General · Language · Problem

Spooky action vs. spooky communication

Quantum reach
Credit: Pixabay/CC0 Public Domain

Is communication at faster than the speed of light possible – actual, instantaneous communication across vast distances using entangled (so-called) particles?

After a conversation with an acquaintance last June … He was reading Katie Mack’s book re cosmology, and said that entanglement permitted instantaneous communication. He referenced information theory. And that information requires no “embodiment.”

Anyway, Chad Orzel‘s 2015 article (below) explains the historical debunking of the notion.

And in another of my posts (“What the heck is quantum entanglement?“), there’s a point that “there can be correlation without communication.”

Orzel’s key point is about quantum randomness, indeterministic quantum states. Namely, that to encode a message you need to deterministically set the value of a “carrier” by measurement; but for an entangled particle that value is not predefined – the value you measure is probabilistic.

In order to send a message (e.g., a non-random bit sequence), the sender needs to pre-define the information and deterministically encode it – something which quantum state randomness defeats.

So, the values measured by the “receiver” – while the same as (correlated with) what the sender measures – are random. An incoherent message, eh.


So, is there a difference between saying “spooky action at a distance” and “spooky communication at a distance“?

The standard rebuttal to the claim – that “spooky action at a distance” allows spooky communication at a distance – is that such a claim violates the theory of relativity regarding the speed of light. And that Alice & Bob would need to confirm correlated information via standard light-speed means.

No physical object, message or field line can travel faster than the speed of light in a vacuum.

Such standard reasoning addresses the puzzle but begs for something deeper than a prohibition.

In a prior post, “How to create entangled photon pairs” (April 25, 2017), physicist Chad Orzel explains why “spooky communication at a distance” (transmitting information faster that light) is not an outcome of entanglement’s “spooky action at a distance.”

• Forbes > “How Quantum Randomness Saves Relativity” by Chad Orzel (Aug 11, 2015) – Why photon state correlation does not permit faster-than-light communication.

The notion that quantum particles do not have well-defined states prior to measurement was just too radical a departure from classical physics for Einstein to stomach …

Without that indeterminacy, quantum physics would allow the sending of messages faster than the speed of light, with disastrous consequences for the whole idea of causality, which is central to the operation of physics.

This [entanglement] might seem like it opens the possibility of faster-than light communication between Alice and Bob. They simply share entangled photons with each other, and then measure their polarizations, calling one outcome “0” and the other “1.” This lets them transmit messages in binary code, and violate the restriction from relativity that nothing can exceed the speed of light.

But this is where quantum randomness, … steps in to save the day. The simplest possible scheme you could imagine – Alice assigns a “1” to vertical polarization and a “0” to horizontal polarization – doesn’t work at all, because there’s no way to force a particular outcome. When Alice sets her polarization analyzer to vertical, there’s a 50% chance that her photon will turn out vertical, and a 50% probability that it turns out horizontal. Bob’s photons will be perfectly correlated with Alice’s, but all that means is that they each have the same totally random string of 0’s and 1’s. No information passes from Alice to Bob.

Orzel then considers another scheme between Alice & Bob, which also is compromised by quantum randomness. A photon cloning scheme also does not work: “an indeterminate state cannot be faithfully duplicated.”

In a 2018 articlePaul Sutter also explains why photon state correlation does not permit faster-than-light communication.

The resolution to Einstein’s question comes via an excruciatingly careful examination of who knows what and when. … while the process of disentanglement happens instantaneously, the revelation of it does not. We have to use good old-fashioned no-faster-than-light communication methods to piece together the correlations that quantum entanglement demands.

Looking for some useful visualizations … [1]

Notes

[1] Perhaps via Wigner function visualizations?

• Quora > “Can quantum entanglement be visualized

Mark John Fernee
20+ years as a physicist 4y [2018]
It depends on what you mean by visualized.

Your example of using a beam splitter does not create an entangled state. For a single photon, a beam splitter creates a superposition of two possible paths. An entangled state requires at least two or more quantum objects (photons, electrons or something more exotic). In fact specific properties can be entangled such as spin, polarisation and so on. In that sense, an entangled particle pair usually refers to some specific property of the pair.

Entanglement shows up in correlated measurement outcomes for the entangled particle pair such that the measurement of one part of the pair is enough to know the measurement of the other.

As for a visualisation, well that can be done using a phase space representation known as a Wigner function. A recent study has used the IBM’s 5 qubit quantum computer to determine the Wigner functions for a range of different entangled states, here Simple procedure for phase-space measurement and entanglement validation. Be warned that this is a technical paper and not really suited to conveying concepts to a more general audience. You will notice a number of 3D plots that visualise the entangled states. However, their interpretation would take some time and effort. What’s really cool about this is they used an IBM quantum computer to calculate the results. It’s really just about the Wow factor …