Ironically, I am just in the process of preparing a talk on the history of the atomic theory. Even as late as 1905 when Einstein published his work on brownian motion it was still highly controversial whether atoms were &quot;really real&quot; or just a mathematical contrivance. Some people are able to maintain skepticism even in the face of overwhelming evidence.&gt; Neither the fact that measurements have one outcome, nor Born&#x27;s rules for probability were derived from this decoherence idea yet.That is because it is not the case that measurements have one outcome. Measurements appear to have one outcome from the perspective of a given classical observer, and the outcome observed by any given observer will be classically consistent with all the other observations that observer makes [1]. The Born rule (which, it must be stressed, is not fundamental, but only apparent from the point of view of any given observer) follows from that, not mathematically but logically [2].[1] <a href="https:&#x2F;&#x2F;arxiv.org&#x2F;pdf&#x2F;quant-ph&#x2F;9605002.pdf" rel="nofollow">https:&#x2F;&#x2F;arxiv.org&#x2F;pdf&#x2F;quant-ph&#x2F;9605002.pdf</a>[2] <a href="https:&#x2F;&#x2F;www.scottaaronson.com&#x2F;papers&#x2F;island.pdf" rel="nofollow">https:&#x2F;&#x2F;www.scottaaronson.com&#x2F;papers&#x2F;island.pdf</a>

&gt; And the answer is: the classical world emerges by a process of decoherence, which is to say, by the creation of large (O(10^23)) networks of entanglements which (it can be shown mathematically) have behavior that is indistinguishable from classical systems. This is the motivation for decoherence and for the Everettian view where the psi function is &quot;the only real thing&quot; and everything else is derived. It is often talked about as if actually accomplished. But it was not; the behaviour of quantum systems, even with decoherence, was not yet shown to be indistinguishable from the classical systems. Neither the fact that measurements have one outcome, nor Born&#x27;s rules for probability were derived from this decoherence idea yet. One thing that follows from decoherence hypothesis is decay of coherence elements in density matrices, but this is not enough to get explanation for single outcomes or for Born&#x27;s rules.

Thanks. But I would actually not say that QM &quot;allows retrocausality.&quot; Instead I would say that the whole concept of &quot;causality&quot; simply meaningless in a time-symmetric system, just as the concept of (say) &quot;up&quot; is meaningless in a spatially symmetric system.

Well put. QM allowing retrocausality has been long known.

The headline is, as popular articles on QM often are, misleading. One of the central mysteries of QM has always been: how does the classical world with its arrow of time emerge from the quantum world where the governing equation is time-symmetric? And the answer is: the classical world emerges by a process of decoherence, which is to say, by the creation of large (O(10^23)) networks of entanglements which (it can be shown mathematically) have behavior that is indistinguishable from classical systems. It is very similar to how thermodynamics and the time-irreversibility of the second law emerge from time-reversible Newtonian mechanics (except that the time-asymmetry that emerges from QM is even more fundamental).Time-reversibility is part-and-parcel of quantum computation because the whole point of quantum computation is to keep the system operating in the regime where its behavior is distinguishable from classical. As soon as you lose coherence, you simultaneously lose all of the quantum behavior, including time-reversibility. Time-reversibility isn&#x27;t anything special in this regard. It&#x27;s all part of the same weirdness.It&#x27;s all very interesting, but none of it is news.

This is true but you can use extra qubits that when ignored make the output behave like XOR, so its not as limiting as you might think

