We discuss a feedback mechanism between electronic states in a double quantum dot and the underlying nuclear spin bath. We analyze two pumping cycles for which this feedback provides a force for the Overhauser fields of the two dots to either equilibrate or diverge. Which of these effects is favored depends on the g-factor and Overhauser coupling constant A of the material. The strength of the effect increases with A/V_x, where V_x is the exchange matrix element, and also increases as the external magnetic field B_{ext} decreases.
We use the nuclear spin coherence of $^{31}$P donors in $^{28}$Si to determine flip-flop rates of donor electron spins. Isotopically purified $^{28}$Si crystals minimize the number of $^{29}$Si flip-flops, and measurements at 1.7 K suppress electron spin relaxation. The crystals have donor concentrations ranging from $1.2\times10^{14}$ to $3.3\times10^{15}~\text{P/cm}^3$, allowing us to detect how electron flip-flop rates change with donor density. We also simulate how electron spin flip-flops can cause nuclear spin decoherence. We find that when these flip-flops are the primary cause of decoh…
We study the shot noise (nonequilibrium current fluctuation) associated with dynamic nuclear polarization in a nonequilibrium quantum wire (QW) fabricated in a two-dimensional electron gas. We observe that the spin-polarized conductance quantization of the QW in the integer quantum Hall regime collapses when the QW is voltage biased to be driven to nonequilibrium. By measuring the shot noise, we prove that the spin polarization of electrons in the QW is reduced to $\sim 0.7$ instead of unity as a result of electron-nuclear spin-flip scattering. The result is supported by Knight shift measureme…
We study the effect of contact hyperfine interaction on the nuclear spin diffusion coefficients in semiconductor quantum dots. The diffusion coefficients are calculated with both the method of moment and density matrix. We show that nuclear spin diffusion is strongly suppressed by the nonuniform hyperfine coupling resulting from the confined electron wavefunction. Our results agree with the observed suppression of nuclear spin diffusion in these structures in recent experiments, and clarify the degree of validity of the method of moment in an inhomogeneous system.
We numerically investigate the encoding and retrieval processes for a quantum memory realized in a semiconductor quantum dot, by focusing on the effect of inhomogeneously polarized nuclear spins whose polarization depends on the local hyperfine coupling strength. We find that the performance of the quantum memory is significantly improved by the inhomogeneous nuclear polarization, as compared to the homogeneous one. Moreover, the narrower the nuclear polarization distribution is, the better the performance of the quantum memory is. We ascribe the performance improvement to the full harnessing …
We propose an entanglement mechanism of nuclear spins in quantum dots driven by the electric current accompanied by the spin flip. This situation is relevant to a leakage current in spin-blocked regions where electrons cannot be transported unless their spins are flipped. The current gradually increases the components of larger total spin of nuclei. This correlation among the nuclear spins markedly enhances the spin-flip rate of electrons and hence the leakage current. The enhancement of the current is observable when the residence time of electrons in the quantum dots is shorter than the deph…
We find that detuning an optical pulse train from electronic transitions in quantum dots controls the direction of nuclear spin flips. The optical pulse train generates electron spins that precess about an applied magnetic field, with a spin component parallel to the field only for detuned pulses. This component leads to asymmetry in the nuclear spin flips, providing a way to produce a stable and precise value of the nuclear spin polarization. This effect is observed using two-color, time-resolved Faraday rotation and ellipticity.
We study a thermally induced spin flip of an electron spin located in a semiconductor quantum dot. This interesting effect arises from an intriguing interplay between the Zeeman coupling to an external magnetic field and the hyperfine interaction with the surrounding nuclear spins. By considering a minimal model, we explain the main mechanism driving this spin flip and analyze its dependence on the strength of the external magnetic field, the number of nuclear spins and the ratio of the electron and nuclear Zeeman energies, respectively. Finally we show, that this minimal model can be applied …
Spin-blockaded quantum dots provide a unique setting for studying nuclear-spin dynamics in a nanoscale system. Despite recent experimental progress, observing phase-sensitive phenomena in nuclear spin dynamics remains challenging. Here we point out that such a possibility opens up in the regime where hyperfine exchange directly competes with a purely electronic spin-flip mechanism such as the spin-orbital interaction. Interference between the two spin-flip processes, resulting from long-lived coherence of the nuclear-spin bath, modulates the electron-spin-flip rate, making it sensitive to the …
Optically-induced nuclear spin polarization in a fluorine-doped ZnSe epilayer is studied by time-resolved Kerr rotation using resonant excitation of donor-bound excitons. Excitation with helicity-modulated laser pulses results in a transverse nuclear spin polarization, which is detected as a change of the Larmor precession frequency of the donor-bound electron spins. The frequency shift in dependence on the transverse magnetic field exhibits a pronounced dispersion-like shape with resonances at the fields of nuclear magnetic resonance of the constituent zinc and selenium isotopes. It is studie…
Spectral diffusion arising from $^{29}$Si nuclear spin flip-flops, known to be a primary source of electron spin decoherence in silicon, is also predicted to limit the coherence times of neutral donor nuclear spins in silicon. Here, the impact of this mechanism on $^{31}$P nuclear spin coherence is measured as a function of $^{29}$Si concentration using X-band pulsed electron nuclear double resonance (ENDOR). The $^{31}$P nuclear spin echo decays show that decoherence is controlled by $^{29}$Si flip-flops resulting in both fast (exponential) and slow (non-exponential) spectral diffusion proces…
We demonstrate that the dynamic nuclear polarization (DNP) of phosphorus donors in silicon can be very effective in a magnetic field of 4.6 T and at temperatures below 1 K. The DNP occurs due to the Overhauser effect following a cross relaxation via the forbidden flip-flop or flip-flip transitions. Nuclear polarization values $P>0.98$ were reached after 20 min of pumping with 0.4 $μ$W of microwave power. We evaluated that the ratio of hyperfine state populations increased by three orders of magnitude after 2 hours of pumping, and an extremely pure nuclear spin system containing $<10^{-11}$ of …
We investigate the dynamic nuclear polarization process by frequently injecting polarized electron spins into a quantum dot. Due to the suppression of the direct dipolar and indirect electron-mediated nuclear spin interactions, by the frequently injected electron spins, the analytical predictions under the independent spin approximation agree well with quantum numerical simulations. Our results show that the acquired nuclear polarization is highly inhomogeneous, proportional to the square of the local electron-nuclear hyperfine interaction constant, if the injection frequency is high. Utilizin…
We have theoretically analyzed coherent nuclear-spin dynamics induced by electron transport through a quantum-dot spin valve. The hyperfine interaction between electron and nuclear spins in a quantum dot allows for the transfer of angular momentum from spin-polarized electrons injected from ferromagnetic or half-metal leads to the nuclear spin system under a finite voltage bias. Accounting for a local nuclear-spin dephasing process prevents the system from becoming stuck in collective dark states, allowing a large nuclear polarization to be built up in the long-time limit. After reaching a ste…
We study spin effects in the magneto-conductance of ballistic mesoscopic systems subject to inhomogeneous magnetic fields. We present a numerical approach to the spin-dependent Landauer conductance which generalizes recursive Green function techniques to the case with spin. Based on this method we address spin-flip effects in quantum transport of spin-polarized and -unpolarized electrons through quantum wires and various two-dimensional Aharonov-Bohm geometries. In particular, we investigate the range of validity of a spin switch mechanism recently found which allows for controlling spins indi…
Entanglement generation and detection are two of the most sought-after goals in the field of quantum control. Besides offering a means to probe some of the most peculiar and fundamental aspects of quantum mechanics, entanglement in many-body systems can be used as a tool to reduce fluctuations below the standard quantum limit. For spins, or spin-like systems, such a reduction of fluctuations can be realized with so-called squeezed states. Here we present a scheme for achieving coherent spin squeezing of nuclear spin states in few-electron quantum dots. This work represents a major shift from e…
