Foundations of Quantum Mechanics in the Light of New Technology

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• CONTENTS

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We discuss the operational definition of decoherence in various solid state systems. In particular, we review, in the context of spin-based solid state quantum computation, the introduction of T₁ and T₂ to describe decoherence in a two level system. We provide a perspective on recent experiments involving the manipulation of spin coherence in semiconductors, and discuss specific decoherence and dephasing issues in electron spin-based quantum dot quantum computer architectures.¹

Quantum coherent properties of an artificial two-level system in a Josephson-junction circuit were studied experimentally. Responses of the quantum state to a sequence of gate-voltage pulses were measured in time-ensemble measurements, and free-induction decay and "charge echo" signals were obtained. It was found that the decoherence of the two-level system is dominated by the dephasing due to low-frequency charge fluctuations.

We discuss parameters that quantify efficiency of devices suggested as detectors of the quantum state of Josephson-junction charge and flux qubits. These parameters are used to compare various detectors. We estimate the dephasing rate by a turned-off detector and show that for a dc-SQUID and a SET it vanishes very fast at low temperatures, suggesting their possible use in experiments.

We have fabricated coupled double quantum dots, each with a diameter of around 40nm, using trench isolation. A range of transport measurements have been performed on various double quantum dot systems at temperatures down to 20 mK. Peak splitting in the Coulomb oscillations has been clearly observed at 4.2 K due to the capacitative coupling of the two dots. An advantage of the trench isolation approach to fabricating coupled quantum dots is that secondary structures, such as single electron electrometers can be fabricated close to the dots quite easily. We present measurements showing that an electrometer 50 nm away from the double dot can detect a single hole added to one of the dots.

A strategy to obtain a quantum bit (qubit) having a longer coherence time is discussed. If the main origin of the decoherence is the weak and continuous interaction with a macroscopic bath, we can make one qubit decoherence time longer with plural two-level systems and the appropriate interaction between them. The "appropriate" interactions can be chosen from the policies of usual Quantum Error Correction Codes (QECC). The redundancy introduced by using plural two-level systems gives a Hilbert space that is much larger than that necessary for a single qubit. The two states of lower energies are assigned as the two states of the qubit ("coding space"). The interaction is chosen so as to bring the other states to higher energies. The "errors" correspond to the escapes from the lower energy states to higher energy states. Then the escapes are strongly suppressed by the interaction energy, which results in a longer coherence time when the plural two-level systems are used as a single quantum bit.

The dephasing of a qubit coupled with a point-contact detector is theoretically studied. We calculate the time evolution of the reduced density matrix of qubit by using the perturbation expansion. We show that the dephasing rate is proportional to the temperature at zero bias-voltage, while it is proportional to the bias-voltage when the bias-voltage is large. We also evaluate the dephasing rate by using the real time renormalization group method and show that the higher order processes of the particle-hole excitation enhances dephasing of qubit.

Coulomb blockade effects in capacitively coupled quantum dots can be utilized for constructing an N-qubit system with antiferromagnetic Ising interactions. Starting from the tunneling Hamiltonian, we theoretically show that the Hamiltonian for a weakly coupled quantum-dot array is reduced to that for nuclear magnetic resonance (NMR) spectroscopy. Quantum operations are carried out by applying only electrical pulse sequences. A possible measurement scheme in an N-qubit system is quantitatively discussed.

This paper addresses a quantum channel identification problem: given a parametric family {Γ_θ}_θ of quantum channels, find the best strategy of estimating the true value of the parameter θ. As illustrative examples, we present statistical estimation of a depolarizing channel and a unitary channel acting on a two-level quantum system.

Entanglement swapping in its most fundamental form is the teleportation of a quantum state that is itself entangled to another quantum system. This connection to teleportation provides us with a tool to demonstrate and verify the non-local nature of the teleportation procedure. In this work the non-locality is experimentally confirmed by a violation of Bell's inequality using teleported entanglement.

