Concatenated Greenberger-Horne-Zeilinger Research Articles

Construction of quantum gates for concatenated Greenberger–Horne–Zeilinger-type logic qubit

Concatenated Greenberger–Horne–Zeilinger (C-GHZ) state is a kind of logic qubit which is robust in noisy environment. In this paper, we encode the C-GHZ state as the logic qubit and design two kinds of quantum gates for such logic qubit. The first kind is the single logic-qubit gate which contains the logic-qubit bit-flip gate and phase-flip gate. The second kind is the logic-qubit controlled-not (CNOT) gate. We exploit the single quantum gate for physical qubit, such as bit-flip gate and phase-flip gate, and two-qubit CNOT gate to realize the logic-qubit gate. We also calculated the success probability of such logic-qubit gate based on the imperfect physical quantum gate. This protocol may be useful for future quantum computation.

Purification of the concatenated Greenberger–Horne–Zeilinger state with linear optics

Logic qubit plays an important role in quantum computation. Recent research showed that logic qubit entanglement can be used in long-distance quantum communication. In this paper, we describe an entanglement purification protocol (EPP) for one type of logic qubit entanglement, named concatenated Greenberger–Horne–Zeilinger(C-GHZ) state. Not only the logic bit-flip and phase-flip errors in the C-GHZ state but also the single physical qubit bit-flip and phase-flip errors in the logic qubit can be purified. This EPP does not require the sophisticated controlled-not gate, but only requires some feasible linear optical elements, which is feasible in the current experimental condition.

Robust Preparation of Atomic Concatenated Greenberger–Horne–Zeilinger States via Shortcuts to Adiabaticity

AbstractHere, a protocol for robust preparation of an atomic concatenated Greenberger–Horne–Zeilinger (C‐GHZ) state via shortcuts to adiabaticity (STA) is proposed. The devices for implementing the protocol consist of atoms, cavities, and the optical fibers, which are feasible with current technology. The atoms are trapped in the separated cavities allowing individual control over each atom with classical fields. STA helps to design Rabi frequencies of classical fields so that the atoms can be driven from the initial states to the target states. The numerical simulations show that the protocol holds robustness against atomic spontaneous emissions and photonic leakages. Thus, the protocol may be realized by experiments in the near future.

Polarization entanglement purification for concatenated Greenberger–Horne–Zeilinger state

Entanglement purification plays a fundamental role in long-distance quantum communication. In the paper, we put forward the first polarization entanglement purification protocol (EPP) for one type of nonlocal logic-qubit entanglement, i.e., concatenated Greenberger–Horne–Zeilinger (C-GHZ) state, resorting to the photon–atom interaction in low-quality (Q) cavity. In contrast to existing EPPs, this protocol can purify the bit-flip error and phase-flip error in both physic and logic level. Instead of measuring the photons directly, this protocol only requires to measure the atom states to judge whether the protocol is successful. In this way, the purified logic entangled states can be preserved for further application. Moreover, it makes this EPP repeatable so as to obtain a higher fidelity of logic entangled states. As the logic-qubit entanglement utilizes the quantum error correction (QEC) codes, which has an inherent stability against noise and decoherence, this EPP combined with the QEC codes may provide a double protection for the entanglement from the channel noise and may have potential applications in long-distance quantum communication.

Electronic Entanglement Concentration for the Concatenated Greenberger-Horne-Zeilinger State

Concatenated Greenberger-Horne-Zeilinger (C-GHZ) state, which encodes many physical qubits in a logic qubit will have important applications in both quantum communication and computation. In this paper, we will describe an entanglement concentration protocol (ECP) for electronic C-GHZ state, by exploiting the electronic polarization beam splitters (PBSs) and charge detection. This protocol has several advantages. First, the parties do not need to know the exact coefficients of the initial less-entangled C-GHZ state, which makes this protocol feasible. Second, with the help of charge detection, the distilled maximally entangled C-GHZ state can be remained for future application. Third, this protocol can be repeated to obtain a higher success probability. We hope that this protocol can be useful in future quantum computation based on electrons.

Generation of concatenated Greenberger–Horne–Zeilinger-type entangled coherent state based on linear optics

The concatenated Greenberger---Horne---Zeilinger (C-GHZ) state is a new type of multipartite entangled state, which has potential application in future quantum information. In this paper, we propose a protocol of constructing arbitrary C-GHZ entangled state approximatively. Different from previous protocols, each logic qubit is encoded in the coherent state. This protocol is based on the linear optics, which is feasible in experimental technology. This protocol may be useful in quantum information based on the C-GHZ state.

Generation of an arbitrary concatenated Greenberger–Horne–Zeilinger state with single photons

The concatenated Greenberger–Horne–Zeilinger (C-GHZ) state is a new kind of logic-qubit entangled state, which may have extensive applications in future quantum communication. In this letter, we propose a protocol for constructing an arbitrary C-GHZ state with single photons. We exploit the cross-Kerr nonlinearity for this purpose. This protocol has some advantages over previous protocols. First, it only requires two kinds of cross-Kerr nonlinearities to generate single phase shifts ±θ. Second, it is not necessary to use sophisticated m-photon Toffoli gates. Third, this protocol is deterministic and can be used to generate an arbitrary C-GHZ state. This protocol may be useful in future quantum information processing based on the C-GHZ state.

