Pablo R. Zangara

Universidad Nacional de Córdoba

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Carlos A. Meriles

City College of New York

Daniela Pagliero

City College of New York

Ashok Ajoy

Lawrence Berkeley National Laboratory

Horacio M. Pastawski

Universidad Nacional de Córdoba

Siddharth Dhomkar

Indian Institute of Technology Madras

Jeffrey A. Reimer

Lawrence Berkeley National Laboratory

Jacob Henshaw

Sandia National Laboratories

Alexander Pines

University of California, Berkeley

Rodolfo H. Acosta

Universidad Nacional de Córdoba

Axel D. Dente

Universidad Nacional de Córdoba

Cooperative Institutions

City College of New York

Lawrence Berkeley National Laboratory

University of California, Berkeley

Consejo Nacional de Investigaciones Científicas y Técnicas

Universidad Nacional de Córdoba

The Graduate Center, CUNY

City University of New York

ACT Government

University of Canberra

Centro Científico Tecnológico - Córdoba

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Toward deep-learning-assisted spectrally-resolved imaging of magnetic noise

arXiv (Cornell University) (2022)

Fernando Meneses David F. Wise Daniela Pagliero Pablo R. Zangara Siddharth Dhomkar

Recent progress in the application of color centers to nanoscale spin sensing makes the combined use of noise spectroscopy and scanning probe imaging an attractive route for the characterization of arbitrary material systems. Unfortunately, the traditional approach to characterizing the environmental magnetic field fluctuations from the measured probe signal typically requires the experimenter's input, thus complicating the implementation of automated imaging protocols based on spectrally resolved noise. Here, we probe the response of color centers in diamond in the presence of externally engineered random magnetic signals, and implement a deep neural network to methodically extract information on their associated spectral densities. Building on a long sequence of successive measurements under different types of stimuli, we show that our network manages to efficiently reconstruct the spectral density of the underlying fluctuating magnetic field with good fidelity under a broad set of conditions and with only a minimal measured data set, even in the presence of substantial experimental noise. These proof-of-principle results create opportunities for the application of machine-learning methods to color-center-based nanoscale sensing and imaging.

SIGNAL (programming language)

High fidelity

10.48550/arxiv.2208.01107

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Fragility of superposition states evaluated by the Loschmidt echo

Physical Review A (2013)

Denise Bendersky Pablo R. Zangara Horacio M. Pastawski

We consider the degradation of the dynamics of a Gaussian wave packet in a harmonic oscillator under the presence of an environment. This last is given by a single non-degenerate two level system. We analyze how the binary degree of freedom perturbs the free evolution of the wave packet producing decoherence, which is quantified by the Loschmidt Echo. This magnitude measures the reversibility of a perturbed quantum evolution. In particular, we use it here to study the relative "fragility" of coherent superpositions (cat states) with respect to incoherent ones. This fragility or sensitivity turns to increase exponentially with the energy separation of the two components of the superposition.

Harmonic

Echo (communications protocol)

10.1103/physreva.88.032102

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One-directional polarization transport in electron/nuclear spin chains with loss and gain

Physical review. B./Physical review. B (2021)

Santiago Bussandri Pablo R. Zangara Rodolfo H. Acosta Carlos A. Meriles

Understanding the joint dynamics of electron and nuclear spins is central to core concepts in solid-state magnetic resonance such as spin-lattice relaxation and dynamic nuclear polarization, but a generalization that capitalizes on competing polarization loss and gain channels is still lacking. Here, we study theoretically the non-Hermitian dynamics of hybrid electron/nuclear spin systems in the simultaneous presence of electron spin pumping and spin-lattice relaxation. Focusing on periodic, one-dimensional chains, we find that, by adjusting the electron spin pumping to a critical level, it is possible to steer the flow of nuclear polarization to create site-dependent distributions where either end of the array polarizes in opposite ways, irrespective of the initial state. By contrast, we show that ring-like patterns---where the limit nuclear polarization is uniform---exhibit a non-decaying, externally driven nuclear spin current. Interestingly, cyclic magnetic field modulation can render these processes largely robust to defects in the chain, a response featuring some interesting similarities---and differences---with recent findings in other non-Hermitian physical platforms.