As a nuclear spin model of scalable quantum register, the one-dimensional chain of the magnetic atoms with nuclear spins 1/2 substituting the basic atoms in the plate of nuclear spin free easy-axis 3D antiferromagnet is considered. It is formulated the generalized antiferromagnet Hamiltonian in spin-wave approximation (low temperatures) considering the inhomogeneous external magnetic field, which is directed along the easy axis normally to plane of the plate and has a constant gradient along the nuclear spin chain. Assuming a weak gradient, the asymptotic expression for coefficients of unitary…
Spin-blockaded electronic transport across a double quantum dot (DQD) system represents an important advancement in the area of spin-based quantum information. The basic mechanism underlying the blockade is the formation of a blocking triplet state. The bistability of the leakage current as a function of the applied magnetic field in this regime is believed to arise from the effect of nuclear Overhauser fields on spin-flip transitions between the blocking triplet and the conducting singlet states. The objective of this paper is to present the nuances of considering a two bath model on the expe…
Spin transport via electrons is typically plagued by Joule heating and short decay lengths due to spin-flip scattering. It is known that dissipationless spin currents can arise when using conventional superconducting contacts, yet this has only been experimentally demonstrated when using intricate magnetically inhomogeneous multilayers, or in extreme cases such as half-metals with interfacial magnetic disorder. Moreover, it is unknown how such spin supercurrents decay in the presence of spin-flip scattering. Here, we present a method for generating a spin supercurrent by using only a single ho…
A novel Thomas-Fermi (TF) approach to inhomogeneous superfluid Fermi-systems is presented and shown that it works well also in cases where the Local Density Approximation (LDA) breaks down. The novelty lies in the fact that the semiclassical approximation is applied to the pairing matrix elements not implying a local version of the chemical potential as with LDA. Applications will be given to the generic fact that if a fermionic superfluid in the BCS regime overflows from a narrow container into a much wider one, pairing is substantially reduced at the overflow point. Two examples pertinent to…
The central-spin problem, in which an electron spin interacts with a nuclear spin bath, is a widely studied model of quantum decoherence. Dynamic nuclear polarization (DNP) occurs in central spin systems when electronic angular momentum is transferred to nuclear spins and is exploited in spin-based quantum information processing for coherent electron and nuclear spin control. However, the mechanisms limiting DNP remain only partially understood. Here, we show that spin-orbit coupling quenches DNP in a GaAs double quantum dot, even though spin-orbit coupling in GaAs is weak. Using Landau-Zener …
We study Majorana bound states in a disordered chain of semiconductor quantum dots proximity-coupled to an s-wave superconductor. By calculating its topological quantum number, based on the scattering-matrix method and a tight-binding model, we can identify the topological property of such an inhomogeneous one-dimensional system. We study the robustness of Majorana bound states against disorder in both the spin-independent terms (including the chemical potential and the regular spin-conserving hopping) and the spin-dependent term, i.e., the spin-flip hopping due to the Rashba spin-orbit coupli…
We propose an entanglement mechanism of nuclear spins in quantum dots driven by the electric current. The current accompanied by the spin flip in quantum dots gradually increases components of larger total spin of nuclei. This entangled state drastically enhances the spin relaxation rate of electrons, which can be detected by measuring a leakage current in the spin-blockade region. This mechanism is not relevant in optical experiments which examine the spin relaxation of single excitations.
We theoretically investigate the influence of the fluctuating Overhauser field on the spin of an electron confined to a quantum dot (QD). The fluctuations arise from nuclear angular momentum being exchanged between different nuclei via the nuclear magnetic dipole coupling. We focus on the role of the nuclear electric quadrupole moments (QPMs), which generally cause a reduction in internuclear spin transfer efficiency in the presence of electric field gradients. The effects on the electron spin coherence time are studied by modeling an electron spin echo experiment. We find that the QPMs cause …
We report optical pumping of neutral quantum dots leading to nuclear spin alignment with direction insensitive to polarization and wavelength of light. Measurements of photoluminescence of both "dark" and "bright" excitons in single dots reveal that nuclear spin pumping occurs via a virtual spin-flip transition between these states accompanied by photon emission. The sign of the nuclear spin polarization is determined by asymmetry in the exciton energy spectrum, rather than by the sign of the exciton spin polarization.
