As the atoms flow

July 2017: Borrowing a technique developed for mapping electrical conductance in semiconductor devices, QUIC physicists at ETH imaged cold neutral atoms as they are transported through constrictions narrow enough for quantum effects to come into play. These results highlight the potential of using neutral atoms to simulate electronic transport in nanoscale devices. Through a series of  works, the group of Tilman Esslinger has in recent years established atomic quantum gases as a promising platform for studying quantum transport.

In the approach, two reservoirs holding ultracold atoms (schematically shown here in orange) are connected by a mesoscopic channel, in analogy to quantum-electronic devices. The structures in which the atoms reside and move are defined by optical potentials, providing the basis for a superbly clean and highly controllable quantum system.
Samuel Häusler and his colleagues have now implemented in their setup an analogue of a method known in semiconductor electronics as scanning-gate microscopy [1]. Using this method the team observed currents that originate mainly from quantum tunneling — a regime that is out of reach for its electronic counterpart.
In scanning-gate microscopy, a sharp obstacle is placed in a channel and subsequently its effect of the current is recorded. As the obstacle is moved, spatial dependencies of transport properties can be determined. The method has been introduced for studying electronic transport in semiconductor nanostructures, where electrically conductive tips are used as probes. In the experiments of the ETH physicists, in contrast, neutral atoms are transported through structures defined by light fields and a tightly focused laserbeam serves as the obstacle (shown above in cyan).

With their novel method, Häusler et al. achieved spatial resolutions comparable to the extent of the atomic wave function in the transport channel. Depending on the strength of the tip, the atoms were observed to either flow over or tunnel through it.
The team expects that their flexible scanning-gate microscope for neutral atoms will extent the capabilities of their quantum simulators. In the future, the approach could provide both fresh fundamental insights — for instance into disorder in materials — and a scalable route towards controlling transport at the quantum level in mesoscopic ‘atomtronic’ devices.
[1] S. Häusler, S. Nakajima, M. Lebrat, D. Husmann, S. Krinner, T. Esslinger & J.-P. Brantut, Scanning Gate Microscope for Cold Atomic Gases. Physical Review Letters 119,030403 (2017).


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