22 research outputs found
Effects of dipolar coupling on an entanglement storage device
Quantum computation requires efficient long-term storage devices to preserve
quantum states. An attractive candidate for such storage devices is qubits
connected to a common dissipative environment. The common environment gives
rise to persistent entanglements in these qubit systems. Hence these systems
act efficiently as a storage device of entanglement. However, the existence of
a common environment often requires the physical proximity of the qubits and
hence results in direct dipolar coupling between the qubits. In this work, we
investigate the total effect of the dipolar coupling on the environment-induced
entanglement using a recently-proposed fluctuation-regulated quantum master
equation [A. Chakrabarti and R. Bhattacharyya, Phys. Rev. A 97, 063837 (2018)].
We show that nonsecular part of the dipolar coupling results in reduced
entanglement and hence less efficiency of the storage devices. We also discuss
the properties of efficient storage that mitigates the detrimental effects of
the dipolar coupling on the stored entanglement.Comment: 22 pages, 7 figure
Emergence and stability of discrete time-crystalline phases in open quantum systems
Here we provide a theoretical framework to analyze discrete time-crystalline
phases (DTC) in open quantum many-body systems. As a particular realization, we
choose a quantum many-body system that exhibits cascaded prethermalization .
The analysis uses a fluctuation-regulated quantum master equation. The master
equation captures the dissipative effects of the drive and dipolar coupling on
the dynamics regularized by the thermal fluctuations. We find that the
dissipators from the drive and the dipolar interactions lend stability to the
dynamics and are directly responsible for the robustness. Specifically, we find
that longer fluctuation correlation time enhances the stability of DTC. Our
results are in good agreement with the experiments. Finally, we show and
quantify how the DTC performance degrades with temperature.Comment: 13 pages, 6 figure
Creation of long-lived states in interacting spins coupled to a thermal bath
We study the classic problem concerning the dissipative dynamics of a system
of two interacting spin- particles, coupled to a thermal bath. In
particular, we consider the case of a resonant excitation of the initial
single-spin coherences and examine the consequences of the interplay between
this excitation and the spin-spin dipolar coupling. Using a
fluctuation-regulated Quantum Master Equation (frQME) [Phys. Rev. A 97, 063837
(2018)] we show that the relevant dissipator consists of cross-terms between
the drive and the dipolar Hamiltonian, apart from regular self-terms
responsible for ordinary relaxation effects. The drive-dipole
cross-correlations provide for a novel second-order coupling of single and
two-spin observables in the Zeeman basis, which cannot be classified as a
dissipative effect. We show that the presence of these unique second-order
non-dissipative terms lead to the well-known spin-locked steady-states in
dipolar spin-ensembles. Importantly, the mathematical form of the steady-state
magnetization in the spin-locked phase, obtained from our theory, matches with
the prediction of the traditional spin-temperature based methods used to
describe such phenomena.Comment: 17 pages, 6 figure
A fast method for the measurement of long spin-lattice relaxation times by Single Scan Inversion Recovery experiment
A new method of measuring long spin-lattice relaxation times () is
proposed. Being a single scan technique, the method is at least one order of
magnitude faster than the conventional technique. This method (Single-Scan or
Slice Selected Inversion Recovery or SSIR) relies on the slice selection
technique. The method is experimentally verified and compared with the time
taken by the conventional measurement. Furthermore, it is shown that the
conventional Inversion Recovery (IR) method which suffers from effects of r.f.
inhomogeneity can also be improved by measuring the magnetization of only a
central slice.Comment: 12 pages, 5 figures. Chemical Physics Letters, in pres
Quantum information processing by NMR using a 5-qubit system formed by dipolar coupled spins in an oriented molecule
Quantum Information processing by NMR with small number of qubits is well
established. Scaling to higher number of qubits is hindered by two major
requirements (i) mutual coupling among qubits and (ii) qubit addressability. It
has been demonstrated that mutual coupling can be increased by using residual
dipolar couplings among spins by orienting the spin system in a liquid
crystalline matrix. In such a case, the heteronuclear spins are weakly coupled
but the homonuclear spins become strongly coupled. In such circumstances, the
strongly coupled spins can no longer be treated as qubits. However, it has been
demonstrated elsewhere, that the energy levels of a strongly coupled N
spin-1/2 system can be treated as an N-qubit system. For this purpose the
various transitions have to be identified to well defined energy levels. This
paper consists of two parts. In the first part, the energy level diagram of a
heteronuclear 5-spin system is obtained by using a newly developed
heteronuclear z-cosy (HET-Z-COSY) experiment. In the second part,
implementation of logic gates, preparation of pseudopure states, creation of
entanglement and entanglement transfer is demonstrated, validating the use of
such systems for quantum information processing.Comment: 23 pages, 8 figure
Use of cross-correlated NMR relaxation for the study of motional anisotropy of liquid crystals
A method to measure the rotational diffusion coefficients of a liquid crystal based on the measurements of auto and cross-correlated relaxation is presented here. We report the measurements of cross-correlations between the relaxation processes originating from the chemical shift anisotropy (CSA) of 13C and its dipolar coupling with the attached proton, for various carbons in a liquid crystal. The spectral densities are expressed as a function of motional parameters, using extended Lipari-Szabo model-free approach. These motional parameters were extracted from the measured self-relaxation, cross-relaxation and the cross-correlation rates. The potential of this method is demonstrated by the determination of the rotational diffusion coefficients of N-4-methoxybenzylidene-4-butylaniline (MBBA) at 297 K undergoing magic angle sample spinning
Non-Bloch decay of Rabi oscillations in liquid state NMR
Rabi oscillations are known to exhibit non-Bloch behaviour in anisotropic media. In this letter, we report an experimental observation of non-Bloch decay of Rabi oscillations in isotropic liquid state NMR. To avoid the dephasing due to the radio-frequency inhomogeneities, we develop a modified version of the rotary echo protocol and use it to determine the decay rates of Rabi oscillations. We find that the measured decay rates are proportional to the square of the Rabi frequencies and the proportionality constant is of the order of tens of picoseconds. Further, we show that this non-Bloch nature of the decay rates becomes less prominent with increasing temperature. The implications of the presence of non-Bloch decay rates in liquid state NMR in the context of ensemble quantum computing are also discussed
Use of cross-correlated NMR relaxation for the study of motional anisotropy of liquid crystals
A method to measure the rotational diffusion coefficients of a liquid crystal based on the measurements of auto and cross-correlated relaxation is presented here. We report the measurements of cross-correlations between the relaxation processes originating from the chemical shift anisotropy (CSA) of and its dipolar coupling with the attached proton, for various carbons in a liquid crystal. The spectral densities are expressed as a function of motional parameters, using extended Lipari–Szabo model-free approach. These motional parameters were extracted from the measured self-relaxation, cross-relaxation and the cross-correlation rates. The potential of this method is demonstrated by the determination of the rotational diffusion coefficients of N-4-methoxybenzylidene-4-butylaniline (MBBA) at 297 K undergoing magic angle sample spinning