Harkirat Singh

Experimental detection of quantum information sharing and its quantification in quantum spin systems

We study the macroscopic entanglement properties of a low- dimensional quantum spin system by investigating its magnetic properties at low temperatures and high magnetic fields. The spin system chosen for this is copper nitrate (Cu(NO3)2 ◊2.5H2O), which is a spin chain that exhibits dimerization. The temperature and magnetic field dependence of entanglement from the susceptibility and magnetization data are given, by comparing the experimental results with the theoretical estimates. Extraction of entanglement has been made possible through the macroscopic witness operator, magnetic susceptibility. An explicit comparison of the experimental extraction of entanglement with theoretical estimates is provided. It was found that theory and experiments match over a wide range of temperatures and fields. The spin system studied exhibits quantum phase transition (QPT) at low temperatures when the magnetic field is swept through a critical value. We show explicitly for the first time, using tools used in quantum information processing, that QPT can be captured experimentally using quantum complementary observables, which clearly delineate entangled states from separable ones across the QPT.

Experimental quantification of entanglement through heat capacity

A new experimental realization of heat capacity as an entanglement witness is reported. Entanglement properties of a low-dimensional quantum spin system are investigated by heat capacity measurements performed down to very low temperatures (400mK), for various applied magnetic field values. The experimentally extracted results for the value of heat capacity at zero field matches perfectly with the theoretical estimates of entanglement from model Hamiltonians. The studied sample is a spin half antiferromagnetic system that shows a clear signature of quantum phase transition at very low temperatures when the heat capacity is varied as a function of fields at a fixed temperature. The variation of entanglement as a function of field is then explored in the vicinity of the quantum phase transition to capture the sudden loss of entanglement.

Signature of quantum entanglement in NH4CuPO4·H2O

Entangled solid state systems have gained a great deal of attention due to their fruitful applications in modern quantum technologies. Herein, detection of entanglement content from experimental magnetic susceptibility and specific heat data is reported for NH4CuPO4·H2O in its solid state crystalline form. NH4CuPO4·H2O is a prototype of Heisenberg spin 1/2 dimer system. Temperature dependent magnetic susceptibility and specific data are fitted to an isolated dimer model and the exchange coupling constant is determined. Field dependent magnetization isotherms taken at different temperatures are plotted in a three dimensional plot. Subsequently, entanglement is detected both from susceptibility and specific heat through two different entanglement measures; entanglement witness and entanglement of formation. The temperature evolution of entanglement is studied and the critical temperature is determined up to which entanglement exists. Temperature dependent nature of entanglement extracted from susceptibility...

Probing quantum discord in a Heisenberg dimer compound.

A quantitative estimation of quantum discord is performed for a Heisenberg spin 1/2 dimer compound (NH4CuPO4, H2O) by means of experimental magnetic and thermal measurements. Magnetic susceptibility and specific heat data were collected for NH4CuPO4, H2O and analyzed within the framework of the Heisenberg isolated dimer model. Internal energy as a function of temperature is obtained by integrating the specific heat versus temperature data. Subsequently, quantum discord, total correlations and spin-spin correlation function are quantified from susceptibility and internal energy and plotted as a function of temperature. Violation of Bells inequality is also tested for NH4CuPO4, H2O via both experimental susceptibility and specific heat data signifying the presence of entanglement.

Experimental detection of thermal entanglement in a molecular chain

Detection of entangled states from experimental magnetic susceptibility is reported for an organic radical. The present system exemplifies an ideal spin ½ chain with full isotropy. Experimental susceptibility data are modelled by the formulation described by Bonner and Fisher. An explicit correlation is established between the molecular structure and the spin chain interaction. Using experimental magnetic susceptibility as entanglement witness, entanglement is observed to exist up to 28u2009K in the present system. Evolution of entanglement with magnetic field is also studied and a surface plot is generated, which clearly exhibits the variation of entanglement with magnetic field and temperature.

