We study the macroscopic entanglement properties of a lowdimensional 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(NO 3 ) 2 × 2.5H 2 O), 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.

Initializing a set of qubits to a given quantum state is a basic prerequisite for the physical implementation of quantum-information protocols. Here, we discuss the polarization of the electronic and nuclear spin in a single Nitrogen vacancy center in diamond. Our initialization scheme uses a sequence of laser, microwave and radio-frequency pulses, and we optimize the pumping parameters of the laser pulse. A rate equation model is formulated that explains the effect of the laser pulse on the spin system. We have experimentally determined the population of the relevant spin states as a function of the duration of the laser pulse by measuring Rabi oscillations and Ramsey-type free-induction decays. The experimental data have been analyzed to determine the pumping rates of the rate equation model.

Recent studies dealing with magnetocaloric response of antiferromagnetic (AFM) spin systems have established that being superior coolants they have the potential to replace the paramagnetic salts, which are presently abundant in cooling technology. Presently we report an enhanced magnetocaloric effect (MCE) in one such spin system: N H4CuP O4.H2O (NCP), an AFM spin 1/2 dimer. Experimental magnetization and specific heat data have been presented and these data have been employed to evaluate entropy, magnetic energy and magnetocaloric properties. We witness a sign change in magnetic Grüneisen parameter across the level-crossing field BC . An efficient adiabatic cooling is observed at low temperature by tracing the isentropic curves in temperature-magnetic field plane. Energy-level crossover characteristics in NCP interpreted through MCE analysis are well consistent with the observations made from magnetization and specific heat data.Studying low dimensional quantum antiferromagnets have been at the forefront of both experimental and theoretical condensed matter physics due to novel nature of their ground states. This has led to a number of fascinating physical properties [1][2][3][4][5][6][7][8]. Spin-gapped compounds are a class of quantum spin systems, which possess an energy gap in the excitation spectra [9]. Examples of widely investigated spin-gapped systems include spin-dimers [10,11], spin-Peierls compounds [12,13], spin ladders [14,15], alternating spin chains [16][17][18], frustrated spin systems [19,20] etc. Antiferromagnetically exchange-coupled spin dimers are interesting to study as depending on the nature of their inter-dimer coupling they exhibit various kinds of magnetic excitation under the influence of external tuning parameters like magnetic field. For instance, field induced magnetic ordering has been witnessed and the results have been explained using the theory of Bose-Einstein condensation of magnons for compounds like T lCuCl 3 [10] and BaCuSi 2 O 6 [11]. Appearance of magnetization plateaus and Wigner crystallization of magnons are investigated for the dimerized system SrCu 2 (BO 3 ) 2 [21,22]. Spin dimer systems with significantly weak interdimer interaction can be considered as independent spin clusters. The energy spectrum of a spin 1/2 dimer consists of a singlet ground state and a 3-fold degenerate state which upon application of external magnetic field split into three states and evolve as the field changes. A level crossing between the ground state and the first excited state occurs when the magnetic field is increased through a critical field value. Thus the ground state undergoes a qualitative change and the first excited state becomes the new ground state. Such level crossing happens between two pure quantum states and ideally it is a zero temperature phenomena, although it is possible to capture its evidence by measuring the physical properties of the system at finite temperature. The present report demonstrates investigation of magnetocaloric effect (MCE) at field-induced level ...

Abstract. 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 (400 mK), 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.