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A. K. Khitrin

Publications and source records attributed to A. K. Khitrin.

At least 19 recordsLinked to original sources
arXiv2017

Bright-field microscopy of transparent objects: a ray tracing approach

Formation of a bright-field microscopic image of a transparent phase object is described in terms of elementary geometrical optics. Our approach is based on the premise that image replicates the intensity distribution (real or virtual) at the front focal plane of the objective. The task is therefore reduced to finding the change in intensity at the focal plane caused by the object. This can be done by ray tracing complemented with the requirement of conservation of the number of rays. Despite major simplifications involved in such an analysis, it reproduces some results from the paraxial wave theory. Additionally, our analysis suggests two ways of extracting quantitative phase information from bright-field images: by vertically shifting the focal plane (the approach used in the transport-of-intensity analysis) or by varying the angle of illumination. In principle, information thus obtained should allow reconstruction of the object morphology.

physics.optics↗
arXiv2014

Reversibility of dynamics and multiple-quantum coherences

In spin systems, the decay of the Loschmidt echo in the time-reversal experiment (evolution, perturbation, time-reversed evolution) is linked to the generation of multiple-quantum coherences. The approach is extended to other systems, and the general problem of reversibility of quantum dynamics is analyzed.

quant-ph↗
arXiv2013

Suspended Long-Lived NMR Echo in Solids

We report an observation of extremely long-lived spin states in systems of dipolar-coupled nuclear spins in solids. The 'suspended echo' experiment uses a simple stimulated echo pulse sequence and creates non-equilibrium states which live many orders of magnitude longer than the characteristic time of spin-spin dynamics T2. Large amounts of information can be encoded in such long-lived states, stored in a form of multi-spin correlations, and subsequently retrieved by an application of a single 'reading' pulse.

cond-mat.mes-hall↗
arXiv2011

Coherent Averaging: an Alternative to the Average Hamiltonian Theory

Line-narrowing by periodic modulation of nuclear spin interaction Hamiltonians is the central element of various experimental techniques in NMR spectroscopy. In this study, we present a theoretical formulation of coherent averaging to calculate the heights of narrowed spectral peaks. This concept is experimentally demonstrated using proton spectra of solids obtained under fast magic-angle spinning.

cond-mat.stat-mech↗
arXiv2010

Selective excitation of homogeneous spectral lines

It is possible, for homogeneously broadened lines, to excite selectively the response signals, which are orders of magnitude narrower than the original lines. The new type of echo, which allows detecting such signals, and the formalism, useful for understanding the phenomenon, as well as the experimental examples from NMR spectroscopy are presented.

cond-mat.stat-mech↗
arXiv2008

Experimental Implementation of Projective Measurement in Bell Basis

A scheme for direct projection of a quantum state on Bell states is described. The method is based on constructing an average Hamiltonian with Bell eigenstates and then, projecting the state on these eigenstates. The projection is performed by adding the results of a direct and time-reversed evolution. Experimental demonstration is shown for pairs of dipolar-coupled nuclear spins.

quant-ph↗
arXiv2007

Adiabatic cross-polarization

A scheme of adiabatic cross-polarization is described. It is based on demagnetization - remagnetization, when the Zeeman order of abundant nuclei in the laboratory frame is first adiabatically converted into the dipolar order, and then, into the Zeeman order of rare nuclei. The scheme, implemented with two low-power frequency-sweeping pulses, is very efficient for static samples and can significantly increase polarization of rare nuclei, compared to the conventional Hartmann-Hahn cross-polarization. The experimental examples are presented for solids, liquid crystal, and molecules in a liquid-crystalline solvent.

cond-mat.other↗
arXiv2007

Controlled spontaneous emission

The problem of spontaneous emission is studied by a direct computer simulation of the dynamics of a combined system: atom + radiation field. The parameters of the discrete finite model, including up to 20k field oscillators, have been optimized by a comparison with the exact solution for the case when the oscillators have equidistant frequencies and equal coupling constants. Simulation of the effect of multi-pulse sequence of phase kicks and emission by a pair of atoms shows that both the frequency and the linewidth of the emitted spectrum could be controlled.

quant-ph↗
arXiv2007

Experimental demonstration of stimulated polarization wave in a chain of nuclear spins

