Electrically driven reverse overhauser pumping of nuclear spins in quantum dots

We propose a new mechanism for polarizing nuclear spins in quantum dots, based on periodic modulation of the hyperfine coupling by electric driving at the electron spin resonance frequency. Dynamical nuclear polarization results from resonant excitation rather than hyperfine relaxation mediated by a thermal bath, and thus is not subject to Overhauser-like detailed balance constraints. This allows polarization in the direction opposite to that expected from the Overhauser effect. Competition of the electrically driven and bath-assisted mechanisms can give rise to spatial modulation and sign reversal of polarization on a scale smaller than the electron confinement radius in the dot.     

Optical activity of helical quantum-dot supercrystals

The size of chiral nanoparticles is much smaller than the optical wavelength. As a result, the difference in interaction of enantiomers with circularly polarized light of different handedness is practically unobservable. Due to the large mismatch in scale, the problem of enhancement of enantioselectivity of optical properties of nanoparticles is particularly important for modern photonics. In this work, we show that ordering of achiral nanoparticles into a chiral supercrystal with dimensions comparable to the wavelength of light allows achieving nearly total dissymmetry of optical absorption and demonstrate this using a helical super-crystal made of semiconductor quantum dots as an example. The proposed approach may find numerous applications in various optical and analytical methods used in biomedicine, chemistry, and pharmacology.     

Optical pumping of the electronic and nuclear spin of single charge-tunable quantum dots

We present a comprehensive examination of optical pumping of spins in individual GaAs quantum dots as we change the net charge from positive to neutral to negative with a charge-tunable heterostructure. Negative photoluminescence polarization memory is enhanced by optical pumping of ground state electron spins, which we prove with the first measurements of the Hanle effect on an individual quantum dot. We use the Overhauser effect in a high longitudinal magnetic field to demonstrate efficient optical pumping of nuclear spins for all three charge states of the quantum dot.     

Superradiation of a system of nuclear spins

The superradiation of a system of nuclear spins is investigated taking into account the broadening of the NMR spectrum due to the stochasticity of the local magnetic field. The effect of the local field generated by nuclear spins randomly distributed in space is investigated. In this case it is possible to achieve better agreement with the experimentally observed characteristics of this phenomenon. Specifically, the possibility of a monopulse regime with a high initial inversion of the spin polarization is explained.     

Optical pumping of ions

Radiofrequency spectroscopy has been extended to positive ions with S ground states by means of optical pumping. The ions are stored in buffer gases or ion traps and are polarized directly by optical pumping or indirectly by spin exchange, change exchange, or Penning ionization with optically pumped atoms. The applied methods are described. The experiments can be divided into two categories:Collisional interactions of the polarized ions are investigated, like the exchange processes mentioned above, spin exchange with free electrons, spin depolarization and hyperfine density shifts in rare gases. For the two latter effects drastic differences are observed between atomic and ionic ${}^2\text{S}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ states the explanations of which reveal the influence of the ionic charge on the interactions. The comparison of equivalent processes involving isoelectronic ²S or ²P configurations of atoms and ions provides a test of current collision theories.Precision rf spectroscopy of ionic ground states yields the ${}^2\text{S}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ hyperfine structure splittings of ³He⁺ and group II ions with relative accuracies of up to 10^?9, with further improvement possible. Direct and indirect optical pumping of ${}^1\text{S}{}_0$ ions is used to determine nuclear magnetic moments, diamagnetic shielding coefficients, and chemical shifts. Applications and further developments of the present methods are discussed.     

Self-error-rejecting multipartite entanglement purification for electron systems assisted by quantum-dot spins in optical microcavities

We present a self-error-rejecting multipartite entanglement purification protocol (MEPP) for N-electron-spin entangled states, resorting to the single-side cavity-spin-coupling system. Our MEPP has a high efficiency containing two steps. One is to obtain high-fidelity N-electron-spin entangled systems with error-heralded parity-check devices (PCDs) in the same parity-mode outcome of three electron-spin pairs, as well as M-electron-spin entangled subsystems (2≤M <N) in the different parity-mode outcomes of those. The other is to regain the N-electron-spin entangled systems from M-electron-spin entangled states utilizing entanglement link. Moreover, the quantum circuits of PCDs make our MEPP works faithfully, due to the practical photon-scattering deviations from the finite side leakage of the microcavity, and the limited coupling between a quantum dot and a cavity mode, converted into a failed detection in a heralded way.     

Optical spectroscopy of CdTe/ZnTe quantum-dot superlattices

Studies of CdTe/ZnTe quantum-dot superlattices (self-assembled quantum-dot multilayers) have been carried out by optical spectroscopy methods in a wide range of temperatures. It has been shown that the ZnTe spacer layer thickness affects the properties of these quantum-dot superlattices due to changes in the elastic strain distribution pattern. An additional luminescence band appearing in the spectrum of the structure with the thinnest ZnTe spacer layer exhibits an anomalous shift of the peak position and an unusual behavior of integral intensity with the temperature increase. We assume that the spectrum of CdTe/ZnTe quantum-dot superlattices with the thinnest ZnTe spacer is caused by two kinds of excitonic states—spatially indirect and spatially direct.     

