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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
  • $\begingroup$ Quantum entanglement can be used to create ridiculously good telescopes with distributed optics $\endgroup$ – Dragongeek Nov 14 '19 at 0:32
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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 to much common cryptography! (We do have methods which are not believed to be vulnerable to attack by a quantum computer, such as AES).

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. So you send secret decrypt keys in plaintext down an untappable channel, and much larger volumes of data encrypted with that key down a conventional channel, and can change your encryption key very often and automatically so that the conventional channel never provides large amounts of data using a constant key that might make exploitation of a cryptographic weakness easier.

NB quantum key distribution has nothing at all to do with the Science Fiction FTL communicator - which is almost certainly impossible.

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  • $\begingroup$ 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 '19 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 '19 at 20:12
  • $\begingroup$ This needs qualification in the interest of “open mindedness.” FTL communicator - which is almost certainly impossible will be true only if quantum mechanics finally explains everything. As it stands there is no reason to believe nothing in the entire universe is deterministic - Einstein strongly argues for deterministic features. The non-communication theorem is axiomatic in QM. $\endgroup$ – Vogon Poet Nov 14 '19 at 20:30
  • $\begingroup$ @Vogon_poet I feel that the word "almost" covers that. In addition , the speed of light is the speed of causality. Exceeding it opens up nearly as many paradoxes as time travel. Buf if FTL communication is ever observed ... I'm a scientist, facts rule, theories fall. $\endgroup$ – nigel222 Nov 19 '19 at 9:12
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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
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    $\begingroup$ 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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You don't need to assume any sort of transmission is possible as it already is allowed mathematically in deterministic hidden variable theories being developed to unify the discrepancies between quantum mechanics and general relativity. Quantum mechanics is non-deterministic by definition, as a derivitive of the Heisenberg Uncertainty Principle, and therefore it is a simple axiom that nothing at all is predetermined and nothing at all can be exactly known. So some may quarrel with how your idea is denounced by quantum mechanics - because it is - but quantum mechanics is only one partial explanation of our universe, and everyone getting paid in that area accepts that it is incomplete (which is why we still look for the Grand Unified Theory).

Many have accepted in the discoveries of dark matter, dark energy, and matter-antimatter asymmetry problem that there simply must be some constants and variables in this universe which are beyond our perception. The De Broglie–Bohm theory supports that somehow, fundamentally, there are deterministic quantities at the foundation of this universe. It's not really hard to accept, and it is well supported in mathematical HVT models.

Hidden Variable theory does allow for instantaneous information transmission via quantum entanglement, so your world simply supports the De Broglie–Bohm theory of quantum operations rather than a rigid quantum mechanical doctrine.

But before we go into the instantaneous information transmission application, they are currently being used to transmit code keys for sending secure information. For example, assume I send you a jumble of letters that only make sense when you add or subtract some number from each letter. The problem today is that I have to get you that code somehow. Assume the number is "1011010," now when I send this to you in an email, someone can hack it and decode our secret. However, with entangled particles, we can send beams of entangled photons to you and I, and the "number" in that beam does not exist until one of us measures the light. When I get my beam, I decode a number (which is random, by the way), and get "1011010". With quantum entanglement, I already know two things:

  • You just got the opposite code from me ("0100101")

  • Only you know what that code is - no one could possibly intercept it

This is very valuable in transmitting information with no possibility of being decoded.

Per HVT, you and I can get the same quantum entangled beams, and I could encode quantum counterfactual information onto my photon, and mathematically your entangled photon would also contain that counterfactual information.

The technology would mostly be wasted in computers due to the loss rate and difficulty of entangling a single particle. It's best suited for larger scale, interplanetary links or further, where radio propagation delays become an obstacle.

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