Advantages and Disadvantages of Quantum Computing

Advantages

Simultaneous Operations

Quantum computers can perform multiple simultaneous operations due to the properties of quantum physics, specifically superposition and entanglement. Superposition allows a quantum bit (qubit) to exist in multiple states simultaneously, while entanglement allows two or more qubits to be 'connected' in such a way that the state of each qubit is affected by the others.

This allows quantum computers to perform certain calculations much faster than classical computers by taking advantage of the ability to exist in multiple states simultaneously. For example, if a classical computer were to search a database for a specific entry, it would have to check each entry one at a time. A quantum computer, on the other hand, could check all entries simultaneously.

(Note: This is extremely simplified. The actual process if much more complicated and involves a lot more science.)


Decision Problems: P vs BQP vs NP

Another advantage of quantum computers is that they can solve certain problems much more efficently than regular computers such as finding the prime factors of large numbers (using Shor's algorithm).


P: Polynomial Time BQP: Bounded-error Quantum Polynomial time NP: Nondeterministic Polynomial Time
  • Can be solved efficiently (in polynomial time) by regular computers
  • Examples include: Most linear programming questoins, checking is a number is prime, and checking for connectivity in a graph
  • Can be solved efficiently by quantum computers
  • Error probability must be less than 1/3
  • Solution to these problems can only be verified in polynomial time by regular computers
  • Finding the solution to these problem cannot be done efficiently
  • Examples include the Travelling Salesman Problem (traversing all nodes and returning to start) and Boolean Satisfiability Problem.

In the diagram, overlapping areas represent problems in more than one category.
Eg. All Polynomial time problems can be solve efficiently by quantum computers. The overlap between BQP and NP means that quantum computers can solve some NP problems quickly (such as prime factorization).

p vs np vs bqp

Disadvantages

Accuracy of Calculations Since the state of qubits depends on probabilities, errors can occur during calculations. Error correction measures must be taken such as by using 'logical qubits', a group of physical qubits working together that correct errors as they occur. However, doing this will greatly increase the amount of qubits needed for the computer, thus increasing the cost.
Environmental Requirements Quantum computers must be cooled to extremely low temperatures (close to absolute zero) to ensure that environmental factors don't affect the quantum state of quantum computers. Once again, this will increase the cost of building and maintaining a quantum computer.
Quantum Algorithms Quantum computers cannot use the algorithms used on traditional computers, but need their own special algorithms. Creating and optimizing these algorithms can be very time consuming.