Hyperpolarization

10.1103/physrevb.103.214409

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Loschmidt echo in many-spin systems: a quest for intrinsic decoherence and emergent irreversibility

Physica Scripta (2017)

Pablo R. Zangara Horacio M. Pastawski

If a magnetic polarization excess is locally injected in a crystal of interacting spins, this "excitation" would spread as consequence of spin-spin interactions. Such an apparently irreversible process is known as spin diffusion and it can lead the system back to "equilibrium". Even so, a unitary quantum dynamics would ensure a precise memory of the non-equilibrium initial condition. Then, if at certain time, say $t/2$, an experimental protocol reverses the many-body dynamics, it would drive the system back to the initial non-equilibrium state at time $t$. As a matter of fact, the reversal is always perturbed by small experimental imperfections and/or uncontrolled internal or environmental degrees of freedom. This limits the amount of signal $M(t)$ recovered locally at time $t$. The degradation of $M(t)$ accounts for these perturbations, which can also be seen as the sources of decoherence. This idea defines the Loschmidt echo (LE), which embodies the various time-reversal procedures implemented in nuclear magnetic resonance. Here, we present an invitation to the study of the LE following the pathway induced by the experiments. With such a purpose, we provide a historical and conceptual overview that briefly revisits selected phenomena that underlie the LE dynamics, ultimately leading to the discussion of irreversibility as an emergent phenomenon. In addition, we introduce the LE formalism by means of spin-spin correlation functions and we present new results that could trigger new experiments and theoretical ideas. In particular, we propose to transform an initially localized excitation into a more complex initial state, enabling a dynamically prepared LE. This induces a global definition of the LE in terms of the raw overlap between many-body wave functions. Our results show that as the complexity of the prepared state increases, it becomes more fragile towards small perturbations.

10.1088/1402-4896/aa5bee

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Time fluctuations in isolated quantum systems of interacting particles

Physical Review E (2013)

Pablo R. Zangara Axel D. Dente E. Jonathan Torres-Herrera Horacio M. Pastawski Anı́bal Iucci

Numerically, we study the time fluctuations of few-body observables after relaxation in isolated dynamical quantum systems of interacting particles. Our results suggest that they decay exponentially with system size in both regimes, integrable and chaotic. The integrable systems considered are solvable with the Bethe ansatz and have a highly nondegenerate spectrum. This is in contrast with integrable Hamiltonians mappable to noninteracting ones. We show that the coefficient of the exponential decay depends on the level of delocalization of the initial state with respect to the energy shell.

Bethe ansatz

Quantum system

Ansatz

Exponential decay

10.1103/physreve.88.032913

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Citations (75)

Nuclear spin temperature reversal via continuous radio-frequency driving

Physical review. B./Physical review. B (2021)

Pablo R. Zangara Daniela Pagliero Ashok Ajoy Rodolfo H. Acosta Jeffrey A. Reimer

Optical spin pumping of color centers in diamond is presently attracting broad interest as a platform for dynamic nuclear polarization at room temperature, but the mechanisms involved in the generation and transport of polarization within the host crystal are still partly understood. Here we investigate the impact of continuous radio-frequency (RF) excitation on the generation of nuclear magnetization produced by optical illumination. In the presence of RF excitation far removed from the nuclear Larmor frequency, we witness a magnetic-field-dependent sign reversal of the measured nuclear spin signal when the drive is sufficiently strong, a counterintuitive finding that immediately points to nontrivial spin dynamics. With the help of analytical and numerical modeling, we show our observations indicate a modified form of solid effect, down-converted from the microwave to the radio-frequency range through the driving of hybrid transitions involving one (or more) nuclei and two (or more) electron spins. Our results open intriguing opportunities for the manipulation of many-electron spin systems by exploiting hyperfine couplings as a means to access otherwise forbidden intraband transitions.

Larmor precession

10.1103/physrevb.103.085205

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Two-Electron-Spin Ratchets as a Platform for Microwave-Free Dynamic Nuclear Polarization of Arbitrary Material Targets

Nano Letters (2019)

Pablo R. Zangara Jacob Henshaw Daniela Pagliero Ashok Ajoy Jeffrey A. Reimer

Optically pumped color centers in semiconductor powders can potentially induce high levels of nuclear spin polarization in surrounding solids or fluids at or near ambient conditions, but complications stemming from the random orientation of the particles and the presence of unpolarized paramagnetic defects hinder the flow of polarization beyond the defect's host material. Here, we theoretically study the spin dynamics of interacting nitrogen-vacancy (NV) and substitutional nitrogen (P1) centers in diamond to show that outside protons spin-polarize efficiently upon a magnetic field sweep across the NV–P1 level anticrossing. The process can be interpreted in terms of an NV–P1 spin ratchet, whose handedness, and hence the sign of the resulting nuclear polarization, depends on the relative timing of the optical excitation pulse. Further, we find that the polarization transfer mechanism is robust to NV misalignment relative to the external magnetic field, and efficient over a broad range of electron–electron and electron–nuclear spin couplings, even if proxy spins feature short coherence or spin–lattice relaxation times. Therefore, these results pave the route toward the dynamic nuclear polarization of arbitrary spin targets brought in proximity with a diamond powder under ambient conditions.

