Quantum Computation

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Transcript Quantum Computation

Debasis Sadhukhan
M.Sc. Physics, IIT Bombay
1. Basics of Quantum Computation.
2. Quantum Circuits
3. Quantum Fourier Transform and
it’s applications.
4. Quantum Search Algorithm
1. Basic Quantum Mechanics &
2. A little Background of Computer
Quantum Mechanics
Computer Science
• Church-Turing Thesis
• Computational
Complexity Classes (P,
• Satisfiability (3-SAT & KSAT)
• Reduction
• Reversible Computation
*So, if the state can’t be written in the product state form, then they are
Entangled. They are called to be Entangled State.
*Classical Analogy:
think of :
No classical analog exists. But you can
Harry Potter and Voldemort
Examples: Bell states or EPR pairs
Some of the very important applications are :
1. Super-dense coding
2. Quantum Teleportation
3. Quantum Cryptography
4. Quantum Games
A completely graphical
way to represent
equations and physical
Represent a quantum state as a triangle with attached wires & do
operation on quantum states just manipulating this picture
Standard Quantum
Tensor Network States
*Execution of an classical algorithm require hardware, consist of
many electrical circuits containing wires and logic gates.
*These logic gates are the basic building block of a classical
*Similarly, to execute a quantum algorithm we must have a quantum
computer where the building blocks are quantum gates.
*So, What are the Quantum Gates…?
*As the name suggests, the gates are quantum, the laws of quantum
mechanics must be applicable here.
*So, they must be unitary operator and can be made reversible.
*Note: The target and control qubit are not basis independent i.e. our
target and control qubit may change if we use a different basis .
*In Classical Computation, we have seen NAND and NOR gate as
universal quantum gate. A similar universality is true for quantum
computation also.
*Every classical gates can be created using unitary quantum gates.
In that sense quantum circuits include all the classical circuits.
*So, universality of quantum gates is obvious.
*An algorithm is a well defined procedure or a set of
instructions to perform an information processing task.
*Turing-Church Thesis: Any algorithmic process can
be simulated efficiently using a probabilistic Turing
*Complexity Classes: P , NP
*Quantum algorithms are those that uses quantum
mechanical principles at the time of it’s execution.
Hard to design !
*The final state of the 1st register:
Now, apply Inverse Fourier Transform on the 1st register.
Final state:
Overall Circuit:
*The major applications are
1. Order finding
2. Prime factorization
These can be used to break the
cryptosystem used in classical computer
Period Finding etc.
1. Still, the problem remains in NP class.
2. If we don’t know the exact no of solution, we
may not reach to our solution as no of iteration
explicitly depends on M.
* [1] Michael A. Nielsen and Isaac I. Chuang, Quantum Computation
and Quantum Information, Cambridge University Press(2002).
* [2] Phillip Kaye, Raymond Laflamme and Michele Mosca, An
Introduction to Quantum Computing, Oxford University Press(2007).
* [3] Jamie Smith and Michele Mosca, arXiv:1001.0767v2 [quant-ph]
* [4] Lecture notes of John Preskill, California Institute of Technology: