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Physical Review A

Contrast enhancement by time selection in neutron interferometry

Michael Zawisky, H. Rauch, Yuji Hasegawa

Published Dec 1, 1994• 23 citations
Publisher

Abstract

We have demonstrated in our experiments that important additional information can be won even from a stationary neutron beam by registering the detection time. Time selection was performed by registering the time of arrival of neutrons with respect to reference events. It was thus possible to measure the time-dependent neutron-neutron pair correlation function of a statistical beam. The sequence of time-correlated pairs is more strongly correlated than the Poissonian distribution of the initial events. Using this method it is possible to enhance the contrast and the phase resolution.

DOI
10.1103/physreva.50.5000

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We have demonstrated in our experiments that important additional information can be won even from a stationary neutron beam by registering the detection time. Time selection was performed by registering the time of arrival of neutrons with respect to reference events. It was thus possible to measure the time-dependent neutron-neutron pair correlation function of a statistical beam. The sequence of time-correlated pairs is more strongly correlated than the Poissonian distribution of the initial events. Using this method it is possible to enhance the contrast and the phase resolution.
As shown recently by Zawisky, Rauch, and Hasegawa in neutron interferometry [Phys. Rev. A 50, 5000 (1994)], the interference pattern obtained with a low-intensity particle beam is deeply modified when a selection is made of the time intervals between successive detected events. Experiments of the same type performed with a Stern-Gerlach interferometer using a metastable hydrogen atom beam are described. Results very similar to those of the neutron experiment are obtained, in particular a strong enhancement of the contrast when large time intervals are selected.
Abstract This chapter describes a rather broad selection of neutron interferometry experiments whose theoretical description requires an understanding of the mutual coherence function, and the sensitive experimental techniques utilized. Within the interferometer the coherent beams can be influenced by interactions with fields and material media in space and time causing phase shifts and wave packet delays that substantially affect the outgoing beams. Post-selection experiments involving spectral filtering and time-of-flight analysis are described. The phase-echo effect is described; it is a generalization of the spin-echo effect. Placing statistical or deterministic absorbers within the interferometer affects the outgoing coherence properties of beams quite differently. Low counting rate interference experiments show time pair-correlation effects characteristic of Poissonian statistical beams. Decoherence effects are investigated to study the transition from a quantum to a classical regime.
In the experiment described in this paper, a chopper serves as a source of neutron intensity pulses which pass through a perfect Si-crystal neutron interferometer (NI), where they are split coherently into two parts that recombine and interfere. As the occupation number of a single pulse is much smaller than unity, the results very clearly demonstrate the single-particle interference phenomena of neutron interferometry experiments. While traveling the pulses spread, with faster (slower) neutrons tending toward the leading (trailing) edge. The coherence length of the neutrons depends on the spectral width of their wavelength distribution. When a material with a neutron-nuclear optical potential is placed in one beam path in the NI, there is a loss of fringe visibility (contrast), which depends on the coherence length. By using time-of-flight techniques, we divide the pulse into time segments. Within a given time slice, the wavelength spectrum is narrower than in the pulse as a whole. As a result, contrast remains in the time slices even when it disappears in the overall pulse. We also observe an additional contrast modulation due to the overlap of neighboring pulses.
We present a detailed analysis of the time series of time-stamped neutron counts obtained by single-neutron interferometry. The neutron counting statistics display the usual Poissonian behavior, but the variance of the neutron counts does not. Instead, the variance is found to exhibit a dependence on the phase-shifter setting which can be explained by a probabilistic model that accounts for fluctuations of the phase shift. The time series of the detection events exhibit long-time correlations with amplitudes that also depend on the phase-shifter setting. These correlations appear as damped oscillations with a period of about 2.8 s. By simulation, we show that the correlations of the time differences observed in the experiment can be reproduced by assuming that, for a fixed setting of the phase shifter, the phase shift experienced by the neutrons varies periodically in time with a period of 2.8 s. The same simulations also reproduce the behavior of the variance. Our analysis of the experimental data suggests that time-stamped data of single-particle interference experiments may exhibit transient features that require a description in terms of non-stationary processes, going beyond the standard quantum model of independent random events.
Neutrons are proper tools for testing basic laws of quantum physics since they are massive and can be handled and measured with high efficiency. Suitable postselection experiments demonstrate coherence features of sub-ensembles even when the whole ensemble seems to have lost its coherence. All experiments have the capacity to explain more details by additional pre- and post-selection methods. It will be shown that specific losses are unavoidable in any interaction. Coherence and decoherence are intrinsic quantum effects and can shed light on the measurement problem. Quantum contextuality is a consequence of the entanglement of different degrees of freedom. This makes quantum phenomena more strongly correlated than classical ones. Most experiments have been performed with perfect neutron interferometers and some others by using ultracold neutrons and spin-echo systems. An event by event based interpretation can also be brought into agreement with the experimental results. In many cases parasitic beams carry the same information as the main beam and this relates such measurements to "weak" measurements. The coupling and entanglement of various parameter spaces guide us to a more elaborate discussion of quantum effects.