By Geoff Brumfiel

Suspending a cat between life and death is one of the best-known thought experiments in quantum mechanics.

Now researchers from Germany and Spain are proposing a real experiment to probe whether a virus can exist in a superposition of two quantum states. Such superpositions are typically the domain of smaller, inanimate objects such as atoms. But the team believes that their technique, using finely tuned lasers, will soon allow for the superposition of something much closer to a living organism. They outline the experiment in a paper posted to the arXiv pre-print server.

At its most fundamental level, quantum mechanics says that particles can only exist in discrete states. For example, researchers can measure the direction a particle spins as either 'up' or 'down', but nothing in between. Yet, as long as no one is looking, the particle exists in a combination of both states simultaneously, a strange blend known as a superposition.

In the 1930s, Austrian physicist Erwin Schrödinger described the now-famous cat experiment, intending it as a caution against applying quantum rules to the real, 'classical' world. He imagined placing a cat inside a box that contains a vial of hydrogen cyanide. A hammer, suspended above the vial, would be set to smash it if triggered by the decay of a tiny chunk of radioactive material.

As long as the box is closed, the radioactive material behaves like a spinning particle because it exists in two states -- decayed or not decayed. That means the cat is left in a limbo of being simultaneously dead and not dead. Only when scientists actually measure whether the radioactive substance has decayed will the superposition break down into one or other of the two possibilities.

Oriol Romero-Isart at the Max Plank Institute for Quantum Physics in Garching, Germany, and his colleagues now say they hope to attempt a similar 'Schrödinger's virus' experiment in the lab.

Chilled out virus

The team hope to trap a virus in a vacuum using an electromagnetic field created by a laser. Then, with another laser, the team would slow down the virus's movement until it sits motionless in its lowest possible energy state.

Once the virus is fixed, the team will use a single photon to put the virus into a quantum superposition of two states, where it is either moving or not. Until it is measured, the virus should exist in a superposition of motion and stillness.

The team suggest that tobacco mosaic virus, a rod-shaped plant virus measuring about 50 nanometres wide and almost 1 micrometre long, would be an ideal candidate for the experiment. While there is still debate about whether such viruses can really be classed as alive, the experiment could even be extended to tiny organisms, the scientists say. Microscopic tardigrades, or water-bears, can survive in the vacuum of space for days, and may be suitable for the same sort of Schrödinger treatment.

Other physicists are sceptical about how much the experiment will show. There's no reason to believe that a virus would behave any differently than a similar-sized inanimate object, says Martin Plenio a physicist at Imperial College in London. "I'm absolutely convinced that a virus would behave exactly the same as an inorganic molecule," he says.

Still, he concedes, testing relatively large objects, whether viruses or molecules, could prove interesting. According to quantum mechanics, it should be possible for macroscopic objects like cars and people to enter superpositions, but that never appears to happen. Studying relatively large objects, says Plenio, may help physicists learn where the quantum world ends and the our macroscopic world begins.