Field Of Quantum Information Processing Research Articles

Mode-selective photon-added two-mode squeezed vacuum state and its entanglement properties

A class of non-Gaussian entangled states is introduced by applying the nonlocal single-photon addition to two-mode squeezed vacuum state and the properties of entanglement are numerically investigated according to linear entropy and Einstein–Podolsky–Rosen (EPR) steering. After the nonlocal single-photon addition operation, Wigner function of the generated state appears some negative region and loses its Gaussian property in phase space. In essence, non-Gaussian entangled states are generated after applying nonlocal single-photon addition. Additionally, by studying the linear entropy and EPR steering, we find that single-photon addition can enhance their entanglement degree and non-Gaussian steering can be witnessed in a large range of squeezing parameter for the second-order quadratures, which may provide a well application in the fields of quantum information processing.

Investigating entangled photons to quantify quantum correlations in dual optomechanical cavities.

Quantum correlations that go beyond quantum entanglement have a significant role in the fields of quantum information processing and quantum computation. In essence, when we describe a quantum system using pure states, quantum entanglement and quantum correlation are essentially indistinguishable. However, this equivalence breaks down when dealing with general mixed states. To contribute to understanding this distinction, we have considered a straightforward model for both generating and measuring these quantum correlations. We focus on two macroscopic mechanical resonators situated in separate Fabry–Perot cavities and coupled through the photon hopping process. Our model allows us to conduct a comprehensive examination and quantification of quantum correlations that extend beyond entanglement between these mechanical modes. Our approach begins with the determination of the global covariance matrix, serving as the foundation for calculating two key metrics: the Entropy of Formation and the Gaussian quantum Discord. These metrics enable us to quantify the extent of quantum entanglement and quantum correlations, respectively. Our analysis, based on these quantifiers, reveals that quantum discord serves as a more appropriate metric for characterizing the quantum correlations between the mechanical modes in an optomechanical quantum system, especially in the presence of robust photon hopping.

Port-based entanglement teleportation via noisy resource states

Port-based teleportation (PBT) represents a variation of the standard quantum teleportation and is currently being employed and explored within the field of quantum information processing owing to its various applications. In this study, we focus on PBT protocol when the resource state is disrupted by local Pauli noises. Here, we fully characterise the channel of the noisy PBT protocol using Krauss representation. Especially, by exploiting the application of PBT for entanglement distribution necessary in realizing quantum networks, we investigate entanglement transmission through this protocol for each qubit considering noisy resource states, denoted as port-based entanglement teleportation (PBET). Finally, we derive upper and lower bounds for the teleported entanglement as a function of the initial entanglement and the noises. Our study demonstrates that quantum entanglement can be efficiently distributed by protocols utilizing large-sized resource states in the presence of noise and is expected to serve as a reliable guide for developing optimized PBET protocols. To obtain these results, we address that the order of entanglement of two qubit states is preserved through the local Pauli channel, and identify the boundaries of entanglement loss through this teleportation channel.

Magneto-optical epitaxial bismuth-substituted yttrium iron garnet thin films on a diamagnetic substrate for low temperature applications

The uprising interest in the integration of the magneto-optical iron garnets into the field of quantum information processing imposes requirements for the damping parameter and optical properties at low temperatures. Typically, high quality iron garnet films are mostly grown epitaxially on the paramagnetic substrates that have rare-earth elements in its composition, which leads to an increase of their magnetic damping at temperatures below 100 K. Here, we epitaxially grew magneto-optical ferromagnetic garnet films on a diamagnetic yttrium scandium gallium garnet substrate to demonstrate narrow ferromagnetic resonance (FMR) linewidth at temperature range from 300 K to 4 K. The maximum Faraday angle measured at wavelength of 660 nm is ΘF = 0.34 ⋅103 deg/cm at 65 K. The FMR linewidth of the iron garnet film gets slightly larger as temperature decreases, but still remains at a level of 35 Oe at 4 GHz even at 4 K. The investigated iron garnet thin films grown on diamagnetic substrates can be successfully implemented in low-temperature technological applications that require low damping and high magneto-optical response.

Molecule–plasmon–photon hybridization and applications

We study a potential hybrid quantum system with a plasmonic nanocavity coupled to a vibrating mode of a single molecule and another optical cavity mode. To explore some important and valuable applications in quantum physics, we discuss and evaluate several different applications with respect to the plasmon-mediated quantum interface, the plasmon-assisted engineering of two-mode continuous-variable entanglement, and pursuing an indirect and ultrastrong molecule–photon cooperativity. In addition, governed by the relation of symmetry breaking and quantum phase transitions (QPTs),single-molecule-induced QPTs are also studied in this tripartite hybrid quantum system. This theoretical study strongly supports potential applications of this hybrid system in the field of quantum information processing. It is believed that our investigation of molecule–plasmon–photon hybridization can not only open a new avenue toward quantum manipulation, but also provide a fresh and reliable platform to carry out many applications with high efficiency.

