Travelling faster than light? Can quantum entanglement make the impossible possible?

by Fung Tsz Chai

In the popular Japanese cartoon ‘Doraemon’, the Dokodemo doa (or everywhere door) can transfer any object instantly. We may think that this is absurd in reality because it violates Einstein’s theory of relativity – nothing can travel faster than light. Chinese physicists, however, discovered recently that quantum entanglement could lead to a breakthrough of this limitation1. They worked on experiments repeatedly and finally proved that quantum entangled particles are able to transfer physical states much faster than light1. Does it mean that travelling superluminally (i.e. faster than light) is possible? Does quantum entanglement break all existing physical laws?

What is quantum entanglement?

The term ‘quantum entanglement’ may sound highly complex but it just describesthe link between two particles so that they behave like one system. If two particles are ‘entangled’, they will have correlated physical properties regardless of the distance and the medium between them2. For example, if one of the entangled particles spins up, the other must spin down even if they are located at two opposite ends of the universe3. Two entangled particles are like two people sitting on opposite ends of a seesaw: when one person is in the high position then the other must be in the low position. This effect is always the same independent of the length of the seesaw.

Superluminal entangled particles

After one of a pair of entangled particles changes its physical state, how long does it take the opposite particle to change its state?

Professor Juan Yin, a Chinese physicist from the University of Science and Technology Shanghai, and his colleagues recently measured the speed1. They sent pairs of entangled light particles (called photons) to two sites 15.3km apart from each other1. Two groups of scientists, one at each of the two sites, measured the polarizations (one of a photon’s physical properties) of photons continuously at the same time. After 12 hours of experimenting, the two separate groups compared their results and found that the polarizations of the entangled photons changed at the same time.1 This revealed that entangled particles can influence each other almost spontaneously. Allowing for the errors of measurement apparatus such as clocks, they calculated that the speed must still be at least 10,000 times greater than speed of light1.

What is most surprising about this experiment is that it did not rule out the possibility of instant transfer of physical states between entangled particles. If this is true, how can we apply this weird phenomenon in our everyday life?

Is a Dokodemo doa possible?

Now that Chinese scientists have discovered the superluminal property of entangled photons. does it imply that we can transport objects or communicate information instantly, just like Doeramon’s Dokodemo-doa ? Unfortunately, the answer is ‘no’.

In quantum entanglement, only physical states, not objects, are transferred from one place to the other. During the experiment, the two entangled particles did not actually move faster than light. Therefore, quantum entanglement is not a way to teleport a physical substance instantly.

Worse still, quantum entanglement cannot even help us deliver meaningful information faster than light2,4. In order to transmit messages using the properties of entangled particles, we need to control the physical properties of one of the entangled particles. For example, if we want to send a ‘spin down’ signal to a friend on the other side of the universe, we may try to adjust our entangled photon to a ‘spin up’. However, our friend on the other side of the universe would not measure a ‘spin down’ photon because once we manually interrupt one of the entangled particles, both particles become un-entangled and their physical properties become unrelated, just like breaking a seesaw.

If we really want to use quantum entanglement to deliver data, we have to seek help from classical channels4, such as radio waves. However, all kinds of classical communications are unable to travel faster than light. As a result, superluminal transmissions are still a fantasy.

Quantum teleportation – A secure way to communicate

You may be disappointed that information transmissions using quantum entanglement cannot be superluminal. Yet, quantum entanglement is still a crucial scientific discovery because it has lots of applications. The most important one is quantum teleportation, which is a secure way to deliver messages between 2 distant parties using both classical channels and quantum entanglement5.

The most significant difference between quantum and classical (traditional) teleportation is the way to encrypt data. Traditional encryption uses passwords, so mathematical formulas, numbers and letters are used as the ‘keys’ to decrypt information5. Quantum encryption, on the other hand, abandons the use of passwords. It makes use of the phenomenon that two entangled particles are physically correlated5. Therefore, the entangled particles become the ‘keys’ to encrypt or decrypt messages.

