This proved difficult to realize, however. Instead, a different experiment was proposed involving a ring geometry interrupted by tunnel barriers, with a constant bias voltage "V" relating the potentials of the two halves of the ring. This situation results in an Aharonov–Bohm phase shift as above, and was observed experimentally in 1998, albeit in a setup where the charges do traverse the electric field generated by the bias voltage. The original time dependent electric Aharonov-Bohm effect has not yet found experimental verification. Nano rings were created by accident while intending to make quantum dots. They have interesting optical properties associated with excitons and the Aharonov–Bohm effect. Application of these rings used as light capacitors or buffers includes photonic computing and communications technology. Analysis and measurement of geometric phases in mesoscopic rings is ongoing. It is even suggested they could be used to make a form of slow glass. Several experiments, including some reported in 2012, show Aharonov–Bohm oscillations in charge density wave (CDW) current versus magnetic flux, of dominant period "h"/2"e" through CDW rings up to 85 µm in circumference above 77 K. This behavior is similar to that of the superconducting quantum interference devices (see SQUID). The Aharonov–Bohm effect can be understood from the fact that one can only measure absolute values of the wave function. While this allows for measurement of phase differences through quantum interference experiments, there is no way to specify a wavefunction with constant absolute phase. In the absence of an electromagnetic field one can come close by declaring the eigenfunction of the momentum operator with zero momentum to be the function "1" (ignoring normalization problems) and specifying wave functions relative to this eigenfunction "1". In this representation the i-momentum operator is (up to a factor formula_23) the differential operator formula_24. However, by gauge invariance, it is equally valid to declare the zero momentum eigenfunction to be formula_25 at the cost of representing the i-momentum operator (up to a factor) as formula_26 i.e. with a pure gauge vector potential formula_27. There is no real asymmetry because representing the former in terms of the latter is just as messy as representing the latter in terms of the former. This means that it is physically more natural to describe wave "functions", in the language of differential geometry, as sections in a complex line bundle with a hermitian metric and a U(1)-connection formula_28.