Spin resonance linewidths of bismuth donors in silicon coupled to planar microresonators

Ensembles of bismuth donor spins in silicon are promising storage elements for microwave quantum memories due to their long coherence times which exceed seconds. Operating an efficient quantum memory requires achieving critical coupling between the spin ensemble and a suitable high-quality factor resonator -- this in turn requires a thorough understanding of the lineshapes for the relevant spin resonance transitions, particularly considering the influence of the resonator itself on line broadening. Here, we present pulsed electron spin resonance measurements of ensembles of bismuth donors in natural silicon, above which niobium superconducting resonators have been patterned. By studying spin transitions across a range of frequencies and fields we identify distinct line broadening mechanisms, and in particular those which can be suppressed by operating at magnetic-field-insensitive `clock transitions'. Given the donor concentrations and resonator used here, we measure a cooperativity $C\sim 0.2$ and based on our findings we discuss a route to achieve unit cooperativity, as required for a quantum memory

Random-access quantum memory using chirped pulse phase encoding

Quantum memories capable of faithfully storing and recalling quantum states on-demand are powerful ingredients in bulding quantum networks [arXiv:0806.4195] and quantum information processors [arXiv:1109.3743]. As in conventional computing, key attributes of such memories are high storage density and, crucially, random access, or the ability to read from or write to an arbitrarily chosen register. However, achieving such random access with quantum memories [arXiv:1904.09643] in a dense, hardware-efficient manner remains a challenge, for example requiring dedicated cavities per qubit [arXiv:1109.3743] or pulsed field gradients [arXiv:0908.0101]. Here we introduce a protocol using chirped pulses to encode qubits within an ensemble of quantum two-level systems, offering both random access and naturally supporting dynamical decoupling to enhance the memory lifetime. We demonstrate the protocol in the microwave regime using donor spins in silicon coupled to a superconducting cavity, storing up to four multi-photon microwave pulses and retrieving them on-demand up to 2~ms later. A further advantage is the natural suppression of superradiant echo emission, which we show is critical when approaching unit cooperativity. This approach offers the potential for microwave random access quantum memories with lifetimes exceeding seconds [arXiv:1301.6567, arXiv:2005.09275], while the chirped pulse phase encoding could also be applied in the optical regime to enhance quantum repeaters and networks

Coherent spin dynamics of rare-earth doped crystals in the high-cooperativity regime

Rare-earth doped crystals have long coherence times and the potential to provide quantum interfaces between microwave and optical photons. Such applications benefit from a high cooperativity between the spin ensemble and a microwave cavity -- this motivates an increase in the rare earth ion concentration which in turn impacts the spin coherence lifetime. We measure spin dynamics of two rare-earth spin species, $^{145}$Nd and Yb doped into Y$_{2}$SiO$_{5}$, coupled to a planar microwave resonator in the high cooperativity regime, in the temperature range 1.2 K to 14 mK. We identify relevant decoherence mechanisms including instantaneous diffusion arising from resonant spins and temperature-dependent spectral diffusion from impurity electron and nuclear spins in the environment. We explore two methods to mitigate the effects of spectral diffusion in the Yb system in the low-temperature limit, first, using magnetic fields of up to 1 T to suppress impurity spin dynamics and, second, using transitions with low effective g-factors to reduce sensitivity to such dynamics. Finally, we demonstrate how the `clock transition' present in the $^{171}$Yb system at zero field can be used to increase coherence times up to $T_{2} = 6(1)$ ms.Comment: 8 pages, 5 figure

Hibridinių metalo-formiato karkasų tyrimai elektronų paramagnetinio rezonanso spektroskopija

The dissertation is dedicated to electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) study of structural phase transitions, structural phases and dynamic effects in hybrid metal-formate frameworks. The continuous-wave EPR experiments allowed to characterize the order of the structural phase transitions and motion of the molecular cations in the disordered phases of [(CH3)2NH2][Zn(HCOO)3], [CH3NH2NH2][Zn(HCOO)3] and [NH3(CH2)4NH3][Zn(HCOO)3]2 hybrid frameworks. EPR measurements with an external electric field revealed non-ferroelectric nature of the dimethylammonium zinc-formate, while [NH4][Zn(HCOO)3] framework proved to be ferroelectric. The pulsed EPR spectroscopy allowed to probe the motion of the methyl groups in the ordered phases as well as the lattice dynamics in the vicinity of the structural phase transitions in zinc-formate frameworks. The pulsed ENDOR experiments verified the proton positions in the structural models of these compounds

Electron paramagnetic resonance spectroscopy of hybrid metal-formate frameworks

The dissertation is dedicated to electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) study of structural phase transitions, structural phases and dynamic effects in hybrid metal-formate frameworks. The continuous-wave EPR experiments allowed to characterize the order of the structural phase transitions and motion of the molecular cations in the disordered phases of [(CH3)2NH2][Zn(HCOO)3], [CH3NH2NH2][Zn(HCOO)3] and [NH3(CH2)4NH3][Zn(HCOO)3]2 hybrid frameworks. EPR measurements with an external electric field revealed non-ferroelectric nature of the dimethylammonium zinc-formate, while [NH4][Zn(HCOO)3] framework proved to be ferroelectric. The pulsed EPR spectroscopy allowed to probe the motion of the methyl groups in the ordered phases as well as the lattice dynamics in the vicinity of the structural phase transitions in zinc-formate frameworks. The pulsed ENDOR experiments verified the proton positions in the structural models of these compounds

X- and Q-band EPR with cryogenic amplifiers independent of sample temperature

Inspired by the success of NMR cryoprobes, we recently reported a leap in X-band EPR sensitivity by equipping an ordinary EPR probehead with a cryogenic low-noise microwave amplifier placed closed to the sample in the same cryostat [Šime˙ nas et al. J. Magn. Reson. 322, 106876 (2021)]. Here, we explore, theoretically and experimentally, a more general approach, where the amplifier temperature is independent of the sample temperature. This approach brings a number of important advantages, enabling sensitivity improvement irrespective of sample temperature, as well as making it more practical to combine with ENDOR and Q-band resonators, where space in the sample cryostat is often limited. Our experimental realisation places the cryogenic preamplifier within an external closed-cycle cryostat, and we show CW and pulsed EPR and ENDOR sensitivity improvements at both X- and Q-bands with negligible dependence on sample temperature. The cryoprobe delivers signal-to-noise ratio enhancements that reduce the equivalent pulsed EPR measurement time by 16 at X-band and close to 5 at Q-band. Using the theoretical framework we discuss further improvements of this approach which could be used to achieve even greater sensitivity

Magnetic excitation and readout of methyl group tunnel coherence

ISSN:2375-254

Percolation and Transport Properties in The Mechanically Deformed Composites Filled with Carbon Nanotubes

The conductivity and percolation concentration of the composite material filled with randomly distributed carbon nanotubes were simulated as a function of the mechanical deformation. Nanotubes were modelled as the hard-core ellipsoids of revolution with high aspect ratio. The evident anisotropy was observed in the percolation threshold and conductivity. The minimal mean values of the percolation of 4.6 vol. % and maximal conductivity of 0.74 S/m were found for the isotropic composite. Being slightly aligned, the composite demonstrates lower percolation concentration and conductivity along the orientation of the nanotubes compared to the perpendicular arrangement