A stimulated wave of polarization, which implements a simple mechanism of quantum amplification, is experimentally demonstrated in a chain of four J-coupled nuclear spins, irradiated by a weak radio-frequency transverse field. The "quantum domino" dynamics, a wave of flipped spins triggered by a flip of the first spin, has been observed in fully $^{13}$C-labeled sodium butyrate.

quant-ph↗
arXiv2006

Projective measurement in nuclear magnetic resonance

It is demonstrated that nuclear magnetic resonance experiments using pseudopure spin states can give possible outcomes of projective quantum measurement and probabilities of such outcomes. The physical system is a cluster of six dipolar-coupled nuclear spins of benzene in a liquid-crystalline matrix. For this system with the maximum total spin S=3, the results of measuring $S_X$ are presented for the cases when the state of the system is one of the eigenstates of $S_Z$.

quant-ph↗
arXiv2006

Stimulated wave of polarization in spin chains

Stimulated wave of polarization, triggered by a flip of a single spin, presents a simple model of quantum amplification. Previously, it has been found that such wave can be excited in a 1D Ising chain with nearest-neighbor interactions, irradiated by a weak resonant transverse field. Here we explore models with more realistic Hamiltonians, in particular, with natural dipole-dipole interactions. Results of simulations for 1D spin chains and rings with up to nine spins are presented.

quant-ph↗
arXiv2006

Accurate measurement of ^{13}C - ^{15}N distances with solid-state NMR

Solid-state NMR technique for measureing distances between hetero-nuclei in static powder samples is described. It is based on a two-dimensional single-echo scheme enhanced with adiabatic cross-polarization. As an example, the results for intra-molecular distances in $α$-crystalline form of glycine are presented. The measured NMR distances ^13 C(2) - ^15 N and ^13 C(1) - ^15 N are 1.496 $\pm$ 0.002 Åand 2.50 $\pm$ 0.02 Å, respectively.

cond-mat.other↗
arXiv2005

Resurrection of Schrodinger Cat

Quantum decoherence is the major obstacle in using the potential of engineered quantum dynamics to revolutionize high-precision measurements, sensitive detection, or information processing. Here we experimentally demonstrate that quantum state of a system can be recovered after the state is destroyed by uncontrollable natural decoherence. Physical system is a cluster of seven dipolar-coupled nuclear spins of single-labeled 13C-benzene in liquid crystal. 13C spin plays a role of a device for measuring protons' "cat" state, a superposition of states with six spins up (alive) and six spins down (dead). Information about the state, stored in the 13C spin, is used to bring the protons' subsystem into the alive state, while the excess entropy produced by decoherence is transferred to the "measuring device", the 13C spin.

quant-ph↗
arXiv2005

Twelve-spin "Schrodinger cat"

Pseudopure "cat" state, a superposition of quantum states with all spins up and all spins down, is experimentally demonstrated for a system of twelve dipolar-coupled nuclear spins of fully 13C-labeled benzene molecule oriented in a liquid-crystalline matrix.

quant-ph↗
arXiv2005

Adiabatic cross-polarization via intermediate dipolar-ordered state

It is experimentally demonstrated that an adiabatic demagnetization - remagnetization scheme, where the Zeeman order of abundant nuclei is first adiabatically converted into the dipolar order, and then, into the Zeeman order of rare nuclei, can significantly increase polarization of rare nuclei compared to the conventional cross-polarization technique.

cond-mat.mtrl-sci↗
arXiv2005

Quantum state expansion

It is experimentally demonstrated that an arbitrary quantum state of a single spin 1/2: a|u> + b|d> can be converted into a superposition of the two ferromagnetic states of a spin cluster: a|uu...uu> + b|dd...dd>. The physical system is a cluster of seven dipolar-coupled nuclear spins of single-labeled 13C-benzene molecules in a liquid-crystalline matrix. In this complex system, the pseudopure ground state and the required controlled unitary transformations have been implemented. The experimental scheme can be considered as an explicit model of quantum measurement.

quant-ph↗
arXiv2005

Stimulated wave of polarization in 1D Ising chain

It is demonstrated that in one-dimensional Ising chain with nearest-neighbor interactions, irradiated by a weak resonant transverse field, a stimulated wave of flipped spins can be triggered by a flip of a single spin. This analytically solvable model illustrates mechanisms of quantum amplification and quantum measurement.

quant-ph↗