Controlling Spin-Spin Network Dynamics by Repeated Projective Measurements

Abstract

We show that coupled-spin network manipulations can be made highly effective by repeated projections of the evolving quantum states onto diagonal density-matrix states (populations). As opposed to the intricately crafted pulse trains that are often used to fine-tune a complex network's evolution, the strategy hereby presented derives from the "quantum Zeno effect" and provides a highly robust route to guide the evolution by destroying all unwanted correlations (coherences). We exploit these effects by showing that a relaxationlike behavior is endowed to polarization transfers occurring within a N-spin coupled network. Experimental implementations yield coupling constant determinations for complex spin-coupling topologies, as demonstrated within the field of liquid-state nuclear magnetic resonance.

Quantum Sensing of Noisy and Complex Systems under Dynamical Control

“…Such probes may operate at nanoscales, as magnetometers, thermometers, sensors for imaging or monitoring chemical and biological processes [9,[14][15][16][17][18][19].…”

“…As shown here, it is likewise instrumental for extracting information on the bath characteristics [9,15]. Regardless of the chosen form of control, the controlled-probe dynamics must then be Zeno-like, namely, result in suppressed probe-bath interaction.…”

“…The QZE and AZE correspond to the suppression and enhancement, respectively, of the bath effects via control. Dynamical control methods based on the QZE are aimed at protecting the quantumness (coherence or entanglement) of the probe [45,63,64], but also at diagnosing the bath spectra [9,15,65] and transferring QI via noisy media [43,[66][67][68][69]. By contrast, AZE-based control is useful for fast, versatile and robust cooling of thermalized quantum systems [51][52][53]70,71].…”

“…This, however, is a laborious and time-consuming undertaking. Instead, it may suffice to record only a few parameters of the noise that provide a distinct signature of the bath: the bath structure and the dynamics of the underlying processes (e.g., diffusion, collisions and thermal fluctuations [9,[14][15][16][17][18][19]). We show here that such tasks may be accomplished with high fidelity or success probability by subjecting the probe to an appropriate dynamical control that may turn a sub-optimal into a fully optimal probing/sensing process.…”

“…Noise-spectrum information obtainable by quantum probing/sensing is particularly important not only for QI processing [10][11][12][20][21][22] and quantum metrology [23][24][25][26][27] but also for the design of quantum thermal (heat) machines [28][29][30][31][32][33] and, most intriguingly, for biomedical diagnostics based on NMR and MRI [11,[14][15][16][17][18][19].…”