Computing / Quantum Computing

How the nature of cause and effect will determine the future of quantum technology

An unprecedented, global-scale test of one of quantum theory’s most counterintuitive predictions sheds new light on the nature of reality and how we can exploit it with quantum technologies.

Here’s a curious question: Do certain physical events have no cause, or is there a reason behind every action?

This conundrum lies at the heart of one of the strangest areas of foundational science. And it has puzzled some of the greatest minds in the history of science.

But it also has important consequences for emerging technologies such as quantum computing and quantum cryptography. It may even lie at the heart of an entirely new area of science that is changing our understanding of cause and effect.

Today we get an answer to this question, thanks to the work of Morgan Mitchell at the Barcelona Institute of Science and Technology in Spain, along with dozens of collaborators and more than 100,000 experimenters around the world who have carried out a unique test of one of the most confounding predictions of quantum theory.

Their conclusion is that there needn’t be an explanation for every action. “If human will is free, there are physical events with no causes,” say Mitchell and co. Their research uses evidence-based science to link the metaphysical concept of free will with basic physics for the first time.

First, some background. One of the curious features of quantum mechanics is that it allows quantum particles created at the same point in space and time to share the same existence. This link is known as entanglement and it remains intact no matter how far apart these particles move.

The strange thing about entanglement is that it links one point in the universe with another point without spanning the distance in between. So a measurement on one instantaneously influences the other no matter how far away it is.

That’s a long-standing puzzle because there is no way for one particle to influence the other without sending faster-than-light signals, and physicists are pretty sure this isn’t what’s happening.

But there is another potential explanation. This is that both particles are correlated in a hidden way that physicists do not yet understand. But were it possible to measure this hidden variable, physicists would see how it determines the behavior of both particles.

By this way of thinking, quantum behavior is entirely deterministic and there is a reason for everything that happens on the quantum scale. This hidden variable must be part of a deeper theory of reality.

That raises an obvious question: If there is a deeper theory of reality, how can we find evidence of it?

In the 1960s, John Bell, then an obscure physicist at CERN, became concerned with this problem. Einstein had wrestled with it unsuccessfully back in the 1930s, but successive generations of physicists had swept the problem under the carpet since then, reluctant to deal with the idea that there could be a more fundamental theory than quantum mechanics.

By contrast, Bell grasped the problem by the scruff of the neck. He showed that if a hidden variable theory was the bedrock on which quantum mechanics was built, the universe would behave in a subtly different way than if quantum mechanics were the bedrock alone. And crucially, he showed how this difference could be measured.

Bell’s test measures the properties of two entangled particles—essentially how a measurement on one influences the other. If a hidden variable theory were true, there would be one outcome; if not true, a different outcome.

In the late 1960s, the Bell test was beyond the capabilities of quantum physicists. It required a reliable source of entangled particles, which was impossible to produce in those days. And it required lots of measurements to build up the statistical evidence required to convince physicists.

It wasn’t until 1982 that the technology had advanced enough for a Bell test to be performed. And the experiment clearly showed that hidden variable theories were incompatible with the results. The Bell test showed that the way one entangled particle influenced another was not the result of a hidden variable governed by deterministic principles. In other words, the process of cause and effect could not explain this influence.

This result was so mind-bending and profound that most physicists simply ignored it. But a small band of quantum physicists began to investigate it in more detail.

They worried that the experiment had an important loophole. Bell’s test requires certain measurements to be performed with random settings. For example, an entangled photon could be sent through a polarizing filter set at a randomly chosen angle.

True randomness is important because it does not have an underlying pattern that could be determined by a hidden variable theory. However, if the settings in the test were not random, but instead influenced by a hidden variable, the results would be void and the experiment invalid.

But here’s the difficulty. Guaranteeing true randomness is hard. Physicists can compute seemingly random numbers, but this process depends on the laws of physics and therefore on any hidden variable theory, should it exist. Indeed, if a hidden variable theory is in operation, then it governs the whole universe and every process within it, including any deterministic process used to set up the experiment.

Since 1982, physicists have performed many Bell tests. Indeed, they have become routine in quantum optics labs and a key part of the protocols used in emerging technologies such as quantum cryptography. Every one of these tests suggests that hidden variable theory cannot be true. But at the same time, every test could be a victim of this same loophole.

For Bell, there was one potential way out of this conundrum—to use human free will. In principle, free will allows us to choose any setting for the experiment, regardless of the role of a hidden variable theory. So the ultimate Bell test would involve humans choosing the settings in the experiment to close this freedom-of-choice loophole.

That’s easier said than done. A typical Bell test involves millions of entangled pairs and millions of changes to the experimental settings over a period of a few hours. But a single human controlling these settings could change them no faster than about 3 bits per second. Clearly such an experiment would be impractical.

That’s where Mitchell and his colleagues come in. Their idea was to crowdsource the necessary human influence. So for 12 hours on 30 November 2016, they gathered together 100,000 volunteers—so-called Bellsters—from all over the world to generate random bits that could then be used to control the settings on 13 different tests of Bell’s ideas.

To produce enough data consistently, Mitchell and co. gamified the process of producing bits, giving players scores and rewards for achieving certain targets. The bits were then fed at a constant rate of 1,000 bits per second to labs all around the world that had agreed to perform a Bell test in various ways, using photons as the quantum particles, atoms, and even superconductors in myriad combinations.

By itself, that's an impressive achievement. Engaging 100,000 volunteers from all over the world to work on an experiment at the same time on a single day is a significant accomplishment by any standards. It'll be interesting to see how this kind of crowdsourcing capability can be used in the future.

But the experiment itself is the real focus and the results are unequivocal. The 13 experiments all produced results that strongly refute the possibility of a hidden variable theory. And they close the freedom-of-choice loophole as far as it is possible to do so. “The results show empirically that human agency is incompatible with causal determinism, a question formerly accessible only by metaphysics,” say Mitchell and co.

That is good news for the many emerging quantum technologies that rely on Bell tests, such as quantum teleportation and quantum cryptography. The existence of a hidden variable theory would imply, for example, that quantum cryptography may not be perfectly secure.

Of course, this Big Bell Test isn’t perfect. Humans are governed by the laws of physics in the same way as all other objects. Indeed, we are simply complex machines, no different in principle from any other machine that can twiddle dials and change experimental settings.

So human free will has no special status in the universe, and if hidden variable theory governs the universe, it must also govern our free will too. In that case, human will would not be free but ultimately governed by a deterministic system of hidden variables.

So the Big Bell Test doesn’t close all the loopholes. But it does suggest that if there is a deeper reality beneath quantum mechanics, it will not be accessible to us.

So what of the original question: Do certain physical events have no cause?

The Big Bell Test provides an answer, albeit of a conditional variety. The answer is this: if humans have free will, then some physical events have no cause.

And that is a stepping stone for a whole new set of foundational experiments on the nature of cause and effect. Quantum mechanics—and Bell tests in particular—blur the distinction between cause and effect. So physicists are exploring the boundaries of these ideas to see how they can be used in computing devices, security algorithms, and the like. The early results are promisingly ambiguous, although it will be some time before they find their way into everyday applications.

It is 50 years since Bell put forward his controversial ideas, but Bell tests now lie at the heart of the emerging quantum technology revolution. He would surely be optimistic that further progress will be forthcoming.

Ref: Challenging Local Realism With Human Choices