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Physical Review A

Reconstructing the wave function through the momentum weak value

Junfan Zhu, An Wang, Xiong Liu, Yurong Liu, Zhiyou Zhang, Fuhua Gao

Published Sep 27, 2021• 23 citations
Publisher

Abstract

The wave function is the cornerstone of quantum mechanics, although it is still open to different interpretations. It has been shown that the amplitude and the phase of a wave function are manifested in the imaginary and real parts of the momentum weak value, respectively. Based on this, we develop a weak measurement scheme to reconstruct the wave function through measuring the momentum weak value. A proof-of-principle experiment is made to confirm the feasibility of our measurement scheme, in which the polarization states and the momentum of light are coupled. We believe the experimental protocol has the potential to be applied in diverse fields such as wave-front sensing, edge detection, and optical communication, and measurement of the momentum weak value may offer valuable insight to better understand quantum mechanics.

DOI
10.1103/physreva.104.032221

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The wave function is the cornerstone of quantum mechanics, although it is still open to different interpretations. It has been shown that the amplitude and the phase of a wave function are manifested in the imaginary and real parts of the momentum weak value, respectively. Based on this, we develop a weak measurement scheme to reconstruct the wave function through measuring the momentum weak value. A proof-of-principle experiment is made to confirm the feasibility of our measurement scheme, in which the polarization states and the momentum of light are coupled. We believe the experimental protocol has the potential to be applied in diverse fields such as wave-front sensing, edge detection, and optical communication, and measurement of the momentum weak value may offer valuable insight to better understand quantum mechanics.
The wave function is the cornerstone of quantum theory, and various techniques have been proposed for its direct measurement. The reconstruction of the wave function can be achieved by utilizing the real and imaginary components of momentum weak value, which correspond to amplitude and phase differentiation, respectively. However, these methods are limited in their applicability to wave functions with large phase gradients due to the weak coupling condition, which is a fundamental constraint. In this paper, we propose a method for reconstructing wave functions with large phase gradients based on optical differentiation through weak measurement. We have developed a more generalized theory of phase differentiation and implemented a common optical path and fast measurement for wave functions with large phase gradients. The experimental detection of wave functions with varying phase gradients has validated the reliability of our method. This work expands the range of applications for weak measurement while retaining its high resolution and sensitivity advantages, particularly in the field of wavefront sensing.
The wave function is important for understanding and predicting the behavior of photons. Recent studies have shown that wave function can be reconstructed through weak measurement. But currently, the relationships of wave function, weak value, and probe are single and not complete. Here, we propose a wave function reconstruction method based on complex weak values and complex shift probes. Four relationships of them are built and the wave function can be reconstructed by position and momentum shift probes. The complex weak values can be obtained by adjusting the pre- and post-selection in weak measurement, while the complex shift probes originate from the complex nature of the Fresnel coefficient in total internal reflection. To demonstrate the feasibility of the proposed scheme, an experiment of phase distribution reconstruction is achieved by a real weak value and a momentum shift probe; the experimental results agree well with the theory and show a high contrast. Therefore, the wave function can be conveniently reconstructed by our scheme and the relationships between the wave function, weak values, and probes become more complete. This work provides a valuable reference for the application of weak measurement in image processing, quantitative phase imaging, and label-free biological imaging.
The weak value, the average result of a weak measurement, has proven useful for probing quantum and classical systems. Examples include amplifying small signals, investigating quantum paradoxes, and elucidating fundamental quantum phenomena such as geometric phase. A key characteristic of the weak value is that it can be complex, in contrast to a standard expectation value. However, typically only either the real or imaginary component of the weak value is determined in a given experimental setup. Weak measurements can be used to, in a sense, simultaneously measure noncommuting observables. This principle was used in the direct measurement of the quantum wave function. However, the wave function's real and imaginary components, given by a weak value, are determined in different setups or on separate ensembles of systems, putting the procedure's directness in question. To address these issues, we introduce and experimentally demonstrate a general method to simultaneously read out both components of the weak value in a single experimental apparatus. In particular, we directly measure the polarization state of an ensemble of photons using weak measurement. With our method, each photon contributes to both the real and imaginary parts of the weak-value average. On a fundamental level, this suggests that the full complex weak value is a characteristic of each photon measured.
The measurement of a wave function plays a pivotal role in quantum physics and presents a distinctive challenge in experiment. Recent works have shown that both the real and imaginary components of the wave function can be extracted by employing weak or strong measurements, thereby enabling the determination of its amplitude and phase. Here, we propose a simple approach for reconstructing the wave function utilizing the spin-orbit interaction of light at the air-glass interface. By directly measuring the amplitude and employing spatial differentiation to capture the phase gradient, it becomes possible to successfully reconstruct an unknown wave function. To demonstrate its feasibility, we experimentally measure the pure wave function of photons with a Gaussian state. Furthermore, we conduct measurements on a custom state featuring a targeted phase jump to examine the accuracy of our methodology. The measured results show a distribution with high contrast and considerable accuracy, with a fidelity that can exceed 85%. We believe that this work contributes valuable insights into the practical applications of spin-orbit interaction, including optical image processing, wave-front sensing, and quantitative phase imaging.
A weak measurement does not completely collapse the wave function into one of the components of a superposition. A subsequent projective measurement on the weakly measured wave function can result in an amplification of the result for the original weak measurement. This chapter illustrates the procedure by a measurement of the average trajectories taken by particles in the two-slit experiment, in which the local momentum of a photon is encoded into its polarization, by means of a birefringent calcite crystal. The particle trajectories show the interference pattern. A second experimental example finds the real part and the imaginary part of a particle’s complex wave function. The amplitude and phase of the wave function are expressed in terms of its real and imaginary parts.