An observable measure of entanglement for pure states of multi-qubit systems

Recently, Meyer and Wallach [D.A. Meyer and N.R. Wallach (2002), J. of Math. Phys., 43, pp. 4273] proposed a measure of multi-qubit entanglement that is a function on pure states. We find that this function can be interpreted as a physical quantity related to the average purity of the constituent qubits and show how it can be observed in an efficient manner without the need for full quantum state tomography. A possible realization is described for measuring the entanglement of a chain of atomic qubits trapped in a 3D optical lattice.Comment: 8 pages, 2 figure

Why should anyone care about computing with anyons?

In this article we present a pedagogical introduction of the main ideas and recent advances in the area of topological quantum computation. We give an overview of the concept of anyons and their exotic statistics, present various models that exhibit topological behavior, and we establish their relation to quantum computation. Possible directions for the physical realization of topological systems and the detection of anyonic behavior are elaborated.Comment: 22 pages, 13 figures. Some changes to existing sections, several references added, and a new section on criteria for TQO and TQC in lattice system

Stability of global entanglement in thermal states of spin chains

We investigate the entanglement properties of a one dimensional chain of spin qubits coupled via nearest neighbor interactions. The entanglement measure used is the n-concurrence, which is distinct from other measures on spin chains such as bipartite entanglement in that it can quantify "global" entanglement across the spin chain. Specifically, it computes the overlap of a quantum state with its time-reversed state. As such this measure is well suited to study ground states of spin chain Hamiltonians that are intrinsically time reversal symmetric. We study the robustness of n-concurrence of ground states when the interaction is subject to a time reversal antisymmetric magnetic field perturbation. The n-concurrence in the ground state of the isotropic XX model is computed and it is shown that there is a critical magnetic field strength at which the entanglement experiences a jump discontinuity from the maximum value to zero. The n-concurrence for thermal mixed states is derived and a threshold temperature is computed below which the system has non zero entanglement.Comment: 13 pages, 3 figures. v.2 includes minor corrections and an added section treating the quantum XX model with open boundarie

A Quantum Computer Architecture using Nonlocal Interactions

Several authors have described the basic requirements essential to build a scalable quantum computer. Because many physical implementation schemes for quantum computing rely on nearest neighbor interactions, there is a hidden quantum communication overhead to connect distant nodes of the computer. In this paper we propose a physical solution to this problem which, together with the key building blocks, provides a pathway to a scalable quantum architecture using nonlocal interactions. Our solution involves the concept of a quantum bus that acts as a refreshable entanglement resource to connect distant memory nodes providing an architectural concept for quantum computers analogous to the von Neumann architecture for classical computers.Comment: 4 pages, 2 figures, Slight modifications to satisfy referee, 2 new references, modified acknowledgement. This draft to appear in PRA Rapid Communication

Measurement-based quantum computer in the gapped ground state of a two-body Hamiltonian

We propose a scheme for a ground-code measurement-based quantum computer, which enjoys two major advantages. First, every logical qubit is encoded in the gapped degenerate ground subspace of a spin-1 chain with nearest-neighbor two-body interactions, so that it equips built-in robustness against noise. Second, computation is processed by single-spin measurements along multiple chains dynamically coupled on demand, so as to keep teleporting only logical information into a gap-protected ground state of the residual chains after the interactions with spins to be measured are turned off. We describe implementations using trapped atoms or polar molecules in an optical lattice, where the gap is expected to be as large as 0.2 kHz or 4.8 kHz respectively.Comment: 5 pages, 1 figure; v3 the extended final versio

Quantum error correction on symmetric quantum sensors

Symmetric states of collective angular momentum are good candidates for multi-qubit probe states in quantum sensors because they are easy to prepare and can be controlled without requiring individual addressability. Here, we give quantum error correction protocols for estimating the magnitude of classical fields using symmetric probe states. To achieve this, we first develop a general theory for quantum error correction on the symmetric subspace. This theory, based on the representation theory of the symmetric group, allows us to construct efficient algorithms that can correct any correctible error on any permutation-invariant code. These algorithms involve measurements of total angular momentum, quantum Schur transforms or logical state teleportations, and geometric pulse gates. For deletion errors, we give a simpler quantum error correction algorithm based on primarily on geometric pulse gates. Second, we devise a simple quantum sensing scheme on symmetric probe states that works in spite of a linear rate of deletion errors, and analyze its asymptotic performance. In our scheme, we repeatedly project the probe state onto the codespace while the signal accumulates. When the time spent to accumulate the signal is constant, our scheme can do phase estimation with precision that approaches the best possible in the noiseless setting. Third, we give near-term implementations of our algorithms.Comment: 26 pages, 7 figures, 2 column