Bell States Research Articles

Quantum anonymous one vote veto protocol based on entanglement swapping

AbstractAs a special voting method, one‐vote veto voting also has a wide range of applications. A veto means that when the voting council puts forward a proposal, it cannot pass unless all the voters agree to it. If there is a no vote, the proposal will be rejected, but no one will know how anyone else votes. In most existing quantum anonymous one‐vote veto voting protocols, an absolutely honest third party is generally required to assist the voting. However, it is difficult to find a fully trusted third party in reality. In addition, the existing quantum anonymous one‐vote veto protocol does not consider the attack from the insider voters. Therefore, based on the characteristics of entanglement swapping between the Cat state and Bell state, the authors propose a new quantum anonymous one‐vote veto protocol, which can not only calculate the voting result quickly and effectively but also demonstrate higher security.

A modular entanglement-based quantum computer architecture

Abstract We propose a modular quantum computation architecture based on utilizing multipartite entanglement. Each module consists of a small-scale quantum computer comprising data, memory and entangling qubits. Entangling qubits are used to selectively couple different modules by harnessing some non-controllable, distance-dependent interaction, which is effectively controlled and enhanced via a proper adjusting of the internal state of the qubits. In this way, multipartite entangled states with different entanglement topologies can be shared between modules. These states are stored in memory qubits where they can be further processed so they can eventually be used to deterministically perform certain classes of gates or circuits between modules on demand, including parallel controlled-\textit{Z} gates with arbitrary interaction patterns, multi-qubit gates or whole Clifford circuits, depending on their entanglement structure. The usage of different kinds of multipartite entanglement rather than Bell pairs allows for more efficient and flexible coupling between modules, leading to a scalable quantum computation architecture.

Polarization entanglement enabled by orthogonally stacked van der Waals NbOCl2 crystals

Polarization entanglement holds significant importance for photonic quantum technologies. Recently emerging subwavelength nonlinear quantum light sources, e.g., GaP and LiNbO3 thin films, benefiting from the relaxed phase-matching constraints and volume confinement, have shown intriguing properties, such as high-dimensional hyperentanglement and robust entanglement anti-degradation. Van der Waals (vdW) NbOCl2 crystal, with strong optical nonlinearities, has emerged as a potential candidate for ultrathin quantum light sources. However, polarization entanglement is inaccessible in the NbOCl2 crystal due to its unfavorable nonlinear susceptibility tensor. Here, by leveraging the twist-stacking degree of freedom inherently in vdW systems, we showcase the preparation of polarization entanglement and quantum Bell states.

Generation of Tunable Three‐Photon Entanglement in Cubic Nonlinear Coupled Waveguides

AbstractThe generation of three‐photon states with spatial entanglement in cubic nonlinear coupled waveguides is theoretically investigated using third‐order spontaneous parametric down‐conversion and quantum walks. This approach involves independently pumping two coupled waveguides to generate a path‐encoded three‐photon Greenberger–Horne–Zeilinger (GHZ) state, which then evolves with complex spatial dynamics governed by coupling coefficients and phase mismatch. By appropriate parameter tuning, the generation of robust heralded Bell states is demonstrated, uniform states, and GHZ‐like states at the chip output. This work demonstrates an integrated source of three‐photon spatial entanglement on a simple chip, offering additional reconfigurability for advanced multiphoton quantum applications.

Quantum illumination with high-dimensional Bell states

Quantum illumination with high-dimensional Bell states

Mxene-decorated spinel oxides as innovative activators of peroxymonosulfate for degradation of caffeine in WWTP effluents: Insights into mechanisms

