Grover. Part 2
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Transcript of Grover. Part 2
Grover. Part 2
Components of Grover Loop
• The Oracle -- O
• The Hadamard Transforms -- H
• The Zero State Phase Shift -- Z
O is an Oracle
H is Hadamards
H is Hadamards
Z is Zero State Phase Shift
Grover Iterate
Inputs oracle
We need to initialize in a superposed state
This is action of quantum oracle
This is a typical way how oracle operates
This is a typical way how oracle operation is described
Encodes input combination with changed sign in a superposition of all
Role of Oracle
• We want to encode input combination with changed sign in a superposition of all states.
• This is done by Oracle together with Hadamards.
• We need a circuit to distinguish somehow globally good and bad states.
Vector of Hadamards
Notation Reminder
a Control with value a=1
a Control with value a=0
a
equivalent
This is value of oracle bit
Flips the data phase
All information of oracle is in the phase but how to read it?
This is just an example of a single minterm, but can be any function
Zero State Phase Shift Circuit
Flips the oracle bit when all bits are zero
Rewriting matrix Z to Dirac notation, you can change phase globally
This is state of all zeros
2 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
1 0 0 0
0 -1 0 0
0 0 -1 0
0 0 0 -1
-1 0 0 0
0 -1 0 0
0 0 -1 0
0 0 0 -1
+ =
With accuracy to phase
In each G
This is a global view of Grover. Repeatitions of G
Here you have all components of Grover’s loop
Generality
• Observe that a problem is described only by Oracle.
• So by changing the Oracle you can have your own quantum algorithm.
• You can still improve the Grover loop for particular special cases
proof
Here we explain in detail what
happens inside G. This can be
generalized to G-like circuits
Grover iterate has two tasks: (1) invert the solution states and (2) invert all states about the mean
Here we prove that |> < | used inside HZH calculates the mean
a Vector of mean values
Will be explained in next slide
Explanation of the first part of Grover iterate formula
This proof is easy and it only uses formalisms that we already know.
(( ))
(( ))
From previous slide
What does it mean invert all states about the mean?
For every bit
Amplitudes of bits after Hadamard
Positive or negative amplitudes in other explanations
All possible states
Amplitudes of bits after one stage of G
This value based on previous slide
This slides explains the basic mechanism of the Grover-like algorithms
You can verify it also in simulation
Additional Additional ExerciseExercise
This is a lot calculations, requires matrix multiplication
Here we calculate analytically when to stop
The equations taken from the previous slides “Grover Iterate”
For marked state
For unmarked state
We found k from these equations
recursionWe want to find how many times to iterate
But you can do better if you have knowledge, for instance the upper bound of chromatic number in graph coloring
Grover search example.
• Here is an example of Grover search for n = 3 qubits, where N = 2n =8. – We omit reference to qubit n+1, which is in
state 1 /√2 (|0>−|1>i) and does not change.
• The dimension of the unitary operators for this example is thus 2n = 8 also.)
• (Remember that numbering starts with 0 and ends with 7, so that the -1 here is in the slot for |5>.)
• This matrix reverses the sign on state |5>, and leaves the other states unchanged.
•Suppose the unknown number is |a> = |5>. •The matrix or black box oracle Ufa is
oracle
•The Walsh matrix W8 is
Now we use normalization
The matrix −Uf0 is
This matrix changes the sign on all states except |0>.
Finally, we have the repeated step RsRa in the
Grover algorithm:
oracle
shift
hadamards
After second rotation we get
Summary and our work
When you know anything about the problem (symmetry, observation, bounds, function within some classification class) you can design a better Grover like algorithm but for your data only.
This is enough in real life like CAD or Image Processing, since data are always specific, not the worst case data as in Mathematic proofs
Problem for students
• Build the Grover algorithm for ternary quantum logic.
• First you need to generalize Hadamard transform to Chrestenson transform.
• Next you need to have some kind of ternary reversible gates to build oracle.
• The same gates will be used for Zero State Phase Shift circuit.