Understanding quantum-nonlocality will unlock new technology

Novel research conducted by researchers from the University of Vienna, the Austrian Academy of Sciences, and the Perimeter Institute concludes that quantum-nonlocality is a universal property of the world, regardless of how and at what speed quantum particles move.

Quantum-nonlocality demonstrates the strong correlations between numerous quantum particles, some of which adjust their state immediately when the others are measured, irrespective of the distance between them.

While this phenomenon has been verified for slow moving particles, it has been discussed whether nonlocality is preserved when particles move very quickly at velocities close to the speed of light, and even more so when those velocities are quantum mechanically indefinite.

Now, scientists from the University of Vienna, the Austrian Academy of Sciences, and the Perimeter Institute have explained that nonlocality is a universal property of the world, regardless of how and at what speed quantum particles move.

Their research has been published in the latest issue of Physical Review Letters.

Correlations arise in everyday life and obey two principles – ‘realism’ and ‘locality’. To explain how correlations can arise, suppose that each day of the month you send Alice and Bob a toy engine of a set of two for their collection. You can select each of the engines to be either red or blue or either electric or steam. Your friends are divided by a large distance and do not know about your selection. Once their engines are delivered, they can verify the colour with a device that can differentiate between red and blue or check whether the engine is electric or steam by applying another device. They contrast the measurements made over time to look for specific correlations.

‘Realism’ means that Alice and Bob reveal only what colour or the mechanism of the engine you had chosen in the past, and ‘locality’ means that Alice’s measurement cannot change the colour or the mechanism of Bob’s engine (or vice versa).

Bell’s theorem, published in 1964, is deemed to be one of the most insightful breakthroughs in the foundations of physics, demonstrating that correlations in the quantum world contradict the two principles, which is a phenomenon known as quantum non-locality.

Quantum nonlocality has been validated by copious experiments, the so-called Bell tests, on atoms, ions, and electrons. It has profound philosophical connotations and also reinforces many applications, such as quantum computation and quantum satellite communications. However, in all the experiments, the particles were either at rest or moving at low velocities (which is referred to as “non-relativistic”). One of the unresolved challenges in this field is whether nonlocality is preserved when particles are travelling extremely fast, close to the speed of light or when they are not even travelling at a well-defined speed.

For two quantum particles in a Bell’s test which travel at high speeds, physicists have forecasted that the correlations between the particles are, in principle, reduced. However, if Alice and Bob adapt their measurements in a way that depends on the speed of the particles, the correlations between the outcomes of their measurements are still nonlocal. Now suppose that as well as the particles moving very fast, their velocity is also indefinite: each particle moves in a superposition of distinct velocities concurrently. In such a case, is their description of the world still non-local?

The researchers have indicated that Alice and Bob can create an experiment proving that the world is nonlocal. In order to achieve this, they employed one of the most fundamental principles of physics, namely that physical phenomena do not depend on the frame of reference from which we observe them. For example, corresponding to this principle, any spectator, whether moving or not, will see that an apple falling from a tree will touch the ground.

The team went beyond this and extended the principle to reference frames ‘attached’ to quantum particles. These are known as ‘quantum reference frames.’ The core understanding is that if Alice and Bob could move with the quantum reference frames along with their respective particles, they could perform the usual Bell test, since for them the particles would be at rest. In this way, they can prove quantum nonlocality for any quantum particle, irrespective of whether the velocity is indefinite or close to that of light.

Flaminia Giacomini, one of the study’s authors, explained, “Our result proves that it is possible to design a Bell experiment for particles moving in a quantum superposition at very high speeds,” while the co-author, Lucas Streiter, added, “We have shown that nonlocality is a universal property of our world.”

Their findings are anticipated to widen applications in quantum technologies, such as quantum satellite communications and quantum computation, using relativistic particles.

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