Recent improvements in the ability to generate entangled states of trapped ions have permitted the application of entanglement to several problems. First, entangled ions were used to demonstrate a violation of Bell's inequality, marking the first such demonstration with high detection efficiency. Second, a pair of ions was used to store a quantum bit of information in a decoherence-free subspace, allowing the qubit to resist environmental decoherence. Third, an entangled state was used to demonstrate improved spectroscopic resolution. These results are summarized here.

Quantum entanglement and a teleportation experiment of continuous-variables are briefly reviewed, and the plans of our teleportation and related research are presented.

We discuss deterministic extraction of Bell pairs from a finite number of partially entangled pairs by using local operations and classical communication. The maximum number of Bell pairs extracted with certainty is derived. It is shown that the optimal deterministic entanglement concentration can be performed by successive two-pair collective manipulations instead of manipulating all entangled pairs at once. Finally, this scheme reveals an entanglement measure that quantifies the exactly distillable entanglement.

We analyze the loss of fidelity in continuous variable teleportation due to non-maximal entanglement. It is shown that the quantum state distortions correspond to the measurement back-action of a field amplitude measurement

We investigate the changes to a single photon state caused by the non-maximal entanglement in continuous variable quantum teleportation. It is shown that the teleportation measurement introduces field coherence in the output.

Retrievable, usable quantum information is transferred in a scheme which, in a striking contrast to quantum teleportation, requires no external channel and does not involve the transfer of a quantum state from one subsystem to another. The process uses a three particle system in which the information is transmitted from 3 to 1 even though 1 and 3 never interact but are both entangled with 2. That this scheme implies a previously unexplored form of quantum nonlocality is also demonstrated.

We compute the time evolving reflection probability of a wave packet incident on a potential barrier while its height is reduced. A time interval is found during which the reflection probability is larger (superarrivals) compared to the unperturbed case. To explain this essestially nonclassical effect, a wave function acts as a "field" through which a disturbance resulting from the boundary condition being perturbed propagates at a speed depending upon the rate of reducing the barrier height. This phenomenon suggests a hitherto unexplored application for secure information transfer.

We investigate the condition in order to perform a high fidelity teleportation from experimental viewpoint. Our scheme referred to the analysis suggested by P. Kok et al [4]. and added our ideas to their analysis. As a result, it is possible to demonstrate an experiment of high fidelity teleportation without receiver's feedback using a visible light photon counter and high visibility entangled photon sources.

Recently, seemingly conflicting results were obtained for the efficiency of the transmission of classical signals over quantum channels. In one study, it was shown that entangled states can improve the transmission of classical information in a two-Pauli channel. However, in another study, Bruss et al showed that entanglement cannot possibly improve transmission of classical information in a depolarizing channel. By studying the Holevo information in a general quantum channel, we analyze the efficiency of such transmission of classical states.

We review experiments in which entanglement is engineered and manipulated in a system made of circular Rydberg atoms interacting with microwave photons in a superconducting cavity.

Current quantum cryptography systems are limited by the Poissonian photon statistics of a standard light source: a security loophole is opened up by the possibility of multiple-photon pulses. By replacing the source with a single-photon emitter, transmission rates of secure information can be improved. A single photon source is also essential to implement a linear optics quantum computer. We have investigated the use of single self-assembled InAs/GaAs quantum dots as such single-photon sources, and have seen a hundred-fold reduction in the multi-photon probability as compared to Poissonian pulses. An extension of our experiment should also allow for the generation of triggered, polarizationentangled photon pairs.

We report a new method to generate parametric fluorescence into two small spots. Because the generated twins are concentrated to the spots, we observed a high single count rate and coincidence count rate per pump power. We also observed that the ratio of coincidence count rate to single count rate was 80%, where the loss at filters and the quantum efficiency of detectors were compensated. We also investigated the photon number distribution of the 'single photon pulses' using gated parametric down conversion with several types of detectors including our high-quantum efficiency multi-photon detectors. We found that the unity fraction of the single photon pulses is achievable using a CW laser pumped PDC, where the fraction of the two photon pulses are small enough.

