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Register a new userObservations of near-perfect nonclassical correlation using coherent light
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Complementarity theory is the essence of the Copenhagen interpretation. Since the Hanbury Brown and Twiss experiments, the particle nature of photons has been intensively studied for various quantum phenomena such as anticorrelation and Bell inequality violation in terms of two-photon correlation. Regarding the fundamental question on these quantum features, however, no clear answer exists for how to generate such an entanglement photon pair and what causes the maximum correlation between them. Here, we experimentally demonstrate the physics of anticorrelation on a beam splitter using sub-Poisson distributed coherent photons, where a particular photon number is post-selected using a multiphoton resolving coincidence measurement technique. According to Born rule regarding self-interference in an interferometric scheme, a photon does not interact with others, but can interfere by itself. This is the heart of anticorrelation, where a particular phase relation between paired photons is unveiled for anticorrelation, satisfying the complementarity theory of quantum mechanics.
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We have observed the three-photon correlation in nonclassical light sources by using an indirect measurement scheme based on the dead time effect of photon-counting detectors. We first developed a general theory which enables us to extract the three-photon correlation from the two-photon correlation of an arbitrary light source measured with detectors with finite dead times. We then confirmed the validity of our measurement scheme in experiments done with the cavity-QED microlaser operating with a large intra-cavity mean photon number exhibiting both sub- and super-poissoniaq photon statistics. The experimental results were in a good agreement with the theoretical expectation. Our measurement scheme provides an alternative approach for N-photon correlation measurement employing (N-1) detectors and thus a reduced measurement time for a given signal-to-noise ratio, compared to the usual scheme requiring N detectors.
In the context of the Oppenheim-Horodecki paradigm of nonclassical correlation, a bipartite quantum state is (properly) classically correlated if and only if it is represented by a density matrix having a product eigenbasis. On the basis of this paradigm, we propose a measure of nonclassical correlation by using truncations of a density matrix down to individual eigenspaces. It is computable within polynomial time in the dimension of the Hilbert space albeit imperfect in the detection range. This is in contrast to the measures conventionally used for the paradigm. The computational complexity and mathematical properties of the proposed measure are investigated in detail and the physical picture of its definition is discussed.
It is demonstrated that a weak measurement of the squared quadrature observable may yield negative values for coherent states. This result cannot be reproduced by a classical theory where quadratures are stochastic $c$-numbers. The real part of the weak value is a conditional moment of the Margenau-Hill distribution. The nonclassicality of coherent states can be associated with negative values of the Margenau-Hill distribution. A more general type of weak measurement is considered, where the pointer can be in an arbitrary state, pure or mixed.
We report the observation of nonclassical light generated via photon blockade in a photonic crystal cavity with a strongly coupled quantum dot. By tuning the frequency of the probe laser with respect to the cavity and quantum dot resonance we can probe the system in either photon blockade or photon-induced tunneling regime. The transition from one regime to the other is confirmed by the measurement of the second order correlation that changes from anti-bunching to bunching.
Continuous variable entanglement is a manifestation of nonclassicality of quantum states. In this paper we attempt to analyze whether and under which conditions nonclassicality can be used as an entanglement criterion. We adopt the well-accepted definition of nonclassicality in the form of lack of well-defined positive Glauber Sudarshan P-function describing the state. After demonstrating that the classicality of subsystems is not sufficient for the nonclassicality of the overall state to be identifiable with entanglement, we focus on Gaussian states and find specific local unitary transformations required to arrive at this equivalency. This is followed by the analysis of quantitative relation between nonclassicality and entanglement.
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