An ion interacts with the quantum system and, at the same time, establishes a controlled contact to the environment. (Graphics: Harald Ritsch)
Experimental physicists have put a lot of effort in isolating sensitive measurements from the disruptive influences of the environment. In an international first, Innsbruck quantum physicists have realized a toolbox of elementary building blocks for an open-system quantum simulator, where a controlled coupling to an environment is used in a beneficial way.
In another breakthrough in this field, a team of young scientists in the research groups of Rainer Blatt and Peter Zoller at the Institute for Experimental Physics and Theoretical Physics of the University of Innsbruck and the Institute of Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences have been the first to engineer a comprehensive toolbox for an open-system quantum computer, which will enable researchers to construct more sophisticated quantum simulators for investigating complex problems in quantum physics.
Pumping into the +1 eigenspace of the Z1Z2 stabilizer operator a, Ideally, only the ancilla qubit and the two system qubits #1 and #2 are involved in the circuit. b, An entangling gate acting on these three ions can be achieved by a refocusing technique, where ions #3 and #4 decouple from the dynamics. However, the latter ions still become entangled. However, these residual interactions are not harmful to the pumping, as they do not a ffect the expectation values of the other two-qubit stabilizer operators. c, Dashed operations in the quantum circuit indicate such residual entangling operations.
The control of quantum systems is of fundamental scientific interest and promises powerful applications and technologies. Impressive progress has been achieved in isolating quantum systems from the environment and coherently controlling their dynamics, as demonstrated by the creation and manipulation of entanglement in various physical systems. However, for open quantum systems, engineering the dynamics of many particles by a controlled coupling to an environment remains largely unexplored. Here we realize an experimental toolbox for simulating an open quantum system with up to five quantum bits (qubits). Using a quantum computing architecture with trapped ions, we combine multi-qubit gates with optical pumping to implement coherent operations and dissipative processes. We illustrate our ability to engineer the open-system dynamics through the dissipative preparation of entangled states, the simulation of coherent many-body spin interactions, and the quantum non-demolition measurement of multi-qubit observables. By adding controlled dissipation to coherent operations, this work offers novel prospects for open-system quantum simulation and computation.
I. Bell-state pumping 1 A. Implemented Kraus maps 1 B. Circuit decomposition 1 C. Towards master equation dynamics 2 D. Further experimental details 2 II. Four-qubit stabilizer pumping 3 A. Pumping 3 B. Repeated four-qubit stabilizer pumping 5 C. Pushing "anyons" around 5 D. Pumping into "excited" states 6 III. QND measurement of a four-qubit stabilizer 6 A. Further details 6 B. Quantitative analysis of the performance 7