How’s this for a quantum magic trick? A clever experiment keeps Schrödinger’s cat alive – and dead – after being sawed in half. The stunt could eventually help network quantum circuits into a working computer.

Fortunately, the technique was tested not on a real cat, but on electromagnetic waves, which can be analogous to the cat in Erwin Schrödinger’s famous thought experiment.

Quantum particles are capable of existing in a superposition of states, or two modes of being at once. A photon, for instance, can simultaneously be polarised vertically and horizontally. This superposition holds until someone makes a measurement, at which point the photon picks a state.

Schrödinger argued that if quantum rules applied in the macroscopic world, a cat stuck inside a closed box could be both alive and dead at the same time – at least until you open the box.

Microwave photons trapped in a box can be coaxed into a so-called “cat” state. Normally, electromagnetic waves in the box will oscillate in strength, like a pendulum sweeping back and forth. But it’s possible to introduce the opposite wave into the box, creating a cat state that is doing two seemingly contradictory things at once.

“A mechanical analog of this would be a pendulum that is simultaneously oscillating to the left and to the right,” says Chen Wang, then at Yale University.

### The magician’s assistant

Wang’s experiment goes a step further, though. His team prepared two cavities of aluminium in which microwave photons could bounce around. Then they connected the cavities with a channel: a superconducting sapphire chip and aluminium circuit, across which electrical signals could travel.

Think of that chip like an on-off switch. When the switch is “on” and the channel is open, microwaves inside a cavity connected to it would oscillate at a different frequency than they would if the switch was “off”.

This being the quantum world, though, it is possible to have the linking bridge be both “on” and “off” at the same time. “Once that happens, both cavities will have two frequencies at once,” Wang says.

The magician’s flourish is then to sever the link and show that the two sides are still connected – with one whole, functioning half-alive cat shared between two boxes, like the magician’s assistant smiling and waving after she has been sawed in half.

Wang’s team switched the chip to completely “off” and tested whether the two cavities were still working together. To find out whether he had a cat state, though, he couldn’t just open the box and look.

“You can always ask the question, are you dead or alive?” Wang says. “But this question doesn’t tell you whether it is a true quantum superposition, or whether you prepared half the chance of a dead one and half the chance of a live one.”

Instead, the team had to ask a question that would reveal the cat state without disturbing it. They measured the number of photons in each box, knowing that cat states made from electromagnetic waves should always turn up with an even number of photons.

Measured separately, the two boxes sometimes contained even numbers of photons and sometimes odd. But both boxes added together always turned out even.

“That shows you that when you combine the two boxes, you get a true Schrödinger’s cat state,” Wang says.

### The real payoff

The idea of building a cat state in just one microwave cavity is already a few decades old, and helped win Serge Haroche a Nobel prize, points out Myungshik Kim of Imperial College London.

“You might think oh well, that’s a small extension of what Haroche did,” he says. “But it’s an interesting extension.” Kim suggests linking two cavities in a cat state could help with metrology, the problem of precisely measuring the phase of light.

The real payoff of such an experiment, Wang hopes, is that entangled cavities could be the building blocks of computers that exploit the properties of quantum superpositions to blaze through calculations at lightning speed.

Wang’s current lab, run by Robert Schoelkopf of Yale University, is studying the use of cat states in cavities as qubits, the units that hold information in quantum computing. Cat states could help correct the errors that quickly build up in these small, sensitive systems. Linking such qubits together will be essential.

Journal reference: Science, DOI: 10.1126/science.aaf2941