08 Fakultät Mathematik und Physik

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/9

Browse

Filters

2021 - 202524

Settings

Institute: search.filters.UBSInstitut.3. Physikalisches InstitutStart date: 2021

Search Results

Now showing 1 - 10 of 24
  • Thumbnail Image
    ItemOpen Access
    Nanoscale magnetic resonance spectroscopy with nitrogen-vacancy centers in diamond
    (2021) Paone, Domenico; Wrachtrup, Jörg (Prof. Dr.)
    Stickstoff-Fehlstellen (NV-Zentren) in Diamant bilden interessante Quantensysteme, welche für Quanten-Sensing Protokolle genutzt werden können. In der vorliegenden Arbeit, werden NV-Zentren genutzt, um einzelne Molekülsysteme auszulesen und supraleitende Proben lokal zu charakterisieren. Zusätzlich werden Methoden entwickelt, um die Spineigenschaften der NV-Zentren zu optimieren, welche dann Einfluss auf das Sensorikverhalten des Systems haben.
  • Thumbnail Image
    ItemOpen Access
  • Thumbnail Image
    ItemOpen Access
    Heterodyne sensing of microwaves with a quantum sensor
    (2021) Meinel, Jonas; Vorobyov, Vadim; Yavkin, Boris; Dasari, Durga; Sumiya, Hitoshi; Onoda, Shinobu; Isoya, Junichi; Wrachtrup, Jörg
    Diamond quantum sensors are sensitive to weak microwave magnetic fields resonant to the spin transitions. However, the spectral resolution in such protocols is ultimately limited by the sensor lifetime. Here, we demonstrate a heterodyne detection method for microwaves (MW) leading to a lifetime independent spectral resolution in the GHz range. We reference the MW signal to a local oscillator by generating the initial superposition state from a coherent source. Experimentally, we achieve a spectral resolution below 1 Hz for a 4 GHz signal far below the sensor lifetime limit of kilohertz. Furthermore, we show control over the interaction of the MW-field with the two-level system by applying dressing fields, pulsed Mollow absorption and Floquet dynamics under strong longitudinal radio frequency drive. While pulsed Mollow absorption leads to improved sensitivity, the Floquet dynamics allow robust control, independent from the system’s resonance frequency. Our work is important for future studies in sensing weak microwave signals in a wide frequency range with high spectral resolution.
  • Thumbnail Image
    ItemOpen Access
    Quantum Fourier transform for nanoscale quantum sensing
    (2021) Vorobyov, Vadim; Zaiser, Sebastian; Abt, Nikolas; Meinel, Jonas; Dasari, Durga; Neumann, Philipp; Wrachtrup, Jörg
    The quantum Fourier transformation (QFT) is a key building block for a whole wealth of quantum algorithms. Despite its proven efficiency, only a few proof-of-principle demonstrations have been reported. Here we utilize QFT to enhance the performance of a quantum sensor. We implement the QFT algorithm in a hybrid quantum register consisting of a nitrogen-vacancy (NV) center electron spin and three nuclear spins. The QFT runs on the nuclear spins and serves to process the sensor - i.e., the NV electron spin signal. Specifically, we show the application of QFT for correlation spectroscopy, where the long correlation time benefits the use of the QFT in gaining maximum precision and dynamic range at the same time. We further point out the ability for demultiplexing the nuclear magnetic resonance (NMR) signals using QFT and demonstrate precision scaling with the number of used qubits. Our results mark the application of a complex quantum algorithm in sensing which is of particular interest for high dynamic range quantum sensing and nanoscale NMR spectroscopy experiments.
  • Thumbnail Image
    ItemOpen Access
    Thermodynamics of quantum spin-bath depolarization
    (2023) Dasari, Durga Bhaktavatsala Rao
    We analyze here through exact calculations the thermodynamical effects in depolarizing a quantum spin-bath initially at zero temperature through a quantum probe coupled to an infinite temperature bath by evaluating the heat and entropy changes. We show that the correlations induced in the bath during the depolarizing process does not allow for the entropy of the bath to increase towards its maximal limit. On the contrary, the energy deposited in the bath can be completely extracted in a finite time. We explore these findings through an exactly solvable central spin model, wherein a central spin-1/2 system is homogeneously coupled to a bath of identical spins. Further, we show that, upon destroying these unwanted correlations, we boost the rate of both energy extraction and entropy towards their limiting values. We envisage that these studies are relevant for quantum battery research wherein both charging and discharging processes are key to characterizing the battery performance.
  • Thumbnail Image
    ItemOpen Access
    Cyclic cooling of quantum systems at the saturation limit
    (2021) Zaiser, Sebastian; Cheung, Chun Tung; Yang, Sen; Dasari, Durga Bhaktavatsala Rao; Raeisi, Sadegh; Wrachtrup, Jörg
    The achievable bounds of cooling quantum systems, and the possibility to violate them is not well-explored experimentally. For example, among the common methods to enhance spin polarization (cooling), one utilizes the low temperature and high-magnetic field condition or employs a resonant exchange with highly polarized spins. The achievable polarization, in such cases, is bounded either by Boltzmann distribution or by energy conservation. Heat-bath algorithmic cooling schemes (HBAC), on the other hand, have shown the possibility to surpass the physical limit set by the energy conservation and achieve a higher saturation limit in spin cooling. Despite, the huge theoretical progress, and few principle demonstrations, neither the existence of the limit nor its application in cooling quantum systems towards the maximum achievable limit have been experimentally verified. Here, we show the experimental saturation of the HBAC limit for single nuclear spins, beyond any available polarization in solid-state spin system, the Nitrogen-Vacancy centers in diamond. We benchmark the performance of our experiment over a range of variable reset polarizations (bath temperatures), and discuss the role of quantum coherence in HBAC.
