Periodic Reporting for period 1 - FastoSpintrolux (Fast and Nanoscale Spin Control via Single Flux Quanta in Superconductors)
Reporting period: 2020-10-14 to 2022-10-13
This project “FastoSpintrolux” aims to address two key problems in quantum science and technologies: the speed and spatial precision of coherent control over the quantum states of a qubit (an object which usually have two discrete spin states, i.e. a nitrogen-vacancy defect in diamond in this project). The importance of this subject is manifested in two aspects. First of all, in order to build large and scalable quantum networks, a large number of qubits are involved and could be densely distributed in a small region on microchips or photonic devices. Because of the large micrometer size, conventional microwave devices that are used for spin manipulation are not compatible with the (nanometer) small size and distance between qubits. In general, there is lack of effective tool which can address single qubits with nanometer precision. In addition, due to the interaction with local environment and the finite dephasing time, quantum properties of qubits usually do not last very long, which are detrimental for sufficient interrogations and quantum operations. An increase in the spin control rate will increase the number of coherent operations on the quantum states of qubit, thus improving the fidelity and accuracy of quantum measurements.
Therefore, the overall objective of this project is to demonstrate and develop a novel and efficient method which allows the control over quantum states of single qubit with nanometer spatial precision and fast speed up to gigahertz (10^9 Hz), with the help of laser-manipulated microscopic vortices which is quantized flux in a superconductor and smallest magnetic object available so far. For the benefit of the society, the technology developed in the project not only increase the fidelity in quantum sensing which leads to an improvement in measurement accuracy, but also open new possibilities of efficiently coupling and entangling distant qubits, which are very important in quantum teleportation and communication. For the academic community, the project will fuel novel fundamental studies on the interplay between quantum qubit and microscopic vortices, as well as many-body problems.
Therefore, the overall objective of this project is to demonstrate and develop a novel and efficient method which allows the control over quantum states of single qubit with nanometer spatial precision and fast speed up to gigahertz (10^9 Hz), with the help of laser-manipulated microscopic vortices which is quantized flux in a superconductor and smallest magnetic object available so far. For the benefit of the society, the technology developed in the project not only increase the fidelity in quantum sensing which leads to an improvement in measurement accuracy, but also open new possibilities of efficiently coupling and entangling distant qubits, which are very important in quantum teleportation and communication. For the academic community, the project will fuel novel fundamental studies on the interplay between quantum qubit and microscopic vortices, as well as many-body problems.
By the end of the project “FastoSpintrolux”, the obtained results are:
- optical properties of single NV centers in nanodiamond and bulk chemical-vapor-grown diamond membrane have been characterized, including saturation count, lifetime and single photon emission photon statistics;
- quantum spin properties of single NV centers have been measured, such as coherent spin manipulation and Rabi oscillation, spin dephasing time T2* and T2, and characterization of quantum sensing sensitivity to the external magnetic field;
- imaging single vortices in superconductive niobium film using magneto-optical (MO) imaging method. Study of the effect of garnet indicator on the quality of vortex imaging. Translate and move single vortices in superconductor plane with a focus laser beam;
- Probe vortex pinning and superconductivity in the niobium film with single NV centers.
Exploitation and dissemination: all the data and results obtained during the action “FastoSpintrolux” will be available upon request for further exploitation. The public and colleagues who are interested in quantum sensing or NV magnetometry are welcome to exploit the data in more details.Due to the COVID-19 pandemic, the dissemination of the obtained results at international conferences and to the general public is limited. Results have been presented within the host group and institute in the forms of poster and oral presentations.
Specifically, the work that has been done during the project include:
- build room-temperature and low-temperature confocal microscopes, where the spin properties of single NV centers can be readout and manipulated with microwave.
Because the detection of spin states of single NV centers depends on its fluorescence and the emission yield is not very high, sensitive confocal microscope which can detect single NV center has to be built around an inverted microscope and a cryostat (5K-300K). With a high numerical aperture objective (0.9) and careful alignment, single NV centers can be located with nanometer precision and decent signal-to-noise ratio using the home-built setup.
- characterize optical properties such as saturation count, spectrum, quantum emission, and spin properties such as relaxation time T2.
Optical and quantum spin properties of single NV centers in (nano)diamond are characterized in details using above setups. And information regarding to the properties of single NV centers is obtained such as the saturation intensity, lifetime, spectrum, quantum emission statistics and spin relaxation times. For more details, please check the summery report.
- Static magnetic field sensing using single NV centers. Characterize the sensitivity to the DC magnetic field;
Magnetic sensing experiments with NV centers were performed at room temperature. Sensitivity to the static field is examined and compared for the two diamond host materials: i.e. nanodiamond versus CVD grown bulk diamond. It was shown that sensitivity of NV centers in this project is in the range of a few μT/√Hz, good enough for sensing the milli Tesla range of field around a vortex in superconductive niobium film.
