Archivi tag: quantum entanglement

Testing different aspects of the twin paradox using a physical system

Immaginiamo due gemelli, identici, tranne per il fatto che uno dei due possieda una navicella spaziale. Quest’ultimo decide di fare un viaggio verso una stella distante, diciamo qualche decina di anni-luce, mentre l’altro gemello rimane a terra. Viaggiando con una velocità pari a circa il 75% -85% rispetto a quella della luce, il gemello raggiunge la stella e fa ritorno a terra, incontrando il proprio gemello decisamente più vecchio di lui. Questo fenomeno, noto come paradosso dei due gemelli, è dovuto alla dilatazione dei tempi come viene descritto nella teoria della relatività speciale. Di fatto, Albert Einstein predisse che orologi soggetti ad accelerazioni diverse misurano il tempo in maniera differente. Per quanto strano possa sembrare, l’effetto della dilatazione dei tempi è stato verificato più volte in laboratorio e viene continuamente utilizzato nei sistemi GPS.

The GPS is able to provide you with your position by timing very precisely the signals emitted by satellites, and to this end it needs to take into account the time dilation due to the different accelerations of the satellites. While GPS is one of the most precise systems we have, it can locate your smartphone with an error margin of a few metres. The precision could be improved by using the most precise clocks that we know on Earth, known as quantum clocks because they are ruled by the laws of quantum mechanics. There are plans funded by space agencies to launch these clocks into orbit. It is natural to think that a GPS consisting of quantum clocks would also need to take into account relativistic effects.

However, we do not fully understand how to combine quantum mechanics and relativity. The inability of unifying both theories remains as one of the biggest challenges of modern science.

Predictions in the 1970s said that there is a physical phenomenon that is both quantum and relativistic called the Dynamical Casimir Effect. But it wasn’t until 2011 that an experimental setup could be developed to test the prediction. Here is what theory predicted: if light is trapped between mirrors that move at velocities close to the speed of light, then they will generate more light than there is in the system. Even if initially there is no light between the mirrors, just vacuum, light shows up because the mirror turns the quantum vacuum into particles. This is supposed to happen because vacuum at the quantum level is like a sea of pairs of particles that are constantly emitting and absorbing light. They do this at incredible speeds, but if the mirror moves that fast too some of these particles are reflected by the mirror before disappearing and can be observed. But setting up such a system has proved difficult. In 2011, this difficulty was circumvented in the experiment conducted by Per Delsing at Chalmers University of Technology in Sweden. In this case the mirrors were different. They were magnetic fields inside a Superconducting Quantum Interferometric Device (SQUID), but they behaved exactly like mirrors, making light bounce back and forth. Unlike physical mirrors, these magnetic fields could be moved at incredible speeds. Einstein used to think of clocks as light going back and forth between mirrors. Time can be inferred from the distance between the mirrors divided by the speed of light, which remains constant no matter what. But he never thought about particles being created by motion, a prediction that was made many years after his death.

In a recent work, with colleagues at the University of Nottingham, Chalmers University and University of Warsaw, we have taken inspiration from the 2011 experiment.

We propose using a similar setup to test different aspects of the twin paradox using a physical system, which haven’t been tested so far.

Although it won’t involve human twins, the possibility of achieving enormous speeds and acceleration allows the observation of time dilation in a very short distance. Also, all previous experiments that have tested the theory have involved atomic clocks, which are “point-clocks”, that is, what measures time in these atomic clocks is confined to a tiny point in space. Our experiment would instead use something that has finite length. This is important because, along with time, Einstein’s theory predicts that length of the object changes too. We believe our experiment would test that aspect of the theory for the first time. We have found that particle creation by motion, which was observed in 2011, has an effect on the difference in time between the clock that is moving and the one that is static. Using this setup, while we can reconfirm that time dilation occurs, the more interesting application would be to help build better quantum clocks, by means of a better understanding of the interplay between quantum and relativistic effects.