  • Thumbnail Image
    ItemOpen Access
    Effects of non-stoichiometry on the ground state of the frustrated system Li0.8Ni0.6Sb0.4O2
    (2021) Vavilova, Evgeniya; Salikhov, Timur; Iakovleva, Margarita; Vasilchikova, Tatyana; Zvereva, Elena; Shukaev, Igor; Nalbandyan, Vladimir; Vasiliev, Alexander
    The non-stoichiometric system Li0.8Ni0.6Sb0.4O2 is a Li-deficient derivative of the zigzag honeycomb antiferromagnet Li3Ni2SbO6. Structural and magnetic properties of Li0.8Ni0.6Sb0.4O2 were studied by means of X-ray diffraction, magnetic susceptibility, specific heat, and nuclear magnetic resonance measurements. Powder X-ray diffraction data shows the formation of a new phase, which is Sb-enriched and Li-deficient with respect to the structurally honeycomb-ordered Li3Ni2SbO6. This structural modification manifests in a drastic change of the magnetic properties in comparison to the stoichiometric partner. Bulk static (dc) magnetic susceptibility measurements show an overall antiferromagnetic interaction (Θ = -4 K) between Ni2+ spins (S = 1), while dynamic (ac) susceptibility reveals a transition into a spin glass state at a freezing temperature TSG ~ 8 K. These results were supported by the absence of the λ-anomaly in the specific heat Cp(T) down to 2 K. Moreover, combination of the bulk static susceptibility, heat capacity and 7Li NMR studies indicates a complicated temperature transformation of the magnetic system. We observe a development of a cluster spin glass, where the Ising-like Ni2+ magnetic moments demonstrate a 2D correlated slow short-range dynamics already at 12 K, whereas the formation of 3D short range static ordered clusters occurs far below the spin-glass freezing temperature at T ~ 4 K as it can be seen from the 7Li NMR spectrum.
  • Thumbnail Image
    ItemOpen Access
    Optimizing NV magnetometry for magnetoneurography and magnetomyography applications
    (2023) Zhang, Chen; Zhang, Jixing; Widmann, Matthias; Benke, Magnus; Kübler, Michael; Dasari, Durga; Klotz, Thomas; Gizzi, Leonardo; Röhrle, Oliver; Brenner, Philipp; Wrachtrup, Jörg
    Magnetometers based on color centers in diamond are setting new frontiers for sensing capabilities due to their combined extraordinary performances in sensitivity, bandwidth, dynamic range, and spatial resolution, with stable operability in a wide range of conditions ranging from room to low temperatures. This has allowed for its wide range of applications, from biology and chemical studies to industrial applications. Among the many, sensing of bio-magnetic fields from muscular and neurophysiology has been one of the most attractive applications for NV magnetometry due to its compact and proximal sensing capability. Although SQUID magnetometers and optically pumped magnetometers (OPM) have made huge progress in Magnetomyography (MMG) and Magnetoneurography (MNG), exploring the same with NV magnetometry is scant at best. Given the room temperature operability and gradiometric applications of the NV magnetometer, it could be highly sensitive in the pT/Hz-range even without magnetic shielding, bringing it close to industrial applications. The presented work here elaborates on the performance metrics of these magnetometers to the state-of-the-art techniques by analyzing the sensitivity, dynamic range, and bandwidth, and discusses the potential benefits of using NV magnetometers for MMG and MNG applications.
  • Thumbnail Image
    ItemOpen Access
    Quantum randomness certified by different quantum phenomena
    (2023) Chen, Xing; Wrachtrup, Jörg (Prof. Dr.)
    Quantum random number generation utilizes quantum processes, which involve the collapse of a superposition state upon performing a measurement. In a quantum process, the measurement outcome is fundamentally unpredictable, resulting in true randomness in the generated numbers. We refer to this type of random number generator as a quantum random number generator (QRNG) since quantum processes are involved in generating random numbers. The most common QRNG is the photonic QRNG. In this kind of QRNG, photons from a laser source go into a beamsplitter. After the beamsplitter, the photons are in a superposition state of reflected path and transmitted path. In each path, there is a detector acting as a measurement device. When a measurement is performed, one photon collapses into one detector randomly, resulting in a click in the detector. The click in the transmitted detector is assigned as raw bit 0, and then the click in the reflected detector is assigned as raw bit 1. Ideally, from this QRNG, each random number generated is a quantum random number. Still, in the real world, the randomness in the generated random numbers is not pure quantum randomness since it can have other technical causes other than quantum mechanics. For example, the click events on the two detectors can come from the dark counts, which are considered to be classical noise. We need to utilize some quantum phenomena, which cannot be explained classically, to prove quantumness in the raw bits to guarantee that the random numbers from the QRNG are all generated by quantum processes instead of some unexpected classical noises. After the quantumness is proved, randomness