Entangled Coherent States Research Articles

Secure sharing of one-sided quantum randomness using entangled coherent states

Secure sharing of one-sided quantum randomness using entangled coherent states

Entangled unique coherent line in the ground-state phase diagram of the spin-1/2 XX chain model with three-spin interaction.

Entangled spin coherent states are a type of quantum states that involve two or more spin systems that are correlated in a nonclassical way. These states can improve metrology and information processing, as they can surpass the standard quantum limit, which is the ultimate bound for precision measurements using coherent states. However, finding entangled coherent states in physical systems is challenging because they require precise control and manipulation of the interactions between the modes. In this work we show that entangled unique coherent states can be found in the ground state of the spin-1/2 XX chain model with three-spin interaction, which is an exactly solvable model in quantum magnetism. We use the spin squeezing parameter, the l_{1}-norm of coherence, and the entanglement entropy as tools to detect and characterize these unique coherent states. We find that these unique coherent states exist in a gapless spin liquid phase, where they form a line that separates two regions with different degrees of squeezing. We call this line the entangled unique coherent line, as it corresponds to the almost maximum entanglement between two halves of the system. We also study the critical scaling of the spin squeezing parameter and the entanglement entropy versus the system size.

Asymmetry-enhanced phase sensing via asymmetric entangled coherent states

Asymmetry-enhanced phase sensing via asymmetric entangled coherent states

The entanglement and nonclassical properties of multi-photon amplified added based two-mode entangled coherent states

We explore into the non-classical characteristics of a new entangled coherent state (CS), also known as a multi-photon addition amplified coherent state (MPAACS) gn̂â†m, which is produced by adding any quantity of photons to two-mode coherent states with various gain factor values. A superposition of photon-added amplified two-mode CSs (MPAACS) also exists in this novel entangled state. It is proven that the gain factor and Laguerre polynomials are related to the MPAACS normalization factor. Analytical discussion is also had regarding entanglement and quantum statistical properties. Regarding concurrence, our findings further suggest that by selecting suitable values for the gain factor g, photon added (m,n), and coherent amplitude, entanglement for the output state can occur. Specifically, the enhancement of the entanglement can be found at large g values at fixed (m,n) in comparison to (m,n) values at fixed g and small amplitude with ϕ=0 or pi. Besides that, the analyses of nonclassicality reveal that, when comparing the results for several values (m,n) at fixed g and with several values g at fixed (m,n), the low-order antibunching effect, the cross-correlation function, and the Wigner function all support the sub-Poissonian statistics. When analyses of the nonclassicality and the negative value of the WF for ϕ=0 are larger at fixed parameter (m,n), but larger at g fixed for ϕ=π, the TMAA-TMC states perform better.

Robust phase metrology with hybrid quantum interferometers against particle losses

Entanglement is an important quantum resource to achieve high sensitive quantum metrology. However, the rapid decoherence of quantum entangled states, due to the unavoidable environment noise, result in practically the unwanted sharp drop of the measurement sensitivity. To overcome such a difficulty, here we propose a spin-oscillator hybrid quantum interferometer to achieve the desirable precise estimation of the parameter encoded in the vibrations of the oscillator. Differing from the conventional two-mode quantum interferometers input by the two-mode NOON state or entangled coherent states (ECS), whose achievable sensitivities are strongly limited by the decoherence of the entangled vibrational states, we demonstrate that the present interferometer, input by a spin-dependent two-mode entangled state, possesses a manifest advantage, i.e., the measurement sensitivity of the estimated parameter is not influenced by the decoherence from the spin-oscillator entanglement. This is because that, by applying a spin-oscillator disentangled operation, the information of the estimated parameter encoded originally in the vibrational degrees can be effectively transferred into the spin degree and then can be sensitively estimated by the precise spin-state population measurements. As consequence, the proposed hybrid quantum interferometer possesses a manifest robustness against the particle losses of the vibrational modes. Interestingly, the achieved phase measurement sensitivity can still surpass the SQL obviously, even if relatively large number of particle loss occurs in one of the two modes. The potential application of the proposed spin-oscillator hybrid quantum interferometer is also discussed.