Quantum Simulation of the Shortcut to the Adiabatic Passage Using Nuclear Magnetic Resonance.

Quantum adiabatic shortcut technology provides a technique to accelerate the quantum adiabatic process and has been widely used in various fields of quantum information processing. In this work, we proposed a two-level quantum shortcut adiabatic passage model. Then, exploiting the nuclear magnetic resonance, we experimentally simulated the dynamics of quantum shortcut adiabatic passage using the water molecules.

Certificates of quantum many-body properties assisted by machine learning

Computationally intractable tasks are often encountered in physics and optimization. Such tasks often comprise a cost function to be optimized over a so-called feasible set, which is specified by a set of constraints. This may yield, in general, to difficult and non-convex optimization tasks. A number of standard methods are used to tackle such problems: variational approaches focus on parameterizing a subclass of solutions within the feasible set; in contrast, relaxation techniques have been proposed to approximate it from outside, thus complementing the variational approach by providing ultimate bounds to the global optimal solution. In this work, we propose a novel approach combining the power of relaxation techniques with deep reinforcement learning in order to find the best possible bounds within a limited computational budget. We illustrate the viability of the method in the context of finding the ground state energy of many-body quantum systems, a paradigmatic problem in quantum physics. We benchmark our approach against other classical optimization algorithms such as breadth-first search or Monte-Carlo, and we characterize the effect of transfer learning. We find the latter may be indicative of phase transitions, with a completely autonomous approach. Finally, we provide tools to generalize the approach to other common applications in the field of quantum information processing.

Heating rate measurement and characterization of a prototype surface-electrode trap for optical frequency metrology

We present the characterization of a prototype surface-electrode (SE) trap as a first step towards the realization of a compact, single-ion optical clock based on Yb$^+$. The use of a SE trap will be a key factor to benefit from clean-room fabrication techniques and technological advances made in the field of quantum information processing. We succesfully demonstrated trapping at a 500 $\mu$m electrodes distance and characterized our trap in terms of lifetime and heating rate. This is to our knowledge the highest distance achieved for heating rates measurements in SE traps. This simple 5-wire design realized with simple materials yields a heating rate of $\mathbf{8\times 10^3}$ phonons/s. We provide an analysis of the performances of this prototype trap for optical frequency metrology.

Global and Bipartite Entanglement for Three-Qubit System Local Unitary Classes

Entanglement of multipartite quantum systems is an important resource in the field of quantum information processing. Using a measure of global entanglement and the notion of concurrence, the properties of entanglement for a three-qubit system are studied and classified in terms of local unitary classes. The connection of such classification with the three-qubit entanglement polytope is also discussed.

Spin-wave-based tunable coupler between superconducting flux qubits

Quantum computing and simulation based on superconducting qubits have achieved significant progress and entered the noisy intermediate-scale quantum (NISQ) era recently. Using a third qubit or object as a tunable coupler between qubits is an important step in this process. In this article, we propose a hybrid system made of superconducting qubit and a yttrium iron garnet (YIG) system as an alternative way to realize this. YIG thin films have spin wave (magnon) modes with low dissipation and reliable control for quantum information processing. Here, we propose a scheme to achieve strong coherent coupling between superconducting (SC) flux qubits and magnon modes in YIG thin film. Unlike the direct $\sqrt{N}$ enhancement factor in coupling to the Kittel mode and other spin ensembles, with $N$ the total number of spins, an additional spatial-dependent phase factor needs to be considered when the qubits are magnetically coupled with the magnon modes of finite wavelength. To avoid undesirable cancellation of coupling caused by the symmetrical boundary condition, a CoFeB thin layer is added to one side of the YIG thin film to break the symmetry. Our numerical simulation demonstrates avoided crossing and coherent transfer of quantum information between the flux qubit and the standing spin waves in YIG thin films. We show that the YIG thin film can be used as a tunable coupler between two flux qubits, which have a modified shape with small direct inductive coupling between them. Our results manifest that by cancellation of direct inductive coupling and indirect spin-wave couple of flux qubits we can turn on and off the net coupling between qubits. This bring magnonic YIG thin film into the field of quantum information processing.

Ultrafast quantum key distribution using fully parallelized quantum channels.

The field of quantum information processing offers secure communication protected by the laws of quantum mechanics and is on the verge of finding wider application for the information transfer of sensitive data. To improve cost-efficiency, extensive research is being carried out on the various components required for high data throughput using quantum key distribution (QKD). Aiming for an application-oriented solution, we report the realization of a multichannel QKD system for plug-and-play high-bandwidth secure communication at telecom wavelengths. We designed a rack-sized multichannel superconducting nanowire single photon detector (SNSPD) system, as well as a highly parallelized time-correlated single photon counting (TCSPC) unit. Our system is linked to an FPGA-controlled QKD evaluation setup for continuous operation, allowing us to achieve high secret key rates using a coherent-one-way protocol.