- Imaging single vortices in niobium using magneto-optical imaging. Manipulate single vortices in the superconductor plane with a focused laser beam;
Wide-field magneto-optical imaging is built and used to locate vortices in niobium superconductor. With the home-built setup and Faraday-rotation garnet indicator, it was shown that individual vortices can be imaged and located with micrometer spatial resolution. Vortex contrast, the effect of indicator’s thickness on the vortex contrast and sizes were carefully examined. In addition, it was verified that a focused laser beam with a wavelength of 532 nm can move single vortices to a target position in the superconductor plane. Laser intensity, translation distance and heat effect of laser were examined as well.
- Probe the superconductivity and vortex pinning in the superconductive niobium film with single NV centers.
NV centers in nanodiamonds were directly drop casted on niobium surface in order to minimize the distance between NV centers and superconductor surface. When the sample is cooled down below Tc in the presence of a magnetic field, flux can be trapped at pinning sites in the superconductor, if the applied field is below the critical field of niobium. A NV center in nanodiamond, with its N-V axis orientated randomly, can detect and sense the magnetic field and its components around the core region of single vortices. As is demonstrated in the project, the resonance of NV centers indeed changes due to the presence of vortices and the field screening effect of superconductivity when sample temperature is varied above/below Tc. The results indicate the feasibility of tuning and controlling the resonance and thus the quantum states of spin qubit via vortices.
- optical properties of single NV centers in nanodiamond and bulk chemical-vapor-grown diamond membrane have been characterized, including saturation count, lifetime and single photon emission photon statistics;
- quantum spin properties of single NV centers have been measured, such as coherent spin manipulation and Rabi oscillation, spin dephasing time T2* and T2, and characterization of quantum sensing sensitivity to the external magnetic field;
- imaging single vortices in superconductive niobium film using magneto-optical (MO) imaging method. Study of the effect of garnet indicator on the quality of vortex imaging. Translate and move single vortices in superconductor plane with a focus laser beam;
- Probe vortex pinning and superconductivity in the niobium film with single NV centers.
Exploitation and dissemination: all the data and results obtained during the action “FastoSpintrolux” will be available upon request for further exploitation. The public and colleagues who are interested in quantum sensing or NV magnetometry are welcome to exploit the data in more details.Due to the COVID-19 pandemic, the dissemination of the obtained results at international conferences and to the general public is limited. Results have been presented within the host group and institute in the forms of poster and oral presentations.
Specifically, the work that has been done during the project include:
- build room-temperature and low-temperature confocal microscopes, where the spin properties of single NV centers can be readout and manipulated with microwave.
Because the detection of spin states of single NV centers depends on its fluorescence and the emission yield is not very high, sensitive confocal microscope which can detect single NV center has to be built around an inverted microscope and a cryostat (5K-300K). With a high numerical aperture objective (0.9) and careful alignment, single NV centers can be located with nanometer precision and decent signal-to-noise ratio using the home-built setup.
- characterize optical properties such as saturation count, spectrum, quantum emission, and spin properties such as relaxation time T2.
Optical and quantum spin properties of single NV centers in (nano)diamond are characterized in details using above setups. And information regarding to the properties of single NV centers is obtained such as the saturation intensity, lifetime, spectrum, quantum emission statistics and spin relaxation times. For more details, please check the summery report.
- Static magnetic field sensing using single NV centers. Characterize the sensitivity to the DC magnetic field;
Magnetic sensing experiments with NV centers were performed at room temperature. Sensitivity to the static field is examined and compared for the two diamond host materials: i.e. nanodiamond versus CVD grown bulk diamond. It was shown that sensitivity of NV centers in this project is in the range of a few μT/√Hz, good enough for sensing the milli Tesla range of field around a vortex in superconductive niobium film.
- Imaging single vortices in niobium using magneto-optical imaging. Manipulate single vortices in the superconductor plane with a focused laser beam;
Wide-field magneto-optical imaging is built and used to locate vortices in niobium superconductor. With the home-built setup and Faraday-rotation garnet indicator, it was shown that individual vortices can be imaged and located with micrometer spatial resolution. Vortex contrast, the effect of indicator’s thickness on the vortex contrast and sizes were carefully examined. In addition, it was verified that a focused laser beam with a wavelength of 532 nm can move single vortices to a target position in the superconductor plane. Laser intensity, translation distance and heat effect of laser were examined as well.
- Probe the superconductivity and vortex pinning in the superconductive niobium film with single NV centers.
NV centers in nanodiamonds were directly drop casted on niobium surface in order to minimize the distance between NV centers and superconductor surface. When the sample is cooled down below Tc in the presence of a magnetic field, flux can be trapped at pinning sites in the superconductor, if the applied field is below the critical field of niobium. A NV center in nanodiamond, with its N-V axis orientated randomly, can detect and sense the magnetic field and its components around the core region of single vortices. As is demonstrated in the project, the resonance of NV centers indeed changes due to the presence of vortices and the field screening effect of superconductivity when sample temperature is varied above/below Tc. The results indicate the feasibility of tuning and controlling the resonance and thus the quantum states of spin qubit via vortices.
The results obtained so far pave the way for the future exploration of coupling vortices with single spin qubit, and use those smallest “nano-magnets” to swiftly flip spin qubit with nanometer precision. In addition, the sensitivity of NV centers to the presence of vortices and inhomogeneity in the superconductor film demonstrated in the project opens new possibilities on the investigation of the interplay between macroscopic quantum object and spin qubits.