Coherently manipulating single photons in nanophotonic structures with unidirectional propagation is one of the central goals for integrated quantum information processing. Photonic devices constructed by a single atom coupled to a whispering-gallery-mode microresonator (WGMM) with nonideal chiral photon-atom interactions inevitably induce undesired photon scattering. Here, an external scatterer is introduced to the WGMM to cancel the reflection of signal photons by an atom due to nonideal chiral interactions. By properly tuning the positions of the scatterer, destructive and constructive interferences between reflected photons of different pathways are utilized to control the reflection properties of single incident photons. The results show that the reflection probabilities can be suppressed by applying the interplay between chirality and backscattering. Constructing perfect destructive interferences directly leads to unidirectional propagation even when the photon-atom interactions are not ideal. Because the amplitudes of the reflected photons produced by the scatterer can be enhanced to meet requirements of perfect destructive interfering processes, unidirectional propagation is preserved against dissipations.
Coherently manipulating single photons in nanophotonic structures with unidirectional propagation is one of the central goals for integrated quantum information processing. Photonic devices constructed by a single atom coupled to a whispering-gallery-mode microresonator (WGMM) with nonideal chiral photon-atom interactions inevitably induce undesired photon scattering. Here, an external scatterer is introduced to the WGMM to cancel the reflection of signal photons by an atom due to nonideal chiral interactions. By properly tuning the positions of the scatterer, destructive and constructive interferences between reflected photons of different pathways are utilized to control the reflection properties of single incident photons. The results show that the reflection probabilities can be suppressed by applying the interplay between chirality and backscattering. Constructing perfect destructive interferences directly leads to unidirectional propagation even when the photon-atom interactions are not ideal. Because the amplitudes of the reflected photons produced by the scatterer can be enhanced to meet requirements of perfect destructive interfering processes, unidirectional propagation is preserved against dissipations.
Abstract Unidirectional propagation of photons originated from perfect chirality meets the critical requirement for building a high-performance quantum network. However, it not only requires that the circular dipole emitter is precisely located at points of circularly polarized electric fields, which leads to non-reciprocal interactions for photons with opposite propagation directions, but also the light-emitter interaction strength should be strong enough to guarantee a π phase shift. Unfortunately, these perfect chirality points are scarce and accessible points with elliptically polarized fields result in non-ideal photon-emitter chiral interactions and emitters radiating photons bidirectionally. Meanwhile, reflection properties, phase shifts, and non-reciprocal interactions are sensitive to frequency detunings and dissipations. Here, without engineering the dipole and optimizing the distribution of the field, a scatter such as a nanotip placed at the evanescent field of a whispering gallery mode resonator (WGMR) is adopted to control the transporting properties of single photons under non-ideal chiral interactions. By properly adjusting the relative position between the nanotip and the atom or the overlap between the nanotip and the mode volume of the WGMR, amplitudes of reflected photons in different pathways are changed. Consequently, complete destructive interference appears and thus no photons are reflected. The corresponding phase shifts of π and non-reciprocal interactions are guaranteed simultaneously. Significantly, the perfect chirality reconstructed here is robust against frequency detunings and dissipations. Therefore, the atom-WGMR-nanotip structure can be regarded as a compound chiral atom with radiating photons in only one direction.
Single-photon transport in nonideal chiral photon-atom interaction structures generally contains information backflow and thus limits the capabilities to transfer information between distant emitters in cascaded quantum networks. Here, in the nonideal chiral case, a V-type atom coupled to a waveguide is proposed to realize completely unidirectional transmission of the single photons in a superposition state of different frequencies. A microwave field is introduced to drive the two excited states of the atom and results in photon conversion between two transitions. By adjusting the Rabi frequency and the phase of the external driving field, the transport behaviors of incident photons with specific frequencies can be optimized to complete transmission or reflection. Based on the constructive interferences between photons from different traveling paths, the transmission probabilities of the specific-frequency photons could be enhanced. Due to photon conversions with compensating the dissipations and ensuring the complete destructive interference, the ideal unidirectional transmission contrast can be maintained to $\ifmmode\pm\else\textpm\fi{}1$, even when the atom has dissipations into the nonwaveguide modes.
