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A mixed state is described by a density matrix ρ, that is a non-negative trace-class operator of trace 1 on the tensor product . The partial trace of ρ with respect to the system B, denoted by , is called the reduced state of ρ on system A.
Consider a quantum system that can be divided into two parts, A and B, such that independent measurements can be made on either part. The state space of the entire quantum system is then the tensor product of the spaces for the two parts. :=. Let ρ AB be a density matrix acting on states in H AB.
Consider a vector of the tensor product . in the form of Schmidt decomposition = =. Form the rank 1 matrix =.Then the partial trace of , with respect to either system A or B, is a diagonal matrix whose non-zero diagonal elements are | |.
the reduced state of ρ on system A, ρ A, is obtained by taking the partial trace of ρ with respect to the B system: =. The partial trace operation is a CPTP map, therefore a quantum channel in the Schrödinger picture. [5] In the Heisenberg picture, the dual map of this channel is
In mathematics, specifically functional analysis, a trace-class operator is a linear operator for which a trace may be defined, such that the trace is a finite number independent of the choice of basis used to compute the trace. This trace of trace-class operators generalizes the trace of matrices studied in linear algebra.
The tensor product of two vector spaces is a vector space that is defined up to an isomorphism.There are several equivalent ways to define it. Most consist of defining explicitly a vector space that is called a tensor product, and, generally, the equivalence proof results almost immediately from the basic properties of the vector spaces that are so defined.
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In this case, even entangled states can be written as a mixture of tensor products of single-party aphysical states, very similar to the form of separable states. In the qubit case, M k {\displaystyle M_{k}} are physical density matrices, which is consistent with the fact that for two qubits all PPT states are separable.