Nuclear spins are polarized electrically in a breakdown regime of an odd-integer quantum Hall effect (QHE). Electron excitation to the upper Landau subband with the opposite spin polarity flips nuclear spins through the hyperfine interaction. The polarized nuclear spins reduce the spin-splitting energy and accelerate the QHE breakdown. The Knight shift of the nuclear spins is also measured by tuning electron density during the irradiation of radio-frequency magnetic fields.
The flip-flop qubit, encoded in the states with antiparallel donor-bound electron and donor nuclear spins in silicon, showcases long coherence times, good controllability, and, in contrast to other donor-spin-based schemes, long-distance coupling. Electron spin control near the interface, however, is likely to shorten the relaxation time by many orders of magnitude, reducing the overall qubit quality factor. Here, we theoretically study the multilevel system that is formed by the interacting electron and nuclear spins and derive analytical effective two-level Hamiltonians with and without peri…
Nuclear spin polarization is typically generated in GaAs quantum point contacts (QPCs) when an out-of-plane magnetic field gives rise to spin-polarized quantum Hall edge states, and a voltage bias drives transitions between the edge states via electron-nuclear flip-flop scattering. Here, we report a similar effect for QPCs in an in-plane magnetic field, where currents are spin polarized but edge states are not formed. The nuclear polarization gives rise to hysteresis in the d.c. transport characteristics, with relaxation timescales around 100 seconds. The dependence of anomalous QPC conductanc…
Hyperfine coupling of electron spins to nuclear spins is studied for a GaAs-based double quantum dot in the spin blockade regime where the electron conduction is mostly blocked by Pauli effect unless the electron spin state in the double dot is changed. A current flowing through the double dot shows time-dependent oscillations with a period of as long as 200 sec in a certain DC magnetic field range. The oscillatory behavior is significantly diminished by application of an AC magnetic field whose frequency can induce nuclear magnetic resonance for 71Ga and 69Ga, respectively. A possible nuclear…
We have fabricated and characterized the Landau level spin diode in GaAs two dimensional hole system. We used the hole Landau level spin diode to probe the hyperfine coupling between the hole and nuclear spins and found no detectable net nuclear polarization, indicating that hole-nuclear spin flip-flop processes are suppressed by at least three orders of magnitude compared to GaAs electron systems.
The spins of atoms and atom-like systems are among the most coherent objects in which to store quantum information. However, the need to address them using oscillating magnetic fields hinders their integration with quantum electronic devices. Here we circumvent this hurdle by operating a single-atom `flip-flop' qubit in silicon, where quantum information is encoded in the electron-nuclear states of a phosphorus donor. The qubit is controlled using local electric fields at microwave frequencies, produced within a metal-oxide-semiconductor device. The electrical drive is mediated by the modulati…
We formulate a generalized Chalker--Coddington network model that describes the effect of nuclear spins on the two-dimensional electron gas in the quantum Hall regime. We find exact analytical expression for the transmission coefficient of a charged particle through a saddle point potential in presence of perpendicular magnetic field that takes into account spin-flip processes. Spin-flip scattering creates a metallic state in a finite range around the critical energy of quantum Hall transition. As a result we find that the usual insulating phases with Hall conductance $σ_{xy}=0, 1, 2$ are sepa…
Nuclear quadrupole interaction extends the limits imposed by hyperfine interaction on the spin coherence of the electron and nuclei in a quantum dot. The strain-induced nuclear quadrupole interaction suppresses the nuclear spin flip and makes possible the zero-field dynamic nuclear polarization in self-organized InP/InGaP quantum dots. The direction of the effective nuclear magnetic field is fixed in space, thus quenching the magnetic depolarization of the electron spin in the quantum dot. The quadrupole interaction suppresses the zero-field electron spin decoherence also for the case of non-p…
We measure the dynamics of nuclear spins in a self-assembled quantum dot at a magnetic field of 5 Tesla and identify two distinct mechanisms responsible for the decay of the Overhauser field. We attribute a temperature-independent decay which lasts ~100 seconds to intra-dot diffusion induced by hyperfine-mediated indirect nuclear spin interaction. In addition, we observe a gate-voltage and temperature dependent decay stemming from co-tunneling mediated nuclear spin flip processes. By adjusting the gate-voltage and lowering the electron temperature to ~200 milliKelvin, we prolong the correspond…
