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Optical and Quantum Sensing and Precision Metrology II

Toward quantum many-body dynamics in NV center ensembles

Nir Bar‐Gill, Demitry Farfurnik

Published Mar 1, 2019• 0 citations
Publisher

Abstract

The study of quantum many-body spin physics in realistic solid-state platforms has been a long-standing goal in quantum and condensed-matter physics. We demonstrate separate steps required to reach this goal using nitrogen-vacancy (NV) centers in diamond. First, standard (TEM) electron irradiation is used for the enhancement of N to NV conversion efficiencies by over an order-of-magnitude. Second, robust pulsed and continuous dynamical decoupling (DD) techniques enable the preservation of arbitrary states of the ensemble. These combined efforts could lead to the desired interaction-dominated regime. Finally, we simulate the effects of continuous and pulsed microwave (MW) control on the resulting NV-NV many body dynamics in a realistic spin-bath environment. We emphasize that dominant interaction sources could be identified and decoupled by the application of proper pulse sequences, and the modification of such sequences could lead to the creation engineered interaction Hamiltonians. Such interaction Hamiltonians could pave the way toward the creation of non-classical states, e.g. spin-squeezed states, which were not yet demonstrated in the solid-state, and could eventually lead to magnetic sensing beyond the standard quantum limit (SQL).