In the frame of the environmental issues related to the efficiency of wastewaters treatment, the generation of advanced oxidation processes (AOPs) by 2D materials appears one of the most promising solutions. In this study, a novel catalytic system for peroxymonosulfate activation (PMS) was designed based on MXene (Ti3C2Tx) decorated with spinel oxides Co3O4, Fe3O4 and CoFe2O4 catalysts. Their efficiency in caffeine (CAF) degradation via PMS activation was assessed. The insertion of spinel oxides inside the multilayer structure of MXene along with their uniform surface decoration was demonstrated by SEM and TEM analyses and it also avoided the aggregation of the magnetic particles, thus increasing their efficiency. Among the different catalysts, the MXene/CoFe2O4 (MXCF) stood out as the most effective, mainly due to the Fe and Co redox cycles. The complete degradation of CAF was achieved in the dark within 10 min at natural pH using 0.2 g/L of MXCF and 0.5 mM of PMS. The novelty of current study lies in the efficient activation of PMS by, for the first time, MXCF in the dark along with mechanistic elucidation of PMS activation. The important role of Co3+/Co2+ and Fe3+/Fe2+ redox cycles alongside surface bound functional groups were highlighted. Radical scavenging and EPR experiments confirmed •OH and 1O2 as the main ROS involved in the CAF degradation. The CAF degradation pathways pointed to hydroxylation and imidazole ring opening mechanisms and MXCF catalyst also exhibited high efficiency in the degradation of sulfamethoxazole and phenol via PMS activation. To further highlight the relevance of the obtained results, treatment of tertiary effluents of wastewaters treatment plant (WWTP) in Bratislava contaminated by CAF exhibited a complete pollutant degradation after 3 h by supplying 0.2 g/L of catalyst and 2 mM PMS in the dark.

Complete Bell state measurement for polarization-path high-dimensional hyperentangled photons

Complete Bell state measurement for polarization-path high-dimensional hyperentangled photons

Efficient Quantum Private Comparison with Unitary Operations

Quantum private comparison (QPC) is a crucial component of quantum multiparty computing (QMPC), allowing parties to compare their private inputs while ensuring that no sensitive information is disclosed. Many existing QPC protocols that utilize Bell states encounter efficiency challenges. In this paper, we present a novel and efficient QPC protocol that capitalizes on the distinct characteristics of Bell states to enable secure comparisons. Our method transforms private inputs into unitary operations on shared Bell states, which are then returned to a third party to obtain the comparison results. This approach enhances efficiency and decreases the reliance on complex quantum resources. A single Bell state can compare two classical bits, achieving a qubit efficiency of 100%. We illustrate the feasibility of the protocol through a simulation on the IBM Quantum Cloud Platform. The security analysis confirms that our protocol is resistant to both eavesdropping and attacks from participants.

Entanglement and iSWAP gate between molecular qubits.

Quantum computation and simulation rely on long-lived qubits with controllable interactions. Trapped polar molecules have been proposed as a promising quantum computing platform, offering scalability and single-particle addressability while still leveraging inherent complexity and strong couplings of molecules1-5. Recent progress in the single quantum state preparation and coherence of the hyperfine-rotational states of individually trapped molecules allows them to serve as promising qubits6-11, with intermolecular dipolar interactions creating entanglement12,13. However, universal two-qubit gates have not been demonstrated with molecules. Here we harness intrinsic molecular resources to implement a two-qubit iSWAP gate using individually trapped X1Σ+ NaCs molecules. By allowing the molecules to interact for 664 μs at a distance of 1.9 μm, we create a maximally entangled Bell state with a fidelity of 94(3)% in trials in which both molecules are present. Using motion-rotation coupling, we measure residual excitation of the lowest few motional states along the axial trapping direction and find them to be the primary source of decoherence. Finally, we identify two non-interacting hyperfine states within the ground rotational level in which we encode a qubit. The interaction is toggled by transferring between interacting and non-interacting states to realize an iSWAP gate. We verify the gate performance by measuring its logical truth table.

