Saturday, November 21, 2009

Quantum Nonlocality and Superluminal Communication

Superluminal (faster than light) travelling is important to the colonisation of space in the future. The theory of relativity seems to forbid that as it means travelling back in time too. However, quantum nonlocality of nature is experimentally demonstrated with the violation of Bell’s inequality, thus raising the question: Does quantum nonlocality allows superluminal communication ?

*Superluminal Travelling, Communication and Time Travel will be regarded as the same and of equal footing here as they can be made to be the same. Superluminal Communication=Backward Time Travel +Forward Time travel +Displacement.*

Before going further, here’s a brief background in the world of physics. The field of theoretical physics is regarded highly as it is the foundation of all electrical, nuclear and nano technology. Most of these technologies highly make use of just one theory: quantum physics.

Ironically, the famous Albert Einstein who provided the first physical insight into quantum physics also highly dislikes the theory. He tried to show that quantum physics, with its inherent stochastic (probabilistic) mechanism, is not the most fundamental theory of the Universe. His final attack against quantum physics is known as the Einstein-Podolsky-Rosen (EPR) paradox. It shows that either quantum physics is incomplete or there has to be superluminal information transfer , which seems to be against the theory of relativity (A. Einstein May 15 1935). There has to be a local hidden variable theory then to replace quantum physics that requires this superluminal information transfer. The question of what is the true underlying mechanism of nature remained unsolved for almost 30 years, until Bell’s inequality came into the picture in 1964. Bell’s inequality enables physicists to verify, through experiment, how nature works in the fundamental level.

*In the context of this text, information transfer is not the same as communication. Information transfer is just the correlation between entangled pairs, and it does not necessary allows communication through them as will be seen. Instantaneous information transfer between the two entangled pairs is the nonlocality in quantum physics.*

Bell’s inequality is based on quantum entanglement which is explained below. In quantum theory, when two (or more) quantum objects interact with each other, they are in an entangled state. This means that the two objects lose their individual states and only the pair, considered together can have well defined states. Quantum physics is a stochastic theory because it does not give a well defined property of an object. Only when a measurement is made, will the property be fixed, based on a certain probability determined by its state. If a measurement of say, momentum is done on one particle, it will randomly choose a value of momentum, and the other entangled particle will have a well defined momentum (because of momentum conservation) instantaneously. If the pair does not interact with external objects, thus still remains entangled, no matter how far they are in space, once a measurement of a property is done on one particle, the other particle will instantaneously gain the information of what has been done on the first particle to fix its property!

Bell’s inequality is formulated in such a way that if the particle pair transmitted information faster than the speed of light as required by quantum entanglement, then the equality is violated. And current experiments verified that the inequality is violated, nature is nonlocal (instantaneous information transfer).
Thus it becomes interesting to know does quantum nonlocality allows superluminal communication?

The question is important as superluminal travelling and communication is essential for the colonisation space in a short time. Currently the Earth is undertaking a large toll of greenhouse gases and global warming is slowly but surely spelling massive damage to human civilisation in this century and the next. Environmental efforts maybe one of the solution to this problem but it is a dim hope as the call for it has been around for decades and comparatively little effort has been done in that direction. Moving to a new planet on the other hand, is an attractive alternative. However with the current state of space travel technology, it will be generations before any humans can make it safely to even the nearest star.

If we are able to master the technology of superluminal travelling, the whole galaxy, the whole universe is opened up to us. Space colonisation will be as easy as building a new city on Earth. Looking at the limited amount of time left before global warming takes its toll on research grants, it is imperative that the technology of superluminal travelling be developed as soon as possible. However, before going into the technology, it must be known whether superluminal travelling is theoretically allowed or not. Hence the question of interest.

Returning to physics, quantum entanglement seems to be superluminal communication itself. Many experiments (including Delayed “Choice” Quantum Eraser (Yoon-Ho Kim 3 January 2000), Bell’s inequality and even Quantum Teleportation (Zeilinger 1999)) have been designed to make use of entanglement and all of them failed to enable superluminal communication (to be fair, almost none of them were designed in mind to enable superluminal communication). These lead to a theorem called the no communication theorem.

To illustrate this further, some definitions are long overdue. Let’s call the two observers at two sides of a quantum entangled pair Alice and Bob respectively. Alice and Bob are also sharing a continuous beam of entangled pairs from a source. The only input is from the choice of which property of the quantum object to measure. The only output is from the values of the measured property. For Bob to communicate with Alice using the quantum entangled pair, Alice must be able to differentiate from her output (for some input she put in) what input Bob put into his entangled pair and vice-versa.

Even through Bob is free to choose his input; his output is governed by probability and beyond the control of anyone. The output of Alice is coupled to Bob’s output by quantum entanglement, thus it reflects the probability of Bob’s output. Since the output of Alice is random and beyond Bob’s control whatever his input is, Alice can never gain any useful information about Bob by just looking at her output. Hence the no communication theorem holds.