Quantification of entanglement from magnetic susceptibility for a Heisenberg spin 1/2 system

Abstract We report temperature and magnetic field dependent magnetization and quantification of entanglement from the experimental data for dichloro (thiazole) copper (II), a Heisenberg spin chain system. The plot of magnetic susceptibility vs. temperature indicates an infinite spin chain. Isothermal magnetization measurements (as functions of magnetic field) were performed at various temperatures below the antiferromagnetic (AFM) ordering, where the AFM correlations persist significantly. These magnetization curves are fitted to the Bonner–Fisher model. Magnetic susceptibility is used as an entanglement witness to quantify the amount of entanglement in the system.

Experimental evidences of singlet to triplet transition in a spin cluster compound

Abstract Experimental realization of magnetic field induced singlet to triplet transition is reported for NH 4 CuPO 4 ·H 2 O, a two spin cluster material with isotropic Heisenberg interaction. Experimental magnetization and specific heat data have been collected as a function of temperature and magnetic field. Experimental data have been analyzed in terms of Heisenberg dimer model. Two quantum complementary observables representing local and non-local properties of the spins are constructed using the experimental data and a clear evidence of singlet to triplet transition is observed through partial quantum information sharing when the magnetic field is swept through a particular value. Signature of this transition has also been captured when specific heat is measured as a function of magnetic field. Furthermore, using the experimental specific heat data, magnetic energy values are calculated and their variations are captured as a function of magnetic field and temperature.

Investigation of thermodynamic properties of Cu(NH3)4SO4·H2O, a Heisenberg spin chain compound

Abstract Detailed experimental investigation of thermal and magnetic properties are presented for Cu(NH 3 ) 4 SO 4 ·H 2 O, an ideal uniform Heisenberg spin ½ chain compound. A comparison of these properties with relevant spin models is also presented. Temperature dependent magnetic susceptibility and specific heat data have been compared with the exact solution for uniform Heisenberg chain model derived by means of Bethe ansatz technique. Magnetization isotherms measured as a function of field are analyzed using the numerical results simulated by Quantum Monte Carlo technique. Specific heat as a function of magnetic field (up to 7T) and temperature (down to 2xa0K) is reported. Subsequently, the data are compared with the corresponding theoretical curves for infinite Heisenberg spin ½ chain model with Jxa0=xa06xa0K. Moreover, internal energy and entropy are calculated by analyzing the experimental specific heat data. Magnetic field and temperature dependent behavior of entropy and internal energy are in good agreement with the theoretical predictions.

Violation of Bell’s inequality in a spin 1/2 quantum magnet

Violation of Bell’s inequality test has been established as an efficient tool to determine the presence of entanglement in quantum spin 1/2 magnets. Herein, macroscopic thermodynamic quantities, namely, magnetic susceptibility and specific heat have been employed to perform Bell’s inequality test for [NH4CuPO4, H2O], a spin 1/2 antiferromagnet with nearest neighbor interactions. The mean value of the Bell operator is quantified and plotted as a function of temperature. The threshold temperature is determined above which the Bell’s inequality is not violated and a good consistency is found between the analyses done on magnetic and thermal data.

Study of entanglement in a quantum antiferromagnet

The absolute temperature (T) and magnetic field (B) dependent magnetization(M) and study of entanglement out of the experimental data is reported for dichloro(thiazole)copper(II), a Heisenberg spin chain system. The magnetic susceptibility (χ) vs. T plot assures the behavior of infinite spin chain. The M vs. B data has been taken at different temperature as long as the antiferromagnetic(AFM) correlations persist significantly. M, B and T have been plotted in a 3D plot showing explicitly the dependence of the three parameters on one another. Since the concurrence is a measure of entanglement, the value of concurrence is extracted from the χ vs. T data and thats how the information about entanglement is extracted from the macroscopic magnetic susceptibility.