Local quantum uncertainty and non-commutativity measure discord in two-mode photon-added entangled coherent states

Local quantum uncertainty and non-commutativity measure discord in two-mode photon-added entangled coherent states

Quantum Electrodynamics of Intense Laser-Matter Interactions: A Tool for Quantum State Engineering

Intense laser-matter interactions are at the center of interest in research and technology since the development of high-power lasers. They have been widely used for fundamental studies in atomic, molecular, and optical physics, and they are at the core of attosecond physics and ultrafast optoelectronics. Although the majority of these studies have been successfully described using classical electromagnetic fields, recent investigations based on fully quantized approaches have shown that intense laser-atom interactions can be used for the generation of controllable high-photon-number entangled coherent states and coherent state superpositions. In this tutorial, we provide a comprehensive fully quantized description of intense laser-atom interactions. We elaborate on the processes of high-harmonic generation, above-threshold ionization, and we discuss new phenomena that cannot be revealed within the context of semiclassical theories. We provide the description for conditioning the light field on different electronic processes, and their consequences for quantum state engineering of light. Finally, we discuss the extension of the approach to more complex materials, and the impact to quantum technologies for a new photonic platform composed of the symbiosis of attosecond physics and quantum information science.12 MoreReceived 30 June 2022DOI:https://doi.org/10.1103/PRXQuantum.4.010201Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum description of light-matter interactionQuantum engineeringAtomic, Molecular & OpticalQuantum Information

Generation of a GHZ-type optical entangled coherent state without measurements

Typically, multipartite entangled coherent states are difficult to be extended and produced without measurement. We here propose a way to deterministically generate a GHZ (Greenberger–Horne–Zeilinger)-type entangled coherent state of cavities, utilizing a system consisting of a single superconducting qutrit (i.e., a three-level quantum system) and multiple microwave cavities. Due to the use of only a coupler qutrit, the architecture of the circuit system is quite simple. More importantly, our proposal does not require measurement on the state of qutrit compared with the previous proposals. Since the qutrit's third energy level is not populated during the operation, decoherence from the higher energy level is greatly minimized. Furthermore, the entire operation time is independent of the number of microwave cavities. As an example, our numerical simulations show that high-fidelity generation of a three-cavity GHZ-type entangled coherent state is feasible with present circuit quantum electrodynamics technology. This proposal is universal and can be applied to other physical systems, such as microwave or optical cavities, which are coupled to a single natural or artificial three-level atom.

Realization of Quantum Swap Gate and Generation of Entangled Coherent States

The cross fusion of quantum mechanics and information science forms quantum information science. Quantum logic gates and quantum entanglement are very important building blocks in quantum information processing. In this paper, we propose one-step schemes for realizing quantum swap gates and generating two-mode entangled coherent states via circuit QED. In our scheme, due to the adiabatic elimination of the excited state of the qutrit under the condition of large detuning, the decoherence of the spontaneous emission of the qutrit can be ignored. The fidelity of the quantum swap gate remains at a very high level. In addition, we also explore the nonclassical properties of two-mode entangled coherent states prepared in our scheme by addressing the second-order correlation function and intermodal squeezing. In particular, two classes of entangled coherent states demonstrate distinct entanglement and nonclassical behavior.

Atomic Marginal Distribution and Squeezing Phenomena of Correlated Two Modes Interacting with a Three-Level Atom in the Presence of an External Classical Field

The influence of the external classical field on a correlated two-mode of the electromagnetic field interacting with a three-level atom in the Λ structure is studied. A rotation of the atomic basis is used to remove the classical field terms. The time-dependent wave function is obtained by solving the Schrödinger equation. The influence of the classical field on the phenomenon of revival, collapse, squeezing, and marginal atomic distribution are discussed. In our analysis, the cavity field is prepared in the entangled pair coherent states and the atomic system in the upper state. The results showed that the occupation of the atomic level is significantly affected by the addition of the classical field. The presence of the classical field reduces the squeezing intervals and the extreme values of the atomic marginal distribution.

High Photon Number Entangled States and Coherent State Superposition from the Extreme Ultraviolet to the Far Infrared.

We present a theoretical demonstration on the generation of entangled coherent states and of coherent state superpositions, with photon numbers and frequencies orders of magnitude higher than those provided by the current technology. This is achieved by utilizing a quantum mechanical multimode description of the single- and two-color intense laser field driven process of high harmonic generation in atoms. It is found that all field modes involved in the high harmonic generation process are entangled, and upon performing a quantum operation, lead to the generation of high photon number optical cat states spanning from the far infrared to the extreme ultraviolet spectral region. This provides direct insights into the quantum mechanical properties of the optical field in the intense laser matter interaction. Finally, these states can be considered as a new resource for fundamental tests of quantum theory, quantum information processing, or sensing with nonclassical states of light.

