First evidence for entanglement of three macroscopic objects has been
seen in a superconducting circuit built at the University of Maryland.
By examining an electrical circuit operating at temperatures near absolute
zero, the researchers have found new evidence that the laws of quantum
mechanics apply not just to microscopic particles such as atoms and
electrons, but also to large electronic devices called superconducting
quantum bits (qubits).
While researchers have previously created superconducting
qubits, and other groups have entangled two macroscopic objects (Update
558), this research is the first to observe the quantum interaction
of three macroscopic components: a niobium inductor-capacitor (LC) circuit
plus a pair of Josephson junctions, each a sandwich of two superconductors
separated by an insulator. Remarkably, all three macroscopic devices
seem to act, when cold enough, like huge atoms. The LC circuit coupled
the Josephson junctions in such a way as to transfer quantized oscillations
of current in one junction to the other junction. The LC circuit was
more than a simple connector; its condition depended upon the two Josephson
junctions in a way defined by the laws of quantum mechanics.
obtained evidence of the entanglement indirectly, through the use of
microwave pulses that probed the Josephson junctions; future experiments
will seek to directly control the junctions and obtain evidence more
directly. Superconducting circuits such as this one provide a promising
route towards a practical quantum computer, which would require the
entanglement of many qubits.
Scaling up superconducting devices to many-qubit
systems should be possible once single superconducting qubits are perfected,
according to team member Frederick Strauch, (now at NIST, 301-975-5159,
Frederick.Strauch@nist.gov). The challenge will be to fabricate sufficiently
high-quality circuits so that the superconducting qubits achieve the
very low noise levels necessary for quantum computing. (Xu
et al., Physical Review Letters, 21 January 2005)