Correlated dynamics of three-body Rydberg superatoms

Generation and preservation of quantum entanglement are among the primary tasks in quantum information processing. State stabilization via quantum bath engineering offers a resource-efficient approach to achieve this objective. However, current methods for engineering dissipative channels to stabilize target entangled states often require specialized hardware designs, complicating experimental realization and hindering their compatibility with scalable quantum computation architectures. In this work, we propose and experimentally demonstrate a stabilization protocol readily implementable in the mainstream integrated superconducting quantum circuits. The approach utilizes a Raman process involving a resonant (or nearly resonant) superconducting qubit array and the qubits' dedicated readout resonators to effectively develop nonlocal dissipative channels. Leveraging individual controllability of the qubits and resonators, the protocol stabilizes two-qubit Bell states with a fidelity of 90.7%, marking the highest reported value in solid-state platforms to date. Furthermore, by extending this strategy to include three qubits, an entangled W state is achieved with a fidelity of 86.2%. This demonstrates the protocol's extensibility beyond two-qubit systems. Notably, the protocol is of practical interest since it only utilizes existing hardware common to standard operations in the underlying superconducting circuits, thereby facilitating the exploration of many-body quantum entanglement with dissipative resources.

Quantum controlled-phase-flip (CPF) gate between a flying photon qubit and a stationary atomic qubit could allow the linking of distant computational nodes in a quantum network. Here we present a scheme to realize quantum CPF gate between a flying optical photon and an atomic ensemble based on cavity input-output process and Rydberg blockade. When a flying single-photon pulse is reflected off the cavity containing a Rydberg atomic ensemble, the dark resonance and Rydberg blockade induce a conditional phase shift for the photon pulse, thus we can achieve the CPF gate between the photon and the atomic ensemble. Assisted by Rydberg blockade interaction, our scheme works in the N-atoms strong-coupling regime and significantly relaxes the requirement of strong coupling of single atom to photon in the optical cavity.

While for single-mode bosonic fields, one talks about nonclassicality of quantum states, which plays an important role in various quantum information processing tasks, for composite systems, one talks about quantum correlations such as quantum entanglement and quantum discord, which are crucial and indispensable for implementation of quantum communication and computation. Both nonclassicality and quantum correlations are quantum features of physical systems and fundamental resources which can be exploited to bring supremacy over classical systems. A natural question arises on the relation and interconversion between these two resources. In this work, we investigate the conversion of single-mode nonclassicality into quantum correlations via beamsplitters. Our work is based on the idea of Asbó et al. (Phys. Rev. Lett., 94 (2005) 173602) of converting nonclassicality into quantum entanglement via a balanced beamsplitter. Through a paradigmatic setup with Gaussian states fed into the beamsplitters, we demonstrate that quantum correlations between the output states, as exhibited by either entanglement or quantum discord, increase monotonically with nonclassicality of the input Gaussian states, as quantified by an information-theoretic measure in terms of the Wigner-Yanase skew information.

The spectra of three-photon laser excitation (according to the 5S1/2 → 5P3/2 → 6S1/2 → 37P3/2 scheme) of mesoscopic ensembles of cold Rb Rydberg atoms in a magneto-optical trap, using cw single-frequency lasers in each stage, have been investigated. The ensembles consisted of 1 to 5 atoms, and their presence was recorded by the selective field ionisation method with postselection according to the number of atoms. The dependence of the spectral shape on the excitation laser pulse duration has been studied. For 2-μs pulses, the minimum spectral width was found to be 1.3 MHz; it was determined by the laser linewidth and the spectral width of laser pulse. For pulses shorter than 0.5 μs, additional spectral broadening and occurrence of Rabi oscillations in the wings of three-photon resonances have been observed, a fact indicative of implementation of coherent three-photon laser excitation of Rydberg atoms. An analysis of the spectra within the four-level theoretical model based on optical Bloch equations showed good agreement between the experimental and theoretical results. The dependence of the shape of three-photon excitation spectrum on the number of recorded atoms was also studied. An increase in the average number of atoms leads to the occurrence of a dip in the single-atom excitation spectra; this dip is due to the specificity of the excitation and detection statistics of Rydberg atoms. The results obtained are important for applications of Rydberg atoms in quantum informatics.