Pursuing high-fidelity control of spin qubits in natural Si/SiGe quantum dot

Electron spins in silicon quantum dots are a promising platform for fault-tolerant quantum computing. Low-frequency noise, including nuclear spin fluctuations and charge noise, is a primary factor limiting gate fidelities. Suppressing this noise is crucial for high-fidelity qubit operations. Here, we report on a two-qubit quantum device in natural silicon with universal qubit control, designed to investigate the upper limits of gate fidelities in a non-purified Si/SiGe quantum dot device. By employing advanced device structures, qubit manipulation techniques, and optimization methods, we have achieved single-qubit gate fidelities exceeding 99% and a two-qubit controlled-Z (CZ) gate fidelity of 91%. Decoupled CZ gates are used to prepare Bell states with an average fidelity of 91%, typically exceeding previously reported values in natural silicon devices. These results underscore that even natural silicon has the potential to achieve high-fidelity gate operations, particularly with further optimization methods to suppress low-frequency noise.

Wavepacket interference of two photons through a beam splitter: from temporal entanglement to wavepacket shaping

Quantum interferences based on beam splitting are widely used for entanglement. However, the quantitative measurement of the entanglement in terms of temporal modes and wavepacket shaping facilitated by this entanglement remain unexplored. Here we analytically study the interference of two photons with different temporal shapes through a beam splitter (BS) and then propose its application in temporal entanglement and shaping of photons. The temporal entanglement described by Von Neumann entropy is determined by the splitting ratio of BS and temporal indistinguishability of input photons. We found that maximum mode entanglement can be achieved with a 50/50 BS configuration, enabling the generation of a Bell state encoded in temporal modes, independent of the exact form of the input photons. Then, detecting one of the entangled photons at a specific time enables the probabilistic shaping of the other photon. This process can shape the exponentially decaying (ED) wavepacket into the ED sine shapes, which can be further shaped into Gaussian shapes with a fidelity exceeding 99%. The temporal entanglement and shaping of photons based on interference may solve the shape mismatch issues in large-scale optical quantum networks.

Quantum Entanglement an Indispensable Tool of Quantum Teleportation

One of the most crucial procedures in quantum information is quantum teleportation. Quantum teleportation, which makes use of the physical resource of entanglement, is a fundamental primitive in many quantum information tasks and is a crucial component of quantum technologies. It is essential to the ongoing development of quantum networks, quantum computing, and quantum communication. Here, the practical applications of Bell states, algebra of entanglement, and quantum dense coding are highlighted. The fundamentals of quantum entanglement are described, along with its characteristics and the Einstein-Poldolsky-Rosen dilemma. By separating its distinctive properties, this paper demonstrated that teleportation will be used practically for quantum key distribution in the very near future. It also revealed the fundamental theoretical concepts behind quantum teleportation.

Ceric Ammonium Nitrate Oxidation of a MnII Complex Generates a Bis(μ-oxo)dimanganese(IV,IV) Product via a MnIV-Oxo Intermediate.

The oxidation of manganese complexes using ceric ammonium nitrate (CAN) is often complicated by the fact that cerium(IV) can serve as both an oxidant and a Lewis acid. In this work, we explore the reaction of CAN with the MnII complex [MnII(OTf)(DMMN4py)](OTf) (DMMN4py = N,N-bis(4-methoxy-3,5-dimethyl-2-pyridylmethyl)-N-bis(2-pyridyl)methylamine). We chose this complex as multiple oxidation products, including oxomanganese(IV) and bis(μ-oxo)dimanganese(III,IV) complexes, have previously been reported. We envisioned that knowledge of the spectral properties of these intermediates would aid in understanding the potential complexities of CAN oxidation reactions. The oxidation of [MnII(OTf)(DMMN4py)](OTf) with 2.0 equiv CAN in 9:1 (v/v) MeCN:H2O at 25 °C transiently forms the MnIV-oxo complex, but this species is formed in low yields and is unstable. The MnIV-oxo complex evolves to a new intermediate that had not been previously observed. At lower temperatures, the formation of this new intermediate is preceded by formation of the previously reported bis(μ-oxo)dimanganese(III,IV) complex. EPR and X-ray absorption experiments for the new intermediate provide strong evidence for its formulation as [MnIVMnIV(μ-O)2(DMMN4py)2]4+. This work establishes dinuclear MnIVMnIV species as products that must be considered in CAN oxidation reactions of MnII complexes and shows that both mononuclear MnIV-oxo and dinuclear MnIIIMnIV complexes can be intermediates in such reactions.

