Superposition Of Spin States Research Articles

Efficient Adiabatic Spin-Dependent Kicks in an Atom Interferometer.

We present an atom interferometry technique in which the beam splitter is split into two separate operations. A microwave pulse first creates a spin-state superposition, before optical adiabatic passage spatially separates the arms of that superposition. Despite using a thermal atom sample in a small (600 μm) interferometry beam, this procedure delivers an efficiency of 99% per ℏk of momentum separation. Utilizing this efficiency, we first demonstrate interferometry with up to 16ℏk momentum splitting and free-fall limited interrogation times. We then realize a single-source gradiometer, in which two interferometers measuring a relative phase originate from the same atomic wave function. Finally, we demonstrate a resonant interferometer with over 100 adiabatic passages, and thus over 400ℏk total momentum transferred.

The 0.7-Anomaly in Quantum Point Contact; Many-Body or Single-Electron Effect?

Apart from usual quantization steps on the ballistic conductance of quasi-one-dimensional conductor, an additional plateau-like feature appears at a fraction of about 0.7 below the first conductance step in GaAs-based quantum point contacts (QPCs). Despite a tremendous amount of research on this anomalous feature, its origin remains still unclear. Here, a unique model of this anomaly is proposed relying on fundamental principles of quantum mechanics. It is noticed that just after opening a quasi-1D conducting channel in the QPC a single electron travels the channel at a time, and such electron can be—in principle—observed. The act of observation destroys superposition of spin states, in which the electron otherwise exists, and this suppresses their quantum interference. It is shown that then the QPC-conductance is reduced by a factor of 0.74. “Visibility” of electron is enhanced if the electron spends some time in the channel due to resonant transmission. Electron’s resonance can also explain an unusual temperature behavior of the anomaly as well as its recently discovered feature: oscillatory modulation as a function of the channel length and electrostatic potential. A recipe for experimental verification of the model is given.

Features of chemical bonding within the Fe(NO)2 fragment for crystalline bis(thiosulfate) tetranitrosyl diiron tetramethylammonium salt as an example according to high-resolution X-ray diffraction data

Results of the study of the experimental charge density function for a crystal of the watersoluble {(СH3)4N+}2[Fe2(S2O3)2(NO)4] iron complex are presented. An inequality of two NO ligands at the same metal atom follows from the results of topological analysis of the charge density reconstructed particularly by using the “invariom” model. The inequality also reproduces that in the earlier studied complex Fe(SC2H3N3)(SC2H2N3)(NO)2]•0.5H2O, which is not caused by a superposition of spin states. Crystal packing analysis demonstrates the absence of a significant influence of solvation effects on the observed features of chemical bonding in the Fe(NO)2 fragment.

Entanglement of electronic subbands and coherent superposition of spin states in a Rashba nanoloop

The present work is concerned with an analysis of the entanglement between the electronic coherent superpositions of spin states and subbands in a quasi-one-dimensional Rashba nanoloop acted upon by a strong perpendicular magnetic field. We explicitly include the confining potential and the Rashba spin-orbit coupling into the Hamiltonian and then proceed to calculate the von Neumann entropy, a measure of entanglement, as a function of time. An analysis of the von Neumann entropy demonstrates that, as expected, the dynamics of entanglement strongly depends upon the initial state and electronic subband excitations. When the initial state is a pure one formed by a subband excitation and the z-component of spin states, the entanglement exhibits periodic oscillations with local minima (dips). On the other hand, when the initial state is formed by the subband states and a coherent superposition of spin states, the entanglement still periodically oscillates, exhibiting stronger correlations, along with elimination of the dips. Moreover, in the long run, the entanglement for the latter case undergoes the phenomenon of collapse-revivals. This behaviour is absent for the first case of the initial states. We also show that the degree of entanglement strongly depends upon the electronic subband excitations in both cases.

