Simon’s Algorithm¶
Overview¶
This module emulates Simon’s Algorithm.
Simon’s problem is summarized as follows. A function \(f :\{0,1\}^n\rightarrow\{0,1\}^n\) is promised to be either onetoone, or twotoone with some nonzero \(n\)bit mask \(s\). The latter condition means that for any two different \(n\)bit numbers \(x\) and \(y\), \(f(x)=f(y)\) if and only if \(x\oplus y = s\). The problem then is to determine whether \(f\) is onetoone or twotoone, and, if the latter, what the mask \(s\) is, in as few queries to \(f\) as possible.
The problem statement and algorithm can be explored further, at a high level, in reference [1]. The implementation of the algorithm in this module, however, follows [2].
Algorithm and Details¶
This algorithm involves a quantum component and a classical component. The quantum part follows similarly to other blackbox oracle algorithms. First, assume a blackbox oracle \(U_f\) is available with the property $$U_f\vert x\rangle\vert y\rangle = \vert x\rangle\vert y\oplus f(x)\rangle$$
where the top \(n\) qubits \(\vert x \rangle\) are the input, and the bottom \(n\) qubits \(\vert y \rangle\) are called ancilla qubits.
The input qubits are prepared with the ancilla qubits into the state $$(H^{\otimes n} \otimes I^{\otimes n})\vert 0\rangle^{\otimes n}\vert 0\rangle^{\otimes n} = \vert +\rangle^{\otimes n}\vert 0\rangle^{\otimes n}$$ and sent through a blackbox gate \(U_f\). Then, the HadamardWalsh transform \(H^{\otimes n}\) is applied to the \(n\) input qubits, resulting in the state given by $$(H^{\otimes n} \otimes I^{\otimes n})U_f\vert +\rangle^{\otimes n}\vert 0\rangle^{\otimes n}$$
It turns out the resulting \(n\) input qubits are in a uniform random state over the space killed by (modulo \(2\), bitwise) dot product with \(s\). This covers the onetoone case as well, if one considers it to be the degenerate \(s=0\) case.
Suppose we then measured the \(n\) input qubits, calling the bitstring output \(y\). The above property then requires \(s\cdot y = 0\). The space of \(y\) that satisfies this is \(n1\) dimensional. By running this quantum subroutine several times, \(n1\) nonzero linearly independent bitstrings \(y_i\), \(i = 0, \ldots, n2\), can be found, each orthogonal to \(s\).
This gives a system of \(n1\) equations, with \(n\) unknowns for finding \(s\). One final nonzero bitstring \(y^{\prime}\) can be classically found that is linearly independent to the other \(y_i\), but with the property that \(s\cdot y^{\prime} = 1\). The combination of \(y^{\prime}\) and the \(y_i\) give a system of \(n\) independent equations that can then be solved for \(s\).
By using a clever implementation of Gaussian Elimination and Back Substitution for mod2 equations, as outlined in Reference [2], \(s\) can be found relatively quickly. By then sending separate input states \(\vert 0\rangle\) and \(\vert s\rangle\) through the blackbox \(U_f\), we can find whether or not \(f(0) = f(s)\) (in fact, any pair \(\vert x\rangle\) and \(\vert x\oplus s\rangle\) will do as well). If so, we conclude \(f\) is twotoone with mask \(s\); otherwise, \(f\) is onetoone.
Overall, this algorithm can be solved in \(O(n^3)\), i.e., polynomial, time, whereas the best classical algorithm requires exponential time.
Source Code Docs¶
Here you can find documentation for the different submodules in simon.
grove.simon.simon¶
Module for Simon’s Algorithm. For more information, see [Simon1995], [Loceff2015], [Watrous2006]
[Simon1995]  Simon, D.R. (1995), “On the power of quantum computation”, 35th Annual Symposium on Foundations of Computer Science, Proceedings, p. 116123. 
[Loceff2015]  Loceff, M. (2015), “A Course in Quantum Computing for the Community College”, Volume 1, Chapter 18, p 484541. 
[Watrous2006]  Watrous, J. (2006), “Simon’s Algorithm”, University of Calgary CPSC 519/619: Quantum Computation, Lecture 6. 

class
grove.simon.simon.
Simon
¶ Bases:
object
This class contains an implementation of Simon’s algorithm using pyQuil. For more references see the documentation

find_mask
(cxn, bitstring_map)¶ Runs Simon’s mask_array algorithm to find the mask.
Parameters:  cxn (QVMConnection) – the connection to the Rigetti cloud to run pyQuil programs
 String] bitstring_map (Dict[String,) – a truth table describing the boolean function, whose period is to be found.
Returns: Returns the mask of the bitstring map or raises an Exception if the mask cannot be found.
Return type: String


grove.simon.simon.
create_1to1_bitmap
(mask)¶ A helper to create a bit map function (as a dictionary) for a given mask. E.g. for a mask \(m = 10\) the return is a dictionary:
>>> create_1to1_bitmap('10') ... { ... '00': '10', ... '01': '11', ... '10': '00', ... '11': '01' ... }
Parameters: mask (String) – binary mask as a string of 0’s and 1’s Returns: dictionary containing a mapping of all possible bit strings of the same length as the mask’s string and their mapped bitstring value Return type: Dict[String, String]

grove.simon.simon.
create_valid_2to1_bitmap
(mask, random_seed=None)¶ A helper to create a 2to1 binary function that is invariant with respect to the application of a specified XOR bitmask. This property must be satisfied if a 2to1 function is to be used in Simon’s algorithm
More explicitly, such a 2to1 function \(f\) must satisfy \(f(x) = f(x \oplus m)\) where \(m\) is a bit mask and \(\oplus\) denotes the bit wise XOR operation. An example of such a function is the truthtable
x f(x) 000 101 001 010 010 000 011 110 100 000 101 110 110 101 111 010 Note that, e.g. both 000 and 110 map to the same value 101 and \(000 \oplus 110 = 110\). The same holds true for other pairs.
Parameters:  mask (String) – mask input that defines the periodicity of f
 random_seed (Integer) – (optional) integer to set numpy.random.seed parameter.
Returns: dictionary containing the truth table of a valid 2to1 boolean function
Return type: Dict[String, String]
References
[1]  http://pages.cs.wisc.edu/~dieter/Courses/2010fCS880/Scribes/05/lecture05.pdf 
[2]  (1, 2) http://lapastillaroja.net/wpcontent/uploads/2016/09/Intro_to_QC_Vol_1_Loceff.pdf 