Feasible logic Bell-state analysis with linear optics.

We describe a feasible logic Bell-state analysis protocol by employing the logic entanglement to be the robust concatenated Greenberger-Horne-Zeilinger (C-GHZ) state. This protocol only uses polarization beam splitters and half-wave plates, which are available in current experimental technology. We can conveniently identify two of the logic Bell states. This protocol can be easily generalized to the arbitrary C-GHZ state analysis. We can also distinguish two N-logic-qubit C-GHZ states. As the previous theory and experiment both showed that the C-GHZ state has the robustness feature, this logic Bell-state analysis and C-GHZ state analysis may be essential for linear-optical quantum computation protocols whose building blocks are logic-qubit entangled state.

Efficient entanglement concentration for concatenated Greenberger–Horne–Zeilinger state with the cross-Kerr nonlinearity

Concatenated Greenberger---Horne---Zeilinger (C-GHZ) state, which encodes physical qubits in a logic qubit, has great application in the future quantum communication. We present an efficient entanglement concentration protocol (ECP) for recovering less-entangled C-GHZ state into the maximally entangled C-GHZ state with the help of cross-Kerr nonlinearities and photon detectors. With the help of the cross-Kerr nonlinearity, the obtained maximally entangled C-GHZ state can be remained for other applications. Moreover, the ECP can be used repeatedly, which can increase the success probability largely. Based on the advantages above, our ECP may be useful in the future long-distance quantum communication.

Two-step complete polarization logic Bell-state analysis.

The Bell state plays a significant role in the fundamental tests of quantum mechanics, such as the nonlocality of the quantum world. The Bell-state analysis is of vice importance in quantum communication. Existing Bell-state analysis protocols usually focus on the Bell-state encoding in the physical qubit directly. In this paper, we will describe an alternative approach to realize the near complete logic Bell-state analysis for the polarized concatenated Greenberger-Horne-Zeilinger (C-GHZ) state with two logic qubits. We show that the logic Bell-state can be distinguished in two steps with the help of the parity-check measurement (PCM) constructed by the cross-Kerr nonlinearity. This approach can be also used to distinguish arbitrary C-GHZ state with N logic qubits. As both the recent theoretical and experiment work showed that the C-GHZ state has its robust feature in practical noisy environment, this protocol may be useful in future long-distance quantum communication based on the logic-qubit entanglement.

Entanglement analysis for macroscopic Schrödinger's Cat state

Macroscopic entanglement, or say the Schrödinger's Cat state has attracted much attention for a long time. Recently, the first theoretical work of Fröwis and Dür (Phys. Rev. Lett., 106 (2011) 110402) and the first experiment of Lu et al. (Nat. Photon., 8 (2014) 364) both showed that, a new type of Schrödinger's Cat state, the logic-qubit entanglement (concatenated Greenberger-Horne-Zeilinger (C-GHZ) state) is immune and robust to the noise, and is possible to be applied in future large-scale quantum networks. In this paper, we describe a protocol of entanglement analysis for this kind of Schrödinger's Cat state. Both the Bell-state type of logic-qubit entanglement and multipartite C-GHZ state can be completely distinguished. Based on the entanglement analysis, an arbitrary unknown macroscopic Schrödinger's Cat superposed state can be teleportated and we can also perform the macroscopic entanglement swapping. Our protocol shows that it is possible to realize long-distance quantum communication and large-scale quantum network based on logic-qubit entanglement.

Experimental realization of a concatenated Greenberger–Horne–Zeilinger state for macroscopic quantum superpositions

With the help of two photonic controlled-NOT gates, a three-logical-qubit concatenated Greenberger–Horne–Zeilinger (C-GHZ) state encoded by a six-photon graph state is experimentally created. Observation of the dynamics of distillability evolving under a collective noisy environment revealed that the C-GHZ state is more robust than the conventional GHZ state.

Stability of encoded macroscopic quantum superpositions

The multipartite Greenberger-Horne-Zeilinger (GHZ) state is a paradigmatic example of a highly entangled multipartite states with distinct quantum features. However, the GHZ state is very sensitive to generic decoherence processes, where its quantum features and in particular its entanglement diminish rapidly, thereby hindering possible practical applications e.g. in the context of quantum metrology. In this paper, we discuss GHZ-like quantum states with a block-local structure and show that they exhibit a drastically increased stability against noise for certain choices of block-encoding. We analyze in detail the decay of the interference terms, the entanglement in terms of distillable entanglement and Negativity as well as the notion of macroscopicity as measured by the so-called q-index, and provide general bounds on these quantities. We focus on an encoding where logical qubits are themselves encoded as GHZ states, which leads to so-called concatenated GHZ (C-GHZ) states. We compare the stability of C-GHZ states with other types of encodings, thereby showing the superior stability of the C-GHZ states. Analytic results are complemented by numerical studies, where tensor network techniques are used to investigate the quantum properties of multipartite entangled states under the influence of decoherence.