Researchers at Tampere University and their collaborators from Germany and India have experimentally confirmed that angular momentum is conserved when a single photon is converted into a pair – validating a key principle of physics at the quantum level for the first time. This breakthrough opens new possibilities for creating complex quantum states useful in computing, communication, and sensing.
Conservation laws are the heart of our natural scientific understanding as they govern which processes are allowed or forbidden. A simple example is that of colliding billiard balls, where the motion – and with it, their linear momentum – is transferred from one ball to another. A similar conservation rule also exists for rotating objects, which have angular momentum. Interestingly, light can also have an angular momentum, e.g., orbital angular momentum (OAM), which is connected to the light's spatial structure.
In the quantum realm, this implies that single particles of light, so-called photons, have well-defined quanta of OAM, which need to be conserved in light-matter interactions. In a recent study in Physical Review Letters, researchers from Tampere University and their collaborators, have now pushed the test of these conservation laws to absolute quantum limit. They explore if the conservation of OAM quanta holds when a single photon is split into a photon pair.
One minus one equals zero
The conservation rule dictates, e.g., that when a photon with zero OAM is split into two photons, the OAM quanta of both photons must add to zero. Hence, if one of the newly generated photons is found to have one OAM quanta, its partner photon must have the opposite, i.e., negative OAM quanta. Or in other words, the simple formula 1 + (-1) = 0 needs to hold. While these conservation rules have been tested and utilized in a myriad of optics experiments with a laser, they have never been tested for a single photon.
"Our experiments show that the OAM is indeed conserved even when the process is driven by a single photon. This confirms a key conservation law at the most fundamental level, which is ultimately based on the symmetry of the process," explains Dr. Lea Kopf, who is the lead author of the study.
Finding the photonic needle in the laboratory haystack
The team's experiments rely on delicate measurements as the required nonlinear optical processes are very inefficient. Only every billionth photon is converted to a photon pair, such that measuring the conservation of OAM for single photons resembles the proverbial search for the needle in the haystack.
An extremely stable optical setup, low background noise, a detections scheme with the highest possible efficiency, and a lot of experimental endurance enabled the researchers to record enough successful conversions such that they could confirm the fundamental conservation law.
In addition to confirming OAM conservation, the team observed first indications of quantum entanglement in the generated photon pairs, which suggests that the technique can be extended to create more complex photonic quantum states.
"This work is not only of fundamental importance, but it also takes us a significant step closer to generating novel quantum states, where the photons are entangled in all possible ways, i.e., in space, time, and polarization," adds Prof. Robert Fickler, who leads the Experimental Quantum Optics group where the experiment was performed.
Looking forward, the researchers plan to improve the overall efficiency of their scheme and develop better strategies for measuring the generated quantum state such that in the future these photonic needles can be found easier in the laboratory haystack. Moreover, the researchers aim at leveraging the generated multi-photon quantum states for novel fundamental quantum tests and quantum photonics applications such as quantum communication and network schemes.
Read the whole article in Physical Review Letters.
The Experimental Quantum Optics (EQO) group is an international research group investigating structured light, its interaction with matter on the single quantum level, as well as structured matter waves. In the broad sense, the group is interested in answering fundamental questions of quantum physics, such as high-dimensional entanglement and light-matter analogies, as well as in developing novel schemes for future quantum technologies.
