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Cambridge University Press eBooks

Two-photon interferometry, the quantum measurement problem, and more

Marlan O. Scully, Muhammad Suhail Zubairy

Published Sep 4, 1997• 0 citations
Publisher

Abstract

As was demonstrated in the previous chapter, the process of observation and acquisition of information or at least the possibility of ‘knowing’ (whether or not we bother to ‘look’) can profoundly change the outcome of an experiment. For example, in the case of the micromaser which-path detector, we do not need to ‘look at’ or ‘interrogate’ the masers in order to lose the interference cross term; it is enough that we could have known. Experiments along these lines provide a dramatic example of the importance of which-path, or ‘Welcher-Weg’, information. The present chapter treats the Welcher-Weg quantum eraser problem from a different vantage. We first consider the interference of light as it is scattered from simple atomic systems consisting of single atoms located at two neighboring sites. From this simple model, we can gain a wealth of insight into such problems as complementarity, delayed choice, and the quantum eraser via field–field and photon–photon correlation functions, i.e., via G (1) ( r , t ) and G (2) ( r , r ′ t , t ′). The chapter concludes with a demonstration that such considerations can, in principle, even lead to new kinds of high-resolution spectroscopy.

DOI
10.1017/cbo9780511813993.023

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As was demonstrated in the previous chapter, the process of observation and acquisition of information or at least the possibility of ‘knowing’ (whether or not we bother to ‘look’) can profoundly change the outcome of an experiment. For example, in the case of the micromaser which-path detector, we do not need to ‘look at’ or ‘interrogate’ the masers in order to lose the interference cross term; it is enough that we could have known. Experiments along these lines provide a dramatic example of the importance of which-path, or ‘Welcher-Weg’, information. The present chapter treats the Welcher-Weg quantum eraser problem from a different vantage. We first consider the interference of light as it is scattered from simple atomic systems consisting of single atoms located at two neighboring sites. From this simple model, we can gain a wealth of insight into such problems as complementarity, delayed choice, and the quantum eraser via field–field and photon–photon correlation functions, i.e., via G (1) ( r , t ) and G (2) ( r , r ′ t , t ′). The chapter concludes with a demonstration that such considerations can, in principle, even lead to new kinds of high-resolution spectroscopy.
The "welcher Weg" (which-path) detector as described by Scully et al. [Nature 351, 111 (1991)] employs a pair of initially empty micromaser cavities placed in front of a double-slit apparatus in an atomic interferometer. Laser excited atoms spontaneously emit a photon into either cavity thereby marking the atoms' path and thereby destroying the interference. I propose an alternative method wherein at least one of the cavities is prepared in a coherent state with a strong amplitude. Which-path information is obtained by a nonresonant, dispersive type of atom-field interaction associated with quantum nondemolition measurements. Ground-state atoms passing through the cavity remain in the ground state but impart a phase shift to the cavity fields. Velocity selection is shown to affect the visibility of the fringes. An associated quantum eraser is also discussed.
We discuss complementarity and uncertainty in a gedanken Which-Way (Welcher-Weg) experiment in a Mach-Zehnder interferometer. Although a Welcher-Weg measurement can be performed with only a negligible amount of momentum change in the detector itself, the change in the interference pattern must correspond to a change in the momentum distribution of the particle. Entanglement between the Mach-Zehnder interferometer and the Welcher-Weg measurement introduces orthogonality that disables the interference of the particle, but simultaneously, it also generates a momentum transfer between the particle and other parts of the whole entangled system (including macroscopic parts). This analysis can be related to the Delayed Choice interpretation. We show where the momentum change takes place and we conclude that entanglement is the key to understand how complementarity (i.e. Which Way versus interference) and the position-momentum uncertainty relation are interwoven.
Introduction In recent years, numerous quantum optics experiments have demonstrated novel quantum effects based on quantum amplitude superposition. Some of these empirical observations raise disturbing questions about the nature of reality, particularly concerning quantum nonlocality. In this chapter we discuss some experiments that suggest (to us, at least) that the information void, that uncharted regime between signal preparation and outcome detection, is not well modeled by classical spacetime. Something strange seems to be lurking there. For the purposes of exposition, the sequence of experiments we discuss in this chapter does not follow the historical sequence of those experiments, We discuss first the delayed-choice quantum eraser experiment of Kim et al. (Kim et al., 2000), referred to here as KIM. This leads us to define a heuristic measure of which-path information that is central to the theme of this chapter. This leads to a discussion of Wheeler's thought experiments on the conundrum of delayed choice in physics, particularly evident in the phenomenon of galactic lensing (Wheeler, 1983), and its empirical implementation, the delayed-choice interferometer experiment of Jacques et al., referred to as JACQUES. Our final experiment in this sequence is the double-slit quantum eraser of Walborn et al. (Walborn et al., 2002), referred to as WALBORN. That experiment gives a significant challenge for quantized detector networks (QDN) to reproduce its empirical results. Such experiments have led to suggestions that interference patterns formed by particles impacting on a screen may be influenced in some way by decisions made long after those particles had landed on that screen. Our objective in this chapter is to show by a detailed QDN analysis of those experiments that quantum principles do not support these suggestions. Nevertheless, something bizarre seems to be there. There are two complementary aspects in these experiments that have deeply worried physicists. The first involves spatial nonlocality and is the point of Wheeler's concerns. Historically, it has been a great source of debate among physicists.
A novel quantum optical interference experiment based on a delayed correlation technique and involving photons prepared in a polarization-entangled state is presented. The resulting phenomenology is found to be determined by the simultaneous realization of several non-classical interference processes of distinct nature albeit strictly interrelated. A formal interpretation of the results is attempted by a standard quantum analysis and by the adoption of a set of interfering Feynman diagrams. In addition, a physical interpre-tation at a deeper level is undertaken by introducing for the system the basic theoretical background of a new concept in quantum optics: entangled interferometry. As a relevant application of this new paradigm, a novel two-photon welcher-weg argument within a delayed-choice experiment is presented. It leads to an unexpected complementarity puzzle which expresses the manifestation of the EPR non-locality within the general context of interferometry. PACS numbers: 03.65.Bz, 42.50.-p, 42.50.Wm.
We present various experiments demonstrating the mutual exclusivity of observing interference and which-path information, as demanded by Bohr's complementarity principle. Using photon pairs created in parametric down-conversion, no which-path measurements need to be performed on the interfering photon itself. Instead the other photon can be used to introduce distinguishability, which consequently destroys interference. However, a suitable measurement erases this distinguishability and interference can be recovered.