Quantum Computing Concepts – Quantum Algorithms

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The reason why a quantum computer can massively outperform a classical one is that it can run special algorithms which operate on quantum superpositions of the input values, and arrive at the final result in a much smaller number of steps compared to their classical counterpart.

In this episode of Quantum Computing Concepts, Professor Andrea Morello of UNSW Australia illustrates the power of quantum computers with an example of great practical use: the quantum search algorithm.

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[coolworx]

Ok... I have to come clean.
I'm grokking somewhere around 20% of the content, but I'm 100% fascinated with that leopard skin shirt.

[georgedumoulin]

That last sentence actually made me "see it"... thank you!

[saxenaamitmca]

If we know the number already ( charlie matches|101> ) the state we want to amplify how is the search working for unknown states.

[dirmajanse]

Thank you Andrea Morello, your videos about quantum computing helped me a lot! Really clear explanations. Best, Dirma

[geoablesd]

Wasn't expecting leopard print this video, but I'm suitably impressed!

[MaxParadiz]

You set out with the mission of finding out which name is associated with the code 101. Then, you make it so that the amplitude of 101 is high enough so that you will measure that value with 95% probability... You perform a measurement and get the output "101".

So... when do you find out that "Charlie" is associated with that number?

[Markus]

At 2:10 you say a global operation is performed on all qubits such that the qubit we are looking for has a negative value. What is this operation? Correct me if I'm wrong, but in my opinion this is the crucial step of this whole calculation.

[rollaz]

Grandissimo professore, anche se questo è l'unico video dove indossa una camicia orrenda, rispetto alle altre.
Oppure questa è una sovrapposizione quantica di tutte le altre indossate negli altri video.

[AngelinaCruz]

I've watched this video 📹 like a zillion times.

[JNCressey]

What's the point of measuring and getting '101'? '101' is the phone number we're taking as input. We want to find its position in the database, which is the 8th place or address '111', so we can read off the name of charlie.

[crazieeez]

What quantum gate is used to give 101 a higher amplitude ?

[xy]

it took my a couple tries just to get over that shirrtt harhar

[EloTimeYT]

So we get an approximate solution, but much faster than the exakt one. Seems like numeric algorithms took over one more time.

[akhilsankar]

people should stop worrying about his shirt and start to understand the logic and appreciate his time.

[yr]

It's not that complicated, let me explain...

Normally with 3 bits you are storing 3 separate pieces of information (0 or 1 for each bit). These bits don't care what the other bits are set to, they also don't care if you measure them and they will give you the same measurement 100% of the time.

When you get down to the quantum level, an important detail changes; particles no longer have a single definite location. Instead, they are spread across multiple orbitals.


When you combine these particles into a super position, the orbitals combine into a new orbital and therefore the probabilities of where they exist also change.

Now here's the key, with 3 qbits, there are 8 possible measurements, each with their own probability. These are not the probabilities that you're used to working with; these probabilities will depend on how they are measured.
i.e,
001 = a
010 = b
011 = c
100 = d
101 = e
110 = f
111 = g
000 = h

Let's say that there is a way to measure these probabilities such that a>b>c>d>e>f>g>h. This type of measurement would correspond to 1 particular phone number.

When he talks about applying a global operation at 2:14, I believe he's essentially measuring the orbitals in a way such that he gets the probabilities for the sequence that he's looking for (i.e a>b>c>d>e>f>g>h). If he measured the qbits in a different way, he would get a different arrangement of probabilities (i.e c>b>a..etc.).

Therefore, suppose we know that:
111 - Beth
000 - Dave
101 - Charlie
etc.

And we have some set of numbers represented by abcdefgh. We want to find who abcdefgh belongs to.

We get our 3 qbits into superposition and then do a measurement that highlights the abcdefgh sequence. Remember as soon as we measure the qbits, they collapse to a particular value. This measurement results in 101 qbits being set. Now we know that abcdefgh corresponds to Charlie.

If we measured them in a different way, it might have collapsed to a different number.

So in summary:
To find which qbits correspond to a target sequence
0. Assign each element of the target sequence (i.e a) to a particular qbit sequence (i.e 001)
1. Get all the qbits into a super position
2. Measure the qbits in such a way that you pick out the probabilities you had setup in step 0
3. Now observe what value the qbits have collapsed to

[fullmetalsvr]

Ignore the quantum physics part, seems like he is stuck in the 70's or 80's?

[blue]

what is so special about a Quantum Computer. why cant a normal computer perform such an algorithm?

[richardgomes]

Your wide variety of shirts is distracting... :-)

[clarkg]

I only got 19 sec's in, couldn't stop focusing on that shirt. If you want to impress someone with your words, don't distract them with your apparel. Get your priorities straight.

[roayahelal]

this is a bad explication
non- quantams can do it
vlookups in excel lol