Long-lived quantum memory with nuclear atomic spins

We propose to store nonclassical states of light into the macroscopic collective nuclear spin (10(18) atoms) of a 3He vapor, using metastability exchange collisions. These collisions, commonly used to transfer orientation from the metastable state 2 3S1 to the ground state of 3He, can also transfer quantum correlations. This gives a possible experimental scheme to map a squeezed vacuum field state onto a nuclear spin state with very long storage times (hours).     

Dynamic nuclear polarization with single electron spins

We polarize nuclear spins in a GaAs double quantum dot by controlling two-electron spin states near the anticrossing of the singlet (S) and m(S)= +1 triplet (T+) using pulsed gates. An initialized S state is cyclically brought into resonance with the T+ state, where hyperfine fields drive rapid rotations between S and T+, "flipping" an electron spin and "flopping" a nuclear spin. The resulting Overhauser field approaches 80 mT, in agreement with a simple rate-equation model. A self-limiting pulse sequence is developed that allows the steady-state nuclear polarization to be set using a gate voltage.     

Optical pumping with laser-induced-fluorescence

We describe a new procedure for optical pumping that is based on laser-induced fluorescence (LIF). The procedure is demonstrated by optically exciting a sample of Rb⁸⁵ atoms, which then creates a population imbalance between the ground state hyperfine levels of Rb⁸⁷ by “LIF depopulation pumping”. Though optical pumping with this technique increases the intensity dependent light-shift coefficient (i.e., ac Stark shift) of the Rb⁸⁷ 0-0 hyperfine transition, it reduces the frequency dependent light-shift coefficient by at least an order of magnitude. Since the stabilization of the diode laser wavelength is a significant challenge in the development of laser-pumped gas-cell atomic clocks, it is anticipated that optical pumping with LIF will be of benefit to atomic clock technology.     

Indirect interaction of nuclear spins in chiral magnets

The effective Hamiltonian of the Suhl-Nakamura interaction of nuclear spins in a helimagnet has been constructed for the case where an external magnetic field is applied perpendicular to the axis of the helicoid. The contribution from the intraboundary and intradomain spin excitations to the parameter and effective radius of this interaction has been calculated. The second moment and local broadening of the NMR absorption line, which are determined by the indirect interaction of the nuclear spins, have also been calculated.     

Optical pumping for investigation of nuclear properties of short-lived isotopes/isomers: A review

Optical Pumping (OP) is an efficient technique to produce oriented nuclei by absorption of polarized light by atoms. With the advent of tunable lasers, both resolution and sensitivity of the OP technique has greatly increased. The optically induced nuclear orientation produces changes in the angular distribution of nuclear emissions. The β-asymmetry or γ-anisotropy of the optically oriented nuclei is frequency dependent and the hyperfine structure of short-lived isotopes/isomers may be studied by monitoring the asymmetry/anisotropy as a function of laser tuning. This brief review describes the recent developments in the OP technique for studying the short-lived isotopes/isomers and lists all the nuclei studied so far by this technique.     

Optical pumping into many-body entanglement

We propose a scheme of optical pumping by which a system of atoms coupled to harmonic oscillators is driven to an entangled steady state through the atomic spontaneous emission. It is shown that the optical pumping can be tailored so that the many-body atomic state asymptotically reaches an arbitrary stabilizer state regardless of the initial state. The proposed scheme can be suited to various physical systems. In particular, the ion-trap realization is well within current technology.     

Ultra-sensitive magnetometry based on free precession of nuclear spins

We discuss the design and performance of a very sensitive low-field magnetometer based on the detection of free spin precession of gaseous, nuclear polarized ³He or ¹²⁹Xe samples with a SQUID as magnetic flux detector. The device will be employed to control fluctuating magnetic fields and gradients in a new experiment searching for a permanent electric dipole moment of the neutron. Furthermore, with the detection of the free precession of co-located ³He/¹²⁹Xe nuclear spins it can be used as ultra-sensitive probe for non-magnetic spin interactions, since the magnetic dipole interaction (Zeeman-term) drops out. Characteristic spin precession times T₂ ^* of up to 60 h were measured. The achieved signal-to-noise ratio of more than 5000:1 leads to an expected sensitivity level (Cramer-Rao lower bound) of δB≈1 fT after an integration time of 220 s and of δB≈10^-4 fT after one day. By means of a co-located ³He/¹²⁹Xe magnetometer, noise sources inherent in the magnetometer could be investigated, showing that CRLB is fulfilled, at least down to δB≈10^-2 fT. The reason for such a high sensitivity is that free precessing ³He (¹²⁹Xe) nuclear spins are almost completely decoupled from the environment. Therefore, this type of magnetometer is particularly attractive for precision field measurements where long-term stability is required.     

Investigation of the Leggett-Garg inequality for precessing nuclear spins

We report experimental implementation of a protocol for testing the Leggett-Garg inequality (LGI) for nuclear spins precessing in an external magnetic field. The implementation involves certain controlled operations, performed in parallel on pairs of spin-1/2 nuclei (target and probe) from molecules of a nuclear magnetic resonance ensemble, which enable evaluation of temporal correlations from an LG string. Our experiment demonstrates violation of the LGI for time intervals between successive measurements, over which the effects of relaxation on the quantum state of target spin are negligible. Further, it is observed that the temporal correlations decay, and the same target spin appears to display macrorealistic behavior consistent with LGI.