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arXiv:1804.06472 · Physical Review A

Information-reality complementarity in photonic weak measurements

Luca Mancino, Marco Sbroscia, Emanuele Roccia, Ilaria Gianani, Valeria Cimini, Mauro Paternostro, Marco Barbieri

Published Jun 7, 2018• 22 citations
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Abstract

The emergence of realistic properties is a key problem in understanding the quantum-to-classical transition. In this respect, measurements represent a way to interface quantum systems with the macroscopic world: these can be driven in the weak regime, where a reduced back-action can be imparted by choosing meter states able to extract different amounts of information. Here we explore the implications of such weak measurement for the variation of realistic properties of two-level quantum systems pre- and postmeasurement, and extend our investigations to the case of open systems implementing the measurements.

DOI
10.1103/physreva.97.062108

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The emergence of realistic properties is a key problem in understanding the quantum-to-classical transition. In this respect, measurements represent a way to interface quantum systems with the macroscopic world: these can be driven in the weak regime, where a reduced back-action can be imparted by choosing meter states able to extract different amounts of information. Here we explore the implications of such weak measurement for the variation of realistic properties of two-level quantum systems pre- and postmeasurement, and extend our investigations to the case of open systems implementing the measurements.
Information about the internal properties of a system can only be obtained through interactions of the system with an external meter. However, such interactions generally result in entanglement between the system and the meter, making it difficult to trace the measurement result back to a specific value of the physical property in the system. It is therefore possible that the outcomes of quantum measurements depend in a nontrivial manner on the dynamics of the measurement interaction, possibly providing a physical explanation for the role of measurement contexts in quantum mechanics. Here we show that the effects of the measurement interaction on the meter can be described entirely in terms of the quantum coherent system dynamics associated with the back-action on the system. For sufficiently small back-action uncertainties, the physical property of the system is described by a weak value obtained from the Hamilton-Jacobi equation of the back-action dynamics. At higher measurement resolutions, the observed values are determined by quantum interferences between different amounts of back-action. Eigenvalues emerge when the quantum interferences between different back-actions correspond to a Fourier transform in the back-action parameter. We conclude that the values of physical properties obtained in quantum measurements originate from the quantum coherent properties of the back-action dynamics generated by that physical property during an interaction. Measurement outcomes represent elements of the dynamics and cannot be explained by measurement independent elements of reality.
Many quantum paradoxes based on a realistic view of weak values were discussed in the last decades. They lead to astonishing conclusions such as the measurement of a spin component of a spin-1/2 particle resulting in $100\ensuremath{\hbar}$, the separation of a photon from its polarization, and the possibility of having three particles in two boxes without any two particles being in the same box, among others. Here, we show that the realistic view of the weak values present in these (and other) works is equivalent to a realistic (and highly controversial) view of quantum measurements, where a measurement reveals the underlying reality of the measured quantity. We discuss that all quantum paradoxes based on weak values simply disappear if we deny these realistic views of quantum measurements and weak values. Our work thus aims to demonstrate the strong assumptions and the corresponding problems present in the interpretation of these quantum paradoxes.
In the weak measurement regime, post-selection can result in the observation of anomalous weak values, seemingly contradicting the eigenvalue statistics observed when the measurement interaction is strong. Here, we investigate the dependence of meter statistics on measurement strength in a post-selected measurement. We find that the meter statistics in the intermediate regime between weak and strong measurements is nearly independent of measurement strength and show that, in this regime, the system performs a measurement of momentum on the meter. Our results show that post-selection is extremely sensitive to the action of the meter on the system, highlighting the difference between the weak measurement regime where this back-action is negligible and the average meter position is a valid measurement result, and the intermediate regime where the meter statistics should not be misinterpreted as a measurement outcome. The transition between weak values and eigenvalues is facilitated by a reversal of the roles of the system and the meter, where the post-selection acts as a readout of information about the meter. The intermediate measurement regime thus reveals that the apparent contradictions between anomalous weak values and eigenvalues originate from the non-classical effects of back-action on the post-selected meter statistics.
The concept of information offers a more complete description of complementarity than the traditional approach based on observables. We present the first experimental test of information complementarity for two-qubit pure states, achieving close agreement with theory. We also explore the distribution of information in a comprehensive range of mixed states. Our results highlight the strange and subtle properties of even the simplest quantum systems; for example, entanglement can be increased by reducing the correlations between two subsystems.
This thesis covers research in two disjoint research areas: \n \nThe ontological model program (formerly hidden-variables program) for quantum theory has a long and noble tradition in the quantum foundations literature. By postulating a physical reality beyond the quantum state, we gain intuition on quantum phenomena and also come to understand constraints on realist interpretations of quantum theory. Bell's theorem tells us that such an underlying reality must be non-local, while the Kochen-Specker contextuality theorem abuses the classical notion that measurement should simply reveal pre-existing properties of reality. Recent research programs suggest that it is beneficial to view the quantum state as representing purely information. We show that the only current model which does this in a satisfactory manner is unable to reproduce all the statistics of quantum measurements. A recent generalization of the notion of contextuality has allowed for proofs of contextuality which differ from the original Kochen-Specker notion. We add a new result which shows that measurements in a model where the quantum state represents information must be contextual. Additionally, we refine the generalized notion of contextuality into strong and weak forms in order to parse the relationship between new and old results. \n \nEntanglement resource theory is a highly successful investigation of the usefulness of entanglement for information processing tasks. In this thesis we apply the ideas from entanglement resource theory to another resource: state distinguishability. We show analogies between distinguishability resource theory and entanglement resource theory. In particular, the analogy includes: measures which are monotonic under a class of transformations; units of a resource; and bounds on measures in terms of the amount of the unit resource needed to form states and the amount of unit resource that can be extracted from states. We show that the pairs of states which can be reversibly converted into \\emph{classical states} are exactly the pairs of simultaneously diagonalizable states. Lastly, we characterize the trace-distance distinguishability of formation on a qubit system.
One of the most remarkable features of quantum physics is that attributes of quantum objects, such as the wavelike and particlelike behaviors of single photons, can be complementary in the sense that they are equally real but cannot be observed simultaneously. Quantum measurements, serving as windows providing views into the abstract edifice of quantum theory, are basic tools for manifesting the intrinsic behaviors of quantum objects. However, a quantitative formulation of complementarity that highlights its manifestations in general measurement scenarios remains elusive. Here we develop a general framework for demonstrating quantum complementarity in the form of information exclusion relations (IERs), which incorporates the wave-particle duality relations as particular examples. Moreover, we explore the applications of our theory in entanglement detection and elucidate that our IERs lead to an extended form of entropic uncertainty relations, providing insights into the connection between quantum complementarity and the preparation uncertainty.