The Conversation: How to test the twin paradox without using a spaceship

arXiv: The twin paradox with macroscopic clocks in superconducting circuits

Why nature is quantum?

Sappiamo che gli scienziati sono come dei bambini perchè si pongono sempre la domanda sul ‘perché’ delle cose. Tra queste, una domanda a cui si deve ancora rispondere riguarda come mai la natura si comporta secondo le leggi della fisica quantistica. Oggi, due ricercatori, Corsin Pfister e Stephanie Wehner del Centre for Quantum Technologies presso la National University di Singapore stanno cercando di affrontare la questione in un articolo pubblicato su Nature Communications.

We know that things that follow quantum rules, such as atoms, electrons or the photons that make up light, are full of surprises. They can exist in more than one place at once, for instance, or exist in a shared state where the properties of two particles show what Einstein called “spooky action at a distance“, no matter what their physical separation. Because such things have been confirmed in experiments, researchers are confident the theory is right. But it would still be easier to swallow if it could be shown that quantum physics itself sprang from intuitive underlying principles. One way to approach this problem is to imagine all the theories one could possibly come up with to describe nature, and then work out what principles help to single out quantum physics. A good start is to assume that information follows Einstein’s special relativity and cannot travel faster than light. However, this alone isn’t enough to define quantum physics as the only way nature might behave. Corsin and Stephanie think they have come across a new useful principle. “We have found a principle that is very good at ruling out other theories“, says Corsin. In short, the principle to be assumed is that if a measurement yields no information, then the system being measured has not been disturbed. Quantum physicists accept that gaining information from quantum systems causes disturbance. Corsin and Stephanie suggest that in a sensible world the reverse should be true, too. If you learn nothing from measuring a system, then you can’t have disturbed it. Consider the famous Schrodinger’s cat paradox, a thought experiment in which a cat in a box simultaneously exists in two states (this is known as a ‘quantum superposition’). According to quantum theory it is possible that the cat is both dead and alive until the cat’s state of health is ‘measured’ by opening the box. When the box is opened, allowing the health of the cat to be measured, the superposition collapses and the cat ends up definitively dead or alive. The measurement has disturbed the cat. This is a property of quantum systems in general. Perform a measurement for which you can’t know the outcome in advance, and the system changes to match the outcome you get. What happens if you look a second time? The researchers assume the system is not evolving in time or affected by any outside influence, which means the quantum state stays collapsed. You would then expect the second measurement to yield the same result as the first. After all, “If you look into the box and find a dead cat, you don’t expect to look again later and find the cat has been resurrected“, says Stephanie. “You could say we’ve formalised the principle of accepting the facts“. Corsin and Stephanie show that this principle rules out various theories of nature. They note particularly that a class of theories they call ‘discrete’ are incompatible with the principle. These theories hold that quantum particles can take up only a finite number of states, rather than choose from an infinite, continuous range of possibilities. The possibility of such a discrete ‘state space’ has been linked to quantum gravitational theories proposing similar discreteness in spacetime, where the fabric of the Universe is made up of tiny brick-like elements rather than being a smooth, continuous sheet. As is often the case in research, Corsin and Stephanie reached this point having set out to solve an entirely different problem altogether. Corsin was trying to find a general way to describe the effects of measurements on states, a problem that he found impossible to solve. In an attempt to make progress, he wrote down features that a ‘sensible’ answer should have. This property of information gain versus disturbance was on the list. He then noticed that if he imposed the property as a principle, some theories would fail. Corsin and Stephanie are keen to point out it’s still not the whole answer to the big ‘why’ question: theories other than quantum physics, including classical physics, are compatible with the principle. But as researchers compile lists of principles that each rule out some theories to reach a set that singles out quantum physics, the principle of information gain versus disturbance seems like a good one to include.

See also: Is nature – deep down – actually discrete?

CQT: New principle may help explain why nature is quantum
arXiv: If no information gain implies no disturbance, then any discrete physical theory is classical