Girish S. Agarwal, Kishore T. Kapale
We show deterministic generation of Werner states as a steady state of the collective decay dynamics of a pair of neutral atom coupled to a leaky cavity and strong coherent drive. We also show how the scheme can be extended to generate $2N$-particle analogue of the bipartite Werner states.
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We investigate the concurrence and Bell violation of the standard Werner state or Werner-like states in the presence of collective dephasing. It is shown that the standard Werner state and certain kinds of Werner-like states are robust against the collective dephasing, and some kinds of Werner-like states is fragile and becomes completely disentangled in a finite-time. The threshold time of complete disentanglement of the fragile Werner-like states is given. The influence of external driving field on the finite-time disentanglement of the standard Werner state or Werner-like states is discusse…
We study the influence of noisy environment on the evolution of two-atomic system in the presence of collective damping. Generation of Werner states as asymptotic states of evolution is described. We also show that for some initial states the amount of entanglement is preserved during the evolution.
The collective behavior of ensembles of atoms has been studied in-depth since the seminal paper of Dicke [R. H. Dicke, Phys. Rev. 93, 99 (1954)], where he demonstrated that a group of emitters in collective states is able to radiate with increased intensity and modified decay rates in particular directions, a phenomenon which he called superradiance. Here, we show that the fundamental setup of two atoms coupled to a single-mode cavity can be distinctly exceeding the free-space superradiant behavior, a phenomenon which we call hyperradiance. The effect is accompanied by strong quantum fluctuati…
Pairs of spin-1/2 particles are prepared in a Werner state (namely, a mixture of singlet and random components). If the random component is large enough, the statistical results of spin measurements that may be performed on each pair separately can be reproduced by an algorithm involving local ``hidden'' variables. However, if several such pairs are tested simultaneously, a violation of the Clauser-Horne-Shimony-Holt inequality may occur, and no local hidden variable model is compatible with the results.
Entanglement properties of superconducting qubits coupled to a semi-infinite transmission line
本文研究两个超导量子比特耦合到半无限传输线时的纠缠动力学，提出实验友好的伪Werner态，并给出在超导波导量子电动力学系统中生成该态和观测纠缠突然死亡与复苏的方案。
Tight Universal Bounds on Quantum Data Hiding with Multipartite Werner States
该研究解决了多部分Werner态量子数据隐藏的最优安全界问题，证明区分偏差为O(n^2/d)，并将认证隐藏范围从n=O(d^{1/4})扩展到n=O(√d)，同时展示了表示论方法在量子性质测试中的价值。
Using collective atom phase control a population inversion-free photon echo scheme has been studied for quantum memory applications. For the inversion-free photon echoes a double rephasing method is combined with optical locking, where the optical locking coherently controls each atom phase excited by weak data pulses. As a result, intensity flattened, normally ordered, storage time-extended, and noise-free photon echoes are obtained.
We study multipartite entanglement in non-equilibrium quantum phase transition (NEQPT) attainable in a coherently driven atomic ensemble undergoing collective decay. The NEQPT arises in the steady state of the system as the drive field strength is varied in comparison with the decay parameter. A close connection is observed between the critical behavior and the multi-partite entanglement in the system determined via the von Neumann entropy. The derivative of the von Neumann entropy shows Lambda-type behavior typical of the second order phase transitions. We further show that the calculated bi-…
We show that it is possible to ``store'' quantum states of single-photon fields by mapping them onto {\it collective} meta-stable states of an optically dense, coherently driven medium inside an optical resonator. An adiabatic technique is suggested which allows to transfer non-classical correlations from traveling-wave single-photon wave-packets into atomic states and vise versa with nearly 100% efficiency. In contrast to previous approaches involving single atoms, the present technique does not require the strong coupling regime corresponding to high-Q micro-cavities. Instead, intracavity El…
Quantum metrology with critical driven-dissipative collective spin system
该研究提出基于相干驱动N个自旋1/2粒子并受压缩集体自旋衰减影响的临界耗散量子计量方案，利用耗散相变增强参数估计灵敏度，并证明稳态为纠缠自旋压缩态，可实现低于散粒噪声极限的测量。
We study the dynamics of a single excitation coherently shared amongst an ensemble of atoms and coupled to a one-dimensional wave guide. The coupling between the matter and the light field gives rise to collective phenomena such as superradiant states with an enhanced initial decay rate, but also to the coherent exchange of the excitation between the atoms. We find that the competition between the two phenomena provides a characteristic dynamics for the decay of the excitations, and remarkably exhibits an algebraic behavior, instead of the expected standard exponential one, for a large number …
Quantum correlated steady states under competing collective and individual decay
研究量子相关稳态在集体和个体衰减竞争下的表现，发现多体态混合和动态切换现象，揭示了集体耗散在去相关个体衰减下的出现机制。
A near perfect, storage time-extended photon echo-based quantum memory protocol has been analyzed by solving the Maxwell-Bloch equations for a backward scheme in a three-level system. The backward photon echo scheme is combined with a controlled coherence conversion process via control Rabi flopping to a third state, where the control Rabi flopping collectively shifts the phase of the ensemble coherence. The propagation direction of photon echoes is coherently determined by the phase matching condition between the data (quantum) and the control (classical) pulses. Herein we discuss the classic…