Internal Boundary Between Entanglement and Separability Within a Quantum State

Quantum states are the key mathematical objects in quantum mechanics, and entanglement lies at the heart of the nascent fields of quantum information processing and computation. However, there has not been a general, necessary and sufficient, and operational separability condition to determine whether an arbitrary quantum state is entangled or separable. In this paper, we show that whether a quantum state is entangled or not is determined by a threshold within the quantum state. We first introduce the concept of finer and optimal separable states based on the properties of separable states in the role of higher-level witnesses. Then we show that any bipartite quantum state can be decomposed into a convex mixture of its optimal entangled state and its optimal separable state. Furthermore, we show that whether an arbitrary quantum state is entangled or separable, as well as positive partial transposition (PPT) or not, is determined by the robustness of its optimal entangled state to its optimal separable state with reference to a crucial threshold. Moreover, for an arbitrary quantum state, we provide operational algorithms to obtain its optimal entangled state, its optimal separable state, its best separable approximation (BSA) decomposition, and its best PPT approximation decomposition. And the open question of how to calculate the BSA in high-dimension systems is partially done.

Dynamic decoupling for multi-level systems

<sec>Dynamical decoupling refers to a family of techniques that are widely used to suppress decoherence in various quantum systems, caused by quasi-static environmental noise. They have broad applications in the field of quantum information processing. Conventional dynamical decoupling targets at noise in two-level system such as qubits and often consists of specifically engineered sequences of <inline-formula><tex-math id="M1">\begin{document}$ \pi $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20222398_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20222398_M1.png"/></alternatives></inline-formula> pulses that swap between two different states. On the other hand, researchers do not limit their study within simple two-levels systems any more, but go and seek for even more efficient quantum hardware. A variety of quantum algorithms and schemes of quantum control using multi-level systems, such as qutrits and qudits, for quantum information processing have been proposed and implemented successfully. However, decoherence in such a multi-level system is inherently more sophisticated than that in two-level systems. So far there has been little systematic research on how to tackle decoherence problems in such systems.</sec><sec>In this work, we propose several sequences of dynamical decoupling for multi-level systems that only rely on <inline-formula><tex-math id="M2">\begin{document}$ \pi $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20222398_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20222398_M2.png"/></alternatives></inline-formula> pulses linking neighboring levels, which is easy to implement experimentally. Our results show that these sequences can efficiently suppress quasi-static noise presented in multi-level systems. In addition, by calculating the corresponding filter functions of these sequences, we are able to further analyze their effect on generic Gaussian noise that may not be quasi-static. We also give a physical explanation of the noise filtering mechanism of these sequences by considering their control functions. Other topics discussed in our work include power spectral density and correlation of noise in multi-level systems. Our work may be regarded as a first step towards a more systematic investigation of dynamical decoupling techniques applicable to multi-level systems.</sec>

Propagation of Gaussian vortex beams in electromagnetically induced transparency media.

Electromagnetically induced transparency (EIT) is an important phenomenon in quantum optics, and has a wide range of applications in the fields of quantum information processing and quantum precision metrology. Recently, with the rapid progress of the generation and detection of structured light, the EIT with structured light has attracted enormous interests and offers new and novel functionalities and applications. Here, we theoretically study the propagation and evolution of Gaussian vortex beams, a typical type of structured light, in an EIT medium with Λ-type three-level atoms. Based on the generalized Huygens-Fresnel principle, we derive the analytical expressions of fully and partially coherent Gaussian vortex beams propagating in the EIT medium, and study the evolution of the intensity and phase distributions of the beams and their dependencies on parameters such topological charge, coherence length, Rabi frequency, etc. It is shown that both the fully and partially coherent Gaussian vortex beams undergo focusing and diverging periodically during propagation. The phase singularity of the fully coherent beam keeps unchanged, while the phase singularity of the partially coherent beam experiences splitting and recombination periodically. In addition, new phase singularities with opposite topological charge are generated in the latter case. Our results not only advance the study of the interaction between structured light and coherent media, but also pave the avenue for manipulating structured light via EIT.

Normal mode splitting and four-wave mixing in Kerr-nonlinear optical system containing coupled double quantum dot

We theoretically investigate the four wave mixing (FWM) and normal mode splitting (NMS) in coupled double quantum dots cavity system in the presence of a third-order Kerr nonlinear medium. The FWM generated in the system has narrow spectral width and this can be tuned by nonlinear medium strength, the coupling strength between two coupled quantum dots and pump strength. Further, we study NMS that can also be tuned by various physical parameters and show that energy exchange between different modes is possible. The proposed model have potential applications in the field of quantum information processing.

