# MIT 6 050J - Quantum Information (3 pages)

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Lecture Notes

- Pages:
- 3
- School:
- Massachusetts Institute of Technology
- Course:
- 6 050j - Information and Entropy

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Chapter 13 Quantum Information In Chapter 10 of these notes the multi state model for quantum systems was presented This model was then applied to systems intended for energy conversion in Chapter 11 and Chapter 12 Now it is time to apply it to systems intended for information processing The science and technology of quantum information is very new The concept of the quantum bit named the qubit was first presented in the form needed here in 1995 There are still many unanswered questions for example the quantum version of the channel capacity theorem is not known As a result the field is in a state of flux and there are gaps in our knowledge which may become apparent in this chapter 13 1 Quantum Information Storage We have used the bit as the mathematical model of the simplest classical system that can store information Similarly we need a quantum model which will be called the qubit At its simplest a qubit can be thought of as a small physical object with two states which can be placed in one of those states and which can subsequently be accessed by a measurement instrument that will reveal that state However quantum mechanics both restricts the types of interactions that can be used to move information to or from the system and permits additional modes of information storage that have no classical counterparts An example of a qubit is the magnetic dipole which was used in Chapters 9 11 and 12 of these notes Other examples of potential technological importance are quantum dots three dimensional wells for trapping electrons and photons particles of light with various polarizations Suppose our system is a single magnetic dipole The dipole can be either up or down and these states have different energies The fact that the system consists of only a single dipole implies that the system is fragile To preserve the state of the system and therefore its information the system must remain isolated The slightest interaction with its environment is enough to change its state

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