Decoherence suppression from disturbance of the environment is an essential task in quantum information processing. We investigate decoherence suppression of a qubit system interacting with a heat bath with phase decoherence by employing the weak measurement (WM) and quantum measurement reversal (QMR) operation. We show explicitly that the qubit decoherence can be efficiently completely suppressed by means of the combination WM and QMR, which is independent of the form of the spectral density of the reservoir and the form of initial input state.

Quantum state transfer (QST) is an important task in quantum information processing. In this study, we describe two approaches for the high-fidelity transfer of a quantum state between two opposite quantum dots attached to a multi-channel quantum network. First, we demonstrate that a high-efficiency QST can be achieved with the coherent time evolution of a quantum system without any external control. Second, we present an approach that uses an alternative mechanism for a high-fidelity QST. By adiabatically varying tunnel couplings, it is possible to implement the complete transmission of a quantum state based on this quantum mechanical mechanism.

Quantum state engineering and state characterization is a key task in quantum information processing in both discrete and continuous variable systems in the optical domain. In particular, quantum states with non-Gaussian (i.e., non-positive) Wigner quasiprobability distribution functions are crucial to universal, fault-tolerant quantum computing with continuous variables. In this talk, we present our recent results on single-photon Fock state tomography using Photon-Number-Resolving (PNR) measurements. We generated a highly pure narrow-band single-photon Fock state by heralding cavity-enhanced spontaneous-parametric-downconversion from a PPKTP optical parametric oscillator. The Wigner function was reconstructed with photon statistics obtained using superconducting transition-edge sensors with an overall system efficiency of 58(2)%. We then discuss quantum state engineering for pure displaced single-photon Fock states, optical cat states, and approximate GKP states using coherent states and single-photon states along with linear optics and PNR measurements. We report our experimental progress for the same.

Quantum state estimation (or state tomography) is an indispensable task in quantum information processing. Because full state tomography that determines all elements of the density matrix is computationally demanding, one usually takes the strategy of assuming a certain model of quantum states and identifying the model parameters. However, it is difficult to make a valid assumption given little prior knowledge on a quantum state of interest, and thus we need a reasonable model selection method for quantum state estimation. Actually, in the classical statistical estimation theory, several types of information criteria have been established and widely used in practice for appropriately choosing a classical statistical model. In this study, we propose quantum information criteria for evaluating the quality of the estimated quantum state in terms of the quantum relative entropy, which is a natural quantum analogue of the classical information criterion defined in terms of Kullback–Leibler divergence. In particular, we derive two quantum information criteria depending on the type of an estimator for the quantum relative entropy; one uses the log-likelihood and the other uses the classical shadow. The general role of information criteria is to predict the performance of an estimated model for unseen data, although it is a function of only sampled data; this generalization capability of the proposed quantum information criteria is evaluated in numerical simulations.

The interaction between atoms in a Bose–Einstein condensate and plasmon-enhanced fields is a step towards the goal of realizing hybrid atom–polariton systems for tasks in quantum information processing.

Quantum state transfer is an important task in quantum information processing. It is known that one can engineer the couplings of a one-dimensional spin chain to achieve the goal of perfect state transfer. To leverage the value of these spin chains, a spin star is potentially useful for connecting different parts of a quantum network. In this work, we extend the spin-chain engineering problem to the problems with a topology of a star network. We show that a permanently coupled spin star can function as a network switch for transferring quantum states selectively from one node to another by varying the local potentials only. Together with one-dimensional chains, this result allows applications of quantum state transfer to be applied to more general quantum networks.

Generation and characterization of entanglement are crucial tasks in quantum information processing. A hypothesis testing scheme for entanglement has been formulated. Three designs were proposed to test the entangled photon states created by the spontaneous parametric down conversion. The time allocations between the measurement vectors were designed to consider the anisotropic deviation of the generated photon states from the maximally entangled states. The designs were evaluated in terms of the p-value based on the observed data. It has been experimentally demonstrated that the optimal time allocation between the coincidence and anti-coincidence measurement vectors improves the entanglement test. A further improvement is also experimentally demonstrated by optimizing the time allocation between the anti-coincidence vectors. Analysis on the data obtained in the experiment verified the advantage of the entanglement test designed by the optimal time allocation.