10.1021/acs.nanolett.8b05114

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Microwave-Free Dynamic Nuclear Polarization via Sudden Thermal Jumps

Physical Review Letters (2022)

Carlos A. Meriles Pablo R. Zangara

Dynamic nuclear polarization (DNP) presently stands as the preferred strategy to enhance the sensitivity of nuclear magnetic resonance measurements, but its application relies on the use of high-frequency microwave to manipulate electron spins, an increasingly demanding task as the applied magnetic field grows. Here we investigate the dynamics of a system hosting a polarizing agent formed by two distinct paramagnetic centers near a level anticrossing. We theoretically show that nuclear spins polarize efficiently under a cyclic protocol that combines alternating thermal jumps and radio-frequency pulses connecting hybrid states with opposite nuclear and electronic spin alignment. Central to this process is the difference between the spin-lattice relaxation times of either electron spin species, transiently driving the electronic spin bath out of equilibrium after each thermal jump. Without the need for microwave excitation, this route to enhanced nuclear polarization may prove convenient, particularly if the polarizing agent is designed to feature electronic level anticrossings at high magnetic fields.

Thermal equilibrium

10.1103/physrevlett.128.037401

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Loschmidt echo in many-spin systems: contrasting time scales of local and global measurements

Philosophical Transactions of the Royal Society A Mathematical Physical and Engineering Sciences (2016)

Pablo R. Zangara Denise Bendersky Patricia R. Levstein Horacio M. Pastawski

A local excitation in a quantum many-spin system evolves deterministically. A time-reversal procedure, involving the inversion of the signs of every energy and interaction, should produce the excitation revival. This idea, experimentally coined in NMR, embodies the concept of the Loschmidt echo (LE). While such an implementation involves a single spin autocorrelation $M_{1,1}$, i.e. a local LE, theoretical efforts have focused on the study of the recovery probability of a complete many-body state, referred here as global or many-body LE $M_{MB}$. Here, we analyze the relation between these magnitudes, in what concerns to their characteristic time scales and their dependence on the number of spins $N$. We show that the global LE can be understood, to some extent, as the simultaneous occurrence of $N$ independent local LEs, i.e. $M_{MB}\sim \left( M_{1,1}\right) ^{N/4}$. This extensive hypothesis is exact for very short times and confirmed numerically beyond such a regime. Furthermore, we discuss a general picture of the decay of $M_{1,1}$ as a consequence of the interplay between the time scale that characterizes the reversible interactions ($T_{2}$) and that of the perturbation ($\tau _{\Sigma }$). Our analysis suggests that the short time decay, characterized by the time scale $\tau _{\Sigma }$, is greatly enhanced by the complex processes that occur beyond $T_{2}$ . This would ultimately lead to the experimentally observed $T_{3},$ which was found to be roughly independent of $\tau _{\Sigma }$ but closely tied to $T_{2}$.

Echo (communications protocol)

10.1098/rsta.2015.0163

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Citations (13)

Multispin-assisted optical pumping of bulk C13 nuclear spin polarization in diamond

Physical review. B./Physical review. B (2018)

Daniela Pagliero ‎K‎. ‎R‎. ‎K‎. ‎Rao Pablo R. Zangara Siddharth Dhomkar Henry H. Wong

One of the most remarkable properties of the nitrogen-vacancy (NV) center in diamond is that optical illumination initializes its electronic spin almost completely, a feature that can be exploited to polarize other spin species in their proximity. Here we use field-cycled nuclear magnetic resonance (NMR) to investigate the mechanisms of spin polarization transfer from NVs to 13C spins in diamond at room temperature. We focus on the dynamics near 51 mT, where a fortuitous combination of energy matching conditions between electron and nuclear spin levels gives rise to alternative polarization transfer channels. By monitoring the 13C spin polarization as a function of the applied magnetic field, we show 13C spin pumping takes place via a multi-spin cross relaxation process involving the NV- spin and the electronic and nuclear spins of neighboring P1 centers. Further, we find that this mechanism is insensitive to the crystal orientation relative to the magnetic field, although the absolute level of 13C polarization - reaching up to ~3% under optimal conditions - can vary substantially depending on the interplay between optical pumping efficiency, photo-generated carriers, and laser-induced heating.

10.1103/physrevb.97.024422

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Citations (47)