Non-Hermitian dynamics can sharply amplify the response of a quantum sensor near eigenstate coalescence, but whether this amplification yields a genuine metrological gain depends on how the non-unitary evolution is physically realised and how its resources are counted. We address this question experimentally by implementing the pseudo-Hermitian qubit sensing protocol of Chu
et al
. [Phys. Rev. Lett. 124, 020501 (2020)] on a two-qubit nuclear magnetic resonance processor via Naimark dilation. Because both the postselected signal and the full two-qubit probability are obtained from a single set of population measurements, the enhanced response and its postselection cost are accessed within the same experiment. We observe the characteristic steep response of the postselected branch, whereas the sensitivity extracted from the absolute two-qubit probability does not surpass the Ramsey benchmark for the same sensor encoding, consistent with the theoretical resource bound derived by Ding
et al
. [Phys. Rev. Lett. 131, 160801 (2023)] for the corresponding dilation scheme. The experiment complements the recent photonic implementation of the direct non-unitary route and provides an experimental benchmark of the Hermitian-dilation route with explicit resource accounting.
Quantum-enhanced sensing with atomic ensembles has predominantly focused on qubit-based protocols, despite the growing ability of many experimental platforms to coherently control and entangle multi-level systems. Here, we investigate quantum-enhanced sensing with qutrit ensembles by introducing three experimentally feasible qutrit generalisations of the one-axis twisting (OAT) model that involve entangling operations only between two levels, while the third level primarily acts as a spectator. We characterize the metrological utility of the dynamically generated states using the quantum Fisher information toolbox. We find numerically that all three models offer considerable freedom in encoding direction for quantum-enhanced sensing, with up to 6 out of 8 possible directions exhibit near-Heisenberg scaling after a short evolution time. We discuss experimental access to this enhanced metrological precision via effective time-reversal protocols. Furthermore, we examine the practical issues of estimation ambiguity and local dissipation, and show that they can be largely overcome by optimizing the sensor operating point. Finally, we show that the measurement incompatibility in estimating multiple parameters simultaneously encoded in different directions with near-Heisenberg scaling of precision is suppressed at the zero operating point as the system size increases. In the process, we find that one of the models enables near-Heisenberg scaling metrology with a pair of commuting generators, a possibility that arises from the $su(3)$ algebra and is thus absent in qubit-ensemble based sensors of collective $SU(2)$ rotations.
Deepshikha Datta, Sayam Chakraborty, J. D. Wilson et al.· 0 citations
Continuous weak measurements of quantum systems are of great relevance in quantum foundations and applications. They can be achieved by probing the quantum system of interest by repeatedly measuring an auxiliary system weakly coupled to it. Here we study two qubits coupled in different points to a common one-dimensional electromagnetic field and simultaneously monitored in the right- and left-propagating output channels by homodyne detection. Using a collision-model description, we derive an analytical Stochastic Master Equation (SME) governing the resulting diffusive quantum trajectories, including the interference between the two measurement channels. For nonlinear functions of the quantum state, such as entropies, averages over quantum trajectories generally differ from the corresponding quantities evaluated on the unconditional state. Through this mechanism, we show that continuous monitoring generates entanglement, absent in the unconditional dynamics, and enhances quantum magic in the qubit pair during the decay. Both resources can be tuned through the optical phase accumulated between the qubits and the phases of homodyne local oscillators. Our results establish continuous homodyne monitoring of multiple emitters as a tunable mechanism for generating quantum resources.
Debmalya Das, Giuseppe Magnifico, Maria Maffei· 0 citations
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