Qubits 30 meters apart used to confirm Einstein was wrong about quantum

[Buffy]

https://arstechnica.com/science/2023/05 ... local/amp/


A new experiment uses superconducting qubits to demonstrate that quantum mechanics violates what's called local realism by allowing two objects to behave as a single quantum system no matter how large the separation between them. The experiment wasn't the first to show that local realism isn't how the Universe works—it's not even the first to do so with qubits.

But it's the first to separate the qubits by enough distance to ensure that light isn't fast enough to travel between them while measurements are made. And it did so by cooling a 30-meter-long aluminum wire to just a few milliKelvin. Because the qubits are so easy to control, the experiment provides a new precision to these sorts of measurements. And the hardware setup may be essential for future quantum computing efforts.

Getting real about realism

Albert Einstein was famously uneasy with some of the consequences of quantum entanglement. If quantum mechanics were right, then a pair of entangled objects would behave as a single quantum system no matter how far apart the objects were. Altering the state of one of them should instantly alter the state of the second, with the change seemingly occurring faster than light could possibly travel between the two objects. This, Einstein argued, almost certainly had to be wrong.

Over the years, people have proposed various versions of what are called hidden variables—physical properties that are shared between the objects, enabling entanglement-like behavior while keeping the information that dictates that behavior localized. Hidden variables preserve what's called "local realism" but turn out not to actually describe our reality.

Physicist John Bell showed that all local variable frameworks limit the degree 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 demonstrate that hidden variables don't explain their behavior.

Initial steps toward this demonstration were bad for hidden variables but allowed loopholes—even though Bell's inequalities were violated, it remained possible that information was traveling between the quantum objects at the speed of light. But over the past few decades, the loopholes have gradually been closed and the Nobel Prizes handed out.

So why return to the experiments? Partly because qubits give us a great deal of control over the system, allowing us to rapidly perform a large number of experiments and probe the behavior of this entanglement. And partly because it's an interesting technical challenge. Superconducting qubits are controlled with microwave radiation, and entangling them requires moving some very low-energy microwave photons between the two. And doing that without environmental noise messing everything up is a serious challenge.

Spooky action at a 30-meter distance

Violating Bell's inequalities is a relatively simple matter of measuring entangled particles repeatedly and showing that their states are correlated. If that correlation exceeds a critical value, then we know hidden variables can't explain this behavior. And superconducting qubits, called transmons, are made so that measurement is trivial, accurate, and fast. So that part's simple.

Getting rid of one of the major loopholes in these measurements is where things get difficult. You need to show that the correlation in the measurements could not have been mediated by information traveling at the speed of light. Since measurements require a bit of time to take place, that means you have to separate the two qubits by enough distance to allow the measurement to complete before light can travel between them. Based on how long the measurements take, the research team behind the new work, working at ETH Zürich, calculated 30 meters would be sufficient.

While that's barely down the hall in a typical lab building, 30 meters is extremely challenging because of the entanglement process, which involves using low-energy microwave photons, which are easily lost in a sea of environmental noise. In practice, this means that anything involved with these photons has to be kept at the same milliKelvin temperatures as the qubits themselves. So the entire 30 meters of aluminum wire that acts as a microwave waveguide needs to be chilled down to a tiny fraction of a degree above absolute zero.

In practice, this meant giving the entire assembly built to keep the wire cool access to the liquid helium refrigeration systems that housed the qubits at each end—and building a separate refrigeration system at the center point of the 30-meter tube. The system also needed flexible internal connections and exterior supports because the whole thing contracts significantly as it cools down.

Still, it all worked impressively well. Because of the performance of the qubits, the researchers could perform over a million individual trials in only 20 minutes. The resulting correlations ended up being above the limit set by Bell's equations by a staggering 22 standard deviations. Put in different terms, the p value of the result was below 10-108.

Things to come?

The two main factors limiting the system's performance are errors in the qubits and loss of the photons used to entangle them. The researchers think they can improve both, potentially making qubits the most stringent test of Bell's inequalities. But the work may become more significant because of how it entangled the qubits.

Everyone working with superconducting qubits says that we will ultimately need to integrate thousands of them into a single quantum computer. Unfortunately, each of these qubits requires a considerable amount of space on a chip, meaning it gets difficult to make chips with more than a few hundred of them. So major players like Google and IBM ultimately plan to link multiple chips into a single computer (something the startup Rigetti is already doing).

For tens of thousands of qubits, however, we're almost certainly going to need so many chips that it gets difficult to keep them all in a single bit of cooling hardware. This means we're going to eventually want to link chips in different refrigeration systems—exactly what was demonstrated here. So this is an important demonstration that we can, in fact, link qubits across these sorts of systems.

Nature, 2023. DOI: