Correlated Methods for Excited States in Periodic Systems

We present the calculations of the ground state and lowest excited states of the one-dimensional periodic Anderson Hamiltonian with two electrons per site and arbitrary magnitude of the repulsive interaction $U$ . We consider finite cells (up to $N=4$ ) and introduce a new method, using modified periodic boundary conditions, to facilitate comparison of calculations with different $N$ . The ground state is found to be a nonmagnetic singlet in all cases. The lowest-energy excitations for adding or subtracting one electron show that the system is insulating and the lowest spin-flip excitations indicate a near instability to antiferromagnetism due to the "nesting" of the Fermi surface in one dimension. The lowest excitations are shown to vary little with $N$ and, for $N=4$ , the results agree well with infinite-cell calculations, both for small $U$ and for the Kondo-lattice regime. The primary results are the continuous variation from $U=0$ to the Kondo-lattice and mixed-valence regimes and the importance of correlations, which lead to the insulating gap and dispersion in the electronic and spin excitations.We present the calculations of the ground state and lowest excited states of the one-dimensional periodic Anderson Hamiltonian with two electrons per site and arbitrary magnitude of the repulsive interaction $U$. We consider finite cells (up to $N=4$) and introduce a new method, using modified periodic boundary conditions, to facilitate comparison of calculations with different $N$. The ground state is found to be a nonmagnetic singlet in all cases. The lowest-energy excitations for adding or subtracting one electron show that the system is insulating and the lowest spin-flip excitations indicate a near instability to antiferromagnetism due to the "nesting" of the Fermi surface in one dimension. The lowest excitations are shown to vary little with $N$ and, for $N=4$, the results agree well with infinite-cell calculations, both for small $U$ and for the Kondo-lattice regime. The primary results are the continuous variation from $U=0$ to the Kondo-lattice and mixed-valence regimes and the importance of correlations, which lead to the insulating gap and dispersion in the electronic and spin excitations.

Physical review. B, Condensed matterDec 1, 1982101 citations

Excited states in finite systems

WORLD SCIENTIFIC eBooksMay 1, 20080 citations

Estimation of properties of low-lying excited states of Hubbard models: A multiconfigurational symmetrized projector quantum Monte Carlo approach

Bhargavi Srinivasan, S. Ramasesha, H. R. Krishnamurthy

We present in detail the recently developed multiconfigurational symmetrized projector quantum Monte Carlo method for excited states of the Hubbard model. We describe the implementation of the Monte Carlo method for a multiconfigurational trial wave function. We give a detailed discussion of issues related to the symmetry of the projection procedure that validates our Monte Carlo procedure for excited states. In this context we discuss various averaging procedures for the Green function and present an analysis of the errors incurred in these procedures. We study the ground-state energy and correlation functions of the one-dimensional Hubbard model at half-filling to confirm these analyses. We then study the energies and correlation functions of excited states of Hubbard chains. Hubbard rings away from half-filling are also studied and the pair binding energies for holes of $4n$ and $4n+2$ systems are compared with the Bethe ansatz results of Fye, Martins, and Scalettar [Phys. Rev. B 42, 6809 (1990)]. Our study of the two-dimensional Hubbard model includes the $4\ifmmode\times\else\texttimes\fi{}2$ ladder and the $3\ifmmode\times\else\texttimes\fi{}4$ lattice with periodic boundary conditions. The $3\ifmmode\times\else\texttimes\fi{}4$ lattice is nonbipartite and amenable to exact diagonalization studies and is, therefore, a good candidate for checks on the method. We are able to reproduce accurately the energies of ground and excited states, both at and away from half-filling. We study the properties of the $4\ifmmode\times\else\texttimes\fi{}2$ Hubbard ladder with bond alternation as the correlation strength and filling are varied. The method reproduces the correlation functions accurately. We also examine the severity of sign problem for one- and two-dimensional systems.We present in detail the recently developed multiconfigurational symmetrized projector quantum Monte Carlo method for excited states of the Hubbard model. We describe the implementation of the Monte Carlo method for a multiconfigurational trial wave function. We give a detailed discussion of issues related to the symmetry of the projection procedure that validates our Monte Carlo procedure for excited states. In this context we discuss various averaging procedures for the Green function and present an analysis of the errors incurred in these procedures. We study the ground-state energy and correlation functions of the one-dimensional Hubbard model at half-filling to confirm these analyses. We then study the energies and correlation functions of excited states of Hubbard chains. Hubbard rings away from half-filling are also studied and the pair binding energies for holes of $4n$ and $4n+2$ systems are compared with the Bethe ansatz results of Fye, Martins, and Scalettar [Phys. Rev. B 42, 6809 (1990)]. Our study of the two-dimensional Hubbard model includes the $4\ifmmode\times\else\texttimes\fi{}2$ ladder and the $3\ifmmode\times\else\texttimes\fi{}4$ lattice with periodic boundary conditions. The $3\ifmmode\times\else\texttimes\fi{}4$ lattice is nonbipartite and amenable to exact diagonalization studies and is, therefore, a good candidate for checks on the method. We are able to reproduce accurately the energies of ground and excited states, both at and away from half-filling. We study the properties of the $4\ifmmode\times\else\texttimes\fi{}2$ Hubbard ladder with bond alternation as the correlation strength and filling are varied. The method reproduces the correlation functions accurately. We also examine the severity of sign problem for one- and two-dimensional systems.

arXiv:cond-mat/9609024Physical review. B, Condensed matterSep 15, 19974 citations

Measuring Effective Temperatures of Qubits Using Correlations

Anatoly Kulikov, Rohit Navarathna, Arkady Fedorov

Initialization of a qubit in a pure state is a prerequisite for quantum computer operation. A plethora of ways to achieve this has been proposed in the last decade, from active reset protocols to advances in materials and shielding. An instrumental tool to evaluate those methods and develop new ones is the ability to measure the population of excited states with high precision and in a short period of time. In this Letter, we propose a new technique of finding the excited state population of a qubit using correlations between two sequential measurements. We experimentally implement the proposed technique using a circuit QED platform and compare its performance with previously developed ones. Unlike other techniques, our method does not require high-fidelity readout and does not involve the excited levels of the system outside of the qubit subspace. We experimentally demonstrated measurement of the spurious qubit population with accuracy of up to 0.01%. This accuracy enabled us to perform "temperature spectroscopy" of the qubit, which helps to shed light on decoherence sources.Initialization of a qubit in a pure state is a prerequisite for quantum computer operation. A plethora of ways to achieve this has been proposed in the last decade, from active reset protocols to advances in materials and shielding. An instrumental tool to evaluate those methods and develop new ones is the ability to measure the population of excited states with high precision and in a short period of time. In this Letter, we propose a new technique of finding the excited state population of a qubit using correlations between two sequential measurements. We experimentally implement the proposed technique using a circuit QED platform and compare its performance with previously developed ones. Unlike other techniques, our method does not require high-fidelity readout and does not involve the excited levels of the system outside of the qubit subspace. We experimentally demonstrated measurement of the spurious qubit population with accuracy of up to 0.01%. This accuracy enabled us to perform "temperature spectroscopy" of the qubit, which helps to shed light on decoherence sources.

arXiv:2001.00323Physical Review LettersJun 15, 202018 citations

Excited states in finite systems

WORLD SCIENTIFIC eBooksApr 1, 20050 citations

U

. We consider finite cells (up to

N=4

) and introduce a new method, using modified periodic boundary conditions, to facilitate comparison of calculations with different

N

. The ground state is found to be a nonmagnetic singlet in all cases. The lowest-energy excitations for adding or subtracting one electron show that the system is insulating and the lowest spin-flip excitations indicate a near instability to antiferromagnetism due to the "nesting" of the Fermi surface in one dimension. The lowest excitations are shown to vary little with

N

and, for

N=4

, the results agree well with infinite-cell calculations, both for small

U

and for the Kondo-lattice regime. The primary results are the continuous variation from

U=0

to the Kondo-lattice and mixed-valence regimes and the importance of correlations, which lead to the insulating gap and dispersion in the electronic and spin excitations.

4n

and

4n+2

systems are compared with the Bethe ansatz results of Fye, Martins, and Scalettar [Phys. Rev. B 42, 6809 (1990)]. Our study of the two-dimensional Hubbard model includes the $4\ifmmode\times\else\texttimes\fi{}2$ ladder and the $3\ifmmode\times\else\texttimes\fi{}4$ lattice with periodic boundary conditions. The $3\ifmmode\times\else\texttimes\fi{}4$ lattice is nonbipartite and amenable to exact diagonalization studies and is, therefore, a good candidate for checks on the method. We are able to reproduce accurately the energies of ground and excited states, both at and away from half-filling. We study the properties of the $4\ifmmode\times\else\texttimes\fi{}2$ Hubbard ladder with bond alternation as the correlation strength and filling are varied. The method reproduces the correlation functions accurately. We also examine the severity of sign problem for one- and two-dimensional systems.