We propose and demonstrate the measurement of photonic de Broglie wavelength of an entangled photon pair state (a biphoton) generated by parametric down-conversion utilizing a normal Mach-Zehnder interferometer. The observed interference manifests the concept of the photonic de Broglie wavelength.

We investigate the non-resonant interaction between the high-density excitons in a quantum well and a single mode cavity field. An analytical expression of the physical spectrum of the excitons is obtained. We study the spectral properties of the excitons which are initially prepared in the number states by the resonant femtosecond pulse pumping experiment. We numerically analyze the physical spectrum and mainly discuss the detuning effect on it.

We discuss the preparation of qubit states of the form C₀|0〉 + C₁|1〉 using quantum scissors device, which exploits projection synthesis and prepares the desired state by truncating a coherent light. A feasible experimental scheme is proposed and optimized to obtain the desired qubit state with the highest attainable fidelity by tuning the intensity of the input coherent light.

When a superfluid medium is forced into rotation, Quantum Mechanics imposes strong constraints on its velocity field. The superfluid is set into motion through the nucleation of lines of singularity or vortex lines, where the density vanishes and around which the circulation of the velocity is quantized. We report the observation of such vortices in a gaseous Bose-Einstein condensate of ⁸⁷Rb atoms, which is stirred using a rotating laser beam. We characterize the phase and the angular momentum of a single vortex, and we discuss the maximal number of vortices which can be nucleated in this system. Finally we address the problem of the critical rotation frequency at which the first vortex appears.

We experimentally studied vortex excitations in a dilute gas Bose-Einstein condensate. First, the phase profile of a vortex excitation was observed using an interferometric technique. Vortices were created by moving a laser beam through a condensate, and they were observed as dislocations in the interference fringes formed by the stirred condensate and a second unperturbed condensate. Secondly, we have observed the formation of highly ordered vortex lattices in a rotating Bose-condensed gas. These triangular lattices contained over 100 vortices with lifetimes of several seconds. Individual vortices persisted up to 40 seconds. The gaseous Bose-Einstein condensates can be a model system for the study of vortex matter.

The dynamics of collapsing and exploding trapped Bose-Einstein condensates triggered by switching the sign of the interaction from repulsive to attractive is studied by numerically integrating the Gross-Pitaevskii equation with atomic loss. Our simulations reproduce some experimental results, and predict new phenomena.

With modified Bogoliubov replacement , we study the fluctuation in number of the trapped condensate particles around the average N₀ » 1, taking interaction among them into account. A criterion is obtained for the stability of the fluctuation, thus also for the Bose-Einstein condensation. In the case of weak interaction, the condensation takes place if the interaction is in a sense repulsive, and probably the condensation does not take place if attractive.

Cohent reflection of a cold atomic beam with macroscopic dimension from a solid surface caused by attractive van der Waals potential has been studied experimentally. The reflectivity of an ultra-cold metastable neon atomic beam from a silicon surface was measured as a function of normal incident velocity. The reflectivity was shown to improve drastically when the flat surface was changed to a grating structure with narrow ridges. As an example of practical applications the atomic beam was manipulated by a reflective hologram encoded on the silicon surface.

We have observed the Aharonov-Casher effect using a dc Stark-free atom interferometer whose wavepackets are in the different magnetic sublevels. A novel atom interferometer using a thermal Ca atomic beam is comprised of two separated light fields, each of which consists of two laser beams with differnt polarizations and frequencies, and yields Ramsey fringes as a function of detuning of the Zeeman shift frequency. Quantitative dependence of the phase shift on the electric field agree with theoretical prediction of AC effect within 5%.

