This is joint work with Vladan Vuletic.
Efficient quantum information processing requires qubits to be initialized to a known logical state. Moreover, such state preparation must often be performed during the course of a quantum computation, for example, to prepare ancilla states necessary for quantum error correction, or for multi-qubit gates. Trapped ion quantum computation needs such state preparation, in order for two-qubit gates to operate with high fidelity. Initially, the ion qubits may be laser sideband-cooled into their motional quantum ground states |n=0ñ, but during the course of a quantum computation the motional state typically heats up, and becomes a thermal mixture instead of the necessary quantum ground state. Standard laser cooling is inappropriate for re-cooling during quantum computation, because it will typically strongly perturb or destroy ion qubits while cooling, because laser cooling employ closed cycle transitions which include the same narrow transitions used for representing qubits. One solution to this need for quantum non-demolition cooling is to employ another species of ion, for sympathetic cooling, but this requires the secondary ion to be moved immediately adjacent to the computational ions, as well as new sets of lasers for cooling the secondary ion.
Quantum non-demolition cooling may also be accomplished with trapped ions, using a new technique, known as resolved sideband cavity cooling, in which laser cooling is done non-resonantly, by virtue of coupling through strong cooperativity to a high finesse cavity. We have experimentally demonstrated the basic principles of this cooling mechanism, for the first time, using a single Sr+ ion trapped within a knife-edge trap, enclosed within a high finesse optical cavity:
The cooling laser in the experiment was tuned to be resonant with the cavity, in which no cooling was obtained, and the ion would heat up through normal processes (green line); the laser was tuned to the blue side of the cavity, in which case cavity interactions would cause additional heating, by scattering photons even bluer than the incident light (blue line); or the laser was tuned to the red side of the cavity, in which case cavity induced cooling was observerd (red line). This cooling resulted in a equilibrium temperature just below the Doppler limit, and definitively shows agreement with published theories for resolved sideband cavity cooling. This work was partially supported in addition by JST The results [3,4] are described in Physical Review Letters, volume 103, page 103001 (2009).