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Physicists propose 'Schrödinger's virus' experiment

Published: 10 September 2009
Physicists propose 'Schrödinger's virus' experiment
Geoff Brumfiel
Laser technique could put virus in two overlapping quantum states.
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 server1.
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
In the 1930s, Austrian physicist Erwin Schrödinger described the nowfamous 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
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
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.
1. Romero-Isart, O. et al. preprint at http://arxiv.org/abs/0909.1469 (2009).