Bidirectional teleportation through an entangled coherent quantum network

A bidirectional quantum teleportation protocol is introduced over a quantum network consisting of more than four members sharing a coherent entangled state, where it is implemented in a perfect or noisy environment. The results show that the amplitude of the coherent state and the decoherence parameter play important roles in maximizing or minimizing this probability. At fixed values of the amplitude and decoherence parameters, the success of probability increases as the number of users increases. The fidelity of the teleported coherent state depends on the type of encoded information, whether entangled/partial or classical information, namely, it depends on the weight parameter. The stable behavior of the fidelity is displayed when users teleport classical information.

Long-distance twin-field quantum key distribution with entangled sources.

Twin-field quantum key distribution (TFQKD), using single-photon-type interference, offers a way to exceed the rate-distance limit without quantum repeaters. However, it still suffers from photon losses and dark counts, which impose an ultimate limit on its transmission distance. In this Letter, we propose a scheme to implement TFQKD with an entangled coherent state source in the middle to increase its range, as well as comparing its performance under coherent attacks with that of TFQKD variants. Simulations show that our protocol has a theoretical distance advantage of 400 km. Moreover, the scheme has great robustness against the misalignment error and finite-size effects. Our work is a promising step toward long-distance secure communication and is greatly compatible with future global quantum networks.

Einstein-Podolsky-Rosen Steering for Mixed Entangled Coherent States.

By using the Born Markovian master equation, we study the relationship among the Einstein–Podolsky–Rosen (EPR) steering, Bell nonlocality, and quantum entanglement of entangled coherent states (ECSs) under decoherence. We illustrate the dynamical behavior of the three types of correlations for various optical field strength regimes. In general, we find that correlation measurements begin at their maximum and decline over time. We find that quantum steering and nonlocality behave similarly in terms of photon number during dynamics. Furthermore, we discover that ECSs with steerability can violate the Bell inequality, and that not every ECS with Bell nonlocality is steerable. In the current work, without the memory stored in the environment, some of the initial states with maximal values of quantum steering, Bell nonlocality, and entanglement can provide a delayed loss of that value during temporal evolution, which is of interest to the current study.

High success standard quantum teleportation using entangled coherent state and two-level atoms in cavities

We propose here a new idea for quantum teleportation of superposed coherent state which circumvents the problem of performing non-unitary \(Z_c=|\alpha \rangle \langle \alpha |-|-\alpha \rangle \langle -\alpha |\) operation on coherent state qubit. The receiver only requires to perform either no unitary operation or a \(\pi \) phase shift in order to recreate the information state. We use entangled resource \(\sim |\alpha ,\frac{\alpha }{\sqrt{2}}\rangle -|-\alpha ,-\frac{\alpha }{\sqrt{2}}\rangle \) in contrast with the usual \(\sim |\alpha ,\alpha \rangle -|-\alpha ,-\alpha \rangle \) -(both states unnormalized). Bob receives state which is then superposition of the states \(|\pm \frac{\alpha }{\sqrt{2}}\rangle \). Bob mixes these with even or odd coherent states involving superposition of states \(|\pm \frac{\alpha }{\sqrt{2}}\rangle \) to obtain a two-mode state which is one of \(\sim |I,0\rangle \pm |0,I\rangle \), \(|I\rangle \) being the information state. Bob then obtains the teleported information by using the interaction of one of these modes in two cavities with resonant two-level atoms. This scheme results in average fidelity of \(\simeq 0.95\) for \(|\alpha |^2 \simeq 10\), which increases with \(|\alpha |^2\) and tends to 1 asymptotically, varying as \(1-\frac{\pi ^2}{16|\alpha |^2}+\frac{\pi ^2(\pi ^2+8)}{256|\alpha |^4}\) for large values of \(|\alpha |^2\).

Entanglement Concentration Protocols for GHZ-type Entangled Coherent State Based on Linear Optics

We proposed two entanglement concentration protocols (ECPs) to obtain maximally entangled Greenberger-Horne-Zeilinger (GHZ)-type entangled coherent state (ECS) from the corresponding partially entangled GHZ-type ECSs. We obtained the first ECP using a partially entangled GHZ-type ECS assisted with a superposition of single-mode coherent state, however the second ECP is designed using two copies of partially entangled GHZ-type ECSs. The success probabilities have also been calculated and discussed for both the ECPs. We have further compared the success probabilities of our first ECP for 3-mode GHZ-type ECS with an ECP of 3-mode W-type ECS and found that our ECP is more efficient (maximal success probabilities) for larger value (\beta=0.7) of state parameter. For the physical realization, two optical circuits (for two ECPs) using linear optical elements, viz 50:50 beam splitter, phase shifter, and photon detectors are provided, which support the future experimental implementation possible with the present technology.