DOI
10.1117/12.2515427

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The study of quantum many-body spin physics in realistic solid-state platforms has been a long-standing goal in quantum and condensed-matter physics. We demonstrate separate steps required to reach this goal using nitrogen-vacancy (NV) centers in diamond. First, standard (TEM) electron irradiation is used for the enhancement of N to NV conversion efficiencies by over an order-of-magnitude. Second, robust pulsed and continuous dynamical decoupling (DD) techniques enable the preservation of arbitrary states of the ensemble. These combined efforts could lead to the desired interaction-dominated regime. Finally, we simulate the effects of continuous and pulsed microwave (MW) control on the resulting NV-NV many body dynamics in a realistic spin-bath environment. We emphasize that dominant interaction sources could be identified and decoupled by the application of proper pulse sequences, and the modification of such sequences could lead to the creation engineered interaction Hamiltonians. Such interaction Hamiltonians could pave the way toward the creation of non-classical states, e.g. spin-squeezed states, which were not yet demonstrated in the solid-state, and could eventually lead to magnetic sensing beyond the standard quantum limit (SQL).
We describe experimental steps toward the observation of many-body dynamics in optically-addressable spin ensembles (NV centers) in diamond. We study the effect of electron irradiation using a 200 keV transmission electron microscope (TEM) on the conversion efficiency and the coherence times of various types of diamond samples with different initial nitrogen concentrations. Using fluorescence images obtained by confocal microscopy, we observe an order of magnitude improvement in the NV concentration (up to ~1011 NV/cm2), without any degradation in their coherence times of ~180 μs (for a Hahn-Echo experiment). We address the potential of this technique for studying many-body physics of ensembles of NV spins: our simulations show that by implementing a simple dynamical decoupling protocol, dipolar interactions could be distinguished from ordinary interactions with the spin-bath environment. These outcomes could contribute to the creation of non-classical spin states, e.g. for quantum sensing.
The studies of many-body dynamics of interacting spin ensembles, as well as quantum sensing in solid state systems, are often limited by the need for high spin concentrations, along with efficient decoupling of the spin ensemble from its environment. In particular, for an ensemble of nitrogen-vacancy (NV) centers in diamond, high conversion efficiencies between nitrogen (P1) defects and NV centers are essential while maintaining long coherence times of an NV ensemble. In this work, we study the effect of electron irradiation on the conversion efficiency and the coherence time of various types of diamond samples with different initial nitrogen concentrations. The samples were irradiated using a 200 keV transmission electron microscope. Our study reveals that the efficiency of NV creation strongly depends on the initial conversion efficiency and on the initial nitrogen concentration. The irradiation of the examined samples exhibits an order of magnitude improvement in the NV concentration (up to ∼1011 NV/cm2), without degradation in their coherence time of ∼180 μs. We address the potential of this technique toward the study of many-body physics of NV ensembles and the creation of non-classical spin states for quantum sensing.
Solid-state spin defects, particularly the nitrogen-vacancy (NV) center in diamond, have emerged as powerful platforms for exploring quantum many-body physics and quantum metrology. Their exceptional coherence properties and strong dipolar interactions offer unique opportunities to simulate complex quantum dynamics and engineer entangled states for enhanced sensing beyond the standard quantum limit. However, achieving scalable entanglement in these systems is notoriously difficult due to intrinsic disorder and the complex nature of long-range dipole-dipole interactions. This thesis presents both theoretical and experimental advancements in harnessing quantum dynamics in interacting spin ensembles for quantum-enhanced metrology. A central result is the experimental realization of spin squeezing in a dense ensemble of NV centers, demonstrating a clear path toward overcoming classical limits in solid-state platforms. To further formalize and scale these phenomena, this work develops theoretical frameworks for generating robust spin squeezing under realistic experimental conditions. We show that finite-temperature easy-plane magnetism can be actively leveraged to create scalable squeezed states for the dipolar spin systems on regular lattices. Building upon this, we further extend the result to spatially disordered quantum dipoles. We demonstrate that strongly coupled spin pairs suppress squeezing through dynamical heating, and we propose a selective shelving protocol to mitigate this problem. Furthermore, we also propose a universal protocol for quantum-enhanced sensing that exploits information scrambling, utilizing the natural, scrambling dynamics of strongly interacting many-body systems to effectively amplify weak external signals. Together, the findings detailed in this thesis establish a foundational paradigm for controlling and utilizing solid-state spin ensembles, paving the way for entanglement-enhanced quantum sensors.
The study of open quantum systems, quantum thermodynamics and quantum many-body spin physics in realistic solid-state platforms, has been a long-standing goal in quantum and condensed-matter physics. In this talk I will address these topics through the platform of nitrogen-vacancy (NV) spins in diamond, in the context of purification (or cooling) of a spin bath as a quantum resource and for enhanced metrology. I will first describe a general theoretical framework we developed for Hamiltonian engineering in an interacting spin system [1]. I will then extend this framework to coupling of the spin ensemble to a spin bath, including both coherent and dissipative dynamics [2]. Using these tools I will present a scheme for efficient purification of the spin bath, surpassing the current state-of-the-art and providing a path toward applications in quantum technologies, such as enhanced MRI sensing.
Improved control and manipulation of quantum systems lies at the heart of modern quantum science and technology, with broad applications ranging from quantum metrology to quantum information science and many-body physics. In this thesis, I will introduce a novel framework for robust Hamiltonian engineering in interacting spin systems, which enables the efficient design of Floquet engineering pulse sequences for qubit and qudit systems that out-perform state-of-the-art control methods by orders of magnitude. As applications of this framework, we utilize a high density ensemble of nitrogen vacancy (NV) centers in diamond, and demonstrate significantly improved control over the disordered, interacting many-body spin system. This enabled the first solid state spin ensemble quantum sensor operating beyond the sensitivity limit imposed by interactions between sensing particles, as well as the observation and understanding of novel dynamical many-body phenomena, including discrete time crystalline order and thermalization dynamics in long-range interacting XXZ spin models.
Nitrogen-Vacancy (NV) centers in diamond are a compelling solid-state platform for quantum sensing due to their long coherence times, room-temperature operation, and optical spin readout. Several groups have recently demonstrated that NV centers, capable of running at room temperature, are practical for undergraduate laboratories. However, Stanford currently has no dedicated NV-center teaching lab for physics undergraduates. Here, I (1) design of the experimental apparatus and initially can build from existing resources at Stanford; (2) experimental validation of basic features of optical pumping, optical-microwave double resonance, magnetic sensitivity; (3) time variable quantum state preparation and detection using various optical and microwave pulse sequences to measure coherence times; (4) experimental design and validation of pulse sequences; (5) practical curriculum and syllabus for a 2-3 week laboratory in Physics 106. On the second front, I characterize a strongly interacting NV ensemble in diamond.