Universal control of four singlet-triplet qubits.

The coherent control of interacting spins in semiconductor quantum dots is of strong interest for quantum information processing and for studying quantum magnetism from the bottom up. Here we present a 2 × 4 germanium quantum dot array with full and controllable interactions between nearest-neighbour spins. As a demonstration of the level of control, we define four singlet-triplet qubits in this system and show two-axis single-qubit control of each qubit and SWAP-style two-qubit gates between all neighbouring qubit pairs, yielding average single-qubit gate fidelities of 99.49(8)-99.84(1)% and Bell state fidelities of 73(1)-90(1)%. Combining these operations, we experimentally implement a circuit designed to generate and distribute entanglement across the array. A remote Bell state with a fidelity of 75(2)% and concurrence of 22(4)% is achieved. These results highlight the potential of singlet-triplet qubits as a competing platform for quantum computing and indicate that scaling up the control of quantum dot spins in extended bilinear arrays can be feasible.

Enhanced Cu-EDTA degradation with trace Cu(II) in limestone via activating PMS

Effective removal of heavy metal complexes from contaminated water remains a challenging task. In this work, a limestone/PMS system was constructed for the efficient degradation of Cu-EDTA. Under the conditions of 3.0 g/L limestone and 1.0 mM PMS, the degradation efficiency of 10.0 mg/L Cu-EDTA reached 99.0 % in 60 min, and the removal efficiency of Cu ions approached 50.0 %. The main active specie revealed by quenching and EPR experiments was the singlet oxygen (1O2). Interestingly, Trace amounts of Cu(II) in limestone react with PMS enhancing the degradation of Cu-EDTA. The results of the HPLC-MS analysis showed that the reactive species attacked the Cu–O and Cu–N bonds, as well as triggered a decarboxylation process, resulting in the decomposition of Cu-EDTA into six intermediates. Meanwhile, SEM and XPS analyses revealed that Cu ions released during the degradation of Cu-EDTA are deposited on the limestone surface as Cu(OH)2 and Cu2(OH)2CO3. In addition, the initial pH and co-existing ions have a weak effect on the system. Finally, a one-month continuous flow experiment showed that the degradation efficiency of Cu-EDTA was about 95.0 % and the removal efficiency of Cu ions was 40.0 %. In conclusion, this study provides a new approach for the destruction of metal complexes and the removal of heavy metals.

Robust Teleportation of a Surface Code and Cascade of Topological Quantum Phase Transitions

Teleportation is a facet where quantum measurements can act as a powerful resource in quantum physics, as local measurements allow us to steer quantum information in a nonlocal way. While this has long been established for a single Bell pair, the teleportation of a many-qubit entangled state using nonmaximally entangled resources presents a fundamentally different challenge. Here, we investigate a tangible protocol for teleporting a long-range entangled surface-code state using elementary Bell measurements and its stability in the presence of coherent errors that weaken the Bell entanglement. We relate the underlying threshold problem to the physics of anyon condensation under weak measurements and map it to a variant of the Ashkin-Teller model of statistical mechanics with Nishimori-type disorder, which gives rise to a cascade of phase transitions. Tuning the angle of the local Bell measurements, we find a continuously varying threshold. Notably, the threshold moves to infinity for the X+Z angle along the self-dual line—indicating that infinitesimally weak entanglement is sufficient in teleporting a self-dual topological surface code. Our teleportation protocol, which can be readily implemented in dynamically configurable Rydberg-atom arrays, thereby gives guidance for a practical demonstration of the power of quantum measurements. Published by the American Physical Society 2024

Pre- and post-compensation to suppress birefringent walk-off effects of entangled photons.