certification protocols based on this quantumness can be formulated to quantify the entropy of the randomness. This thesis aims to present our progress in constructing randomness certification protocols for QRNGs by leveraging different quantum phenomena to ensure the quantumness of generated random numbers. These quantum phenomena include the single-photon antibunching effect, the wave-particle duality of a delayed-choice experiment, non-locality in a Bell test, and nonzero dimension witness of quantum measurements. In the first approach, a single-photon QRNG based on an nitrogen-vacancy (NV) center is implemented, and three different randomness certification protocols are built to certify quantum randomness in the raw data. In the first model, all the experimental events are used as raw bits to extract randomness, and the randomness output speed is 5.10×10^4 bits per second. In the second model, only single photon events are considered as raw bits, the randomness output speed is 4.74×10^4 bits per second. In the third model only tuple detection events below the unity line are considered raw bits, and the randomness generation speed is 34.37 bits per second. Among them, the second protocol, utilizing the single-photon antibunching effect, achieves a source-independent random number generator without compromising the randomness output speed, making it an ideal protocol for a single-photon QRNG. The second method constructs a QRNG based on a delayed-choice experiment without the fair sampling assumption. Using wave-particle duality, the model ensures photons arrive at detectors in superposition states, eliminating the need for fair sampling. By applying this model to a delayed-choice experiment, we can obtain 1,124 uniformly distributed random bits per second. The third approach certifies quantum randomness from loophole-free Bell test data using Bell's theorem and remote state preparation (RSP)-dimension witness. The RSP-dimension witness model significantly increases the randomness output speed from 2.54 bits per day to 40.63 bits per day, marking an important step towards the practical use of Bell tests in randomness generation. Lastly, a QRNG based on a nuclear spin system inside an NV center is studied, including two randomness certification protocols. The first protocol is a direct application of the W2 model from Lunghi2015, and randomness can be generated with a speed 0.87 bits per second. In the second dimension witness model, we develop a randomness certification protocol based on a three-dimensional dimension witness W3 and its randomness output speed is 1.33 bits per second, which is 53% higher than 0.87 bits per second. By harnessing these four different quantum phenomena, we contribute to the growing need for secure, high-quality random numbers in different fields including cryptography, scientific simulations, and algorithm development.
  • Thumbnail Image
    ItemOpen Access
    Silicon vacancies in silicon carbide towards scalable quantum applications
    (2024) Liu, Di; Wrachtrup, Jörg (Prof. Dr.)
    Quantum technologies harness the principles of quantum mechanics to solve complex problems and are expected to realize practical applications that are impossible with classical technologies. In this innovative field, spin defects in solid-state systems offer unique advantages for quantum computing, sensing and communication, due to their long coherence times and compatibility with existing photonic and electronic devices. This thesis focuses on the silicon vacancy (VSi) centers in 4H-SiC, an emerging and promising platform for scalable quantum technologies. 4H-SiC is a wide-bandgap semiconductor, known for its wafer-scale availability and compatibility with complementary metal-oxide semiconductor fabrication techniques, making it an ideal host material for spin defects and enabling scalable quantum technologies. VSi centers in 4H-SiC exhibit long spin coherence times and narrow zero-phonon line (ZPL) emission linewidths at near-infrared wavelengths, making them favorable for low-loss transmission in fiber networks. These excellent spin and optical properties are preserved when VSi centers are integrated into nanostructures. The research presented in this thesis begins with a comprehensive theoretical and experimental investigation of the internal spin-optical dynamics of VSi centers. Notable achievements include establishing the complete electronic fine structure of VSi centers with the involved intersystem-crossing and deshelving mechanisms, and obtaining the previously unknown radiative and non-radiative transition rates with carefully designed spin-dependent measurements. These findings provide crucial parameters, such as spin initialization fidelity, quantum efficiency, and setup collection efficiency, which guide the nanophotonic optimization of VSi centers. Additionally, the thesis demonstrates the viability of VSi centers for scalable quantum applications by showcasing the high indistinguishability of emitted ZPL photons and advanced spectral tuning based on the piezoelectric effect, thereby fully revealing the device compatibility of 4H-SiC. To further enhance the optical properties and collection efficiency of VSi centers, the thesis implements integration techniques such as the fabrication of Fabry-Pérot nanocavities and tapered-fiber waveguide interfaces under cryogenic conditions. A hybrid photonic cavity system is proposed, combining 4H-SiC nanocavities with lithium niobate (LiNb) substrates for realizing individual cavity tuning exploiting the large electro-optic coefficient of LiNb, further pushing the boundaries of scalable on-chip quantum technological implementations.