Norbert Schuch
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 32m comment Relating purity of reduced density matrix to concurrence It is the trace of the square of the red. density matrix. Also, you should use that $|\alpha|^2+|\beta|^2=1$. Then, you can easily see that the two are equal! 7h comment Relating purity of reduced density matrix to concurrence What have you tried? Do you know how $C$ is defined? (It's a property of a two-qubit state!) 2d comment reconstruct the wave-function from one body reduced density matrix? @MikaelKuisma Doesn't the Hohenberg-Kohn theorem require that the state is a ground state of a very special Hamiltonian -- one which is uniquely fixed except for a one-particle potential (which only couples to the one-particle density), such as in the case of interacting electrons in an external potential? Apr 28 revised Lowest energy product state of a local hamiltonian edited body Apr 28 revised Exponential decay of correlation in PEPS added 605 characters in body Apr 28 answered Lowest energy product state of a local hamiltonian Apr 28 answered Exponential decay of correlation in PEPS Apr 28 comment Complexity of quantum simulation @CuriousOne Basically, any quantum mechanical problem can be simulated classically using a polynomial amount of memory (basically, using a path integral), a class known as PSPACE. It is not known whether PSPACE is strictly larger than P. So if you could prove for a quantum problem that it is outside P, you would have also solved a long-standing open problem in complexity theory. Apr 27 comment Can particle quantum spin be described with a wave function? "spin up" and "spin down" can be understood as completely classical properties. We're not talking about relativistic QM here -- it's just another property of an object, such as its position. Apr 27 comment Can particle quantum spin be described with a wave function? A wave function assigns to each possible classical state (such as position, or position and spin) of a particle a complex number. Apr 24 comment Can particle quantum spin be described with a wave function? Of course spin is characterized by the wave function. It is just a wave function defined on a spatial and a spin degree of freedom. Apr 19 comment Trace of an observable @B.Reddy In more detail? Unlikely. Apr 19 revised Trace of an observable added 19 characters in body Apr 19 comment Trace of an observable As it's written, the answer is no. Could it be that one of the $z$'s should be compex conjugated? Apr 18 comment What do operations on single Qubits of Unfactorable Superpositions Do? @DavidZ +1 for the fact that the question would be much more clear if most of it would be removed. Apr 17 reviewed Close Space time curvature and gravity Apr 16 comment Measuring qubit as the presence or absence of a particle Note that encoding information in presence/absense of particles causes problems due to superselection rules (e.g., superpositions of particles typically cannot be created). This is not true for photons, but a similar problem might arise due to the lack of a phase reference. There are quite a few things which can be said about quantum information in the presence of superselection rules. Apr 16 comment Measuring qubit as the presence or absence of a particle What does "What sense does it make for that qubit" even mean? The qubit does not have a feeling of sense. Apr 15 comment Quantum computing entanglement dimensions question I think we strongly disagree. Apr 14 comment Quantum computing entanglement dimensions question So following wikipedia, the number of parameters needed to describe a mixed state is different from the number of parameters needed to describe the description of a mixed state? :-o