The technical details are more subtle than the argument above; however, the general idea remains the same: Nature seems to conspire to cover up any possibility of superluminal communication. Below shows a preview of some technical details of quantum eraser that will automatically “cover up” its trail so that superluminal communication or information travelling back in time is prevented.

“That tricky detail that we omitted earlier is what saves the day: to see the interference of the particles after applying the quantum eraser, we first have to divide them into two groups and observe the groups separately. One group will display the original pattern of fringes; the other will display the inverse of that pattern, with particles landing on what were originally the dark bands and avoiding the places where the bright fringes were. The two groups combined fill in all the gaps, hiding the interference.

The paradox is avoided because we need data from the photon measurement to know which group each particle belongs to. Thus, we cannot observe the fringes until after we have done the photon measurements (thus requiring results from both detectors), because only then do we know how to split the particles into groups.” (Kwiat 2007)

In other words, the stochastic nature of quantum physics prevents superluminal communication. Or it seems to as John Cramer sees it.

In his passion for daring research into this field, he and his team worked on the problem of setting up an experiment to show that superluminal communication is possible with quantum entanglement. This is done in the climate of most other physicist taking for granted that the no communication theorem will rule out any superluminal communication and save relativity and causality from facing the effects of time travel. According to Cramer:

“Recently it has been pointed out, however, that at least some of these proofs (of no communication theorem) are tautological, with their seemingly reasonable assumptions subtly building in the conclusion, and that key assumptions are inconsistent with some aspects of standard quantum mechanics.” (J. G. Cramer n.d.)

Thus he said that there is still a glimmer of hope for superluminal communication. Inspired by the PhD thesis of Birgit Dopfer in 1998, Cramer modified her EPR experiment to create a final experimental setup as shown.

The basic idea of the experimental setup is based on the splitting of a single photon (laser light) into two entangled beams which enters two similar detectors that are capable of distinguishing whether the photon acts like a wave or a particle. On the sending end of the detector, a controllable variable will decide to measure the photon as a wave or as a particle. Then the corresponding entangled photon pair at the receiving end will automatically correlate with the sending end, thus enabling the receiver to see what was varied in the sending end of the arm.

Note that the special feature in the experiment is that the sending arm lags the receiving arm by 50 microseconds. That is the message is received 50 microseconds before it is sent!
(Cramer 2009)

After years of research on this topic, he recently announced his positive results during the John Cramer Symposium on the occasion of his 75th birthday. However, there’s a noticeable lack of paper on this discovery in the physics journals and the electronic preprint website, ArXiv. Therefore one possible way to conduct a research is to recreate Cramer’s experimental setup and see if his results can be independently verified.

Most of the apparatus used is able to be found in a quantum optics laboratory and National University of Singapore have the very facility in Center for Quantum Technology. The main research cost should be the cost of the long optical fibre. The experimental skills are easily available and the learning opportunities from the existing researchers are immense. The procedure of the research is then to setup the experiment as shown above, then to modify the measurements in the sender leg and observe if there is any corresponding signal in the receiver’s leg. The time difference can also be carried out using an atomic clock and thus demonstrating the weird effects from quantum entanglement.

If Cramer’s experiment can be verified, information at least can travel back in time and this opens up many new applications. Solutions from the future can be downloaded into the present thus solving most of the problem faced by society today. The laws of time travel can be brought into experiment instead of remaining in theoretical/philosophical grounds. The world will never be the same again.

However, if there is any loophole or error in Cramer’s theory of backward time travel, and the no communication theorem still holds, then it will be a sober wake up call to humanity as the only way to survive is to care about the Earth.

In conclusion, the question: “Does quantum nonlocality allows superluminal communication?” is one of the most important and urgent question to be answered as it plays a great role in determining the fate of humanity.

Works Cited
A. Einstein, B. Podolsky, N. Rosen. "Can Quantum Mechanical Description of Reality Be Considered Complete?" Physical Review vol. 47, May 15 1935: 777.

Cramer, John G. The Alternate View. (accessed November 20, 2009).

Cramer, John. The John Cramer Symposium. September 10-11, 2009. (accessed November 20, 2009).

Kwiat, Rachel Hillmer and Paul. "A Do-It-Yourself Quantum Eraser." Scientific American. May 2007. (accessed November 20, 2009).

Yoon-Ho Kim, Rong Yu, Sergei P. Kulik, Yanhua Shih, Marlan O. Scully. "Delayed "Choice" Quantum Eraser." Physical Review Letters Vol. 84 No. 1, 3 January 2000: 1-5.

Zeilinger, Anton. "Experiments and Foundations of Quantum Physics." Reviews of Modern Physics Vol 71 No. 2 Centenary , 1999: S288-S297.

P.S. My quantum physics prof had told me straight that quantum physics maybe non-local but it never allows superluminal signalling. That fact is known. So you can regard this post as a pretty fantasy.

P.S.S. This is a nice ending to the post: in 2014, John Cramer finally finished his experiments and published: They found: In the context of the standard quantum formalism, Nature appears to be well protected from the possibility of nonlocal signaling.

1 comment:

Anonymous said...

love your blog!
can you write more about superluminal communication?