Does Schrödinger's cat really exist? You bet. The first ever quantum superposition in an object visible to the naked eye has been observed.
Aaron O'Connell and colleagues at the University of California, Santa Barbara, did not actually produce a cat that was dead and alive at the same time, as Erwin Schrödinger proposed in a notorious thought experiment 75 years ago. But they did show that a tiny resonating strip of metal – only 60 micrometres long, but big enough to be seen without a microscope – can both oscillate and not oscillate at the same time. Alas, you couldn't actually see the effect happening, because that very act of observation would take it out of superposition.
"We talk about quantum weirdness and things being in two places at once, but it all involves atoms and molecules, stuff we don't normally interact with," says O'Connell, who presented the results at the March meeting of the American Physical Society in Portland, Oregon.
Bridge between worlds?
Proving that all objects, whatever their size, obeys the same rules has long been a goal of physicists. But with quantum mechanics it is no trivial matter: the larger an object, the more easily its fragile quantum state is destroyed by the disruptive influence of the world around it. O'Connell's experiments required delicate control and a temperature of just 25 millikelvin to measure the state in the few nanoseconds before it was broken down by disruptive influences from outside.
"It was a close call, but sufficient to see a first quantum signature," says Markus Aspelmeyer of the University of Vienna, Austria, who was not involved in the research.
The key was to connect the resonating strip to a superconducting qubit – a tiny electric circuit that can easily be prepared in a quantum superposition of two energy states. "The qubit acts as a bridge between the microscopic and the macroscopic worlds," says O'Connell. By tuning the frequency at which the qubit cycled between its two states to match the resonant frequency of the metallic strip, the qubit's quantum state could be transferred to the resonator at will.
When measured afterwards, the resonator was sometimes in its non-oscillating ground state and sometimes in an oscillating "excited" state. The number of times it was measured to be in each state followed the probabilistic rules of quantum mechanics.
Next, the cat?
"It's like you have a child's swing that goes back and forth," says O'Connell. "We pushed the swing and didn't push the swing at the same time."
"This is challenging and creative work," says Khaled Karrai of Ludwig-Maximilian University Munich, Germany. "If correct, it is a breakthrough."
Schrödinger's cat would be unlikely to survive the frigid temperatures of such experiments, so it is perhaps not the next milestone to look out for. But now the spooky influence of quantum physics on visible objects has been proved, can we expect to be putting an object as large as a real child's swing into an indeterminate quantum state any time soon? O'Connell thinks so. "I'd say in the near future – in the next 20 years."