In double quantum dots, the exchange interaction between two electron spins renormalizes the excitation energy of pair-flips in the nuclear spin bath, which in turn modifies the non-Markovian bath dynamics. As the energy renormalization varies with the Overhauser field mismatch between the quantum dots, the electron singlet-triplet decoherence resulting from the bath dynamics depends on sampling of nuclear spin states from an ensemble, leading to the transition from exponential decoherence in single-sample dynamics to power-law decay under ensemble averaging. In contrast, the decoherence of a …
Electron spin qubit coherence in quantum dots is ultimately limited by random nuclear spin bath fluctuations. Here we aim to eliminate this randomness by making spin bath evolution deterministic. We introduce spin bath control sequences, which systematically combine Hahn and solid echoes to suppress inhomogeneous broadening and nuclear-nuclear interactions. Experiments on self-assembled quantum dots show a five-fold increase in nuclear spin coherence. Numerical simulations show that these sequences can be used to suppress decoherence via qubit-qubit interaction in point defect and dopant spin …
We present the results of a detailed study of the thermally-assisted-resonant-tunneling relaxation rate of Mn12 acetate as a function of an external, longitudinal magnetic field and find that the data can be fit extremely well to a Lorentzian function. No hint of inhomogeneous broadening is found, even though some is expected from the Mn nuclear hyperfine interaction. This inconsistency implies that the tunneling mechanism cannot be described simply in terms of a random hyperfine field.
We present experimental data and associated theory for correlations in a series of experiments involving repeated Landau-Zener sweeps through the crossing point of a singlet state and a spin aligned triplet state in a GaAs double quantum dot containing two conduction electrons, which are loaded in the singlet state before each sweep, and the final spin is recorded after each sweep. The experiments reported here measure correlations on time scales from 4 $μ$s to 2 ms. When the magnetic field is aligned in a direction such that spin-orbit coupling cannot cause spin flips, the correlation spectru…
We present a mechanism to protect quantum information stored in an ensemble of nuclear spins in a semiconductor quantum dot. When the dot is charged the nuclei interact with the spin of the excess electron through the hyperfine coupling. If this coupling is made off-resonant it leads to an energy gap between the collective storage states and all other states. We show that the energy gap protects the quantum memory from local spin-flip and spin-dephasing noise. Effects of non-perfect initial spin polarization and inhomogeneous hyperfine coupling are discussed.
Many-body correlations can yield key insights into the nature of interacting systems; however, detecting them is often very challenging in many-particle physics, especially in nanoscale systems. Here, taking a phosphorus donor electron spin in a natural-abundance 29Si nuclear spin bath as our model system, we discover both theoretically and experimentally that many-body correlations in nanoscale nuclear spin baths produce identifiable signatures in the decoherence of the central spin under multiple-pulse dynamical decoupling control. We find that when the number of decoupling -pulses is odd, c…
We polarize nuclear spins in a GaAs double quantum dot by controlling two-electron spin states near the anti-crossing 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 volt…
Recently an ensemble of nuclear spins in a quantum dot have been proposed as a long-lived quantum memory. A quantum state of an electron spin in the dot can be faithfully transfered into nuclear spins through controlled hyperfine coupling. Here we study the decoherence of this memory due to nuclear spin dipolar coupling and inhomogeneous hyperfine interaction during the {\it storage} period. We calculated the maximum fidelity of writing, storing and reading operations. Our results show that nuclear spin dynamics can severely limits the performance of the proposed device for quantum information…
We demonstrate fast universal electrical spin manipulation with inhomogeneous magnetic fields. With fast Rabi frequency up to 127 MHz, we leave the conventional regime of strong nuclear-spin influence and observe a spin-flip fidelity > 96%, a distinct chevron Rabi pattern in the spectral-time domain, and spin resonance linewidth limited by the Rabi frequency, not by the dephasing rate. In addition, we establish fast z-rotations up to 54 MHz by directly controlling the spin phase. Our findings will significantly facilitate tomography and error correction with electron spins in quantum dots.
