We also feel very proud and humbled at Quside that our founders had the opportunity to collaborate with Anton Zeilinger’s loophole-free Bell test experiments in 2015. Today, as a company, we are industrializing and scaling the same quantum randomness technology we used back then as we seek broad societal impact via safer connectivity and improved decision making applications. In this blog entry, we reflect on the history of Bell tests and the role of randomness in these experiments.

The origins of Bell test experiments

In the 1930s, a significant discussion emerged in the physics community due to some surprising predictions that quantum physics was making about how the world worked. Quantum physics theory challenged the idea that our world obeys the principles of local realism. What does that mean? Locality and realism are incredibly intuitive concepts in our daily lives and are also widely used in physics. They are so intuitive that no one had ever questioned them before! Locality refers to the fact that one object can only influence another if they are in the same place, or if some medium carries the influence from one to the other. Realism refers to the fact that objects have well defined properties, independently of our knowledge of them. Quantum physics questioned such basic assumptions and predicted that non-local instantaneous interactions were possible and that objects did not have well-defined properties until we measured them.

This “clash in basic principles” led to (heated) discussion among some of the most renowned physicists of the day and led to two main lines of thought. The first group believed that quantum physics was incomplete and therefore new theories were required (Albert Einstein was the most prominent proponent of this argument). The second group believed that it was probably time to give up to these intuitive concepts and embrace the new theory predictions (Niels Bohr was famously in this second category).

With lack of experimental evidence, however, it was left as a philosophical discussion only. Quantum Physics worked well to describe experiments, and everyone was happy about this new tool

John S. Bell and his landmark contribution to quantum information

In 1964, and well after those discussions emerged in the 1930s, John S. Bell devised a thought experiment that would allow to test experimentally whether or not local realism is really how the world works. But in the 1960s, it was not possible to realize those experiments. In a nutshell, a Bell tests consists in 4 main steps:

• Generating a pair of entangled particles,
• Randomly measure them (that is, generate a random choice to select what measurement you apply to each entangled particle),
• Record the measurement choice and the result of the measurement and
• Process the data obtained from multiple iterations of the experiment and run some statistics.

The beautiful point of a Bell test is this one (for simplicity and without lack of generality, let us assume the Bell test inequality proposed by CHSH): if you do a Bell test experiment and run the statistical correlation, two things may happen. If local realism is correct (that is, “Einstein was right”), the result of the correlations would never be larger than 2. In contrast, if the correlation value is larger than 2, then it means that local realism cannot describe our world (ergo, that “Bohr was right”).

Experiments & random numbers

John F. Clauser (one of the laureates of this year’s Nobel Prize) was the first to experimentally run a Bell test experiment. And he saw that the result of the test was “larger than 2”. Thus, that local realism was potentially wrong. Unfortunately, the experiments he was able to run at that time had some loopholes, and improvements were needed to provide enough confidence to refute local realism. Here’s where Alain Aspect (another of the laureates) comes in. He run the first experiments closing the major loophole that the previous experiments had. He also observed “a value larger than 2”, and therefore provided strong experimental evidence that quantum objects had in fact behaviors totally counter intuitive with our previous physical theories and we needed to embrace this new “reality”.

After these early experiments, Bell tests increased in sophistication over the years, increasing little by little the confidence in the results. And here’s where in 2015, a series of experiments run the so-called loophole-free Bell test experiments, which closed all major loopholes in previous Bell tests simultaneously. These were extremely complex and advanced experiments and took a long time to implement properly. Anton Zeilinger (the third laureate of this year’s Nobel Prize) led one of those loophole-free Bell test experiments in 2015. The other two experiments were led by NIST in the US and TU Delft in the Netherlands.

The Quside founding team had the honor of participating in those 3 experiments by contributing the devices that implemented step #2 of a Bell test, as described above. That is, we built the devices that randomly chose which measurement was to be realized in the loophole-free Bell experiments. To build those devices, we use the same phase-diffusion technology than we use today at Quside to build our products. Two items were decisive for this technology to be used in those experiments. The first one was its speed; The ability to generate fresh random numbers very quickly. The second one was the ability to measure the quality through strong modelling and characterization. This is still the essence of our technology and products.

Future applications

Entangled photons and Bell tests are foundational technologies in quantum information and set the basis of future technologies with applications in communications, security, and computation. By means of Bell test experiments, device-independent cryptographic protocols can be realized, which will constitute a new tool for future communication networks.

Wrap!

We are very happy for today’s Nobel Prize announcement and for the decision to recognize the decades of excellent experimental work that laid the foundations for quantum technologies. These developments are at the heart of the second quantum revolution. We at Quside are proud that the core technology embedded in our products was also used in some of these amazing Bell test experiments. Congratulations to the Nobel Laureates this year and to the Quside founders for their contribution to the loophole-free Bell tests in 2015.