Entanglement is one of the most mysterious and fundamental properties of quantum mechanics and allows particles to have a much closer relationship than is possible in classical physics. If two particles are entangled, we can know the state of one particle by measuring the state of the other. However, entangled states are thought to vanish above a certain temperature because of thermal effects that make the system classical in a phenomenon known as "decoherence".

Now, Vedral and colleagues have shown otherwise. The UK-Portugal-Austria team have calculated that an entangled state formed between the photons in a laser pulse and the phonons -- quantum mechanical vibrations of the crystal lattice -- in a mirror can persist at arbitrarily high temperatures. The physicists obtained their results by treating both the laser light and the mirror as simple quantum-mechanical harmonic oscillators. The photons and phonons interact via the so-called light pressure mechanism, in which photons bombarding the mirror exert a pressure on it because of mutual interactions.

The pressure exerted on the mirror depends on the number of photons hitting it: the more photons in the laser, the more pressure they exert on the mirror and the more the mirror vibrates. Vedral and co-workers calculated that if they were to measure five photons in the light field, then there would be five phonons in the motion of the mirror; and if they measured ten photons, then that meant ten phonons, and so forth. This is typical of an entangled state but the difference in the new calculation is that it works for large systems too -- there are millions of photons in the laser beam and more than a billion atoms in the mirror.

The results show that macroscopic entanglement is not that difficult to create. "If our analysis is confirmed in an experiment -- and I see no reason to believe otherwise -- then this would push the limits of the validity of quantum mechanics further," says Vedral. This may also have important implications for quantum computers: "Perhaps we would not need to cool quantum bits (or 'qubits') down to low temperatures in order to use them for quantum computation. Maybe we could have room temperature quantum computers, just like the classical ones of today."