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Introduction of Week 2

This week, we move into the heart of quantum computing. The week is divided into four major Activities: Qubits, Entanglement, Basic Algorithm Idea, and State Variables. Threading through these Activities are seven key concepts.

These concepts are:

  • superposition
  • interference
  • measurement
  • entanglement
  • unitary (reversible) evolution
  • no-cloning theorem
  • decoherence

We met superposition and interference at the end of last week, in the context of ordinary, classical waves in any kind of material. As we learn about qubits, we will see how these waves are carried using state variables such as polarization in photons. When we measure qubits, our results are probabilistic, depending on that wave superposition. When the outcome of measurements is correlated in ways that classical probability can’t explain, we can say that two (or more) qubits are entangled. We can use this entanglement to teleport the information in a qubit from one place to another. We operate on them using unitary, or reversible, operations. The no-cloning theorem shows us why it is impossible to make independent copies of unknown quantum states, with a number of non-intuitive implications. And finally, the delicate nature of quantum effects forces us to deal with decoherence, in which decay of our state causes the system to not give the results we desire.

Keep these seven concepts in mind this week as you learn about qubits, the behavior of multi-qubit systems, and the basic goal of a quantum algorithm. After building this foundation, next week we will go into more detail on several of the most important quantum algorithms.

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This article is from the free online course:

Understanding Quantum Computers

Keio University

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