Note: I&#x27;m not a professional quantum scientist so if any real ones come along please tell me if I&#x27;m getting this right, and please correct any mistakes that you see in my logic.From my understanding so far:One benefit of using qubits is that you can map multiple of them to one traditional bit.Let&#x27;s say we use light for our quantum computer. I like to see a wave of light as a continuous stream of photons, and photons as &quot;an order&quot;. I&#x27;m assuming all waves and photons in the rest of this comment are of the same frequency.If you have a vertically polarised wave of light, it orders you this:<pre><code> &quot;at time 1: jump up. at time 2: jump down. at time 3: jump up, ...&quot;
</code></pre>
If you have a horizontally polarised photon, it orders you this:<pre><code> &quot;at time 1: move left. at time 2: move right. at time 3: move left, ...&quot;
</code></pre>
We can start by saying &quot;horizontal polarization is 0&quot; and &quot;vertical polarization is 1&quot;.<pre><code> H = 0
 V = 1
</code></pre>
If we define XOR using only H and V:<pre><code> H ^ H = H (0)
 H ^ V = V (1)
 V ^ H = V (1)
 V ^ V = H (0)
</code></pre>
Let&#x27;s now add phase to the qubits to enhance them. Phase can be seen as adding a time delay (or &quot;shift&quot;) to all of the photons.Let&#x27;s see the vertically polarized photon again:<pre><code> &quot;at time 1: jump up. at time 2: jump down. at time 3: jump up, ...&quot;
</code></pre>
And at 180 degree phase, you shift all orders by 1 time unit:<pre><code> &quot;at time 1: jump down. at time 2: jump up. at time 3: jump down, ...&quot;
</code></pre>
One fun and useful property of phase is that if you have two photons of the same polarization but they are at phases 180 degrees apart from each other, they will cancel each other out because you cannot move up and down at the same time.Getting back to qubits and XOR:Let&#x27;s say H at 180 phase is I, and V at 180 phase is W.Note that I is still horizontally polarized, and W is still vertically polarized. Let&#x27;s bring them together, where we have 4 types of qubits mapped to 2 types of bits:<pre><code> H = 0
 I = 0
 V = 1
 W = 1
</code></pre>
Let&#x27;s add a step to the XOR operation:- if the right input is V, phase shift the output by 180 degreesThe outputs of our &quot;XOR 2.0: quantum edition&quot;:<pre><code> H ^ H = H, no shift = H (0)
 H ^ V = V, shift it = W (1)
 V ^ H = V, no shift = V (1)
 V ^ V = H, shift it = I (0)
</code></pre>
Now we have output that still matches the original definition of XOR when mapped to traditional bits, but with the bonus of reversibility.<pre><code> H =&gt; 0,0
 W =&gt; 0,1
 V =&gt; 1,0
 I =&gt; 1,1</code></pre>

XOR has 2 inputs and 1 output so cannot be reversible. CNOT is the closest reversible gate

Isn&#x27;t everything reversible if enough information is stored? Instead of COPY [A, B] -&gt; [A, A]; you could have [A, B, 0] -&gt; [A, A, B] which saves the value of B.

You have the right idea, but not the right operation. XOR is reversible.To give a better example: COPY is not reversible. Imagine that you have two qubits [A, B] as an input and you want to output [A, A]. That is not possible with quantum computers.

Microsoft told me that any operation a quantum computer performs HAS to be reversible: something like XOR doesn&#x27;t work because there&#x27;re four possible inputs and only two possible outputs, so the state of the inputs is destroyed. However, something like NOT works fine because the input-output mapping is 1:1.It seems like cheating to say that &quot;reversing cause and effect is no trouble&quot; when the computer was never allowed to perform irreversible operations to begin with. I think I am missing some important information or maybe I am misunderstanding the words used in the article?

Confirmed. It must be some recent change because we&#x27;ve got plenty of submissions from phys.org:<a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;from?site=phys.org" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;from?site=phys.org</a>Does anybody here work for them and can get in touch with the people that can fix it?

Forgive me for this being off topic: if you use a script blocker this site will blow up your CPU unless you have ajax.googleapis.com allowed. My browser became completely unresponsive. When I checked dev. console there were 50,000+ errors (and quickly climbing) on a video JS import. (2018-07-reversing-effect-quantum.html:1314). Pretty buggy.

Do I read this article in the future because I found it on HN today?Or did I find this article on HN today because I read it in the future?

It hasn’t got anything to do with that, what the exceedingly clickbaity headline is actually talking about is that QCs are more able to reverse computation.I’m unclear &#x2F;exactly&#x2F; what they are trying to say as all QC gates are required to be reversible, but it seems that they’re trying to say that to make a computation reversible on a classical computer you in general (eg worst case) need to store all the state (massive memory cost) or recompute from the beginning, or somewhere in between (eg when skipping back in a video, a player is generally going to compute relative to the nearest iframe).In QC this extra memory isn’t needed as every computation &#x2F;must&#x2F; be reversible so you just run the program backwards, and rely on the increased state that can be stored in each qubit (a lot of the exponential =&gt; polynomial performance improvements basically seem to boil down to each individual qubit being able to represent exponentially more state than a single bit).Maybe that’s helpful? I’m not super sure as the places where QC is faster than classical computation is pretty much only places where you can trade off exponential time for exponential state. E.g. problems that you can’t plausibly use a classical computer in the first place.

joe_the_user is going to post a comment

If quantum computers can maintain a viewpoint where cause and effect don&#x27;t matter, it seems like they will be at a point very different from the human viewpoint - where a person inherently has more difficulty predicting the future than determining the past.

So we shall come to computers that may just define reality by computation.

Reversing cause and effect is no trouble for quantum computers