Multiple emitters coherently interacting with an electromagnetic mode give rise to collective effects such as correlated decay and coherent exchange interaction, depending on the separation of the emitters. By diagonalizing the effective non-Hermitian many-body Hamiltonian we reveal the complex-valued eigenvalue spectrum encoding the decay and interaction characteristics. We show that there are significant differences in the emerging effects for an array of interacting anharmonic oscillators compared to those of two-level systems and harmonic oscillators. The bosonic decay rate of the most sup…
We study the quantum dynamics of N coherently driven two-level atoms coupled to an optical resonator. In the strong coupling regime the cavity field generated by atomic scattering interferes destructively with the pump on the atoms. This suppresses atomic excitation and even for strong driving fields prevents atomic saturation, while the stationary intracavity field amplitude is almost independent of the atom number. The magnitude of the interference effect depends on the detuning between laser and cavity field and on the relative atomic positions and is strongest for a wavelength spaced latti…
Coupling a qubit coherently to an ensemble is the basis for collective quantum memories. A driven quantum dot can deterministically excite low-energy collective modes of a nuclear spin ensemble in the presence of lattice strain. We propose to gate a quantum state transfer between this central electron and these low-energy excitations -- spin waves -- in the presence of a strong magnetic field, where the nuclear coherence time is long. We develop a microscopic theory capable of calculating the exact time evolution of the strained electron-nuclear system. With this, we evaluate the operation of …
A collective description of density matrix is presented for identical multi-level atoms, which are either excited initially, driven coherently or pumped incoherently. The density matrix is defined as expectation value of projection or transition operators in a basis of atom's product states. The identical matrix elements are identified with several integers, which specify uniquely the involved operators. To remove the redundancy, these identical elements are treated as single quantity and the equation for this quantity is dervied by mapping the transition or projection operators to a single ve…
We present a study of collective multi-photon blockade in coherently driven atoms in a single mode cavity. Considering two atoms strongly coupled to an optical cavity, we show that the two-photon blockade with two-photon anti-bunching, and the three-photon blockade with three-photon anti-bunching can be observed simultaneously. The three-photon blockade probes both dressed states in the two photon and three photon spaces. The two photon and three photon blockades strongly depend on the location of two atoms in the strong coupling regime. The asymmetry in the atom-cavity coupling constants open…
A recent experiment demonstrated heterodyne detection-based frequency measurements with superradiant pulses from coherently pumped strontium atoms in an optical lattice clock system, while another experiment has analyzed the statistics of superradiant pulses from incoherently pumped calcium atoms in a similar system. In this article, we propose to perform heterodyne detection of the superradiant pulses from the calcium atoms, and analyze theoretically the corresponding atomic ensemble dynamics in terms of the rotation of a collective spin vector and the incoherent quantum jumps among superradi…
We study a coupled array of coherently driven photonic cavities, which maps onto a driven-dissipative XY spin-$\frac{1}{2}$ model with ferromagnetic couplings in the limit of strong optical nonlinearities. Using a site-decoupled mean-field approximation, we identify steady state phases with canted antiferromagnetic order, in addition to limit cycle phases, where oscillatory dynamics persist indefinitely. We also identify collective bistable phases, where the system supports two steady states among spatially uniform, antiferromagnetic, and limit cycle phases. We compare these mean-field results…
We analytically show that probabilistic amplification of a weak coherent state stored inside an atomic medium can be achieved via detection of coherently scattered photons. We show that this is because of collective excitations created among atoms in the ensemble. We describe the physics of the amplification and identify the failure events, which occur during the amplification process. The amplification is realized by coherently mapping a weak coherent state in an ensemble of $Λ$-level atoms followed by detection of multiple Raman scattered photons, conditionally projecting the coherent state …
Optical control of collective states in 1D ordered atomic chains beyond the linear regime
本文提出利用V型三能级原子组成的一维链作为时变超材料，突破亚辐射态与辐射解耦的限制，实现吸收、存储和按需发射，并证明非线性区双激发态可从超辐射态相干转移至亚辐射态，为纠缠的光学表征开辟道路。
A quantum register using collective excitations in a Bose-Einstein condensate
本研究提出一种基于玻色-爱因斯坦凝聚体相干加载自旋光学晶格的量子寄存器方案，以格点空满态为量子比特基矢，可实现高保真量子操作与非破坏测量，能有效抑制原子损耗与退相干。