Photon-based entanglement sources, crucial for obtaining entangled states, are typically generated via spontaneous parametric down-conversion. However, birefringence in nonlinear crystals causes spatial or temporal walk-off, reducing entangled photon quality. The theoretical analysis attributes birefringent walk-off to dispersive materials curbed by pre- and post-compensation. We experimentally validate this method, enhancing polarization-entangled photon quality. We measured the Bell state along with a violation of CHSH-Bell's inequality by ∼366 standard deviations (S = 2.623 ± 0.0017). Our approach, simple and stable, does not complicate setups. Its applicability extends to optimizing various entanglement source schemes, aiding optical quantum technologies such as computation, sensing, and communication.

Naphthalene Diimide and Bis-Heteroleptic Ru(II) Complex-Based Hybrid Molecule with 3-in-1 Functionalities.

Multipurpose applications of a newly developed homobimetallic Ru(II) complex, Ru-NDI[PF6]4, which incorporates 1,10-phenanthroline and triazole-pyridine ligands and linked via a (-CH2-)3 spacer to the reputed anion-π interacting NDI system, are described. Solution-state studies of the bimetallic complex, including EPR, PL, UV-vis, and NMR experiments, reveal two sequential one-electron transfers to the NDI unit, generating NDI⋅- and NDI2- in the presence of F- selectively. This process inhibits the primary electron transfer from Ru(II) to the NDI unit, thereby allowing the 3MLCT-based emission of the complex to be recovered, resulting in a corresponding ten-fold increase in luminescence intensity. DFT and TD-DFT computational studies further elucidate the experimentally observed absorption spectra of the complex. Secondly, CT-DNA binding studies with the complex are performed using various spectroscopic analyses such as UV-vis, PL, and CD. Comparative DNA binding studies employing EB and molecular docking reveal that the binding with CT-DNA occurs through both intercalative and groove binding modalities. Thirdly, the photocatalytic activities of the complex towards C-C, C-N, and C-O bond formation in organic cross-coupling reactions, including the amidation of α-keto acids to amines and the oxidation of alcohol to aldehydes, are also demonstrated.

Quantum teleportation based on the elegant joint measurement

As a generalization of the well-known Bell state measurement (BSM), the elegant joint measurement (EJM) is a kind of novel two-qubit joint measurement, parameterized by a subtle phase factor θ∈[0,π/2]. We investigate here the quantum teleportation based on the EJM that conceptually goes beyond BSM. It is a probabilistic teleportation caused by nonunitary quantum evolution except for the special case of the BSM with θ=π/2. We show in detail the feasible quantum circuits to realize the present scenario, where a few unitary operations and a nonunitary quantum gate are being utilized. This work suggests that it is feasible to extend the customary (BSM-based) teleportation scenarios to the EJM-based ones, and different from the BSM being single input it may provide a continuously variable input setting or even multiple measurement settings for the sender of the quantum state.

Engineering impurity Bell states through coupling with a quantum bath

We theoretically demonstrate the feasibility of creating Bell states in multicomponent ultracold atomic gases by solely using the ability to control the interparticle interactions via Feshbach resonances. For this we consider two distinguishable impurities immersed in an atomic background cloud of a few bosons, with the entire system being confined in a one-dimensional harmonic trap. By analyzing the numerically obtained ground states we demonstrate that the two impurities can form spatially entangled bipolaron states due to mediated interactions from the bosonic bath. Our analysis is based on calculating the correlations between the two impurities in a two-mode basis, which is experimentally accessible by measuring the particle positions in the left or right sides of the trap. While interspecies interactions are crucial in order to create the strongly entangled impurity states, they can also inhibit correlations depending on the ordering of the impurities and three-body impurity-bath correlations. We show how these drawbacks can be mitigated by manipulating the properties of the bath, namely its size, mass, and intraspecies interactions, allowing one to create impurity Bell states over a wide range of impurity-impurity interactions. Published by the American Physical Society 2024