A new experiment uses superconducting qubits to show that quantum mechanics violates so-called local realism by allowing two objects to act as a single quantum system no matter how large the separation between them is. The experiment isn’t the first to show that local realism isn’t how the Universe works — it’s not the first to do so with qubits.
But it is the first to separate the qubits far enough to ensure that light is not strong enough to travel between them while measurements are being made. And it does this by cooling a 30-meter-long aluminum wire to just a few milliKelvins. Because qubits can be easily controlled, the experiment provides a new precision in these types of measurements. And the hardware setup will be essential for future quantum computing efforts.
Being real about realism
Albert Einstein was famously uncomfortable with some of the consequences of quantum entanglement. If quantum mechanics is correct, then a pair of entangled objects behaves as a quantum system no matter how far apart the objects are. Changing the state of one of them must immediately change the state of the second, with the change seemingly happening faster than light can travel between the two objects. This, Einstein argued, was almost certainly wrong.
Over the years, people have proposed different versions of so-called latent variables—physical properties that are shared between objects, that enable the binding-like behavior while preserving the information that dictates in that localized nature. Hidden variables preserve so-called “local realism” but do not truly describe our reality.
Physicist John Bell has shown that all local variable frameworks limit the extent to which the behavior of quantum objects can be correlated. But quantum mechanics predicts that the correlations should be higher than that. By measuring the behavior of pairs of entangled particles, we can determine whether they violate Bell’s equations, and thus clearly show that hidden variables do not explain their behavior. .
The initial steps toward this demonstration were not good for hidden variables but allowed loopholes—even if Bell’s inequality was violated, it remained possible that information could travel between quantum objects at the speed of light. . But over the past few decades, loopholes have gradually been closed and Nobel Prizes have been awarded.
So why return to experiments? In part because qubits give us a lot of control over the system, which allows us to quickly perform many experiments and examine the nature of this entanglement. And partly because it is an interesting technical challenge. Superconducting qubits are controlled by microwave radiation, and their insertion requires the movement of several very low-energy microwave photons between the two. And doing that without environmental noise disturbing everything is a serious challenge.