The interactions of a number of co-propagating stimulated Raman pulses with trapped atoms can make the Ramsey type multiple atom interferometer, as the effective wave number vector k_eff of Raman beams is nearly zero. The interference fringes were obtained by shifting the atomic wave phase using the perturbation of weak magnetic pulse fields applied between the adjacent Raman pulses.

We consider an experimental set-up where two-level atoms are streamed through a microcavity in such a manner that at most one atom is present inside the cavity at any instant of time. The interaction of a single atom with the cavity photons leaves an imprint on the steady-state cavity density operator. The wave function of the next atom that passes through the cavity gets entangled with the cavity photons and subsequent secondary correlations develop between two or more atoms in this way. After leaving the cavity the atoms pass through an electromagnetic field that is tuned to give a π/2 pulse to the atoms with varied phase for different atoms. The atoms are then detected in either of their upper or lower states. The secondary correlations between two or more subsequent atoms can be exploited to formulate Bell-type inequalities for their detection probabilities. We investigate the effects of decoherence on atomic entanglement brought about by both atomic decay and cavity dissipation through interaction with their respective reservoirs. We show by using realistic models for the micromaser as well as the microlaser that effects of decoherence on the Bell sum can be experimentally monitored and observed in a controlled fashion.

The Heisenberg picture approach of the continuous-wave atom laser with an output coupler was investigated theoretically. The mechanism of the output coupler will be considered without using the so-called Born-Markov Approximation. The quantum theoretical treatment of the continuous-wave atom laser in this paper will demonstrate that it is a self-sustained oscillator.

We describe what we learnt these last years on quantum reversal of large magnetic moments, using mainly conventional SQUID or micro-SQUID magnetometry. Beside the case of ferromagnetic nanoparticles with 10³ – 10⁵ atoms (e.g. Co, Ni, Fe, Ferrites), most fruitful systems appeared to be ensembles of magnetic molecules. These molecules, generally arranged in single crystals, carry relatively small magnetic moments (S = 10 in Mn₁₂-ac and Fe₈). They are sufficiently apart from each other not to be coupled by exchange interactions. The ground multiplet is split over an energy barrier of tens of kelvin (≈ 67 K for Mn₁₂) by a strong local crystal field, leading to an Ising-type ground-state. Only weak inter-molecular dipolar interactions are present, as well as intra-molecular interactions, such as hyperfine interactions. Quantum properties of molecule spins are crucially dependent on their magnetic environment of electronic and nuclear spins (the spin bath). Energy fluctuations of the spin bath of about 0.1 K are important, especially at very low temperatures. In particular, they are much larger than the ground-state tunnel splitting of large-spin molecules in low applied fields, of about 10^-8 K or even less (such a low value is due to the presence of large energy barriers). Theoretical predictions are experimentally checked for tunneling effects in the presence of non-equilibrated or equilibrated spin-energy distribution. It is also shown that the phonon-bath plays no role in low field, except when the temperature approaches the cross-over temperature to the thermal activation regime. In fact, spin–phonon transitions can play a role only if the tunnel splitting is not too small in comparison with k_BT. This is the case both for large-spin molecules in a large magnetic field (e.g. Mn₁₂-ac in a few tesla) and for low-spin molecules, as shown with the study of the molecule V₁₅ (Hilbert space dimension as large as 2¹⁵ and spin 1/2). We also give our latest results on the extension of these studies beyond molecular magnetism. Single-ion slow quantum relaxation is observed in rare-earth Ho³⁺ ions highly diluted in an insulating matrix LiYF₄. This relaxation is due to the coherent tunneling of individual Ho³⁺ spins strongly coupled to their nuclear spins, leading to electro-nuclear entangled states at avoided level crossings. In fact tunneling of the spin system is induced by the hyperfine coupling. Together with the important role of the "spin bath", the roles of cross–spin and spin–phonon relaxations are also considered. All these results confirm the emergence of a new field of research: "mesoscopic magnetism".

