## Test procedure for quantum computers

The working group led by Nadav Katz from the University of California in Santa Barbara and, together with colleagues from Riverside, has developed a method that could be used in the future to check whether calculations made on a quantum computer are correct or require error correction. Because errors can always creep in with every calculation carried out by such a computer, it is essential to check the intermediate results in order to finally be able to say whether the result is trustworthy.

The developers are hoping for performance from these new types of calculators that will far exceed anything that has been seen before. In contrast to the electronic brains that are now in use, the quantum units no longer work with the clearly distinguishable two-bit states one and zero, but with so-called quantum bits. These "qubits" can assume both or even several states at the same time, which means that even the most complex calculations can be carried out quickly in highly parallel work steps.

But these quantum mechanical objects are extraordinarily fragile. The smallest disturbance can falsify the result. For this reason, the monitoring of intermediate results is essential, even though the necessary control procedure itself harbors the risk of making the quantum state unusable for further calculations.

Katz and his team now seem to have found a method to gently read intermediate results from qubits. The scientists work with superconducting components called Josephson junctions. There, electrons, which are combined to form so-called Cooper pairs, penetrate a thin insulator layer according to quantum mechanical rules.

Such elements can be used as quantum bits. With a suitable choice of current and voltage, the circuit contains two or more states at the same time, which are expressed in different high energy values, which are trapped in a so-called potential well.

Using a high-frequency AC voltage pulse in the gigahertz range, the experimenters can now lower one of these flanks of the electric prison several times in a row. In this way, high-energy states can be detected with a certain probability of escaping the cage and then measured. They tell the expert what condition the circuit was in without completely destroying the information it contained. The qubit would therefore continue to be available for subsequent calculations.