For The First Time Quantum Entanglement Of Photons Captured In Real-time

In collaboration with Danilo Zia and Fabio Sciarrino from Sapienza University of Rome, researchers at the University of Ottawa recently showcased a novel technique that makes it possible to visualise the wave function of two entangled photons—the fundamental particles that make up light—in real-time.

The idea of entanglement is comparable to picking a shoe at random when compared to a pair of shoes. As soon as you recognise one shoe, you can always tell what the other shoe is like—left or right—no matter where in the cosmos it is. The interesting aspect, though, is the inherent uncertainty surrounding the identification process up until the precise observational moment.

A fundamental concept in quantum mechanics, the wave function offers a thorough understanding of a particle's quantum state. For example, in the shoe example, the shoe's "wave function" could carry data about left or right, size, colour, and other attributes.

Biphoton state holographic reconstruction. Image reconstruction. a, Coincidence image of interference between a reference SPDC state and a state obtained by a pump beam with the shape of a Ying and Yang symbol (shown in the inset). The inset scale is the same as in the main plot. b, Reconstructed amplitude and phase structure of the image imprinted on the unknown pump. Credit: Nature Photonics 

To put it another way, the wave function lets quantum scientists anticipate what will probably happen when they measure different aspects of a quantum entity, like position, velocity, etc.

This ability to predict the future is extremely valuable, particularly in the quickly developing field of quantum technology, where it is possible to test the computer itself by understanding the quantum state that is generated or input into a quantum computer. Furthermore, the incredibly complex quantum states used in quantum computing involve numerous entities that may show strong non-local correlations, or entanglement.

It is difficult to determine the wave function of such a quantum system; this process is called quantum state tomography, or simply quantum tomography. A complete tomography using the conventional methods (which are based on the so-called projective operations) necessitates a high number of measurements, which rises quickly as the system becomes more complex (dimensionality).

The research group's earlier experiments using this method demonstrated that it can take hours or even days to characterize or measure the high-dimensional quantum state of two entangled photons. Furthermore, the quality of the result is highly dependent on the complexity of the experimental setup and is sensitive to noise.

One way to conceptualise the projective measurement approach to quantum tomography is as viewing shadows of a high-dimensional object projected from independent directions on various walls. A researcher can only see shadows, but they can still deduce the shape and state of the entire object from those shadows. For example, in a CT scan (computed tomography scan), a set of 2D images can be used to reconstruct the information of a 3D object.

But there's another method in classical optics for reconstructing a three-dimensional object. This technique, known as digital holography, works by using a reference light to interfere with the light that an object scatters in order to record a single image known as an interferogram.

The group expanded this idea to the case of two photons under the direction of Ebrahim Karimi, associate professor in the Faculty of Science, co-director of the uOttawa Nexus for Quantum Technologies (NexQT) research institute, and holder of the Canada Research Chair in Structured Quantum Waves.

In order to reconstruct a biphoton state, it must first be superimposed over a presumed well-known quantum state, and the spatial distribution of the positions at which two photons arrive simultaneously must then be examined. A coincidence image is a picture taken of two photons arriving at the same time. These photons could originate from the unidentified source or the reference source. According to quantum mechanics, it is impossible to pinpoint the photons' source.

As a result, an interference pattern is produced that can be utilised to piece together the wave function that is unknown. An advanced camera that captures events on individual pixels with nanosecond resolution made this experiment possible.

One of the paper's co-authors, Dr. Alessio D'Errico, a postdoctoral fellow at the University of Ottawa, emphasised the enormous benefits of this novel strategy by saying, "This method is exponentially faster than previous techniques, requiring only minutes or seconds instead of days." Notably, the complexity of the system has no effect on the detection time, which addresses the long-standing scalability issue in projective tomography."

This research has an impact that extends beyond academia. Improvements in quantum state characterization, quantum communication, and the creation of novel quantum imaging methods could all benefit from its potential to hasten the progress of quantum technology.

The study "Interferometric imaging of amplitude and phase of spatial biphoton states" was published in Nature Photonics.

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