Small contact structure between two NiFe wires was fabricated by an electron beam lithography and a lift-off method and the magnetoresistance was measured. The magnetization switching process was artificially controlled by engineering the sample geometry, and a single domain wall was trapped in the small contact area. The contribution of the domain wall to the resistance was negative, which can be attributed to the anisotropic magnetoresistance. At low temperatures, the resistance increases as the temperature decreases, and the resistance with the domain wall shows almost the same temperature dependence as that without the domain wall.

We show that the quantum dynamics of a domain wall in a quasi-one dimensional mesoscopic ferromagnet is equivalent to that of a Bloch particle within a one-band basis. Collective degrees of freedom of the domain wall, namely the center position and the chirality, correspond to position coordinate and periodic momentum operators, respectively, which are mutually canonically conjugate. Due to the periodicity of the momentum of the Bloch particle, it is shown that position coordinate of the domain wall is quantized in unit of a/2S, where a is the lattice constant of the spin chain and S is the magnitude of spin. Various dispersion relations of energy-band for the domain wall are derived from transverse anisotropy or external magnetic field perpendicular to the easy axis. As an expected phenomenon, under a uniform magnetic field, the domain wall oscillates along the chain. This corresponds to the Bloch oscillation. Using the spin-coherent state path integral on the basis of stationary action approximation, we show how this oscillation manifests itself in the transition probability.

On the basis of the density-functional theory, we calculated the interfacial magnetism of the slab model in ferromagnetic tunneling junction Co/Al-oxide/Co. We found that in the Al/Co interface the interfacial Al layer exhibits a positive spin polarization (SP) and that both the sign and value of SP in the Al layer are in agreement with the experimental data. This result suggests that in the junction Co/Al-oxide/Co, the tunneling of s-character electron in Al is favored. Based on a model exhibiting the positive SP at the interfacial Al layer, we calculated the tunneling conductance. It was found that a change from the tunneling magnetoresistance (TMR) scheme to the giant MR (GMR) scheme occurs only for an ultra-thin film insulator.

We present results of spin density functional calculations for the electronic structure of small (N ~ 30 — 50), GaAs-AlGaAs quantum dots in a transverse magnetic field. We demonstrate that two types of reconstruction of the dot ground state, for N even, can occur when the single particle levels near the Fermi surface cross. In the first case, typically at lower B, the dot reconstructs anti-ferromagnetically, with the total spin remaining zero. In the latter case, a more standard Hund coupling occurs and the dot reconstruction results in the total dot spin increasing to unity.

Single-electron transistor (SET) operation of a quantum dot (QD), fabricated in a GaAs/Al_xGa₁-_xAs heterostructure crystal is demonstrated to serve as an extremely high sensitivity detector of far-infrared (FIR). When the single QD is placed in a high magnetic field, the resonant conductance through the SET switches on (off) upon the excitation of just one electron to a higher Landau level inside the QD, thereby enabling us to detect individual events of FIR-photon absorption (λ = 0.17 - 0.22 mm). Additionally, we demonstrate that the SET consisting of two parallel QDs operates as a high sensitivity detector of FIR approaching the single-photon counting level in the absence of magnetic fields.

Josephson junctions hybridized with an array of superconducting islands are studied. Assuming large intra-island Coulomb interaction, a Josephson critical current are calculated as functions of voltages of two leads based on a hard-core Boson model, which corresponds to the quantum XY model with boundary fields. It is shown that the boundary Josephson coupling affects the number of resonant peaks and phase-current relation.

We studied one- (1D) and two-dimensional (2D) arrays of small Josephson junctions in which each junction was shunted by a normal metal resistor. For the arrays with the charging energy E_C comparable to the Josephson coupling energy E_J, we find a crossover from insulating to superconducting behavior as the shunt resistance R_S decreases. The critical value of R_S is close to dR_Q (d: dimensionality, R_Q≡h/4e² = 6.45 kΩ), which is consistent with theories of the dissipation-driven phase transition. We discuss the difference between the phase diagrams in the E_J/E_C – R_Q/R_S plane for 1D and 2D arrays.

