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1.
In quantum mechanics, the expectation value of an operator can be measured by using the projective measurement, ifthe coupling between the measured system and pointer is strong enough. However in the weak coupling regime, the pointercan not show all the eigenvalue of the physical quantity directly due to the overlapping among the pointer states, whichmakes the measurement of the expectation value difficult. In this paper, we propose an expectation value measurementmethod in the weak coupling regime inspired by the weak measurement scheme. Compared to the projective measurement,our scheme has two obvious advantages. Experimentally we use the internal state and motional state of a single trapped40Ca+ to establish the measurement scheme and realize the proof of principle demonstration of the scheme.  相似文献   
2.
To implement generalized quantum measurement (GQM) one has to extend the original Hilbert space.Generally speaking,the additional dimensions of the ancilla space increase as the number of the operators of the GQM n increases.This paper presents a scheme for deterministically implementing all possible n-operator GQMs on a single atomic qubit by using only one 2-dimensional ancillary atomic qubit repeatedly,which remarkably reduces the complexity of the realistic physical system.Here the qubit is encoded in the internal states of an atom trapped in an optical cavity and single-photon pulses are employed to provide the interaction between qubits.It shows that the scheme can be performed remotely,and thus it is suitable for implementing GQM in a quantum network.What is more,the number of the total ancilla dimensions in our scheme achieves the theoretic low bound.  相似文献   
3.
Positive-operator-value measurement (POVM) is the most general class of quantum measurement. We propose a scheme to deterministically implement arbitrary POVMs of single atomic qubit via cavity QED catalysed by only one ancilla atomic qubit. By appropriately entangling two atomic qubits and sequentially measuring the ancilla qubit, any POVM can be implemented step by step. As an application of our scheme, the realization of a specific POVM for optimal unambiguous discrimination (OUD) between two nonorthogonal states is given.  相似文献   
4.
5.
We reinvestigate the measures concerning the notion of duality of a quantum system (the "quanton") in an asymmetry two-way interferometer. A new measure of the which-way information is introduced, which is based on the fidelity ~ instead of the trace distance Q of the final states of the which-way detector. An improved inequality is derived to be V2 ≤ (R0^2 -P2)(1 -F)2, which is more stringent than the previous one of V2 ≤ (1 -P2)(1 - Q2).  相似文献   
6.
Shi-Wei Cui 《中国物理 B》2021,30(11):110311-110311
In order to understand our previous numerical finding that steady-state entanglement along the instability boundary remains unchanged in a three-mode optomechanical system [Phys. Rev. A 101 023838 (2020)], we investigate in detail the boundary entanglement in a simpler two-mode optomechanical system. Studies show that both the mechanism to generate entanglement and the parameter dependence of boundary entanglement are quite similar in these two models. Therefore, the two-mode system has captured the main features in the three-mode system. With the help of analytical calculations and discussing in a much bigger parameter interval, we find that the unchanging behavior previously discovered is actually an extremely slow changing behavior of the boundary entanglement function, and most importantly, this nearly invariant boundary entanglement is a general phenomenon via parametric down conversion process in the weak dissipation regime. This is by itself interesting as threshold quantum signatures in optomechanical phonon lasers, or may have potential value in related applications based on boundary quantum properties.  相似文献   
7.
The generation of various entangled states is an essential task in quantum information processing. Recently, a scheme (PRA 79, 022304) has been suggested for generating Greenberger-Horne-Zeilinger state and cluster state with atomic ensembles based on the Rydberg blockade. Using similar resources as the earlier scheme, here we propose an experimentally feasible scheme of preparing arbitrary four-qubit W class of maximally and non- maximally entangled states with atomic ensembles in a single step. Moreover, we carefully analyze the realistic noises and predict that four-qubit W states can be produced with high fidelity (F - 0.994) via our scheme.  相似文献   
8.
吴春旺  韩阳  邓志姣  梁林梅  李承祖 《中国物理 B》2010,19(1):10313-010313
A simple scheme is proposed to generate the W state of N Λ-type neutral atoms trapped in an optical cavity via Raman transition. Conditional on no photon leakage from the cavity, the N-qubit W state can be prepared perfectly by turning on a classical coupling field for an appropriate time. Compared with the previous ones, our scheme requires neither individual laser addressing of the atoms, nor demand for controlling N atoms to go through an optical cavity simultaneously with a constant velocity. We investigate the influence of cavity decay using the quantum jump approach and show that the preparation time decreases and the success probability increases with atom number because of a collective enhancement of the coupling.  相似文献   
9.
Positive-operator-value-measurement (POVM) is one of the essential components of quantum information processing ( QIP). Recently a 'binary measurement tree' (BST) strategy (PRA 77, 052104) is suggested for implementing arbitrary POVM by sequential two-operator POVMs. We present a simple novel two-operator POVM module via linear optics, which is employed as block to construct a 'binary measurement tree' for implementing arbitrary POVM on single photonic polarization qubit. The total complexity of the experimental setup is significantly reduced in contrast to the previous works. As an example, we give the detailed settings of a well-known POVM.  相似文献   
10.
We experimentally demonstrate the quantum anti-Zeno effect in a two-level system based on a single trapped ion ~(40)Ca~+. In the large detuning regime, we show that the transfer from the ground state to the excited state can be remarkably enhanced by the inserted projection measurements. The inserted measurements in our experiment are realized by the electron shelving technique. Compared to the ideal projection measurement, which makes the quantum state collapse instantaneously, a practical electron shelving process needs a finite time duration. The minimum time for this collapse process is shown to be inversely proportional to the square of the coupling strength between the measurement laser and the system.  相似文献   
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