Using non-relativistic many body quantum field theory, a master equation is derived for the reduced density matrix of a dilute gas of massive particles undergoing scattering interactions with an environment of light particles. The dynamical variable that naturally decoheres (the pointer basis) is essentially the local number density of the dilute gas. Earlier master equations for this sort of system (such as that derived by Joos and Zeh) are recovered on restricting to the one-particle sector for the distinguished system. The derivation shows explicitly that the scattering environment stores information about the system by ``measuring'' the number density. This therefore provides an important example of the general connection between decoherence and records indicated by the decoherent histories approach to quantum theory. It also brings the master equation for this system into a form emphasizing the role of local densities, which is relevant to current work on deriving hydrodynamic equations from quantum theory.
The use of rotational degrees of freedom of quantum systems in quantum technologies is limited by environmental effects under decoherence mechanism. Here, we study the mechanism of decoherence based on a new formalism of elastic scattering for a nonspherical quantum system prepared in a superposition of rotational degrees of freedom. We show that for a dielectric ellipsoid immersed in an environment composed of thermal photons the rotational decoherence rate depends on the angular differences between the two configurations of the system and the seventh power of the temperature which is differe…
Environmental records unlock universal quantum computation from thermal decoherence
HK Institute of Quantum Science & Technology, The University of Hong Kong（香港量子科学研究所，香港大学）；QICI Quantum Information and Computation Initiative, School of Computing and Data Science, The University of Hong Kong（QICI量子信息与计算倡议，计算与数据科学学院，香港大学）
We experimentally and theoretically investigate the scattering of a photonic quantum field from another stored in a strongly interacting atomic Rydberg ensemble. Considering the many-body limit of this problem, we derive an exact solution to the scattering-induced spatial decoherence of multiple stored photons, allowing for a rigorous understanding of the underlying dissipative quantum dynamics. Combined with our experiments, this analysis reveals a correlated coherence-protection process in which the scattering from one excitation can shield all others from spatial decoherence. We discuss how…
The mechanism of decoherence for a quantum system with rotational degrees of freedom is studied. From a simple model of elastic scattering, we show that the non-diagonal density matrix elements of the system exponentially decay. The decay rate depends on the difference of scattering amplitudes for different rotational configurations, leading to the gradual loss of quantum coherence between the pointer states in the system orientational space. For a dielectric ellipsoid immersed in a photon-gas environment (assuming no absorption), the decay rate is found to be proportional to the seventh power…
我们目前的理论和实验研究的超精细基态叠加由于弹性瑞利散射光的非共振较高的激发态的退相干。我们证明，在适当的条件下，弹性瑞利散射可以是退相干的主要来源，在文献中以前的讨论相反。我们表明，弹性散射退相干率的两个水平的弹性散射\textit{amplitudes}之间的差异的平方给出的两个水平，并为某些失谐的光，振幅可以建设性地干涉，即使当弹性散射\textit{rates}从两个水平是相等的。通过计算和测量$^9$ Be $^+$在4.5 T磁场中基态价电子自旋能级叠加的总退相干率，我们证实了这一预言.
Approximate Decoherence, Recoherence and Records in Isolated Quantum Systems
Instituto de Física de Cantabria (IFCA), Universidad de Cantabria–CSIC（坎塔布里亚物理研究所（IFCA），坎塔布里亚大学-西班牙国家研究委员会）；Física Teòrica: Informació i Fenòmens Quàntics, Departament de Física, Universitat Autònoma de Barcelona（巴塞罗那自治大学物理系理论物理：信息与量子现象）；Department of Mathematics/Computer Science/Physics, University of Osnabrück（奥斯纳布吕克大学数学/计算机科学/物理系）；Department Mathematik/Informatik–Abteilung Informatik, Universität zu Köln（科隆大学数学/信息学系-信息学部）；ICREA – Institució Catalana de Recerca i Estudis Avançats（加泰罗尼亚高级研究与教育学院）
Quantum Darwinism recognizes the role of the environment as a communication channel: Decoherence can selectively amplify information about the pointer states of a system of interest (preventing access to complementary information about their superpositions) and can make records of this information accessible to many observers. This redundancy explains the emergence of objective, classical reality in our quantum Universe. Here, we demonstrate that the amplification of information in realistic spin environments can be quantified by the quantum Chernoff information, which characterizes the distin…
Decoherence of a two-state atom driven by coherent light
由驱动二态原子的激光场的量子性质引起的退相干的最近研究[J. Gea-Banacloce，Phys.Rev.A65，022308（2002）; S. J. van Enk和H. J. Kimble，Quantum Inf. and Comp.2，1（2002）]已被Itano [W. M. Itano，Phys.Rev.A68，046301（2003）]，并且提出所有的退相干都是由于自发发射。我们在级联开放量子系统的形式主义内分析这个问题。我们的结论与板野提案一致。我们表明，退相干，然而，可以分为两个部分-由于向前散射和散射的激光模式。以前的作者将前者归因于激光场的量子性质。
Motivated by trapping and cooling experiments with non-spherical nanoparticles, we discuss how their combined rotational and translational quantum state is affected by the interaction with a gaseous environment. Based on the quantum master equation in terms of orientation-dependent scattering amplitudes, we evaluate the localization rate for gas collisions off an anisotropic van der Waals-type potential and for photon scattering off an anisotropic dielectric. We also show how pure angular momentum diffusion arises from these open quantum dynamics in the limit of weak anisotropies.
