Intermittent decoherence blockade Salvatore Lorenzo, 1 Stefano Longhi, 2 Albert Cabot, 3 Roberta Zambrini, 3 and Gian Luca Giorgi 3 1 Dipartimento di Fisica e Chimica, Universit´ a degli Studi di Palermo, via Archirafi 36, I-90123 Palermo, Italy 2 Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci 32, I-20133 Milano, Italy 3 IFISC (UIB-CSIC), Instituto de Fisica Interdisciplinar y Sistemas Complejos - Palma de Mallorca, E-07122. Spain It has long been recognized that emission of radiation from atoms is not an intrinsic property of individual atoms themselves, but it is largely affected by the characteristics of the photonic environment and by the collective interaction among the atoms. A general belief is that preventing full decay and/or decoherence requires the existence of dark states, i.e., dressed light-atom states that do not decay despite the dissipative environment. Here, we show that, contrary to such a common wisdom, decoherence suppression can be intermittently achieved on a limited time scale, without the need for any dark state, when the atom is coupled to a chiral ring environment, leading to a highly non-exponential staircase decay. This effect, that we refer to as intermittent decoherence blockade, arises from periodic destructive interference between light emitted in the present and light emitted in the past, i.e., from delayed coherent quantum feedback. Introduction. Spontaneous emission is a fundamental process in quantum optics and quantum electrodynamics [1–3]. While in the most typical cases it is described by an exponential decay of a quantum (atomic or solid state) system towards its ground state, accompanied by an irreversible emission of a photon [4], the properties of the surrounding photonic environment [5, 6], as well as measurement [7], or collective effects [8], can largely affect spontaneous emission, with consequences ranging form control of single-photon sources to decoherence. Dimension and geometric constraints of the photonic environment (like cavities [5]), as well as engineered sur- rounding media (for instance exhibiting band-gaps [6]), can significantly enhance or inhibit the decay rate of a single emitter. More recently, more complex photonic environments have been shown to be powerful resources for controlling light-emitter interaction in unprecedented ways [9]. Coupling one or more atoms to one-dimensional chi- ral waveguides or topological photonic structures, that break time reversal symmetry, enables to control the di- rectionality of spontaneous emission and to deeply mod- ify photon-mediated interactions, with major applica- tions in the design of integrated non-reciprocal single- photon devices, spin-photon interfaces, and in the synthe- sis of novel quantum states such as entangled spin states and photonic clusters states [10–17]. Likewise, ’giant’ artificial atoms, in which the atomic dimension greatly exceeds the ’photon’ wavelength and the time spent by light to cross the atom can not be neglected, provide a new paradigm of atom-field interaction [18–26]. Since the atom cannot be considered point-like anymore, sponta- neous emission ceases to be exponential and the decay dynamics is described by a differential-delayed equation [19, 21, 24, 26], displaying strictly non-Markovian (mem- ory) effects arising from delayed coherent quantum feed- back [27–29]. Similar memory-like effects are also found in ordinary (point-like) atoms in the presence of mirrors or retardation effects [30–38]. One among the most striking phenomena achieved through complex environments engineering is the pos- sibility to inhibit spontaneous emission and dechoerence under certain geometric conditions, i.e. the stabilization of quantum superposition states in the presence of dissi- pation. Such a decoherence/decay blockade stems from the appearance of dressed light-atom states, commonly known as dark states, or else bound states in the con- tinuum, that do not decay despite the dissipative envi- ronment. The existence of dark states and their ability to prevent quantum decay via destructive interference among different decay channels has been known since long time and studied in several areas of physics [39– 58], along with the related concept of decoherence-free subspaces [59], i.e. regions in Hilbert space which are not affected by decoherence. A fully open question is whether spontaneous emission and decoherence can be inhibited, at least transiently or intermittently, in the ab- sence of any decoherence-free subspace, or even though the atom-light system does not show any dark state. In this Letter we show rather surprisingly that, har- nessing the idea of delayed coherent quantum feedback, a point-like atom emitting in a chiral ring photonic waveg- uide, sustaining slow and fast counter-propagating pho- tonic modes, undergoes intermittent decoherence sup- pression on a fast time scale, displaying an exotic stair- case decay dynamics. Such an effect, that we refer to as intermittent decoherence blockade, arises from peri- odic destructive interference between light emitted in the present, both in fast and slow photonic modes, and light emitted in the past in the fast photonic modes. Decoherence dynamics of an atom coupled to a chi- ral ring. We consider the decay/decoherence dynam- ics of a two-level atom coupled to the radiation modes of an engineered chiral bath with broken time reversal symmetry. The photonic bath realizes a chiral sawtooth waveguide [15, 16], consisting of a bipartite lattice of cav- ities/resonators composed by two sublattices A and B in a ring geometry, and threaded by a synthetic gauge field φ in each plaquette, as schematically depicted in Fig.1(a). Such a model system has been investigated in some re- cent works and can be physically implemented in differ- ent platforms, such as squids, cold atoms, and integrated arXiv:2005.10839v1 [quant-ph] 21 May 2020