Skip to content
Related Articles

Related Articles

Shor’s Factorization Algorithm
  • Difficulty Level : Hard
  • Last Updated : 05 Feb, 2021

Shor’s Factorization Algorithm:

  • Shor’s Factorization Algorithm is proposed by Peter Shor.
  • It suggests that quantum mechanics allows the factorization to be performed in polynomial time, rather than exponential time achieved after using classical algorithms.
  • This could have a drastic impact on the field of data security, a concept based on the prime factorization of large numbers.
  • Many polynomial-time algorithms for integer multiplication (e.g., Euclid’s Algorithm) do exist, but no polynomial-time algorithm for factorization exists.
  • So, Shor came up with an algorithm i.e. Shor’s Factorization Algorithm, an algorithm for factorizing non-prime integers N of L bits.
  • Quantum algorithms are far much better than classical algorithms because they are based on Quantum Fourier Transform.
  • Run time on the classical computer is O[exp (L1/3(log L)2/3)] but that on the quantum computer is O(L3).
  • So, Shor’s Algorithm in principle, shows that a quantum computer is capable of factoring very large numbers in polynomial time.

Shor’s Algorithm depends on:

The Algorithm stands as:

Given an odd composite number N, find an integer d, strictly between 1 and N, that divides N.

Shor’s Algorithm consists of the following two parts:

  • Conversion of the problem of factorizing to the problem of finding the period. This part can be implemented with classical means.
  • Finding the period or Quantum period finding using the Quantum Fourier Transform, and is responsible for quantum speedup, and utilizes quantum parallelism.

In Shor’s Algorithm, the Input is Non-prime number N and the Output is Non-trivial factor of N

INPUT (N) —> SHOR’S ALGORITHM —> OUTPUT (Non-trivial factor of N)

Algorithm: It contains a few steps, at only step 2 the use of quantum computers is required.

  1. Choose any random number let say r, such that r < N so that they are co-primes of each other.
  2. A quantum computer is used to determine the unknown period p of the function fr, N (x) = rx mod N.
  3. If p is an odd integer, then go back to Step 1. Else move to the next step.
  4. Since p is an even integer so, (rp/2 – 1)(rp/2 + 1) = rp – 1 = 0 mod N.
  5. Now, if the value of rp/2 + 1 = 0 mod N, go back to Step 1.
  6. If the value of rp/2 + 1 != 0 mod N, Else move to the next step.
  7. Compute d = gcd(rp/2-1, N).
  8. The answer required is ‘d’.

Classical part (The order finding problem):
This is the classical part of order finding problem. Given that x and N, such that x<N and gcd(x, N) = 1. The order of x is the least positive integer, y such that xy = 1(mod N).

  1. A random number n is picked, such that n < N. Compute gcd(n, N).
  2. This can be done using the Euclid Algorithm.
  3. If gcd(n, N) != 1, then there is a non-trivial factor of N.If (x+p) = nx + p mod N = nxmod N = f(x).
  4. If r is odd, then go back to Step 1.
  5. If np/2 = -1(mod N), then go back to Step 1.
  6. The gcd(np/2 +/- 1, N) is a non-trivial factor of N.

Quantum part:

This is the quantum order finding part. Choose a power of 2, then
Q = 2L such that N2 <= Q <= 2N2

And consider f = {0, 1, 2, …, Q-1}

Where, f(y)=1/(Q)1/2x=0Q-1I f(x) I wxy and w = exp(2π i/Q), i.e. Qth root of unity.

  • Let’s perform Shor’s Algorithm using an example: To factor an odd integer N (let N = 17).
  • Choose an integer Q such that N2 <= Q ≤ 2 N2 ( lets Q = 324).
  • Then, randomly choose any integer n such that gcd(n, N)=1, (let us choose the integer be n=7).
  • Then create two quantum registers (these registers should be entangled so that the collapse of the input registered corresponds to the collapse of the output register)
    • Input Register: must be containing enough qubits to represent numbers as large as Q-1.(i.e., up to 323, so we need 9 qubits).
    • Output Register: must be containing enough qubits to represent numbers as large as (N – 1). (i.e., up to 16, so we require 4 qubits).

Code :


from qiskit import IBMQ
from qiskit.aqua import QuantumInstance
from qiskit.aqua.algorithms import Shor
# Enter your API token here
IBMQ.enable_account('ENTER API TOKEN HERE'
provider = IBMQ.get_provider(hub='ibm-q')
# Specifies the quantum device
backend = provider.get_backend('ibmq_qasm_simulator')
print('\n Shors Algorithm')
# Function to run Shor's algorithm
# where 21 is the integer to be factored
factors = Shor(35)
result_dict =
    backend, shots=1, skip_qobj_validation=False))
 # Get factors from results
result = result_dict['factors']
print('\nPress any key to close')

Output :

Shors Algorithm
- - - - - - - - - - - - -
Press any key to close

Output for the code above showing the factors 5 and 7 for N=35.

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.  To complete your preparation from learning a language to DS Algo and many more,  please refer Complete Interview Preparation Course.

My Personal Notes arrow_drop_up
Recommended Articles
Page :