We describe a new mechanism of decoherence in excited atoms as a result of thermal particles scattering by the atomic nucleus. It is based on the idea that a single scattering will produce a sudden displacement of the nucleus, which will be perceived by the electron in the atom as an instant shift in the electrostatic potential. This will leave the atom's wave-function partially projected into lower-energy states which will lead to decoherence of the atomic state. The decoherence is calculated to increase with the excitation of the atom, making observation of the effect easier in Rydberg atoms…
Tomography of a quantum state is usually based on positive operator-valued measure (POVM) and on their experimental statistics. Among the available reconstructions, the maximum-likelihood (MaxLike) technique is an efficient one. We propose an extension of this technique when the measurement process cannot be simply described by an instantaneous POVM. Instead, the tomography relies on a set of quantum trajectories and their measurement records. This model includes the fact that, in practice, each measurement could be corrupted by imperfections and decoherence, and could also be associated with …
We experimentally investigate a superconducting qubit coupled to the end of an open transmission line, in a regime where the qubit decay rates to the transmission line and to its own environment are comparable. We perform measurements of coherent and incoherent scattering, on- and off-resonant fluorescence, and time-resolved dynamics to determine the decay and decoherence rates of the qubit. In particular, these measurements let us discriminate between non-radiative decay and pure dephasing. We combine and contrast results across all methods and find consistent values for the extracted rates. …
The quantum state of motion of a large and rotating polar molecule can lose coherence through the collisions with gas atoms. We show how the associated quantum master equation for the center of mass can be expressed in terms of the orientationally averaged differential and total scattering cross sections, for which we provide approximate analytic expressions. The master equation is then utilized to quantify collisional decoherence in a interference experiment with polar molecules.
We present the quantum master equation describing the coherent and incoherent dynamics of a rapidly rotating molecule in presence of a thermal background gas. The master equation relates the rate of rotational alignment decay and decoherence to the microscopic scattering amplitudes, which we calculate for anisotropic van der Waals scattering. For large rotational energies, we find excellent agreement of the resulting alignment decay rate with recent superrotor experiments.
Continuous weak measurement allows localizing open quantum systems in state space, and tracing out their quantum trajectory as they evolve in time. Efficient quantum measurement schemes have previously enabled recording quantum trajectories of microwave photon and qubit states. We apply these concepts to a macroscopic mechanical resonator, and follow the quantum trajectory of its motional state conditioned on a continuous optical measurement record. Starting with a thermal mixture, we eventually obtain coherent states of 78% purity--comparable to a displaced thermal state of occupation 0.14. W…
Various models have been proposed in which the Schrödinger equation is modified to account for a decay of spatial coherences of massive objects. While optomechanical systems and matter-wave interferometry with large clusters are promising candidates to test these models, we here show that using available techniques for atoms in optical lattices, some of these models can be efficiently tested. In particular, we compare unconventional decoherence due to quantum gravity as introduced by Ellis and co-workers [Phys. Lett. B ${\bf 221}$, 113 (1989)] and conventional decoherence due to scattering of …
The prospect of controlling entanglement in interacting quantum systems offers a myriad of technological and scientific promises, given the progress in experimental studies in systems such as ultracold trapped gases. This control is often challenging because of decoherence, the process by which environmental interactions create spurious entanglements that can destroy the desired entanglement. Considering the collisional decoherence that is relevant for quantum measurements utilizing scattering in one-dimensional trapped gases, here we derive a relationship between particle masses and wave pack…
Decoherence of transported spin in multichannel spin-orbit coupled spintronic devices: Scattering approach to spin density matrix from the ballistic to the localized regime
Coupled-channels density-matrix approach to low-energy nuclear collision dynamics: A technique for quantifying quantum decoherence effects on reaction observables
Extracting macroscopic properties of a system from microscopic interactions has always been an interesting topic with the most diverse applications. Here, we use the quantum Boltzmann equation to investigate the density matrix evolution of a system of nucleons. Using the quantum field theory tools for constructing the density matrix operators and calculating the interactions is the main advantage of this equation. The right-hand side of this equation involves forward scattering and usual collision terms. As examples of application, we calculate the standard Bloch equations for the nucleon syst…
The goal of this short note is to show that the formulas I derived originally in [Phys. Rev. A 65, 022308 (2002)] regarding the errors introduced in quantum logical operations by the quantum nature of the control fields apply even in the situation discussed recently by Ozeri et al. in quant-ph/0502063, where the decoherence-inducing spontaneous Raman scattering is considerably suppressed.
We theoretically analyze the observations reported in a four-path quantum interference experiment via multiple beam Ramsey interference [Phys. Rev. Lett. 86, 559 (2001)]. In this experiment, a selective scattering of photons from just one interfering path causes decoherence. However, contrary to expectations, there is an increase in the contrast of the interference pattern, demonstrating that path selective decoherence can not only lead to a decrease, but under certain conditions, to an increase of the fringe contrast. Here we explain this seemingly counter-intuitive effect based on a model fo…
