Threesome of photons. One step beyond in quantum interferometry

Beam splitter that divides the incoming beam in two outgoing beams.

Two independent research groups have showed for the first time interference of three indistinguishable photons, pushing photonics applications towards a more powerful stage.

Interference of particles, manifesting their wave behavior, is a purely quantum phenomenon which has no analog in classical mechanics. In particular, interference of one photon with itself goes against daily intuition, but also shows how nature really behaves. Interference of two single photons is a powerful tool that can be used to test the dual behavior of particles or the non-locality of quantum mechanics. It has also many technological applications in quantum information, for instance, single photon interference can be used for qubit generation and manipulation. In Quantum Key Distribution (QKD), Measurement-Device-Independent QKD exploits the interference of photons at a beam splitter, to reduce the requirements of the setup, closing loopholes for practical implementations. MDI-QKD has also been shown to be a good path to extend two-parties QKD to network QKD.

Two independent researches from Canada, Germany, and Austria, and from the UK and Denmark, have demonstrated experimentally three photon interference at a beam splitter, being able to distinguish from single and two photon interference. A beam splitter is an optical element (crystal) that splits the light in two parts. It has two incoming ports and two outgoing ports. Single photons going to a 50:50 beam splitter will have 50% probability of going out through each output port.

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HOM dip. Plot of coincidence counts versus the difference of the time of arrival of both incoming photons. The dip indicates that both photons leave from the same output port, and therefore there are no coincidence counts.

The HOM experiment
Quantum mechanics says that when two indistinguishable photons arrive simultaneously at the two different input ports of the beam splitter (BS) they will interfere and they will leave the BS from the same output port. This behavior cannot be explained from quantum optics, it is just a quantum interference phenomenon. This result has been long observed. It was first shown experimentally by Hong, Ou, and Mandel in 1987. In their experiment they measured the coincidence probability of having an outgoing photon from each output port, as they changed the time of arrival at the BS of one of the incoming photon. They observed a decrease of the coincidence probability just when both identical photons arrived at the same time. Otherwise, the photons begin to be distinguishable, as their timing is different.

This experiment measures the indistinguishability of two incoming photons, and has many applications. Generating or interfering identical photons is used for qbit generation and transfer. Also in Quantum Key Distribution is capital as it can help detect tampering in the communication. For Bell state measurements is also used, to compare incoming photons at a BS and then check quantum mechanics.

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Interference of three photons

Performing an experiment of photon interference is hard. There are many parameters to control. For the photons to be identical they must have the same frequency, width, time, phase, ... Also, there might be contributions to the interference from unwanted events, such as multi-photon pulses, or dark counts coming from stay photons or from false detections from the detectors, or detectors might just not detect an incoming photon.

Experiments usually send pulses from an attenuated laser (with the drawback that some of the pulses will be empty, and some pulses will contain more than one photon). Another way to obtain single photons is through SPDC (spontaneous parametric downconversion), where a high power laser beam is directed to a photonic crystal (PC). The interaction of photons at the PC will make that some photons (very few, about one in a trillion) will be transformed into two photons (conserving energy). These two outgoing photons will be entangled and experimentalists will use them in their experiment. But obtaining three entangled photons needs a different setup. One of these two entangled photons needs to be sent to a new PC to generate a third entangled photon. This is how one of the groups generated their three entangled photons.

Once you have the three identical photons you need to make them interfere at a BS (be it crystal of fiber based) and detect how many photons you have in each output port each time. In this threesome interference there is a parameter, the triad phase, that is zero in one and two photon interference, that has been used to verify the three photon interference.

These experiments show that photon interference can be scaled to more than just two photons, which has many practical applications in quantum information science.

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