Tuesday, September 29, 2009

Quantum Entanglement Visible to the Naked Eye?

Well, not quite, because all you see is nothing more than an electrical circuit. But the circuit itself is doing the quantum entanglement.

This Wired article reports on a paper published in Nature of the violation of Bell's inequality in a Josephson phase qubits[1].

In the new study, researchers used a microwave pulse to attempt to entangle the electrical currents of the two superconductors. If the currents were quantum-mechanically linked, one current would flow clockwise at the time of measurement (assigned a value of 0), while the other would flow counterclockwise when measured (assigned a value of 1), Martinis says. On the other hand, the currents’ directions would be completely independent of each other if everyday, classical physics were at work.

After attempting to entangle the superconducting circuits, Martinis and his team measured the directions of the currents 34.1 million times. When one current flowed clockwise (measured as a 0), the team found, the other flowed counterclockwise (measured as a 1) with very high probability. So the two were linked in a way that only quantum mechanics could explain.

To complete this, here's the 'abstract' of the paper in question.

Abstract: The measurement process plays an awkward role in quantum mechanics, because measurement forces a system to 'choose' between possible outcomes in a fundamentally unpredictable manner. Therefore, hidden classical processes have been considered as possibly predetermining measurement outcomes while preserving their statistical distributions. However, a quantitative measure that can distinguish classically determined correlations from stronger quantum correlations exists in the form of the Bell inequalities, measurements of which provide strong experimental evidence that quantum mechanics provides a complete description. Here we demonstrate the violation of a Bell inequality in a solid-state system. We use a pair of Josephson phase qubits acting as spin-1/2 particles, and show that the qubits can be entangled and measured so as to violate the Clauser–Horne–Shimony–Holt (CHSH) version of the Bell inequality10. We measure a Bell signal of 2.0732 plusminus 0.0003, exceeding the maximum amplitude of 2 for a classical system by 244 standard deviations. In the experiment, we deterministically generate the entangled state, and measure both qubits in a single-shot manner, closing the detection loophole11. Because the Bell inequality was designed to test for non-classical behaviour without assuming the applicability of quantum mechanics to the system in question, this experiment provides further strong evidence that a macroscopic electrical circuit is really a quantum system7.

This experiment definitely closed the detection loophole since they can make a detection of each qubit. But due to the distance of the separation, they cannot close the locality loophole. Still, they think that, in principle, due to the fast measurement that can be made, that loophole can be closed in future experiments.


[1] M. Ansmann et al., Nature v.461, p.504 (2009).

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