We show how an ion trap, configured for the coherent manipulation of external and internal quantum states, can be used to simulate the irreversible dynamics of a collective angular momentum model known as the Dicke model. In the special case of two ions, we show that entanglement is created in the coherently driven steady state with linear driving. For the case of more than two ions we calculate the entanglement between two ions in the steady state of the Dicke model by tracing over all the other ions. The entanglement in the steady state is a maximum for the parameter values corresponding rou…
We apply collective Raman scattering to create, store and retrieve spatially multimode light in warm rubidium-87 vapors. The light is created in a spontaneous Stokes scattering process. This is accompanied by the creation of counterpart collective excitations in the atomic ensemble -- the spin waves. After a certain storage time we coherently convert the spin waves into the light in deterministic anti-Stokes scattering. The whole process can be regarded as a delayed four-wave mixing which produces pairs of correlated, delayed random images. Storage of higher order spatial modes up to microseco…
Recent experimental advances enable the fabrication of photonic lattices in which the light propagates with non-trivial energy dispersions. When interfaced with quantum emitters, such systems yield strong collective spontaneous emission phenomena, such as perfect sub-radiance, in which the decay into the bath is completely suppressed, forming bound-states-in-the-continuum. Since such photonic lattices are generally lossy, an alternative way of probing them consists in coherently driving them to an steady-state from which photoluminescence can be extracted. Here, we formalize connections betwee…
Exploiting the Photonic Non-linearity of Free Space Subwavelength Arrays of Atoms
本文研究亚波长原子阵列的内在光子非线性，提出利用双激发散射在一维阵列中制备单激发暗态并在平行阵列间实现纠缠，同时给出抑制质心涨落误差的方案。
Nanoscale magnetometry via collective many-body dynamics in diamond
该研究利用金刚石中约10^4个NV中心的集体多体动力学，实现了计量增益达7.9(2) dB的磁信号检测与8.8(3) dB的磁噪声传感，开发出可检测低至50纳米空间关联磁噪声的动量空间分辨传感模式，为相关量子传感器应用开辟了道路。
Atoms interact with each other through the electromagnetic field, creating collective states that can radiate faster or slower than a single atom, i.e. super- and sub-radiance. The generation and control of such states by engineering the dipolar interactions between atoms can enable new tools for atomic-based technologies. Atom-atom interactions in free space are limited in range, since the amplitude of the radiated field decreases inversely with distance. When the field is confined to one dimension it enables infinite-range interactions. This has been observed for atoms in an optical cavity, …
Nonclassical Driven-Dissipative Dynamics in Collective Quantum Optics
本论文研究了集体光物质系统中的合作现象，通过双光子共振和分层绝热消除方法揭示了非线性合作效应，并发现频率分辨Purcell效应可稳定纠缠态，为量子传感和技术提供新途径。
We determine the steady-state phases of a driven-dissipative Bose-Hubbard model, describing, e.g., an array of coherently pumped nonlinear cavities with a finite photon lifetime. Within a mean-field master equation approach using exact quantum solutions for the one-site problem, we show that the system exhibits a tunneling-induced transition between monostable and bistable phases. We characterize the corresponding quantum correlations, highlighting the essential differences with respect to the equilibrium case. We also find collective excitations with a flat energy-momentum dispersion over the…
Electron beam driven collective self hybridized exciton polaritons
本研究通过阴极发光光谱发现电子束激发可增强层状半导体中激子极化激元的集体耦合强度，并揭示低能电子导致的非相干激发转变，为调控光-物质相互作用提供新途径。
Scaling law for optimal excitation storage and superradiant release in waveguide QED systems
研究波导量子电动力学系统中最优激发存储和释放的标度律，通过两个与半无限波导耦合的发射器系综，发现可扩展激发存储机制，提出三阶段协议，实现快速存储和可控发射，展示了相干和耗散集体相互作用的联合利用。
Single-qubit detection by collective phase imprinting
本文提出一种基于里德伯修饰系综的平台，通过量子比特控制的集体相移实现单量子比特非破坏性探测，态赋值保真度达99.81%，为量子信息转移与接口技术提供了新方案。
Supersolids are characterized by the counter-intuitive coexistence of superfluid and crystalline order. Here we study a supersolid phase emerging in the steady state of a driven-dissipative system. We consider a transversely pumped Bose-Einstein condensate trapped along the axis of a ring cavity and coherently coupled to a pair of degenerate counter-propagating cavity modes. Above a threshold pump strength the interference of photons scattered into the two cavity modes results in an emergent superradiant lattice, which spontaneously breaks the continuous translational symmetry towards a period…
The average time required for an open quantum system to reach a steady state (the steady-state time) is generally determined through a competition of coherent and incoherent (dissipative) dynamics. Here, we study this competition for a ubiquitous central-spin system, corresponding to a `central' spin-1/2 coherently coupled to ancilla spins and undergoing dissipative spin relaxation. The ancilla system can describe $N$ spins-1/2 or, equivalently, a single large spin of length $I=N/2$. We find exact analytical expressions for the steady-state time in terms of the dissipation rate, resulting in a…
In this work, we study the driven-dissipative dynamics of a coherently-driven spin ensemble with a squeezed, superradiant decay. This decay consists of a sum of both raising and lowering collective spin operators with a tunable weight. The model presents different critical non-equilibrium phases with a gapless Liouvillian that are associated to particular symmetries and that give rise to distinct kinds of non-ergodic dynamics. In Ref. [1] we focus on the case of a strong-symmetry and use this model to introduce and discuss the effect of dissipative freezing, where, regardless of the system siz…
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