WP 2. Qubit preparation, control, and read-out

Lead contractor: CEA-Saclay

Participants: Chalmers, CEA-Saclay, PTB, TU Delft, HUT, KTH, UNIKARL, SNS

WP 2 deals with the fundamental issues of qubit preparation, control, and read-out. These elementary operations must work before any logical operations can be performed in a meningful way. Important tasks involve measurements of decoherence times of various Josephson circuits by designing and applying read-out devices for single-shot and non-destructive measurements. An important part of WP 2 concerns development of theory for designing and interpreting the experiments.

In this work package we will develop and optimize the necessary tools to do the basic operations on single and coupled two-qubit circuits. Sufficiently fast and ³clean² pulses have to be sent down to the control terminals of the squbit in order to prepare the initial state, and to operate the qubits and the logic gates. Furthermore, different types of schemes have to be tested and optimised for the readout of the different types of qubits.

In Cooper pair box and SQUID qubit circuits, at low temperatures the circuit variables (flux and charge) behave quantum mechanically. One can use external potentials on gate electrodes, or magnetic fields, to vary the quantum mechanical Josephson coupling in the system and to tune the coherent superposition of circuit variables. The ability to control this superposition with external "control knobs" is an important step towards implementation of a quantum computing scheme. The first critical step is to develop single-shot measurement techniques having sufficient sensitivity during the available measuring time to cause complete dephasing without relaxation (i.e. without destroying the level population). To interpret the measurements and improve circuit designs we will develop theory for qubit systems and quantum gates. We will also address the issue of non-destructive measurements of qubit states, and investigate effects of quantum leakage on one- and two-qubit operations.