Three-bit quantum gate a step closer to universal quantum computer Quantum research boffins in Australia have demonstrated one of computing's universal gates, the Fredkin gate, operating with qubits instead of bits. The Fredkin gate – described here at Wikipedia – is a reversible three-bit gate that can be used to construct AND, OR, XOR and NOT functions. If the first of the three bits is a 1 …
Ooo look there it is: "Successful operation is heralded by fourfold coincidence events between the control, target, and trigger detectors".
[A coincidence detector is an electrical device used to select whether the result of the experiment is valid or invalid. If it doesn't fire, they ignore the output of the result.]
So what you've done is create an electric Fredkin gate, you could never know the result of the gate till you've measured your coincidence detector result and decided if the experiment result is valid. Trace the logic of the invalid states and you'll see your coincidence gate won't fire. i.e. its an electrical gate.
Hence you've actually built just a bunch of wave phase interactions that emulate the logic of the Fredkin gate with an electrical detector to give you a result.
It's the same issue again and again, if you need a detector to determine a coincidence of 2 or more photons, then the information is passed by the detector, thats detecting the 2 photons. The link (entanglement) between the two+ photons is the detector.
*Every* entanglement experiment has the same fault.
e.g. Quantum Eraser:
https://en.wikipedia.org/wiki/Quantum_eraser_experiment
Simply filters the measurements at Ds for times the photon was detected at Dp via a coincidence detector. But when you put a 45 degree filter infront of Dp you are selecting 45 degrees photons into the Ds double slits, and those photons have an equal probability of passing through both slits, i.e. can create interference fringes.
The link between the Ds photon and Dp photon, is the coincidence detector selecting the subset of photons that match the case we are looking for.
It isn't that you've magically set the phase by entanglement, its that if the phase was different you would ignore the experimental result, because the coincidence detector wouldn't fire.
Same issue here.
Perhaps I can put this in a bigger context.
This is an optical gate, circa 1982, using polarizing angles. It runs at the speed of light. With an electrical wrapper that delivers the result at the speed of electronics.
It is *not* and never will be part of a quantum computer. It does not use entanglement, it does not use superimposition. The theoretical Quantum computer running faster than light is an 'Emperors new clothes' trick. Entanglement is done with a detector to filter out only the experiments that match the desired result.
Everyone slaps 'quantum' this and 'qubit' that on their papers as a form of marketing and talks up how they're one step closer to the Quantum computer. But that's just funding talk.
ugg ugg, grub grub, and mruff mruff all sat round the camp fire, recounting anecdotes over a steadily roasting mastadon leg, gesticulating wildly.
ugg ugg, points to a rolling rock and draws a circular motion with his hands, attempting to explain rotational motion with a series of grunts and whistles.
grub grub seizes on his friend's discovery and draws a circular shape in the sand by his feet. 'wheeeeel', he says.
all three nod their head in unison, aware that humanity may just be on the brink of a stupendous discovery.
finally, after a long pause, mruff mruff chimes in with his brethren, "it's one step closer to a Bugatti Veyron." he says.
That would be here
Key feature is gates that use zero energy to compute.
Which might be quite handy given how power requirements of new processors have risen and zero design to use asynchronous design.
Is it a quantum computer?
The fact it uses quantum data carriers suggests it might be.
TBH I think quantum systems are looking better at massive parallel correlation and search problems, rather like optical systems were touted at as being good for.
The real quantum computer that takes in a "normal" (but very large) computing problem, then solves
it using quantum electronics (whatever that is) seems as far away as ever.
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