We propose a nonadiabatic non-Abelian geometric quantum operation scheme to realize universal quantum computation with mesoscopic Rydberg atoms. A single control atom entangles a mesoscopic ensemble of target atoms through long-range interactions between Rydberg states. We demonstrate theoretically that both the single qubit and two-qubit quantum gates can achieve high fidelities around or above 99.9% in ideal situations. Besides, to address the experimental issue of Rabi frequency fluctuation (Rabi error) in Rydberg atom and ensemble, we apply the dynamical-invariant-based zero systematic-error sensitivity (ZSS) optimal control theory to the proposed scheme. Our numerical simulations show that the average fidelity could be 99.98% for single ensemble qubit gate and 99.94% for two-qubit gate even when the Rabi frequency of the gate laser acquires 10% fluctuations. We also find that the optimized scheme can also reduce errors caused by higher-order perturbation terms in deriving the Hamiltonian of the ensemble atoms. To address the experimental issue of decoherence error between the ground state and Rydberg levels in Rydberg ensemble, we introduce a dispersive coupling regime between Rydberg and ground levels, based on which the Rydberg state is adiabatically discarded. The numerical simulation demonstrate that the quantum gate is enhanced. By combining strong Rydberg atom interactions, nonadiabatic geometric quantum computation, dynamical invariant and optimal control theory together, our scheme shows a new route to construct fast and robust quantum gates with mesoscopic atomic ensembles. Our study contributes to the ongoing effort in developing quantum information processing with Rydberg atoms trapped in optical lattices or tweezer arrays.

Quantum information science is one of the frontier areas of science and technology. It is also an interdisciplinary area of research where scientists from physics, mathematics, and computer science can contribute alike. One of the main goals is how well one can process information using laws of quantum theory. Quantum information theory aims to process information that is not amenable with classical devices. At the same time this also provides new insights into the nature of the quantum world. Quantum information processing includes quantum computation, quantum communication, quantum cryptography and various information processing tasks. Not only does this provide a fundamental arena to investigate quantum phenomena but also gives new technological benefits like quantum teleportation, remote state preparation and secure communications. In India there is a growing interest in this emerging field. In view of this we had organized an International Program on Quantum Information (IPQI-2010) at the Institute of Physics, Bhubaneswar during January 4–January 30, 2010. In this programme, there were experts from all over the world and topics of focus were on various issues related to quantum information science and general quantum theory, characterization and quantification of entanglement, quantum channels and quantum operations, role of entanglement in quantum algorithms and quantum computation, developing new quantum algorithms, understanding of capacities of quantum channels, and new communication tasks in the multiparty case.

Controlling the evolution of photonic quantum states is crucial for most quantum information processing and metrology tasks. Due to its importance, many mechanisms of quantum state evolution have been tested in detail and are well understood; however, the fundamental phase anomaly of evolving waves, called the Gouy phase, has had a limited number of studies in the context of elementary quantum states of light, especially in the case of photon number states. Here we outline a simple method for calculating the quantum state evolution upon propagation and demonstrate experimentally how this quantum Gouy phase affects two-photon quantum states. Our results show that the increased phase sensitivity of multi-photon states also extends to this fundamental phase anomaly and has to be taken into account to fully understand the state evolution. We further demonstrate how the Gouy phase can be used as a tool for manipulating quantum states of any bosonic system in future quantum technologies, outline a possible application in quantum-enhanced sensing, and dispel a common misconception attributing the increased phase sensitivity of multi-photon quantum states solely to an effective de Broglie wavelength.

Vector beams (VBs) with potential applications are successfully utilized in many fields as light sources with a spatially-varying polarization profile in recent years. Here, we study the transmission of a VB by manipulating atomic polarization via the optical pumping effect. By using hybridly and radially polarized beams as pump and probe beams in a counter-propagating configuration, we observe a four-petal pattern intensity distribution of probe beam, and the four-petal pattern rotates with the polarization state orientation of the pump beam. The results show a polarization dependent absorption in the atomic media. We experimentally demonstrate the absorption characteristics under different polarization combinations of pump and probe beams. The Jones matrix method is used to explain this phenomenon and the simulations are consistent with the experimental observation. Our results may provide a sound foundation for applications in optical manipulation and quantum information in atomic ensembles.