Quantum encryption is more secure than traditional encryption in three ways. Firstly, the quantum encryption ‘keys’ are random and they change very rapidly over time. There are no ways for hackers to predict or find out what the ‘keys’ are by using Mathematics. Moreover, there are no deliveries of objects between entangled particles3. Therefore, it is impossible for the third party to steal the quantum ‘keys’. Furthermore, even if hackers had super-advanced technologies to interrupt entangled particles (which is impossible up to now), they would not obtain any meaningful information about the quantum ‘keys’6. This is because once the entangled particles are disturbed, the entanglement breaks down, and so they are no longer ‘keys’.

Although China is leading the world in quantum entanglement technology, its practical use is still limited at present. After solving some technical problems and limitations encountered in quantum teleportation, it may be adopted by the Chinese government for military communications. This will prevent leakage of military information to other countries, so national security can be protected. In the long term, quantum teleportation can be applied to various communication devices such as mobile phones and computers so that the privacy of Chinese citizens can also be secured.

Limitation of quantum teleportation

Quantum teleportation cannot be practically applied nowadays because entangled particles cannot function if they are separated too far away3. Although theory suggests that particles remain entangled regardless of their distance, the flaws of instruments such as optical fibers make this impossible in reality3.

Chinese physicists and engineers are tackling this obstacle. In 2012, physicists at the University of Science and Technology of China (Shanghai) smashed the quantum teleportation record when they created two separated entangled photons at a distance of 97km successfully3. Since the old record of 16km was also achieved by another group of Chinese physicists7, there is no doubt that the development of quantum teleportation in China will continue to out-perform other countries.

Quantum entanglement is a scientific phenomenon that is changing how scientists view well-known existing physical laws. Although the interactions of entangled particles are mysterious, they could turn out to be the key to secure communications. Though it may be disappointing that we cannot travel faster than light using entanglement at the moment, physicists are discovering more and more new, strange physical laws and it is still possible that Einstein’s theory of relativity is ultimately wrong. So, who knows? Maybe we will be able to travel superluminally one day or even have our very own personal Dokodemo doas.

References:

1. Liu NL, Zhang Q, Peng CZ, Pan JW. Bounding the speed of ‘spooky action at a distance’. [Internet] 2013 Jun 18. Shanghai Branch, National Laboratory for Physical Sciences at Microscale, and Department of Modern Physics, University of Science and Technology of China. [cited 2014 Feb 14]. Available from: http://arxiv.org/pdf/1303.0614v2.pdf

2. Slezak M. Teleportation record heralds secure global network. [Internet] 2012 May 15. New Scientist. [cited 2014 Feb 13]. Available from: http://www.newscientist.com/article/dn21811-teleportation-record-heralds-secure-global-network.html#.UvzCFffyyt8

3. Brown M. Chinese Physicists Smash Quantum Teleportation Record. [Internet] 2012 May 5. WIRED UK. [cited 2014 Feb 13]. Available from: www.wired.com/wiredscience/2012/05/quantum-teleportation-distance/

4. Mochon C. Introduction to Quantum Teleportation. [Internet] 2006 Aug 11. Waterloo: Perimeter Institute for Theoretical Physics. [cited 2014 Feb 14]. Available from: http://lightlike.com/teleport/teletalk.pdf

5. Shurkin J. Quantum Teleportation in Space Explored as Message Encryption Solution. [Internet] 2013 Mar 15. Scientific American. [cited 2014 Feb 15]. Available from:  http://www.scientificamerican.com/article/quantum-teleportation-in-space-explored-as-message-encryption-solution/

6. Bashar M, Chowdhury M, Islam R, Rahman M, Das S. A Review and Prospects of Quantum Teleportation. [Internet] 2009 September 2. MASAUM Journal of Basic and Applied Sciences. [cited 2014 April 19]. Available from: http://qudev.ethz.ch/content/courses/phys4/studentspresentations/qip/quantumTeleportationReviewMASAUM.pdf

7. Lin E. Quantum teleportation achieved over 16 km. [Internet] 2010 May 20. PHYS ORG. [cited 2014 April 19] Available from: http://phys.org/news193551675.html

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