In an article published in the magazine Nature Physics, a team of researchers from Britain, the USA, Germany and Australia demonstrated for the first time the two cracks experiment in the timeline. Instead of spatial dispersion on a screen, the temporal crack created a dispersion in the light spectrum, and from the combination of another crack an interference pattern similar to the spatial pattern from the classic experiment was created. The experiment opens a window for the construction of time-varying optical instruments in a reliable and stable manner
Traditionally, optics is based on physical structures and spatial phenomena. For example, an optical waveguide relies on areas of high refractive index ratio to capture the light rays in the fiber. In recent years, a new approach has been developed in the development of optical instruments: instead of engineering spatial patterns, physicists create temporal patterns that affect the properties of light. This change creates a "temporary surface" that the light reacts to similar to a "surface" that separates two materials with different refractive indices. In an article recently published in the prestigious journal Nature Physics, physicist Romain Tirol from Imperial College London and his partners developed an experimental system in which they created a temporary crack to examine the famous Yang experiment in the timeline. Young's experiment first demonstrated the wave properties of light and later served as a typical experimental system that demonstrates the quantum properties of particles in nature. The idea of temporal instrumentation is not new - in the past, scientists predicted that it would be possible to predict with their help temporal refraction and temporal reflection of light, temporally polarized reflection of light (Brewster angle) and more. Alongside the theoretical predictions, there are few experimental demonstrations, this in light of the difficulty in building a stable and reliable instrument that changes its properties very quickly. In the latest experiment conducted by researchers from the UK, Germany, USA and Australia, the researchers rose to the challenge and developed a temporary crack, a device that changes its refractive index at high speed. With his help, they conducted an experiment that replaced the role of the spatial frequency of the light waves with the temporal frequency and recreated the Yang experiment in the timeline.
Young's spatial two-slit experiment
In Yang's famous experiment known as the two-slit experiment, a unique interference pattern is created that confirms the wave properties of light. More precisely, the experiment demonstrates two phenomena, scattering and interference of light, and by combining these phenomena the unique pattern is created.
Scattering of light is clearly demonstrated by a single slit. In the screen behind the slot, the light will be seen from the wind, and will be in areas where the barrier would not allow it to reach if it were moving in a straight line. The scattering process occurs when the light meets the slit and moves at different angles. The dispersion can be explained using quantum mechanics and the uncertainty principle: as soon as the light hits the slit, it is confined to a very small area (usually proportional to its wavelength), because the position and momentum of the light cannot be known at the same time with high precision, the light will have to change its momentum. Why does the direction of momentum change and not its magnitude? The reason for this comes from the connection between conservation laws and symmetry in nature. Systems that demonstrate symmetry for shifts in time, i.e. systems that do not change their properties as a function of time, conserve their internal energy. Therefore, the light will not change the magnitude of its frequency, only the direction of its movement. (Da, in a vacuum the spatial frequency, which determines the spatial shape of the wave, and the temporal frequency of light, which determines how quickly it changes its value at each moment, are identical up to the speed of light. We will not refer to this fact here, but the change in the magnitude of the momentum, And not only its direction causes a change in the temporal frequency, and therefore the energy that the light carries with it).
Entanglement occurs when two or more waves overlap in position. For example, in the two-slit experiment, the waves scattered from each slit meet on the screen. The light intensity measured on the screen will be equal to the sum of the wave amplitudes squared. This principle is known as superposition. In other words, when waves meet in opposite phase from each other, meaning when the high and low point of each of the waves meet, no light will be seen. In the opposite situation when the light rays meet in the same phase, for example maximum meets maximum, light will be measured.
Young's transient experiment
The temporal analogy to Young's crack experiment replaces the location and spatial frequency of light with the time period in which it is measured and its temporal frequency. In fact, in Tyrol's experiment, light hits a surface that becomes transparent for a brief moment. The device forces the light to appear on the screen in a very specific period of time. From the principle of uncertainty between time and energy, the shorter the measured time period, the higher the uncertainty in energy. In this case, the energy, or the temporal frequency of the light will change and as a result a wide variety of colors will appear on the screen. This time, the system changes its properties in time and therefore the energy can change. After that, the researchers conducted an experiment in which they created two temporary cracks, meaning they opened and closed the barrier twice with short time intervals. Instead of seeing a spatial interference pattern, the researchers saw an interference pattern in the temporal frequency, or in other words in the measured frequency spectrum.
The technological challenges of the experimenters were many. To create a temporary crack, the researchers used the non-linear Kerr effect that changes the refractive index of light depending on the electrical voltage induced on it. The surface that the light hit was made of a layer of indium and tin oxide with a layer of gold underneath. The researchers calibrated the oxide layer around a zero refractive index (in a situation where light does not penetrate the material at all) and under the irradiation of light pulses, the refractive index changed slightly. While the refractive index changed its value, the light wave hit the surface and returned to the measuring devices.
Young's crack experiment is a classic experiment, and Tyrol's experiment could become one if the scientific community embraces the many promises that can emerge from time-varying optical systems.
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