Isolated electron spins in semiconductor nanostructures are promising qubit candidates for a solid state quantum computer, There have seen truly impressive experimental progresses in the study of single spins in the past two years. In this paper we analytically solve the {\it Non-Markovian} single electron spin dynamics due to inhomogeneous hyperfine couplings with surrounding nuclei in a quantum dot. We use the equation-of-motion method assisted with a large field expansion in a full quantum mechanical treatment. We recover the exact solution for fully polarized nuclei. By considering virtual…
We study the effect of bias voltage on the nuclear spin polarization of a ballistic wire, which contains electrons and nuclei interacting via hyperfine interaction. In equilibrium, the localized nuclear spins are helically polarized due to the electron-mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. Focusing here on non-equilibrium, we find that an applied bias voltage induces a uniform polarization, from both helically polarized and unpolarized spins available for spin flips. Once a macroscopic uniform polarization in the nuclei is established, the nuclear spin helix rotates with f…
The control of nuclear spins in quantum dots is essential to explore their many-body dynamics and exploit their prospects for quantum information processing. We present a unique combination of dynamic nuclear spin polarization and electric-dipole-induced spin resonance in an electrostatically defined double quantum dot (DQD) exposed to the strongly inhomogeneous field of two on-chip nanomagnets. Our experiments provide direct and unrivaled access to the nuclear spin polarization distribution and allow us to establish and characterize multiple fixed points. Further, we demonstrate polarization …
We theoretically investigate the spin dynamics of a heavy hole confined to an unstrained III-V semiconductor quantum dot and interacting with a narrowed nuclear-spin bath. We show that band hybridization leads to an exponential decay of hole-spin superpositions due to hyperfine-mediated nuclear pair flips, and that the accordant single-hole-spin decoherence time T2 can be tuned over many orders of magnitude by changing external parameters. In particular, we show that, under experimentally accessible conditions, it is possible to suppress hyperfine-mediated nuclear-pair-flip processes so strong…
The electron-nuclei (hyperfine) interaction is central to spin qubits in solid state systems. It can be a severe decoherence source but also allows dynamic access to the nuclear spin states. We study a double quantum dot exposed to an on-chip single-domain nanomagnet and show that its inhomogeneous magnetic field crucially modifies the complex nuclear spin dynamics such that the Overhauser field tends to compensate external magnetic fields. This turns out to be beneficial for polarizing the nuclear spin ensemble. We reach a nuclear spin polarization of ~50%, unrivaled in lateral dots, and expl…
Coherent spin states in semiconductor quantum dots offer promise as electrically controllable quantum bits (qubits) with scalable fabrication. For few-electron quantum dots made from gallium arsenide (GaAs), fluctuating nuclear spins in the host lattice are the dominant source of spin decoherence. We report a method of preparing the nuclear spin environment that suppresses the relevant component of nuclear spin fluctuations below its equilibrium value by a factor of ~ 70, extending the inhomogeneous dephasing time for the two-electron spin state beyond 1 microsecond. The nuclear state can be r…
The class of Ruddlesden-Popper type (PEA)$_2$PbI$_4$ perovskites comprises two-dimensional (2D) structures whose optical properties are determined by excitons with a large binding energy of about 260 meV. It complements the family of other 2D semiconductor materials by having the band structure typical for lead halide perovskites, that can be considered as inverted compared to conventional III-V and II-VI semiconductors. Accordingly, novel spin phenomena can be expected for them. Spin-flip Raman scattering is used here to measure the Zeeman splitting of electrons and holes in a magnetic field …