Development of an UHV electron microscope and results on gold QPC are summarized. Quantum pointcontact(QPC) of gold was made by an STM installed in the UHV electron microsope to study conductance quantization of gold QPCs in relation with their structure simultaneously. The conductance changed in steps in the unit of 2e²/h in case that gold nanowires were formed at the QPC, where e is the electron charge and h, Planck constant. Long gold nanowires were found to have chiral structure with gold atomic rows which coil around the axis of the wire. A single strand of gold atoms synthesized was found to be stable even when their atomic bond distance reaches to 0.4nm.

Atomic-scale structures on a hydrogen-terminated Si(100)-2x1-H surface are investigated by scanning tunneling microscopy/spectroscopy (STM/STS). Relaxation of atomic-scale dangling-bond (DB) structures can be well described by a Jahn-Teller distortion. When Ga atoms were deposited on the surface at 100 K, we observed a characteristic one-dimensional structure (Ga-bar structure). The Ga-bar structure is an STM image of a rapidly-migrating Ga atom confined in a linear potential well.

We report a new form of exposure control method of scanning-probe nanolithography. It is a hybrid current-voltage control, which is a combination of the conventional constant-voltage and constant-current control methods. We have used the method to fabricate complex resist patterns with a dot-resolution of 60 nm using a 50-nm thick resist film. We found that the new control method is particularly suitable for drawing dot patterns.

We study the spectral properties of one-dimensional quantum wire with a single defect. We reveal the existence of the non-trivial topological structures in the spectral space of the system, which are behind the exotic quantum phenomena that have lately been found in the system.

Magnetotransport phenomena in unidirectional lateral superlattice (LSL) systems of short and ultrashort periods have been studied. Commensurability oscillation of magnetoresitance has been observed in the composite fermion (CF) regime near the Landau level filling ν = 3/2, which is found to be consistent with the one expected for fully spin-polarized CFs subjected to spatial modulation of effective magnetic field. Quenching of spin gap at odd integer fillings has been observed in narrow quantum well samples with ultrashort period LSL potential. This is attributed to combined effect of suppression of exchange term due to the modulation potential and the quantum-well-thickness dependence of the g-factor. Reproducible quasipeirodic fluctuation of magnettoresistance has been observed in one of these samples. The phenomenon resembles the so-called universal conductance fluctuation in mesoscopic systems, although the present sample is of macroscopic size.

We theoretically investigate the effect of the interband scattering on the transport through atomic-size point contacts of multi-band metals. The conductance is evaluated, taking account of the atomic structures by tight-binding models. In the case of single s band, the conductance is almost quantized in units of 2e²/h. In the multi-band model with broad and narrow s bands, the conductance quantization is also observed. With broad s and narrow p bands, on the other hand, the conductance is strongly suppressed with an increase in the interband scattering. This is because the hybridization between s and p orbitals is significantly different in the leads and in the contact region, which forbids the smooth transport of electrons between them.

The tunnel magnetoresistance (TMR) of ferromagnetic single-electron transistors made of Ni and Co has been measured. We found that the TMR is enhanced by a factor about 10 between 4.2 K and 25mK in almost all devices including those whose junction resistance is much higher than the quantum resistance. This indicates that the theories based on the higher-order tunneling are not sufficient to explain the enhancement.

We found that the probe-arrangement greatly affects quantum coherence in mesoscopic samples. The present result poses important questions on pictures on quantum decoherence and intrinsic decoherence at absolute zero.

