Abstract Strong interactions between defect spins in many-body solid-state quantum systems are a crucial resource for exploring non-classical states. However, they face the key challenge of controlling interactions between the defect spins, since they are spatially fixed inside the host lattice. In this work, we present a dressed state approach to control the effective dipolar coupling between solid-state spins and demonstrate this scheme experimentally using two strongly-coupled nitrogen vacancy (NV) centers in diamond. Through Ramsey spectroscopy on the sensor spin, we detect the change of the effective dipolar field generated by the control spin prepared in different dressed states. To observe the change of interaction dynamics, we deploy spin-lock-based polarization transfer measurements between the two NV spins in different dressed states. This scheme allows us to control the distribution of interaction strengths in strongly interacting spin systems, which can be a valuable tool for generating multi-spin correlated states for quantum-enhanced sensing.
Abstract Strong interactions between defect spins in many-body solid-state quantum systems are a crucial resource for exploring non-classical states. However, they face the key challenge of controlling interactions between the defect spins, since they are spatially fixed inside the host lattice. In this work, we present a dressed state approach to control the effective dipolar coupling between solid-state spins and demonstrate this scheme experimentally using two strongly-coupled nitrogen vacancy (NV) centers in diamond. Through Ramsey spectroscopy on the sensor spin, we detect the change of the effective dipolar field generated by the control spin prepared in different dressed states. To observe the change of interaction dynamics, we deploy spin-lock-based polarization transfer measurements between the two NV spins in different dressed states. This scheme allows us to control the distribution of interaction strengths in strongly interacting spin systems, which can be a valuable tool for generating multi-spin correlated states for quantum-enhanced sensing.
In quantum science applications, ranging from many-body physics to quantum metrology, dipolar interactions in spin ensembles are controlled via Floquet engineering. However, this technique typically reduces the interaction strength between spins, and effectively weakens the coupling to a target sensing field, limiting the metrological sensitivity. In this work, we develop and demonstrate a method for direct tuning of the native interaction in an ensemble of nitrogen-vacancy (NV) centers in diamond. Our approach utilizes dressed-state qubit encoding under a magnetic field perpendicular to the crystal lattice orientation. This method leads to a 3.2× enhancement of the dimensionless coherence parameter JT2 compared to state-of-the-art Floquet engineering, and a 2.6× (8.3dB) enhanced sensitivity in AC magnetometry. Utilizing the extended coherence we experimentally probe spin transport at intermediate to late times. Our results provide a powerful Hamiltonian engineering tool for future studies with NV ensembles and other interacting higher-spin (S>21) systems.
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.
Solid-state spin ensembles have emerged as a leading platform for micro- and nanoscale sensing, combining high sensitivity, nanoscale spatial resolution, and operation under ambient conditions. At high spin densities, magnetic dipole–dipole interactions typically limit sensing performance by reducing coherence, but can in principle be harnessed to enhance sensitivity. Realizing such interaction-enhanced sensing in practice, however, remains challenging due to the intrinsic anisotropy of dipolar interactions and the random spatial distribution of spins. This thesis presents a series of experimental works that culminate in a practical interaction-enhanced magnetometer based on ensembles of nitrogen-vacancy (NV) centers in diamond. Our approach is based on collective, one-axis-twisting-like (OAT-like) dynamics engineered from native dipolar interactions using controlled nanoscale spin textures, in combination with a dressed-state qubit encoding that significantly extends the sensor coherence time. These results open the door to practical deployment of interaction-enhanced quantum sensors for nanoscale biological imaging and material characterization.
Under ambient conditions, spin impurities in solid-state systems are found in thermally-mixed states and are optically "dark", i.e., the spin states cannot be optically controlled. Nitrogen-vacancy (NV) centers in diamond are an exception in that the electronic spin states are "bright", i.e., they can be polarized by optical pumping, coherently manipulated with spin-resonance techniques, and read out optically, all at room temperature. Here we demonstrate a dressed-state, double-resonance scheme to transfer polarization from bright NV electronic spins to dark substitutional-Nitrogen (P1) electronic spins in diamond. This polarization-transfer mechanism could be used to cool a mesoscopic bath of dark spins to near-zero temperature, thus providing a resource for quantum information and sensing, and aiding studies of quantum effects in many-body spin systems.
We observe coherent spin exchange between identical electronic spins in the solid state, a key step towards full quantum control of electronic spin registers in room temperature solids. In a diamond substrate, a single nitrogen vacancy (NV) center coherently couples to two adjacent S=1/2 dark electron spins via the magnetic dipolar interaction. We quantify NV-electron and electron-electron couplings via detailed spectroscopy, with good agreement to a model of strongly interacting spins. The electron-electron coupling enables an observation of coherent flip-flop dynamics between electronic spins in the solid state, which occur conditionally on the state of the NV. Finally, as a demonstration of coherent control, we selectively couple and transfer polarization between the NV and the pair of electron spins. Our observations enable the realization of fast quantum gate operations and quantum state transfer in a scalable, room temperature, quantum processor.
Spin qubit coherence is a fundamental resource for the realization of quantum technologies. For solid-state platforms, spin decoherence is dominated by the magneto-active environment in the lattice, limiting their applicability. While standard dynamical decoupling techniques, such as the Hahn echo, extend central spin coherence, they fail to suppress the fast noise arising from strong dipolar interactions within the bath. Here, we present a decoupling mechanism, Hybrid-LG, that suppresses intra-bath dipolar interactions -- thus, fast noise acting on spin qubits- and demonstrate its effectiveness in extending spin coherence through efficient in-house CCE simulations. Specifically, we investigate one of the most widely exploited solid-state quantum platforms: an ensemble of nitrogen-vacancy (NV) centers in diamond coupled to a large and dense bath of substitutional nitrogen paramagnetic impurities (P1 centers). Our results reveal at least a twofold enhancement in NV coherence time relative to standard techniques including P1 center driving, without requiring additional control power.