Efficient, reliable characterization of quantum states is a basic and important step in most tasks in quantum information processing, but standard approaches, including quantum tomography, are notoriously inefficient. This research offers solutions to the problem of verifying nonstabilizer states by proposing efficient, practical protocols for verifying arbitrary $n$-qubit Dicke states. These protocols require only two distinct settings based on Pauli measurements, and thus are readily applicable with current experimental techniques, and are able to verify the robust Dicke states of hundreds of qubits.
Efficient, reliable characterization of quantum states is a basic and important step in most tasks in quantum information processing, but standard approaches, including quantum tomography, are notoriously inefficient. This research offers solutions to the problem of verifying nonstabilizer states by proposing efficient, practical protocols for verifying arbitrary n-qubit Dicke states. These protocols require only two distinct settings based on Pauli measurements, and thus are readily applicable with current experimental techniques, and are able to verify the robust Dicke states of hundreds of qubits.
Dicke states are typical examples of quantum states with genuine multipartite entanglement. They are valuable resources in many quantum information processing tasks, including multiparty quantum communication and quantum metrology. Phased Dicke states are a generalization of Dicke states and include antisymmetric basis states as a special example. These states are useful in atomic and molecular physics besides quantum information processing. Here we propose practical and efficient protocols based on adaptive local projective measurements for verifying all phased Dicke states, including W states and qudit Dicke states. To verify any n-partite phased Dicke state within infidelity $\ensuremath{\epsilon}$ and significance level $\ensuremath{\delta}$, the number of tests required is only $O(n{\ensuremath{\epsilon}}^{\ensuremath{-}1}ln{\ensuremath{\delta}}^{\ensuremath{-}1})$, which is linear in n and is exponentially more efficient than traditional tomographic approaches. In the case of W states, the number of tests can be further reduced to $O(\sqrt{n}\phantom{\rule{0.16em}{0ex}}{\ensuremath{\epsilon}}^{\ensuremath{-}1}ln{\ensuremath{\delta}}^{\ensuremath{-}1})$. Moreover, we construct an optimal protocol for any antisymmetric basis state; the number of tests required decreases (rather than increases) monotonically with n. This is the only optimal protocol known for multipartite nonstabilizer states.
In quantum information transformation and quantum computation, the most critical issues are security and accuracy. These features, therefore, stimulate research on quantum state characterization. A characterization tool, Quantum state tomography, reconstructs the density matrix of an unknown quantum state. Theoretically, reconstructing an unknown state using this method can be arbitrarily accurate. However, this is less practical owing to the huge burden of measurements and data processing for large numbers of qubits. Even comprising an efficient estimator and a precise algorithm, an optimal tomographic framework can also be overburdened owing to the exponential growth of the measurements. Moreover, the consequential postprocessing of huge amounts of data challenges the capacity of computers. Thus, it is crucial to build an efficient framework that requires fewer measurements but yields an expected accuracy. To this end, we built a tomography schema by which only a few Pauli measurements enable an accurate tomographic reconstruction. Subsequently, this schema was verified as efficient and accurate through numerical simulations on the tomography of multi-qubit quantum states. Furthermore, this schema was proven to be robust through numerical simulations on a noisy superconducting qubit system. Therefore, the tomography schema paves an alternatively effective way to reconstruct the density matrix of a quantum state owing to its efficiency and accuracy, which are essential for quantum state tomography.
Graph and hypergraph states are of wide interest in quantum information processing as well as fundamental physics, and efficient verification of these states is key to various applications. The authors propose a simple recipe for verifying hypergraph states that requires only two distinct Pauli measurements for each party, and is dramatically more efficient than conventional protocols based on local measurements. This approach enables verification of hypergraph states and genuine multipartite entanglement of thousands of qubits, even in an adversarial scenario.
Quantum processes, such as quantum circuits, quantum memories, and quantum channels, are essential ingredients in almost all quantum information processing tasks. However, the characterization of these processes remains a daunting task due to the exponentially increasing amount of resources required by traditional methods. Here, by first proposing the concept of quantum process verification, we establish two efficient and practical protocols for verifying quantum processes which can provide an exponential improvement over the standard quantum process tomography and a quadratic improvement over the method of direct fidelity estimation. The efficacy of our protocols is illustrated with the verification of various quantum gates as well as the processes of well-known quantum circuits. Moreover, our protocols are readily applicable with current experimental techniques since only local measurements are required. In addition, we show that our protocols for verifying quantum processes can be easily adapted to verify quantum measurements.
Entanglement is one of the most fascinating features of quantum theory, as it fundamentally distinguishes quantum systems from classical systems. It not only might play a role in natural processes, but provides an especially powerful resource for a plethora of quantum information protocols, including quantum state teleportation, superdense coding as well as quantum computation schemes. However, for larger systems involving more than two parties, it is a challenging task to verify and characterize entanglement. In this thesis, efficient means for detecting genuine multipartite entanglement in multiqubit quantum systems will be derived, demonstrated, and discussed. Beyond that, methods for quantum state and process tomography will be introduced, which allow to analyze and investigate prepared multipartite states or even the performance of whole setups. Some typical obstacles for reliable state tomography will be pointed out, together with new means to understand experimentally obtained data. Finally, the measurement problem, which describes the unavoidable backaction of a measurement process to the quantum state under investigation, will be illustrated. Together with the discussion of different methods of quantum measurements, the optimal measurement instruments for binary qubit measurements will be derived. These optimal measurement instruments allow to perform a quantum measurement, providing a tunable amount of information about the quantum state, while avoiding any unnecessary disturbance to the state. This set of optimal instruments will be compared to other measurement schemes, such as measurements based on the optimal quantum cloning protocol.
Efficient and reliable verification of quantum states is central to quantum information processing applications. If using well-characterized measurement devices, effective methods have been developed for quantum state verification (QSV). In reality, however, measurement devices are generally imperfect or untrusted, which limits significantly the application of standard QSV protocols. Here, we propose the measurement-device-independent QSV (MDI-QSV) scheme for practice. With the help of trusted quantum inputs, we have developed a systematical approach to design MDI-QSV strategies for an arbitrary pure target state. We find that the number of required measurements has an optimal scaling with required accuracy and confidence level, similar to the standard QSV where trusted measurement devices are available. Our results offer a sample-efficient and realistic method for quantum state verification with virtues of a measurement-device-independent manner, and are within reach of current technology.