****NON-TECHNICAL ABSTRACT**** Controlling and manipulating nature at the quantum level is one of the greatest challenges facing both theoretical and experimental physics. The combination of quantum physics with information science has made possible the use of quantum systems to perform calculations and tasks of a complexity unattainable by systems that behave classically. The most important challenge facing quantum information science is to reliably control a scalable quantum system, which provides the ability to build quantum devices, while staving off decoherence (the process that leads to the loss of the quantum properties). The focus of this project is to study decoherence of a small quantum system interacting with a larger environment, whose characteristics are in part under control. The goal is to achieve a deeper physical understanding of decoherence. The ability to vary the environment's properties is critical to achieve a better understanding of decoherence, which is a complex many-body non-equilibrium quantum phenomenon. In turn, a better understanding may lead to techniques for mitigating decoherence and to improved quantum devices. The project will focus on a system, the Nitrogen-Vacancy color center in diamond, which has emerged as a highly promising quantum device for computation, magnetic sensing and bioimaging. This project will support the training of a postdoctoral fellow in an exciting and multidisciplinary research field. This research project receives support from the Division of Materials Research and the Physics Division.
The goal of this project is to study decoherence of a central spin by a spin bath with varying and controllable characteristics. The project will focus on the electronic spin bath in diamond nano-crystals and its effects on the Nitrogen-Vacancy (NV) color center, to achieve a deeper physical understanding as well as to potentially lead to practical applications. Theoretical and experimental efforts towards a full understanding of decoherence mechanisms have been hindered by the very complexity of the dynamics. The ability to manipulate the mesoscopic bath will be exploited to perform a systematic study of the central-spin problem. Specifically, schemes for decoupling the environment from the central spin, for refocusing its internal evolution as well as for polarizing the environment spins will be developed and tested experimentally Potential applications of the control and polarization techniques range from precision measurement and bio-imaging to quantum communication and computation. For example, control and polarization of the bath would not only improve the sensitivity of recently proposed NV-based magnetic sensors, but also allow using the bath itself as a means to achieve sensitivity at the Heisenberg limit. The proposed research program will also provide interdisciplinary training of a postdoctoral fellow in condensed matter physics, nanoscience, optical imaging techniques and quantum information science, as well as in areas of potential applications, such as surface science and bioimaging. This research project receives support from the Division of Materials Research and the Physics Division.
Recent technological advances have opened the possibility to exploit quantum systems for tasks ranging from quantum computation and communication to precise measurement at the nanoscale. A critical challenge to developing novel devices based on quantum systems is to understand and stave off the deleterious effects of noise and perturbations that destroy their quantum properties. The main goal of this project was to characterize the spin environment acting on a central spin of interest, by experiments and numerical simulations. We focused on spin systems since they emerged as promising quantum sensors and quantum bits (qubits) and can take advantage of well-developed control techniques in magnetic resonance. We studied in particular an optically active electronic spin defect in diamond (the Nitrogen-Vacancy –NV– center) and a bath comprised of other Nitrogen spin defects (P1 centers). Specifically, we applied dynamical decoupling (sequences of resonant microwave pulses with precise timing) to perform "noise spectroscopy", revealing the electronic spin bath spectrum in samples with different concentration of defects and isotopic purifications. From the experimental results it emerged that a simple picture of an isolated electronic bath is not enough to account for all the effects observed: the nuclear spin bath acts locally on the electronics spin, modifying their dynamics. This effect is going to be very important in many applications, from NV-magnetometry to P1 polarization. Given the complexity of the spin bath, a very accurate mapping of its spectrum is required. We thus devised an efficient strategy for spectroscopy (and more generally, for waveform reconstruction) with a single quantum probe based on digital –Walsh– functions. This strategy can be used not only to gather information about the bath of a central spin, but also for quantum metrology. Once the spin bath is well characterized, it becomes possible to exploit it for tasks such as quantum information transport in a quantum computing architecture or for quantum-enhanced sensing. In this project we successfully achieved an important first step, by observing polarization transfer from the central spin to the spin bath. As the NV spin can be polarized optically with high efficiency, the polarization can be transferred to the electronic spin bath. We further studied polarization transfer in a spin system both theoretically and experimentally using fluorine spins in a crystal of fluorapatite. Thanks to its geometry, this crystal can be considered as a collection one-dimensional spin chains. Polarization transfer is an important phenomenon in many disciplines, and chiefly in magnetic resonance experiments, where it underlies both relaxation as well as dynamic polarization. Novel insight into spin diffusion would lead to improved strategies for DNP, a popular technique in protein structure determination with NMR. In addition, polarization transfer could be the basis for transferring quantum information between distributed nodes of a quantum computer. By studying theoretically polarization transfer in 1D and 3D systems, we found conditions that enhance the fidelity and speed of transport, even in low-polarized states. In addition, we were able to observe experimentally the polarization transfer from a single polarized spin to a spin chain, by using NMR techniques in fluorapatite Since quantum state transfer creates many-body correlated states that might exhibit an enhanced sensitivity to noise, we studied experimentally their coherence properties using the spin chains in fluorapatite. We leveraged the low dimensionality of the system studied to gain insight into both the many-body states created by the dynamics and their subsequent decay. We were thus able to derive a simple analytical model that captures the essential features of the multi-spin decay, and compares well with the experimental data. We found that the restriction to one-dimensional geometries brings a complex dynamics, but also longer coherence times that in 3D systems, thus pointing to advantages to be found in particular geometries for larger quantum information architectures. Beyond the characterization of the spin bath of a central spin, the research performed in this project has several broader impacts. First, novel insight into the spin bath can inform decisions on how to best engineer materials for various applications (e.g. metrology). Second, the Walsh waveform estimation method has much broader applications, such as the reconstruction of biological magnetic signals that could have far-reaching impacts in the fields of bio-imaging and neuroscience. The work on polarization transfer has broader impacts on quantum information. In addition, the project has contributed in providing research experience and the development of new research skills for postdocs and graduate students involved in the research. Thanks to the interdisciplinary character of the project, training encompassed quantum optics, electronics and magnetic resonance experimental techniques and theoretical modeling.
