In order to evaluate a quantum correlation between two measurement errors in a joint measurement, we performed correlation measurements of entangled photon pairs with two identified joint measurements, where two different non-commuting polarizations are jointly measured for each photon. The joint measurements were realized by filtering the polarization oriented the direction, which is halfway in two observables. The experimentally observed correlations corresponded to the ideal correlations expected for the entangled state, except for the correlation between the products of the two non-commuting polarizations. This result exceeds the limits that apply to classical error statistics and suggests that the correlation between the measurement errors in each local measurement is imaginary.
We introduce a measure Q of bipartite quantum correlations for arbitrary two-qubit states, expressed as a state-independent function of the density matrix elements. The amount of quantum correlations can be quantified experimentally by measuring the expectation value of a small set of observables on up to four copies of the state, without the need for a full tomography. We extend the measure to 2×d systems, providing its explicit form in terms of observables and applying it to the relevant class of multiqubit states employed in the deterministic quantum computation with one quantum bit model. The number of required measurements to determine Q in our scheme does not increase with d. Our results provide an experimentally friendly framework to estimate quantitatively the degree of general quantum correlations in composite systems.
The objectivity of quantum measurement is treated as an emergent phenomenon with
N observers who can agree to the same result of measurement, and meanwhile, they can identify their records with each other. In this many-observer world (MOW), an objective quantum measurement is dealt with as a multipartite [
(N+1)-body] quantum correlation among the measured system and
N observers when its bipartite reductions are the same classical correlations. With this conceptual clarification, we find that, an objective quantum measurement is implemented if and only if the MOW is initially factorized in a pure state and then the total system can evolve into a generalized GHZ state with respect to the orthogonal basis preferred by each observer. Especially, such objective quantum measurement is recast in ideal classical correlation when the observer world is macroscopic for
N→∞.