Modern Physics 5/10/11 Spring 2011 Ben Miller, Alexander DeCarli, Kevin Shaw

A laser (usually ultraviolet for its high frequency) sends a photon through a nonlinear crystal such as Beta Barium Borate The photon bumps an electron to an excited state When the electron comes back down and releases its photon, there is a chance it will split If it splits, the two photons are equally half of the energy These two photons are entangled The overlapping of the cones represents the entanglement The two photons are also polarized opposite of one another

Einstein called this " Spooky action at a distance."

 Bell-state Quantum Eraser  The split photons are opposite in polarization  The double-slit selectively filter between polarizations (e.g. right slit allows clockwise)  A filter in front of Detector A for polarizations as well  If the A filter restricts the polarized light, then the polarization entering the double-slit is known and no interference  If the A filter allows all light, then polarization entering the double-slit is not known and interference shows up in both detectors.  How do photons at B know that polarization is no longer restricted at A?

 Quantum Entanglement comes from the ERP paradox paper.  The paper was written by Albert Einstein, Nathan Rosen and Boris Podosky in  ERP is a topic in physics concerned with measuring and describing microscopic systems.  The three men felt that quantum mechanical theory was incomplete.  By incomplete, they were talking about entanglement but did not have a name for it.

 Erwin Schrodinger read this paper and wrote to Einstein talking about the idea and called it “entanglement.”  Schrodinger later wrote a paper that defined the idea of entanglement.  Both Einstein and Schrodinger were dissatisfied with the idea.  In 1964 entanglement was tested and disproved by John Bell because it violated certain systems but since then other experiments have proved it to be true.  Each experiment had its flaws though.

 Quantum Communication  Quantum Teleportation  Quantum Cryptography  A quantum system in an entangled state can be used as a quantum information channel to perform tasks that are faster than classical systems.

 Typically, entanglement experiments involve entangling pairs of photons and observing the changes in one effecting the changes in the other  Italian physicists thought of an idea where the effects of entanglement could be easily detected  A pair of photons could be entangled and then separated. One of the photons could then be amplified into a shower of thousands of other photons, all entangled to the lone other photon  Nicolas Gisin from the University of Geneva in Switzerland decided to test this with humans.  The beam of macro photons could be shown in one of two positions on a wall depending on the polarization of the lone microscopic photon, which defined the group.  The human tests were successful and matched with the results of a photon detector  A flaw was discovered in which detection of the photon would still occur after the entanglement connection was supposedly broken, suggesting a flaw in amplification and the inherent flaws in any detector  This flaw also hints that this particular experiment may not have been a micro-macro entanglement condition, but work is being done to enhance amplification with lasers  Clearly, humans can not be used

 "Superdense coding"  We typically use bits in computer processing, or in this case, classical bits  In Quantum Mechanics, information can be stored using qubits, which describe a quantum state  Information can be obtained via measurement of the qubit  In theory, qubits can contain other dimensions of information, but the predictability of determining information is only completely effective on a 1:1 scale of information from classical to quantum  This means that effectively, a qubit can only reliable store as much as a classical bit  Useless? Not with entanglement. Qubits can be entangled in pairs and therefore two classical bits per qubit can be reached.  This is a doubling of efficiency known as "superdense coding"

 - Has to do with transmitting a qubit from one location to another without the qubit being moved through free space - This can be used in the idea of a quantum computer, which would take advantage of changes in quantum states in order to rapidly send and process data - With the qubit's use of other dimension, more advanced algorithms can be used, in theory, to solve specific problems significantly faster and more effective than any classical computer - However, it is important to note that a classical computer can simulate a quantum one, therefore a quantum computer would not be able to solve a problem that a classical computer could not. - Typically, qubits are used to define and alter particle spin

 Ben Miller  Alexander DeCarli  Kevin Shaw

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Article on quantum entanglement at high temperatures.