LY12434

Quantum dance of a single atom with a miniature Bose-Einstein condensate

Predicting the movements of three interacting particles is a hallmark example for a difficult problem in classical mechanics - under most circumstances it is unsolvable. For example the mutually dependent orbits of the moon, the earth and the sun can only be predicted up to a certain accuracy. In the quantum world understanding such few-body systems is no simpler and the experimental investigation of their inner workings is very often complicated by their microscopic scales. An important example for quantum few-body systems are atomic nuclei, that consist of a small number of protons and neutrons; large particle accelerators are needed to explore atomic nuclei experimentally.

In our work, we accurately investigate the effects of interactions in an accessible model few-body quantum system with large length and low energy scales; ultracold bosonic Rubidium and fermionic Potassium atoms are trapped at the sites of on optical lattice, an artificial crystal made of laser light. On each site of the crystal a miniature Bose-Einstein condensate of Rubidium atoms interacts with a single fermionic Potassium atom. Using a novel, tricky detection scheme we accurately observe the interplay of interactions, finding that the presence of a single fermionic atom can also mediate interactions between the bosons. Our system may help to better understand theoretical models of atomic nuclei and shows the feasibility of advanced schemes of quantum computation and quantum memory, where one atomic species is used for data storage and the other one for the actual computation.


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EY10611

Spotting efficient broadcasters and receivers

Interactions such as ``who dated whom'', ``who phoned whom'', ``who emailed whom'', and ``who Facebook-friended whom'' do not remain static. They evolve over time. This can have important implications when we try to understand the spread of rumours, opinions, malware or diseases. If Amy meets Bob in the morning and Bob meets Coleen in the afternoon, then an infection, or a piece of gossip, can be passed from Amy to Coleen, but not from Coleen to Amy. If we don't keep track of the order of those interactions, then we can't make accurate summaries. In this work, we show how to incorporate time's arrow into the types of computer algorithms that are becoming widely usedin the field of Social Network Analysis. We apply this new technique to email data within the Enron corporation and cell phone data in a university laboratory, showing how to spot the efficient broadcasters (good places to plant a rumour) and receivers (good places to find out the latest rumour).

LW12245

Atomic friction: when experiments meet simulations

Sliding at the nanometer scale usually does not occur smoothly. Often, the two surfaces stick together and then slip unstably with atomic periodicity. Despite its crucial role in revealing the molecular origins of friction, atomic stick-slip frictional behavior is not fully understood. This is largely due to the fact that the detailed information of the sliding interface is buried and not easily accessible. Now, a collaborative research team has recently that key aspects of atomic stick-slip behavior observed experimentally for an ultra-sharp platinum tip sliding over an atomically flat gold surface can be reproduced consistently in simulations. The sliding conditions are optimally matched in the two systems and a special computational method was adopted to substantially slow down the model sliding speeds to approach those of the experiment. Comparisons of the results with theoretical models confirm that, at lower speeds, atomic friction is a thermally driven process. During this process, the atoms from the tip are hopping to nearby locations with the help of thermal vibration and with a frequency limited by their inertia. However, at high speeds, where most conventional atomistic simulations are run, frictional processes are not in this thermally-activated regime. This provides new guidance for comparing simulations and experiments focused on atomic-scale friction.

LA13215

Binary Bonds

By pulling on an individual atom with an atomically sharp probe, the smallest toggle switch achievable in silicon has been realised. The switch is flipped by reorienting a single chemical bond through mechanical force alone. Although atom switches have previously been reported by a number of research groups, in each case the switching mechanism has required, or involved, a flow of electrons. In this paper we show that a silicon atom can be toggled back and forth between an ‘up’ and a ‘down’ position by directly modifying the bond it makes with its nearest neighbour. This is achieved through manipulation of the force between two atoms – an electric current or a bias voltage is not necessary – and results in binary control of bond angle. The ability to switch the orientation of a single chemical bond in this way is an important new addition to the nanotechnologist’s toolbox.