IT MAY soon be possible to extract information from a quantum object – and even manipulate it – without simultaneously destroying its delicate quantum state. The result would be a boon for quantum computing, which requires control over such states. It would also defy a thought experiment dreamed up by physicist Erwin Schrödinger: in principle it is now possible to peek inside his box without endangering the life of the precarious pussycat inside.

States that are mutually exclusive in classical physics can exist simultaneously in the weird world of quantum mechanics – a situation called a superposition. To illustrate this effect, Schrödinger imagined putting a cat in a box along with a device that would release poison to kill it, depending on the random decay of a radioactive atom. Because the atom’s quantum state only takes a definite value when someone looks at it, the cat is both dead and alive until the box is opened.

Superpositions are fragile, however. Outside disturbances, including observations, tend to destroy the “coherence” of these states, forcing the system to collapse into just one of the possibilities. The larger the system, the harder it is to isolate it from outside influences.

In 2010, physicists put the largest system yet into a superposition: a 40-micrometre-long strip of piezoelectric material, which expands and contracts in response to voltage changes. They put it into a superposition of both minimal and more vigorous oscillation, but the method they used to observe the system caused it to lose this dual state.


Another team now proposes going a step further, putting a wire of about the same size in a superposition and offering a scheme to observe, and even manipulate it, without destroying the weird quantum state. Kurt Jacobs at the University of Massachusetts, Boston, and his team describe their idea in a study to appear in Physical Review A.

The first step is to put the wire into a superposition in which vibrations simultaneously displace it by equal amounts in opposite directions, like a guitar string that gets plucked in two directions at once. Next, an electric charge can be added to the wire, creating an electromagnetic field that can be detected by a sensor (see diagram).

Even though the sensor cannot pinpoint the position of the charge – and therefore the wire – it can detect how far the charge is from a neutral, “unplucked” position. That reveals some information about the system – essentially providing a glimpse inside the box containing Schrödinger’s cat. The key is that it avoids opening the box completely, which would destroy the superposition, says Jacobs: “I extract information, but in a way that I don’t learn too much.”

Previous schemes to glimpse inside the box involved partially destroying the superposition and then trying to restore it. “In our paper, the crucial thing is that the measurement does not destroy the coherence,” says Jacobs.

The team also proposes adjusting the tension of the wire to change the size of the vibrations, an adjustment that would not destroy the fragile superposition.

Carrying out this experiment is still a few years away, the team says – the sensors needed to make the subtle measurements must be made less vulnerable to interfering noise. If the experiment eventually proves a success, it would be a step towards quantum computing.

Quantum computers, which have yet to be built, would be able to do many more calculations simultaneously than conventional computers can. Their capacity will stem from the ability of quantum systems to be in more than one state simultaneously. Making such a computer requires being able to read and alter the state of quantum systems, processes that the new experiment aims to achieve. “This proposal could prove very useful,” says Aephraim Steinberg at the University of Toronto in Canada.

Teleporting schrödinger’s cat OBSERVING an object in more than one quantum state at once – in a superposition – is still an elusive goal (see main story), but teleporting such an object is now old hat. Noriyuki Lee at the University of Tokyo, Japan, and colleagues have managed to make light in a superposition of states vanish in one place and reappear in another. They took advantage of entanglement, a quantum property that creates a spooky connection between separate objects which acts even at a distance. They entangled two light beams, so that measuring one affected the outcome of measuring the other. After mixing one of the entangled beams with pulses of light in a superposition of many quantum states, they were able to recreate the superposition in the second entangled beam (Science, DOI: 10.1126/science.1201034). “It shows that the controlled manipulation of quantum objects has… achieved objectives that seemed impossible just a few years ago,” says Philippe Grangier at the Institute of Optics in Palaiseau, France.