It is demonstrated that the now well-established `flip-flop' mechanism of spin exchange between electrons and nuclei in the quantum Hall effect can be reversed. We use a sample geometry which utilizes separately contacted edge states to establish a local nuclear spin polarization --close to the maximum value achievable-- by driving a current between electron states of different spin orientation. When the externally applied current is switched off, the sample exhibits an output voltage of up to a few tenths of a meV, which decays with a time constant typical for the nuclear spin relaxation. The…
We study the electron spin decoherence of encapsulated atomic hydrogen in octasilsesquioxane cages induced by the 1H and 29Si nuclear spin bath. By applying the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence we significantly suppress the low-frequency noise due to nuclear spin flip-flops up to the point where a maximum T2 = 56 us is observed. Moreover, dynamical decoupling with the CPMG sequence reveals the existence of two sources of high-frequency noise: first, a fluctuating magnetic field with the proton Larmor frequency, equivalent to classical magnetic field noise imposed by the 1H nucle…
Control of electron spin decoherence in contact with a mesoscopic bath of many interacting nuclear spins in an InAs quantum dot is studied by solving the coupled quantum dynamics. The nuclear spin bath, because of its bifurcated evolution predicated on the electron spin up or down state, measures the which-state information of the electron spin and hence diminishes its coherence. The many-body dynamics of nuclear spin bath is solved with a pair-correlation approximation. In the relevant timescale, nuclear pair-wise flip-flops, as elementary excitations in the mesoscopic bath, can be mapped int…
A highly asymmetric dynamic nuclear spin pumping is observed in a single self assembled InGaAs quantum dot subject to resonant optical pumping of the neutral exciton transition leading to a large maximum polarization of 54%. This dynamic nuclear polarization is found to be much stronger following pumping of the higher energy Zeeman state. Time-resolved measurements allow us to directly monitor the buildup of the nuclear spin polarization in real time and to quantitatively study the dynamics of the process. A strong dependence of the observed dynamic nuclear polarization on the applied magnetic…
The Carr-Purcell Meiboom-Gill (CPMG) experiment has been used for decades to measure nuclear-spin transverse ($T_2$) relaxation times. In the presence of magnetic-field inhomogeneities, the limit of short interpulse spacings yields the intrinsic $T_2$ time. Here we show that the signal decay in such experiments exhibits fundamentally different behaviors between liquids and gases. In gases, CPMG unexpectedly fails to eliminate the inhomogeneous broadening due to the non-Fickian nature of the motional averaging.
A pure spin current is predicted to occur when an external magnetic field and a linearly inhomogeneous spin-only field are appropriately aligned. Under these conditions (such as originate from nuclear contact hyperfine fields that do not affect orbital motion) a linear, spin-dependent dispersion for free electrons emerges from the Landau Hamiltonian. The result is that spins of opposite orientation flow in opposite directions giving rise to a pure spin current. A classical model of the spin and charge dynamics reveals intuitive aspects of the full quantum mechanical solution. We propose optica…
We study the quantum dynamics of a single mode/particle interacting inhomogeneously with a large number of particles and introduce an effective approach to find the accessible Hilbert space where the dynamics takes place. Two relevant examples are given: the inhomogeneous Tavis-Cummings model (e.g., N atomic qubits coupled to a single cavity mode, or to a motional mode in trapped ions) and the inhomogeneous coupling of an electron spin to N nuclear spins in a quantum dot.
We theoretically consider solid state nuclear spins in a semiconductor nanostructure environment as long-lived, high-fidelity quantum memory. In particular, we calculate, in the limit of a strong applied magnetic field, the fidelity versus time of P donor nuclear spins in random bath environments of Si and GaAs, and the lifetime of excited intrinsic spins in polarized Si and GaAs environments. In the former situation, the nuclear spin dephases due to spectral diffusion induced by the dipolar interaction among nuclei in the bath. We calculate the decay of nuclear spin quantum memory in the cont…