B-factory experiments in search for CP violation in neutral B-meson decays and long baseline neutrino oscillation experiments are among the most important particle physics experiments either ongoing or in preparation. Quantum-mechanical oscillation phenomena play the central role in these experiments. Very recently, large CP violation has been discovered in neutral B-meson decays by the Belle experiment at KEK and the BaBar experiment at SLAC. The first long baseline neutrino oscillation experiment, K2K, is being conducted between KEK and Super-Kamiokande at Kamioka over a distance of 250 km. Preliminary results are in favor of muon-neutrino oscillation. This article focuses on the experiments in Japan, Belle and K2K.

Quantized magnetic vortices inside high-T_c superconductors were observed using the phase shift of electron waves passing through the vortex magnetic fields with our recently developed 1-MV field-emission electron microscope, which has the brightest electron beam yet attained. We were able to investigate the microscopic pinning mechanism of columnar defects, which are produced by irradiation of high-energy [heavy ions] and are regarded as one of the most effective pinning centers in high-T_c superconnductors. In particular, we determined under which conditions individual vortex lines are trapped along tilted columnar defects in Bi-2212 thin films, which results in a strong pinning effect.

Femtosecond laser frequency comb techniques are vastly simplifying the art of measuring the frequency of light. A single mode-locked femtosecond laser is now sufficient to synthesize hundreds of thousands of evenly spaced spectral lines, spanning much of the visible and near infrared region. The mode frequencies are absolutely known in terms of the pulse repetition rate and the carrier-envelope phase slippage rate, which are both accessible to radio frequency counters. Such a universal optical frequency comb synthesizer can serve as a clockwork in atomic clocks, based on atoms, ions or molecules oscillating at optical frequencies.

A gravitational wave (GW) is a rippie of the space-time propagating with the speed of light; its existence was predicted by Einstein in his theory of general relativity. It is now very promising that huge laser interferometers which are being constructed all over the world can really detect GWs coming from catastrophic astrophysical events such as coalescence of binary neutron stars and supernova; we can expect the detection within several years. In Japan, there is a project, called TAMA, in which a 300-m interferometer has been constructed and is being improved. The current status of the TAMA project as well as other projects for GW detection is reported.

The astronomers Hanbury Brown and Twiss (HBT) were the first to observe in 1956 that the fluctuations in the counting rate of photons originating from uncorrelated point sources become, within the coherently illuminated area, slightly enhanced compared to a random sequence of classical particles^1,2,3. This at a first glance mysterious formation of correlations in the process of propagation turns out to be a consequence of quantum interference between two indistinguishable photons and Bose-Einstein statistics⁴. The latter requires that the composite wave function is a symmetrized superposition of the two possible paths. For fermions, by virtue of the Pauli principle, no two particles are allowed to be in the same state. The corresponding antisymmetrized two particle wave function excludes overlapping wave trains, i.e., simultaneous arrivals of two fermions at contiguous, coherently illuminated detectors are forbidden. These anticorrelations have been observed for the first time for a beam of free electrons in spite of the low mean number (degeneracy) of ~10^-4 electrons per cell in phase space. With respect to the low degeneracy, our experiment is the fermionic twin of HBT's experiment with photons.

The authors have proposed and tested a new type of multilayer cold-neutron interferometer based on a pair of etalons. The range of experimental application of conventional multilayer cold-neutron interferometer was limited due to the small spatial separation between the two coherent beams. Using etalons with an air gap of 20μm in spacing we have observed interference fringes with the contrast of (3.5 ± 0.7)%. The present results have demonstrated the feasibility of developing a cold neutron interferometer with a large path separation to carry out high precision measurements and new types of experiment.

Loss of interference takes place due to entanglement and resembles the collapse of the wave packet. We propose a simple collision experiment to examine this effect with apparatuses already available.

It is pointed out that quantum information theory has a nice application to black hole physics, because the quantum state is an entangled state of the particles inside and outside of the black hole just like the Einstein-Podolsky-Rosen pair. We show in particular that the increase of the generalized entropy by quantum process outside the horizon of a black hole is more than the Holevo bound of mutual information between a message prepared by an agent located outside the horizon and that received by an observer at infinity.