The chiral light-matter interaction, which shows great potential in applications ranging from photonic devices to quantum information processing, can be achieved with the development of recent experimental advances of the fabrication technologies on nanoscales. Traditionally, the chiral optics based on spin-momentum locking in micro- and nanophotonics could be observed only for specific optical modes, such as the transverse magnetic mode. Here in this study, we investigate that the chirality of a whispering-gallery-mode (WGM) microresonator can be well controlled with assistance from coupled Rydberg-atom ensembles. The presented asymmetric backscattering also originates from interferences between scattered optical modes in the WGM microresonator, which is proposed by Wiersig [Phys. Rev. A 84, 063828 (2011)]. Therefore the flow of light can be well controlled, and the statistical properties of photons could be changed by regulating this asymmetric coupling. Most importantly, its universal chiral property is not limited by specific optical modes which may also be used as optical diodes and routers.
Chirality in micro- and nanophotonic structures is crucial for both fundamental research and applied technology. Here, we experimentally demonstrate chiral absorption via backscattering in a single whispering-gallery microresonator under simultaneous excitation from both ports. Remarkably, this scheme does not rely on any nonlinear effects or the breaking of parity or time-reversal symmetry. These intriguing phenomena occur due to the interference between the clockwise- and counterclockwise-propagating light fields induced by the backscattering. Furthermore, we also achieve the chiral optical states in a coupled-microresonator system, which can be utilized to manipulate the electromagnetically induced transparency or absorption effect. Our work proposes a linear and all-optical scheme to realize chiral optical states, which can help control the light flow in photonic systems with a high operation rate.
Summary form only given. Controlling the interaction of light and matter is the basis for diverse applications ranging from light technology to quantum information processing. Nowadays, many of these applications are based on nanophotonic structures. It turns out that the confinement of light in such nanostructures imposes an inherent link between its local polarization and its propagation direction, also referred to as spin-momentum locking of light [1]. Remarkably, this leads to chiral, i.e., propagation direction-dependent effects in the emission and absorption of light, and elementary processes of light-matter interaction are fundamentally altered. For example, when coupling plasmonic particles or atoms to evanescent fields, the intrinsic mirror symmetry of the particles' emission can be broken. In our group, we observed this effect in the interaction between single rubidium atoms and the evanescent part of a light field that is confined by continuous total internal reflection in a whispering-gallery-mode microresonator [2]. In the following, this allowed us to realize chiral nanophotonic interfaces in which the emission direction of light into the structure is controlled by the polarization of the excitation light [3] or by the internal quantum state of the emitter [4], respectively. Moreover, we employed this chiral interaction to demonstrate an integrated optical isolator [5] as well as an integrated optical circulator [6] which operate at the single-photon level and which exhibit low loss. The latter are the first two examples of a new class of nonreciprocal nanophotonic devices which exploit the chiral interaction between single quantum emitters and transversally confined photons.
Nanophotonic components allow the control of the flow of light in integrated optical environments. Thereby, the light’s strong confinement leads to an inherent link between its local polarization and propagation direction which fundamentally alters the physics of light-matter interaction and gives rise to phenomena such as directional emission and direction-dependent coupling strengths [1]. I will present the underlying principles of this chiral light-matter interaction and its consequences for integrated applications [1]. In particular, I will show how we employ this effect to control the direction of spontaneous emission [2] and to realize low-loss nonreciprocal transmission at the single-photon level through a silica nanofiber [3]. We use two different approaches where either an ensemble of spin-polarized atoms is weakly coupled to a nanofiber or a single atom is strongly coupled to the nanofiber via a whispering-gallery-mode resonator. The resulting optical isolators show a strong imbalance between the transmissions in forward and reverse direction and, at the same time, a forward transmissions exceeding 70%. We extended this system to a 4-port device, where a single atom routes photons nonreciprocally from one fiber port to the next. This realizes a quantum optical circulator [4] which can even be prepared in a superposition of its operational modes. The demonstrated systems exemplify a new class of (quantum) nanophotonic devices that are ideally suited for photonic quantum information processing and quantum simulation. [1] Nature 541, 473, (2017). [2] Science 346, 67 (2014). [3] Phys. Rev. X 5, 041036 (2015). [4] Science 354, 1577 (2016).