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Key Concepts in Quantum Computation and Communication

Scale of Quantum Computers

We are working on quantum computer architectures for two reasons associated with the scale of devices: first, we now have the ability to manipulate systems at the level of individual atoms and particles, and doing so might bring us advances in computational power for solving some problems; second, advances in Moore's Law will bring traditional VLSI into the atomic scale in the coming years. The video below gives an idea of the small scale of the phenomena we are talking about.

Probability

An important tool in quantum mechanics is basic probability; the canonical example of a probabilistic classical variable is the roll of a die, as shown in the animation below.

Superposition

In superposition, a quantum system may be partially in one state, and partially in another. One aspect of this is that the state of a quantum system in superposition is indeterminate; which state we will find is a matter of probability.

Interference

The animation below shows interference of waves. The waves add up in some places, and subtract in others. The same phenomenon applies at the quantum mechanical level, and can be used to modify the probability of a given state being measured. Managing interference is the heart of quantum computing algorithms.

The video below shows interference of ocean waves off of Inamuragasaki, near Kamakura, Japan. The waves add up in some places, and subtract in others.

Measurement

In measurement, the superposition of quantum system collapses to a single value.

Using Quantum Interferometry

The video below shows how quantum effects in interferometry and the state collapse caused by which-path measurement can be used to do counter-intuitive things, including detecting whether a photon-sensitive bomb is live or a dud.

Updated Version, no subtitles

Original (ORF) Version

Generating Entanglement

The video below shows how a single photon, or a stronger laser pulse, can interact with atoms held in two cavities, to generate entanglement. After interacting with the two atoms, the photon is measured. Typically, this measurement would tell us the parity of the two photons (even or odd). If the two atoms each started in a superposition state, some results will herald entanglement.

Entanglement and Teleportation

Entanglement can be used to teleport qubits from one location to another.

Using Physical Quantum Effects: Quantum Key Distribution

Quantum effects can be used for either numeric quantum computation, or physical operations such as detection of eavesdropping.

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