In quantum systems of a macroscopic size V, such as interacting many particles and quantum computers with many qubits, there exist pure states such that fluctuations of some intensive operator  is anomalously large, , which is much larger than that assumed in thermodynamics, . By making full use of the locality, we show, starting from Hamiltonians of macroscopic degrees of freedom, that such states decohere at anomalously fast rates when they are weakly perturbed from environments.

The behavior of macroscopic system is investigated with a coarse-grained position operator based on the SO(3,1), whose macroscopic nature induces decoherence leading to classical behavior.

We consider an assembly of N indistinguishable two-level atoms, for which a concept of spin-squeezing and entanglement measures are well defined. We explicitly show that a spin-squeezed state has pair-wise entanglement and a squeezing parameter serves as a measure of entanglement in that case.

A theoretical interpretation of a set of neutron scattering experiments on protons and deuterons is presented in terms of quantum entanglement of spatial and spin degrees of freedom. A soluble model for the scattering of pairs of particles is developed, and it is applied to hydrogen in different condensed matter environments. Decoherence limits the lifetime of quantum entanglement to times of the order of 10^-15s.

Previous experiments on NbH_0.8 and PdH_0.6 have shown large anomalies in the cross sections for protons, when studied by neutron Compton scattering. Here, these investigations are extended to the metallic hydrides YH₂, YH₃, YD₂, YD₃, and Y(H_xD_1-x)₃. Considerably reduced cross sections for hydrogen are observed both in YH₂ and YH₃, but only minor ones for YD₂ and YD₃. The scattering time depends on the neutron scattering angle, which allows a time-differential analysis where the time window lies around one femtosecond. The anomalies persist longer in YH₂ and YH₃ than in NbH_0.8 and PdH_o.6 The reduced cross sections are interpreted as a result of quantum entanglement between protons, surviving for a few fs in the solids.

In earlier neutron Compton scattering (NCS) experiments on H₂O/D₂O mixtures at T ≈ 298 K we observed, for the first time, a striking "anomalous" decrease of the ratio σ_H/σ_D of the total scattering cross sections of H and D. This "anomaly" was found to depend strongly on the H/D composition of the liquid. Extending recent NCS results obtained from solid polystyrene, we present here new results concerning the quantum dynamics and dissociation of C-H bonds (at T ≈ 298 K) in: (a) liquid benzene and C₆H₆/C₆D₆ mixtures; (b) fully protonated and partially deuterated polystyrene; and (c) liquid mixtures of H-acetone (CH₃COCH₃) and D-acetone (CD₃COCD₃). The considered NCS effect was given a theoretical explanation based on short-lived protonic quantum entanglement (QE) and decoherence. The variety of the new results suggests that, in the short-time scale of the NCS experiment, protonic quantum dynamics is strongly correlated with that of electronic degrees of freedom participating in the various chemical bonds.

We show that the Fourier transform of the velocity distribution of a charged particle moving in the field free zone surrounding a solenoid changes abruptly when the particle passes by the solenoid. This change can be attributed to the exchange of a conserved gauge invariant quantity between the particle and the solenoid, and is responsible for the Aharonov-Bohm effect.

Within the semiclassical approximation, it is shown that the classical trajectories that appear in the semiclassical evaluations of Feynman kernels are weak values [Aharonov, Albert and Vaidman, Phys. Rev. Lett. 60 (1988) p. 1351] when the interference among the classical trajectories are negligible. The classical trajectories, including complex-valued ones, are accordingly observable by weak measurements, in principle. Furthermore, this explains why weak values can take "anomalous" values that lie outside the range of the eigenvalues of the corresponding operators, without relying on quantum coherence nor entanglement.

We discuss the effects of decoherence parameters introduced in the correlated two neutral kaon system and show that their magnitudes are limited by the magnitudes of CP violation and of the strangeness non-conserving ΔS = ±2 transitions.

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