Visualizing the mysterious dance: Quantum entanglement of photons captured in real-time

Credit: Nature Photonics (2023). DOI: 10.1038/s41566-023-01272-3

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 visualize 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 recognize 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 time.

A fundamental concept in quantum physics, the wave function offers a thorough comprehension of a particle's quantum state. For example, in the shoe example, the shoe's "wave function" might convey data about left or right, size, color, and other attributes.

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, including position, velocity, etc.

This ability to forecast the future is extremely significant, 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 interactions, 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's complexity (dimensionality) increases.

The research group's earlier studies 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 outcome is contingent upon the intricacy of the experimental setting and is very susceptible to noise.

One way to conceptualize the projective measurement technique 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, yet they can nevertheless deduce the shape and status of the entire thing from those shadows. For example, in a CT scan (computed tomography scan), a sequence 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 situation of two photons under the direction of Abraham 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 sites 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 utilized to piece together the wave function that is unknown. An sophisticated 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, emphasized 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 scaling 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.

Nature Photonics published the paper "Interferometric imaging of amplitude and phase of spatial biphoton states."

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