Quantum dialogue mediated by EPR-type entangled coherent states

Talking with each other on the telephone is convenient but insecure because the conversation content can be eavesdropped perfectly. Quantum dialogue protocols have thus been devised to enable two parties to talk with a reasonable level of security suited to urgent situations without the need of a prior quantum key distribution. Existing protocols use either discrete-variable or continuous-variable entangled states each of which has its own pros and cons. Here we employ Einstein–Podolky–Rosen-type entangled coherent states with fixed and large enough amplitudes which are intermediate between discrete- and continuous-variable states. The outstanding pros is the possibility of unambiguous and efficient identification of a given entangled coherent state which is necessary for the decoding process. Single-mode gates required for the encoding process are executable as well, possibly with the assistance of additional resources. Two types of control methods are introduced to protect the quantum dialogue from outsider’s attacks. The information leakage problem is also discussed showing that it hardly influences the protocol security for a long enough dialogue.

Ququats as superposition of coherent states and their application in quantum information processing

Superposition of optical coherent states (SCS) [Formula: see text], possessing opposite phases, plays an important role as qubits in quantum information processing tasks like quantum computation, teleportation, key distribution, etc. and are of fundamental importance in testing quantum mechanics. Passage of such SCS from a 50:50 beam splitter leads to generation of entangled coherent states. Recently, ququats and qutrits defined in four- and three-dimensional Hilbert space, respectively, have attracted much attention as they offer advantage in secure quantum communication. However, practical utilization of these advantages essentially requires physical realization of quantum optical ququats and qutrits. Here, we define four new multi-photonic states (MPS) with [Formula: see text] (here, [Formula: see text] or 3 and [Formula: see text]) numbers of photon and show how the SCS can be used to encode ququat using these MPS as basis vectors of a four-dimensional Hilbert space. When these MPS fall upon a 50:50 beam splitter, the resulting states are bipartite four-component entangled coherent states (BFECS) equivalently representing the entangled ququats. We briefly discuss the photon statistical properties of such MPS and BFECS. We show that these MPS and BFECS can be synthesized using even coherent states as input to an interferometer. We give a simple linear optical protocol for almost perfect teleportation of a ququat encoded in SCS with the aid of BFECS as quantum channel. We also describe how these ququats can be used for realization of higher-dimensional BB84 protocol to increase the security of quantum key distribution. Finally, we discuss the possible advantages of using SCS as ququats and BFECS as quantum channel in different quantum information processing tasks.

Quantum illumination radar with entangled coherent states

There has been a great interest in quantum metrology (e.g., quantum interferometric radar) due to its applications in sub-Rayleigh ranging and remote sensing. Despite interferometric radar has received vast amount of attentions over the past two decades, very few researches has been conducted on another type of quantum radar: quantum illumination radar, or more precisely quantum target detection. It is, in general, used to interrogate whether the low-reflectivity target in a noisy thermal bath is existed using quantum light. The entanglement properties of its emitted light source give it a unique detection advantage over the classical radar. Entangled coherent state (ECS), as a class of quantum states with high entanglement robustness in noisy environments, has been widely used in several fields of quantum science such as quantum informatics, quantum metrology . In this paper, we investigate the target detection performance of quantum illumination radar based on three different types of ECS states. We employ the two-mode squeezed vacuum state (TMSV) and the coherent state as benchmarks to compare and analyze the relationship between the entanglement strength of the three types of ECS states and their quantum illumination detection performance. We found that the detection performance of the three ECS states is better than that of the coherent state. However, it is inferior to that of the TMSV state when the target is of low reflectivity. The emitted photon number is much smaller than the background noise (we call this as “good” illumination conditions). On the contrary, quantum illumination radar has no obvious advantage over coherent state radar for target detection under other illumination conditions; further, the detection performance of these three types of ECS states is not evidently related to that of the TMSV state and the coherent state. Finally, we reveal that the target detection performance of quantum illumination for the first two types of ECS states can be determined by their entanglement strength under “good” illumination conditions by adjusting the inter-modal phase of these two ECS states while keeping the emitted photon number constant. Under other illumination conditions, there is no evidence to demonstrate the entanglement strength of ECS states being associated with their target detection performance.

Bidirectional teleportation using coherent states

In this work, we propose a simple scheme of bidirectional teleportation for transferring unknown coherent superposition states. It consists of exchanging two states by using tripartite entangled coherent states between two parts. This proposed scheme is essentially based on optical devices like beam splitters, phase shifters and photon number measurement detectors. The probability of success and the mean fidelity of quantum teleportation in the presence of quantum noise are evaluated. However, the quantum Fisher information is studied to estimate some parameters such as the weight and the phase of sent states.