Observation of Scalar Aharonov-Bohm Effect with Longitudinally Polarized Neutrons

Abstract

We have carried out a neutron interferometry experiment using longitudinally polarized neutrons to observe the scalar Aharonov-Bohm effect. The neutrons inside the interferometer are polarized parallel to an applied pulsed magnetic field B(t). The pulsed B field is spatially uniform so it exerts no force on the neutrons. Its direction also precludes the presence of any classical torque to change the neutron polarization. [S0031-9007(98)05764-0] PACS numbers: 03.65. Bz, One distinction that sets quantum mechanics apart from classical mechanics is the treatment of a potential. In classical mechanics, the presence of a potential can be inferred only from the motion of the particles under the influence of the force it generates. The motion of particles through a region of uniform potential is, therefore, no different from that in empty space. In quantum mechanics, however, particles passing through a potential, uniform or not, acquire a quantum mechanical phase shift through their interaction with the potential. For instance, the phase shift acquired by electrons passing through a region of space containing a magnetic vector potential A and a scalar electric potential V is given by the action integralThis phase shift can be detected by interferometric techniques, as first pointed out clearly by Aharonov and Bohm (AB) [1]. The vector AB effect arises from the vector potential A(r) in the spatial part of the action integral, while the scalar AB effect comes from the potential V(t) in the temporal part. However, the experimental realization of the scalar Aharonov-Bohm (SAB) effect has proven to be challenging due to the technical difficulties in electron interferometry. [5]. In this experiment, the magnetic moments m of unpolarized thermal neutrons were subjected to a spatially uniform, but time-dependent magnetic induction B͑t͒ B͑t͒x. The scalar interaction E 2m ? B͑t͒ produces a quantum mechanical phase shift that is measurable by neutron interferometry. In this experiment, a spinindependent phase shifter was used to establish separate control over the phase shifts for the spin-up and the spindown neutron states. However, the use of unpolarized neutrons gave rise to the interpretational objection that each neutron experiences a classical torque [6], t m 3 B͑t͒, and, therefore, the observed phase shift is not strictly SAB, but an effect also observable by classical polarimetry [7]. We have now carried out a similar SAB experiment, but using neutrons polarized along the B(t) field. In this arrangement, there is neither a classical torque nor (as before) a classical force exerted on the neutrons. The experimental setup is shown schematically in Fig. 1. The setup consists of two main parts: the neutron polarizer and the neutron interferometer. The neutron polarizer includes a double-bounce neutron reflector made from a perfect silicon crystal and a magnetic prism assembly. The principle behind the polarizer is birefringence; i.e., the polarization dependence of the neutron 0031-9007͞98͞80(15)͞3165(4)$15.00

The role of inert objects in quantum mechanical phase

“…Let us examine a hypothetical experiment which is similar to the one described in [7]. All parts of this experiment are like those of [7], except the neutron which is replaced by a "classical neutron" whose structure is described below.…”

“…As stated in [7], the neutron is free of classical effects like force and torque. In spite of this, its quantum properties vary due to the external magnetic field.…”

“…All parts of this experiment are like those of [7], except the neutron which is replaced by a "classical neutron" whose structure is described below. …”

“…In the experiment reported in [7], polarized neutrons drift through an external time dependent magnetic field. The neutron's velocity and spin as well as the external magnetic field are parallel to each other.…”

“…Recent experiments utilize this relation between interference pattern and the Lagrangian (1) [6,7] for understanding the dynamics of the system. The present work analyses results of [7] and shows why quantum mechanical properties of the neutron distinguish it from an ordinary classical current loop.…”