Bound state in a giant atom-modulated resonators system

It is of fundamental interest in controlling the light–matter interaction for a long time in the field of quantum information processing. Here, we explore a model by coupling a giant atom with the dynamically-modulated coupled-resonator waveguide and find the bound state, where the light shows the localization effect and the atomic decay into resonator modes is inhibited, excited by a propagating photon. An analytical treatment based on the separation of the propagating states and localized states of light has been proposed and provides inspiring explanation of our finding, i.e., there supports a quantum channel where the propagating photon can be converted to the localized state through the quantum interference from light–atom interactions in three resonators at different frequency detunings. Our work therefore shows the potential for actively localizing the photon in a modulated coupled-resonator waveguide system interacting with the giant atom, and also points out a way to study the light–atom interaction in a synthetic frequency dimension that holds the similar Hamiltonian.

Angular dependence of spatial frequency modulation in diffusion media

An optical field will undergo coherent diffusion when it is mapped into thermal-motioned atoms, e.g., in a slow or storage light process. As was demonstrated before, such diffusion effect is equivalent to a spatial low-pass filter attenuating the high spatial frequency (SF) components of the optical field. Here, employing electromagnetically induced transparency (EIT) based light storage in hot atomic vapor, we demonstrate that the angular deviation between the control and probe beams could be utilized as a degree of freedom to modulate the SF of the probe beam. The principle is to change the diffusion-induced low-pass filter into a band-pass filter, whose SF response can be tuned by varying the direction and magnitude of the angular deviation. Transverse multimode light fields, such as optical images and Laguerre-Gaussian modes are utilized to study such SF modulation. Our findings could be broadly applied to the fields of quantum information processing, all-optical image manipulation and imaging through diffusive media.

Optimal metrology with programmable quantum sensors.

Quantum sensors are an established technology that has created new opportunities for precision sensing across the breadth of science. Using entanglement for quantum enhancement will allow us to construct the next generation of sensors that can approach the fundamental limits of precision allowed by quantum physics. However, determining how state-of-the-art sensing platforms may be used to converge to these ultimate limits is an outstanding challenge. Here we merge concepts from the field of quantum information processing with metrology, and successfully implement experimentally a programmable quantum sensor operating close to the fundamental limits imposed by the laws of quantum mechanics. We achieve this by using low-depth, parametrized quantum circuits implementing optimal input states and measurement operators for a sensing task on a trapped-ion experiment. With 26 ions, we approach the fundamental sensing limit up to a factor of 1.45± 0.01, outperforming conventional spin-squeezing with a factor of 1.87±0.03. Our approach reduces the number of averages to reach a given Allan deviation by a factor of 1.59±0.06 compared with traditional methods not using entanglement-enabled protocols. We further perform on-device quantum-classical feedback optimization to 'self-calibrate' the programmable quantum sensor with comparable performance. This ability illustrates that this next generation of quantum sensor can be used without previous knowledge of the device or its noise environment.

Quantum non-demolition measurement based on an SU(1,1)-SU(2)-concatenated atom-light hybrid interferometer

Quantum non-demolition (QND) measurement is an important tool in the fields of quantum information processing and quantum optics. The atom-light hybrid interferometer is of great interest due to its combination of an atomic spin wave and an optical wave, which can be utilized for photon number QND measurement via the AC-Stark effect. In this paper, we present an SU(1,1)-SU(2)-concatenated atom-light hybrid interferometer, and theoretically study QND measurement of the photon number. Compared to the traditional SU(2) interferometer, the signal-to-noise ratio in a balanced case is improved by a gain factor of the nonlinear Raman process (NRP) in this proposed interferometer. Furthermore, the condition of high-quality QND measurement is analyzed. In the presence of losses, the measurement quality is reduced. We can adjust the gain parameter of the NRP in the readout stage to reduce the impact due to losses. Moreover, this scheme is a multiarm interferometer, which has the potential of multiparameter estimation with many important applications in the detection of vector fields, quantum imaging, and so on.

Engineering of Hong-Ou-Mandel interference with effective noise

The Hong-Ou-Mandel effect lies at the heart of quantum interferometry, having multiple applications in the field of quantum information processing and no classical counterpart. Despite its popularity, only a few works have considered polarization-frequency interaction within the interferometer. In this paper, we fill this gap. Our system of interest is a general biphoton polarization state that experiences effective dephasing noise by becoming entangled with the same photons' frequency state, as the photons propagate through birefringent media. The photons then meet at a beam splitter, where either coincidence or bunching occurs, after which the polarization-frequency interaction continues on the output paths. Along with performing extensive theoretical analysis on the coincidence probability and different polarization states, we outline multiple interesting applications that range from constructing Bell states to an alternative delayed choice quantum eraser.