscopic systems. The applied aspect is that there are predictions that, if coherent quantum phase slips can exist (and this paper demonstrates that they do), you can use that to build certain useful devices.”

Astafiev agrees: “The phenomenon that we demonstrated is fundamental. As fundamental, I would imagine, as the Josephson effect. Josephson physics has proved very rich and there are many very useful devices based on the Josephson effect.” He believes it should be possible to exploit coherent quantum phase slip to build devices analogous to those based on the Josephson junction. In particular, Astafiev, who has a specific interest in quantum computing, hopes that qubits based on coherent quantum phase slip may not be prone to “charge noise” – a type of noise caused by the presence of an insulator that tends to cause quantum decoherence in Josephson qubits.

The research is published in Nature.

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