Building the diffusion operator

Part of the quantum computing series
Part 1: Quantum Computing in a Nutshell
Part 2: More on quantum gates
Part 3: Deutch-Josza Algorithm
Part 4: Grover’s algorithm
Part 5: Building the quantum oracle

Let’s more precisely define the Grover diffusion operator D we used for Grover’s algorithm, and see why it functions to flip amplitudes over the average amplitude.

First off, here’s a useful bit of shorthand we’ll use throughout the post. We define the uniform superposition over states as |s⟩:

Screen Shot 2018-07-17 at 1.32.40 PM

We previously wrote that flipping an amplitude ax over the average of all amplitudes ā involved the transformation ax → 2ā – ax. This can be understood by a simple geometric argument:


Now, the primary challenge is to figure out how to build a quantum gate that returns the average amplitude of a state. In other words, we want to find an operator A such that acting on a state ⟩ gives:

Screen Shot 2018-07-17 at 2.25.12 PM

If we can find this operator, then we can just define D as follows:

Screen Shot 2018-07-17 at 1.47.26 PM

It turns out that we can define A solely in terms of the uniform superposition.

Screen Shot 2018-07-17 at 2.00.08 PM

As a matrix, A would look like:

Screen Shot 2018-07-17 at 3.03.23 PM.png

Proof that this satisfies the definition:

Screen Shot 2018-07-17 at 2.41.44 PM.pngScreen Shot 2018-07-17 at 2.38.38 PM

Thus we have our full definition of D!

Screen Shot 2018-07-17 at 2.44.58 PM.png

As a matrix, D looks like:

Screen Shot 2018-07-17 at 3.05.26 PM

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