Berlin 2024 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 92: Quantum Dynamics, Decoherence and Quantum Information (joint session DY/TT)

TT 92.8: Talk

Friday, March 22, 2024, 12:30–12:45, A 151

Engineering unsteerable quantum states with active feedback — •Samuel Morales¹, Yuval Gefen², Igor Gornyi^3,4, Alex Zazunov¹, and Reinhold Egger¹ — ¹Institut für Theoretische Physik, Heinrich-Heine-Universität, 40225 Düsseldorf, Germany — ²Department of Condensed Matter Physics, Weizmann Institute, 7610001 Rehovot, Israel — ³Institute for Quantum Materials and Technologies, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany — ⁴Institut für Theorie der Kondensierten Materie, Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany

We propose active steering protocols for quantum state preparation in quantum circuits where each system qubit is connected to a single detector qubit, employing a simple coupling selected from a small set of steering operators. The decision is made such that the expected cost-function gain in one time step is maximized. We apply these protocols to several many-qubit models. Our results are underlined by three remarkable insights. First, we show that the standard fidelity does not give a useful cost function; instead, successful steering is achieved by including local fidelity terms. Second, although the steering dynamics acts on each system qubit separately, entanglement in the generated target state is introduced, and can be tuned at will, by performing Bell measurements on detector qubit pairs after every time step. This implements a weak-measurement variant of entanglement swapping. Third, numerical simulations suggest that the active steering protocol can reach arbitrarily designated target states, including passively unsteerable states such as the N-qubit W state.

Keywords: Quantum steering; Open systems; State preparation; Measurements; Feedback