We already have them. They are the future, and they are beautiful. Quantum Entangled Particles. One spins one way. The other, another way. Unobstructed they spin, in complete opposition to each other, over infinite distances. Scary.

The question

What could we use them for, assuming that we could transmit spin changes between them, using them like bits, and that they are expensive?

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    $\begingroup$ Clarification: are these entangled particles according to the currently known laws of quantum physics, or are they capable of any additional feat. In particular, from your phrasing "using them like bits," that suggests a classical behavior which is very powerful and forbidden by the current model of entangled particles in QM. An entangled pair does not encode a bit, it encodes a qbit, which despite having a similar name, is quite fundamentally different in nature. $\endgroup$ – Cort Ammon Jan 30 '16 at 1:57
  • $\begingroup$ See the current issue of Scientific American. $\endgroup$ – JDługosz Jan 31 '16 at 0:06
  • $\begingroup$ Haha. Forgot about "qbits." I meant like bits from computer-speak. $\endgroup$ – Caleb Woodman Feb 8 '16 at 17:14

Quantum computation. There are some problems where creating a set of entangled entities (qubits, quantum bits) and performing operations on them will obtain answers very much faster than by conventional computation. For example, 5000 entangled qubits could obtain the prime factors of any number up to 5000 bits with just two factors. Bye-bye most of today's cryptography!

State of the art is something like five qubits. Yea, we can factorize fifteen with a quantum computer. Whether computing with big numbers of entangled bits is possible is either an open question or an ultra top secret.

Also untappable communications. Quantum key distribution is a real-world technology already being used. Tens of bits per second isn't great but you can be absolutely certain that nobody is eavesdropping on your low bandwidth quantum channel.
NB this has nothing at all to do with the Science Fiction FTL communicator - which is almost certainly impossible.

  • $\begingroup$ Practical quantum computers are likely to be somewhat less efficient than you might think. See this paper: How to factor 2048 bit RSA integers in 8 hours using 20 million noisy qubits. Adding qubits alone ain't enough, because keeping everything coherent is very hard. Also, symmetric key cryptosystems will be much less affected, and quantum-resistant asymmetric cryptosystems are an active area of research. $\endgroup$ – Starfish Prime Oct 15 at 19:54
  • $\begingroup$ "Will obtain answers very much faster than by conventional computation": that depends very strongly on the problem to which one wants to find answers. Some problems can be solved quickly by quantum algorithms. Most interesting problems won't see any significant speedup. "Bye-bye most of today's cryptography": no, not really. We already have good crytographic algorithms which are known to be impervious to quantum compueting. For example, the ubiquitous AES is not afraid of quantum computing at all. $\endgroup$ – AlexP Oct 15 at 20:12

I was planning to use them in my book as coded communications devices —various encased entangled electrons would be placed in a wrist device, allowing the user to have private encrypted messages with others.

It was my understanding that nothing would be traveling faster than light. Just that changes made on one end, would automatically be reflected within the entangled electron on the other end. Those changes could be the basis of a code to communicate instantaneous messages.

Of course, those entangled electrons would have to have previously travel to those separate locations based on whatever form of travel you are using.

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    $\begingroup$ The issue here is the same as with bowlturner's answer - you can't use entangled particles to communicate. $\endgroup$ – HDE 226868 Jan 30 '16 at 23:05
  • $\begingroup$ A quantum channel as used in quantum key distribution is what you want. It is untappable because each transmitted bit is represented as a single quantum state. If someone eavesdrop then they destroy the states and as far as the intended recipient is concerned the line is dead. It has been demonstrated line-of-sight in the atmosphere and is in use down fiber optic cables between cities. Entanglement is involved in generating the signal. State of the art has plenty of scope for improvement. $\endgroup$ – nigel222 Jan 30 '16 at 23:54
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    $\begingroup$ No, that's not how it works. There's existing Answers on Physics. please don't write what you just indicated, but understand what you can really do with them, first. $\endgroup$ – JDługosz Jan 31 '16 at 0:08
  • $\begingroup$ Appropriate xkcd cartoon $\endgroup$ – JDługosz Jan 31 '16 at 6:34
  • $\begingroup$ Physicists disagree about implications and possibilities of quantum entanglement. Many admit they don't fully understand it. If you want to use a theoretical possibility for say, a Sci-Fi novel, it is not unreasonable. $\endgroup$ – GreyGamin Jan 31 '16 at 8:48

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