Observing the Phase Space Trajectory of an Entangled Matter Wave Packet

We observe the phase space trajectory of an entangled wave packet of a trapped ion with high precision. The application of a spin-dependent light force on a superposition of spin states allows for coherent splitting of the matter wave packet such that two distinct components in phase space emerge. We observe such motion with a precision of better than 9% of the wave packet extension in both momentum and position, corresponding to a 0.8 nm position resolution. We accurately study the effect of the initial ion temperature on the quantum entanglement dynamics. Furthermore, we map out the phonon distributions throughout the action of the displacement force. Our investigation shows corrections to simplified models of the system evolution. The precise knowledge of these dynamics may improve quantum gates for ion crystals and lead to entangled matter wave states with large displacements.

Spins in few-electron quantum dots

The canonical example of a quantum-mechanical two-level system is spin. The simplest picture of spin is a magnetic moment pointing up or down. The full quantum properties of spin become apparent in phenomena such as superpositions of spin states, entanglement among spins, and quantum measurements. Many of these phenomena have been observed in experiments performed on ensembles of particles with spin. Only in recent years have systems been realized in which individual electrons can be trapped and their quantum properties can be studied, thus avoiding unnecessary ensemble averaging. This review describes experiments performed with quantum dots, which are nanometer-scale boxes defined in a semiconductor host material. Quantum dots can hold a precise but tunable number of electron spins starting with 0, 1, 2, etc. Electrical contacts can be made for charge transport measurements and electrostatic gates can be used for controlling the dot potential. This system provides virtually full control over individual electrons. This new, enabling technology is stimulating research on individual spins. This review describes the physics of spins in quantum dots containing one or two electrons, from an experimentalist's viewpoint. Various methods for extracting spin properties from experiment are presented, restricted exclusively to electrical measurements. Furthermore, experimental techniques are discussed that allow for (1) the rotation of an electron spin into a superposition of up and down, (2) the measurement of the quantum state of an individual spin, and (3) the control of the interaction between two neighboring spins by the Heisenberg exchange interaction. Finally, the physics of the relevant relaxation and dephasing mechanisms is reviewed and experimental results are compared with theories for spin-orbit and hyperfine interactions. All these subjects are directly relevant for the fields of quantum information processing and spintronics with single spins (i.e., single spintronics).

Polarized neutron interferometry: A survey

A brief summarizing review is given of all those neutron interferometric experiments performed hitherto which explicitly use the spin- 1 2 particle properties of the neutron. It covers topics as the verification of the 4π-periodicity of spinors, the cooperative action of nuclear phase shift and spin-rotation on the neutron wave function, the demonstration of the quantum mechanical principles of fermion spin-state superposition and the more recent double resonance experiments where the two interfering beams propagate through spatially separated oscillatory magnetic fields. Finally a proposal will be presented also for a so-called “late-choice” experiment with polarized neutrons.

A proposal for an additional verification of the spin-state superposition law

We propose an experiment which could be performed by slightly modifying the experimental set up based on a neutron interferometer and used for the verification of the spin-state superposition law. The proposed experiment would complement the previous one, both from the point of view of the verification of the spin-state superposition law as well as from the point of view of the Wigner dilemma in the theory of measurement.

Direct verification of the quantum spin-state superposition law

Coherent superposition of two neutron waves, with mutually opposite polarisation direction, is used to demonstrate the quantum-mechanical properties of spin-state superposition for fermions.

Explicit experimental verification of quantum spin-state superposition

An interferometric experiment with polarized neutrons is described that demonstrates the basic principles of coherent spin-state superposition in an explicit way.

Electron spin polarisation effects of ISC into a superposition of spin states: Triplet excitons in biphenyl-tetracyanobenzene 1:1 complex

The behaviour in a magnetic field of the spin polarisation of triplet excitons in biphenyl-tetracyanobenzene is accounted for by the generation of the triplet state into a superposition of spin states. The molecular conformation in the excited singlet state is discussed.

Intersystem crossing into a superposition of spin states? The system tetramethylpyrazine in durene

We have performed an experiment which shows that upon excitation into S 1 the triplet in aza-aromatic molecules is created in a plane through the axis of the lone pair orbital on the nitrogen and the normal to the molecule. In molecules where this nπ plane does not coincide with the zero-field spin planes this leads to interesting consequences since the spin levels then share a common channel in the intersystem crossing process.