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Einselection from Incompatible Decoherence Channels Einselection from incompatible decoherence channels Alexandre Feller,1, ∗ Guillaume Coeuret Cauquil,1 and Benjamin Roussel1, y 1Advanced Concepts Team, European Space Agency, Noordwijk, 2201 AZ, Netherlands Decoherence of quantum systems from entanglement with an unmonitored environment is to date the most compelling explanation of the emergence of a classical picture from a quantum world. While it is well understood for a single Lindblad operator, the role in the einselection process of a complex system-environment interaction remains to be clarified. In this paper, we analyze an open quantum dynamics inspired by CQED experiments with two non-commuting Lindblad operators modeling decoherence in the number basis and dissipative decoherence in the coherent state basis. We study and solve exactly the problem using quantum trajectories and phase-space techniques. The einselection optimization problem, which we consider to be about finding states minimizing the variation of some entanglement witness at a given energy, is studied numerically. We show that Fock states remain the most robust states to decoherence up to a critical coupling. I. INTRODUCTION herence channels [12{15]. Interesting physical effects are coming from the subtle interplay between those incom- patible channels. For instance, the physics of a quantum The strangeness of the quantum world comes from the magnetic impurity in a magnetic medium can be modeled principle of superposition and entanglement. Explaining as an open quantum system problem in which the envi- the absence of those characteristic features in the classical ronment probes all three Pauli matrices, instead of only world is the key point of the quantum-to-classical tran- one as in the standard spin-boson model [16, 17]. De- sition problem. The fairly recent theoretical and experi- pending on the relative values of the coupling constants, mental progresses have largely reshaped our understand- different classical regimes emerge at low energy with one ing of the emergence of a classical world from quantum channel dominating the others. What's more, for some theory alone. Indeed, decoherence theory [1] has greatly fine tuned cases, the more interesting phenomena of de- clarified how quantum coherence is apparently lost for an coherence frustration occurs [17] where all channels con- observer through the entanglement of the system with a tribute to suppress the loss of coherence of the system at large unmonitored environment. At the same time, this all energy scales. This kind of subtleties in the einselec- interaction with the environment allows to understand tion process have also been studied in circuit QED [18{ the emergence of specific classical pointer states, a pro- 22] where, once again, the emergent classical picture is cess called einselection. strongly dependent on the structure of the environment However, in the case of complex environments, the and the relative strength of its decoherence channels. physics of decoherence can be much more involved. It is then necessary to consider the structure of the envi- Thus, in the case of several incompatible decoherence ronment. A recent approach exploring this path, called channels, the problem of the emergence of a privileged quantum Darwinism [2,3], considers an environment basis is far from trivial. In this paper, we analyze the composed of elementary fragments accessing a partial einselection process of a system in contact with two com- information about the system. While a lot of insights peting decoherence channels inspired by cavity QED ex- were originally model-based [4{8], it has been proved periments. The system a mode of the electromagnetic that some basic assumptions of the quantum Darwinism field confined in a high-quality cavity. The two sources approach, like the existence of a common objective ob- of decoherence, one in the coherent state family and the servable, come directly from the theoretical framework of other in the number basis, comes for the imperfections quantum theory [9{11]. of this cavity and the atoms used to probe the field that arXiv:2001.10851v2 [quant-ph] 9 Apr 2020 Another possible source of complexity comes from the are sent through the cavity. We base our analysis on the possibility that the environment can measure different Lindblad equation to describe the effective open quan- observables of the system. Formally, the effective dynam- tum dynamics. The problem is solved exactly using the ics of the system will be described by many Lindblad op- quantum trajectory, the characteristic function and the erators which may be non commuting. When right eigen- quantum channel approaches allowing to properly char- vectors of a Lindblad operator exist, which correspond to acterize the different parameter regimes. These methods exact pointer states, we say that the operator describes allow to go beyond the formal solution [14, 15] by offering a decoherence channel. Non-commuting Lindblad oper- a clearer physical representation of the dynamics and the ators will give rise to what we call incompatible deco- relevant timescales of the the problem. Still, the einse- lection problem, which is about finding the most robust (approximate) states to the interaction with the environ- ment, has to be solved numerically. We choose those ∗Electronic address: [email protected] states as the pure states that minimize the short-time yElectronic address: [email protected] variation of some entanglement witness (the purity) with 2 the environment at a given fixed energy. Depending on y bk; b the relative values of the coupling constant, we find the (a) k gk intuitive result that the einselected states interpolate be- χ(t) Atom Atom’s EM env (a) tween Fock and coherent states. However, we uncover !eg !k the remarkable fact that Fock states remain exactly the Cavity optimal einselected states up to a critical value of the (c) y g ck; c couplings that can be obtained analytically. Finally, in !c k k the long-term dynamics, the einselection process appears Cavity’s EM env (c) to be much more complicated. !k The paper is structured as follows. In Section II, we present a summary of techniques to derive the reduced FIG. 1: Dynamics of the cavity. The cavity is coupled to dynamics from the full system-environment unitary evo- its own electromagnetic environment and to an atom, itself lution. This section can be skipped if the reader accepts coupled to its own electromagnetic environment. We suppose Eq. (4). The core results are presented in Section III that the two electromagnetic baths are not coupled. Even- where the model is solved exactly. The einselection pro- tually, the coupling of the cavity to its bath leads to photon cess is analyzed in SectionIV and we present numerical losses. The coupling with the atom, in the dispersive regime, evaluation of the Wigner function of the system which leads to decoherence on the photon basis. are used to properly analyze the different einselection regimes. We conclude in SectionV by discussing how our analysis could be generalized and understood from mode on the coherent state basis of the electromagnetic a more abstract perspective through the algebraic struc- field [26], and introduces dissipation. ture of the jump operators. We thus have two natural decoherence channels in this experimental setup, one which selects photon number states and the other coherent states. However, those two basis are incompatible in the sense that they are asso- II. MOTIVATIONS FROM CQED ciated to complementary operators, the phase and num- ber operators. This strong incompatibility between those By managing to entangle a single mode of the electro- classical states motivates the question on which classical magnetic field and an atom, cavity quantum electrody- pictures, if any, emerge from such a constrained dynam- namics (CQED) is a nice experimental setup to explore ics. the foundations of quantum theory, quantum information Before analyzing this problem in details, let's first put and the physics of the quantum to classical transition. the previous discussion on firmer ground by deriving an One implementation of CQED [23] uses an electromag- open quantum system dynamics of the Lindblad form. netic mode trapped in high quality factor mirrors as a We use a Born-Markov approximation of an effective system which is probed by a train of atoms acting as two- Hamiltonian describing a CQED experiment represented level systems (qubits) conveniently prepared. The setup on Fig.1. As a starting point, consider the following can work in different regimes where the qubit is in reso- Hamiltonian: nance or not with the field. The off-resonance functioning mode, also called the dispersive regime, is particularly in- H = HS + HE + HSE ; (1a) teresting since the atoms can be thought as small trans- HS = ~!c N; (1b) parent dielectric medium with respect to the field with 1 X (c) X (a) an index of refraction depending on its state. The atom H = ! σ + ! cy c + ! by b E 2~ eg z ~ k k k ~ k k k is able to register some phase information about the field k k making it a small measurement device. In fact, a train X (a) y + σz · ~g (b + bk) ; (1c) of atoms can be thought as a non-destructive measure- k k k ment device probing the number of photons in the field. X (c) y Focusing our attention on the field itself, the atoms have HSE = ~χ(t) N · σz + a · gk ck + h:c: ; (1d) to be considered as part of the environment of the field k (even if it is well controlled by the experimentalist) giv- with (a; b; c) ladder operators of harmonic oscillators as- ing rise to a decoherence channel having the Fock states sociated to the system and two environments respec- as pointer states [23{25]. Similar experiments can also tively, N = aya the number operator for the system oscil- now be performed on circuit QED platforms [22].
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