First Evidence For Entanglement of Three Macroscopic Objects

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.

The researchers 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)