TraceDistanceCoherence

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TraceDistanceCoherence
Computes the trace distance of coherence of a quantum state

Other toolboxes required CVX
Related functions L1NormCoherence
RelEntCoherence
RobustnessCoherence
Function category Coherence and incoherence
Usable within CVX? no

TraceDistanceCoherence is a function that computes the trace distance of coherence of a quantum state $\rho$, defined as follows:

\[C_{\text{tr}}(\rho) := \min_{D \in \mathcal{I}}\big\{ \|\rho - D\|_{\text{tr}}\big\},\]

where $\mathcal{I}$ is the set of incoherent quantum states (i.e., the set of density matrices that are diagonal in the standard basis).

Syntax

  • TDC = TraceDistanceCoherence(RHO)
  • [TDC,D] = TraceDistanceCoherence(RHO)

Argument descriptions

Input arguments

  • RHO: A state (either pure or mixed) to have its trace distance of coherence computed.

Output arguments

  • TDC: The trace distance of coherence of RHO.
  • D: A vector such that diag(D) is the closest incoherent state to RHO.

Examples

Maximally coherent states

The largest possible value of the trace distance of coherence on $d$-dimensional states is $2 - 2/d$, and is attained exactly by the "maximally coherent states": pure states whose entries all have the same absolute value.

>> d = 5;
>> v = ones(d,1)/sqrt(d); % this is a maximally coherent state
>> TraceDistanceCoherence(v)

ans =

    1.6000

>> 2 - 2/d

ans =

    1.6000

Source code

Click here to view this function's source code on github.