Ultrathin niobium nitride films enable basic quantum experiments and new quantum devices
A fundamental quantum experiment – the proof of the Aharonov-Casher-Effect – has been successfully realized at the National Physics Laboratory (NPL) in London by making use of superconducting niobium nitride films developed at Leibniz IPHT. The fabricated niobium nitride nanowires exhibit coherent quantum behavior in a dual way to the well-known Josephson junction which may lead to a novel class of quantum instruments.
Von S. Linzen // E. Il’ichev // M. Ziegler // U. Hübner // L. Fritzsch // S. George // H.-G. Meyer // H. Schmidt // R. Stolz
Josephson junctions are the basic nonlinear elements for a variety of cryoelectronic devices. They range from the well-known SQUID (Superconducting Quantum Interference Device) with its broad application in clinical diagnostics, geophysical ore exploration or large-scale geo-archaeology up to the Qubit (Quantum Bit) as key component for an implementation of solid-state quantum computing. The physics behind arises mainly from the tunneling of charges through an e.g. dielectric barrier between the two superconducting electrodes of the Josephson junction. The dual quantum effect, the tunneling of magnetic flux across a continuous superconductor, and resulting applications were controversially discussed within the last years. Now, these so-called phase-slip junctions were experimentally realized by means of ultrathin niobium nitride (NbN) films fabricated by atomic layer deposition (ALD) at the Leibniz IPHT clean room. An ALD-NbN nanowire structure, including two phase-slip junctions and a tiny charge island (see figure 1), was patterned at the NPL in London in order to proof the Aharonov-Casher-Effect as a basic quantum experiment in which a magnetic flux dynamics, encircling an electrical charge, demonstrates an interference behavior. The successful experiment represents both, the feasibility of the NbN phase-slip junctions themselves and the first experimental realization of a quantum device on base of this new kind of tunnel junctions – the Charge Quantum Interference Device (CQUID) . Thus, an important step towards a new class of quantum devices was done. These findings may potentially lead to novel applications, for instance in quantum computing and metrology. For the latter the development of a current standard by means of the phase-slip junctions, in analogy to the existing voltage standard based on Josephson junctions, is discussed.
The functionality of the NbN phase-slip junctions bases on the unique structural and electrical properties of the thin superconducting films prepared by ALD. In contrary to epitaxial or highly crystalline thin films required for high quality Josephson junctions, only a specific kind of structural disorder  in combination with film thicknesses smaller than 5 nm enable a flux tunneling and hence the function of the NbN phase-slip junctions at low temperatures. The film thickness of the CQUID junctions was well-adjusted at 3.3 nm which is very close to a sharp superconductor to isolator transition observable for the ALD-NbN, see figure 2. In this way, the kinetic inductance below the critical temperature Tc corresponding to the specific normal resistance above Tc was maximized as further requirement of the CQUID realization.
The careful optimization of the complex ALD-NbN process  at Leibniz IPHT within a couple of years was preceded the development of the described phase-slip junctions and the successful quantum experiment. The application potential of the ALD-NbN thin films, however, is even broader. The realization of single photon detectors (SNSPDs) for high resolution spectroscopy also benefits from the unique low temperature properties of ALD-NbN, see Leibniz IPHT annual report 2016.
Electron microscope image of the experimentally realized CQUID (left) and scheme of the Aharonov casher effect (right), which shows the movement of a flux quantum Φ0 around a charge q and the tunneling through the superconductor. The blue colored structures represent the ALD niobnitride film. Source: NPL / Leibniz IPHT.