Testing the Symmetrization Principle with a Pair of Trapped Ions

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Quantum physics introduces the notion that two or more objects, such as electrons or protons, can be indistinguishable, i.e. they have no properties which would allow one to tell them apart. While the mathematical consequences of this idea form the basis of the stability of all matter we know of, such as nuclei and atoms, it is very difficult to directly verify that two objects are truly ndistinguishable. For this, one must create a situation where two (or more) indistinguishable objects exist at each location at the same time. Such a scenario occurs in molecules, causing the molecule to not absorb and emit light of a certain color. This project will bring this effect from the atomic scale to a more human length scale (comparable to the diameter of human hair) by controllably exchanging the position of multiple Calcium ions spaced 10 micrometers apart. The demonstration of this important quantum effect under very controlled conditions over relatively large length scales will not only test the principles of quantum mechanics in a new parameter regime, but also bring quantum physics to scales accessible to human senses, thereby inspiring students as well as the general public.The goal of this project is to demonstrate interference between two quantum particles with individual single-particle wavefunctions that never overlap. For this, two Calcium-40 ions will be prepared in a quickly rotating quantum state. A laser pulse will impart angular momentum to the ion string, thereby splitting it into two wavepackets. After the two wavepackets have rotated with respect to each other by 180 degrees, they will be combined again with a second light pulse. This procedure interferes one ion with the other and vice versa. It has not been previously demonstrated that ions can interfere with each other. This project aims at showing that ions can be made indistinguishable, even if they are always separated by several micrometers. This unusual situation leads to a vanishing overlap between the two single-particle wavefunctions, allowing the researchers to conduct a controlled test of the exchange principle by directly measuring the phase under particle exchange. Thus it becomes feasible to verify that the fermionic Calcium-40 ions acquire a phase factor of -1 under particle exchange. Finally, the project will study under which circumstances the two ions remain indistinguishable, thereby shedding light on the types of processes which actually can distinguish between identical particles.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.