Researchers from the Indian Institute of Technology (IIT) Kanpur and Leiden University in the Netherlands have developed a novel method to measure the orbital angular momentum (OAM), or the amount of twist, of light. Their method could significantly accelerate data transfer rates and even improve quantum computations and communications.
To understand OAM, let's consider how light usually works. Classically speaking, light travels in waves that can vibrate in different directions in space, like a skipping rope being shaken in different directions. We call this polarisation. OAM is different; instead of vibrating in 2 dimensions, the light wave twists around its direction of travel, like a corkscrew in 3D space. This twist creates an orbital motion, and the amount of twist is what we call the orbital angular momentum.
OAM can have one or many twists, and each twist represents a different mode or channel. The twisted light is identified by an integer called an OAM mode index, which tells us how twisted or spiral-shaped the light beam is. Detecting these modes with equal sensitivity is tricky because the beam can have many possible twists. Detecting the OAM of a light beam reliably can be a challenge. Many detectors could only see a small number of OAM modes, while others were better at seeing specific modes, meaning they favored one twist over another, leading to errors.
In the new study, the researchers devised a method to precisely detect the OAM for multiple modes over a wide range. First, they used lasers and a tool called a Spatial Light Modulator, a unique hologram that can control the intensity, phase, or polarisation of light in a spatially varying manner, to generate beams of light that carry OAM.
The light beam with OAM is then split into two paths in a device called an interferometer. One of the paths has a device called an image rotator. The image rotator developed by the team uses a unique set of mirrors that rotates the image of the beam without losing much of the light. This rotation is crucial for systematically measuring how twisted the beam is, and the amount of rotation is carefully controlled.
The two paths are then combined again, creating an interference pattern. A type of super-sensitive camera called an Electron-Multiplied Charge-Coupled Device, or EMCCD, records the interference pattern, which changes depending on the OAM of the original light beam. The team then applied mathematical transformations, like a Fourier transformation, to turn the camera readings into precise OAM measurements.
This way their new set-up could measure the exact OAM spectrum of the light, meaning it can identify all the different twists present and how strong they are, just by analysing the interference pattern. It can detect a wide range of OAM modes, up to 100 different twists. The method allows for the detection of all modes equally well, so you get an accurate picture of the OAM spectrum. The method proved to be incredibly accurate, with a fidelity, a measure of how close the measurement is to the actual value, of over 98%.
The team also employed a two-shot technique to bypass noise contributions. In any measurement, noise refers to unwanted signals that interfere with the signal being measured. In the case of the OAM detector, noise can come from several sources, including ambient light and electronic fluctuations. Their new method involves taking two separate measurements of the interference pattern, with the phase or the timing of the wave set to two different values. These two values are chosen to give opposite signals from all sources of signal, and are kept separate by a phase value equal to pi. Now, instead of analysing each measurement individually, the detector calculates the difference between the two measurements. This two-shot method reduces the impact of constant noise in the OAM detector, improving the accuracy of the OAM spectrum measurement.
So why do we need to detect the OAM and all its modes? Traditional computing and communication rely on bits, which can be either 0 or 1. OAM allows for what’s called high-dimensional states, which are bits that can be in more than just two states, opening up exciting possibilities in areas like quantum computing. The extra dimensions also allow for OAM beams to carry significantly more data than conventional light sources. Studies have shown that a beam of light split into eight different circular polarities has demonstrated the capacity to transfer up to 2.5 terabits of data per second. OAM states can also be generated in coherent superpositions and can be entangled, two qualities essential for quantum computing. Quantum cryptography, for example, uses the laws of quantum mechanics to create unbreakable codes.
The new OAM detector could spur faster data transfer and enhance quantum technologies, from secure communications to better sensors. While the detector is a significant achievement, there are still challenges to overcome. One challenge is related to the alignment of the detectors and mirrors in the setup, which requires careful control and can be time-consuming. Another challenge is related to the types of light it can read. The current device has been demonstrated to measure the diagonal elements of a light beam, but an extension of the device is needed to measure the off-diagonal elements that contain information about phase coherence, which relates to the stability of the phases of the two beams.
As the demand for more data and better security grows, so does the need for sophisticated light manipulation methods. By giving us a precise, more uniform picture of how light can twist and turn, this research offers a promising leap forward in photonics and quantum science and opens up a whole new world of possibilities for communication, computing, and scientific research.
This research article was written with the help of generative AI and edited by an editor at Research Matters.