Fabry-Perot interferometer with quantum mirrors: nonlinear light transport and rectification

Passive magnetic-free broadband optical isolator based on unidirectional self-induced transparency

Achieving a broadband nonreciprocal device without gain and any external bias is very challenging and highly desirable for modern photonic technologies and quantum networks. Here we theoretically propose a passive and magnetic-free all-optical isolator for a femtosecond laser pulse by exploiting a new mechanism of unidirectional self-induced transparency, obtained with a nonlinear medium followed by a normal absorbing medium at one side. The transmission contrast between the forward and backward directions can reach 14.3 dB for a 2π - 5 fs laser pulse. The 20 dB bandwidth is about 56 nm, already comparable with a magneto-optical isolator. This work provides a new mechanism which may benefit non-magnetic isolation of ultrashort laser pulses.

Microwave quantum diode

The fragile nature of quantum circuits is a major bottleneck to scalable quantum applications. Operating at cryogenic temperatures, quantum circuits are highly vulnerable to amplifier backaction and external noise. Non-reciprocal microwave devices such as circulators and isolators are used for this purpose. These devices have a considerable footprint in cryostats, limiting the scalability of quantum circuits. As a proof-of-concept, here we report a compact microwave diode architecture, which exploits the non-linearity of a superconducting flux qubit. At the qubit degeneracy point we experimentally demonstrate a significant difference between the power levels transmitted in opposite directions. The observations align with the proposed theoretical model. At - 99 dBm input power, and near the qubit-resonator avoided crossing region, we report the transmission rectification ratio exceeding 90% for a 50 MHz wide frequency range from 6.81 GHz to 6.86 GHz, and over 60% for the 250 MHz range from 6.67 GHz to 6.91 GHz. The presented architecture is compact, and easily scalable towards multiple readout channels, potentially opening up diverse opportunities in quantum information, microwave read-out and optomechanics.

Non-Hermitian Waveguide Cavity QED with Tunable Atomic Mirrors

Optical mirrors determine cavity properties by means of light reflection. Imperfect reflection gives rise to open cavities with photon loss. We study an open cavity made of atom-dimer mirrors with a tunable reflection spectrum. We find that the atomic cavity shows anti-PT symmetry. The anti-PT phase transition controlled by atomic couplings in mirrors indicates the emergence of two degenerate cavity supermodes. Interestingly, a threshold of mirror reflection is identified for realizing strong coherent cavity-atom coupling. This reflection threshold reveals the criterion of atomic mirrors to produce a good cavity. Moreover, cavity quantum electrodynamics with a probe atom shows mirror-tuned properties, including reflection-dependent polaritons formed by the cavity and probe atom. Our Letter presents a non-Hermitian theory of an anti-PT atomic cavity, which may have applications in quantum optics and quantum computation.

Waveguide Mach-Zehnder interferometer to enhance the sensitivity of quantum parameter estimation

The waveguide Fabry-Perot interferometer (FPI) (see, e.g., in Phys. Rev. Lett.113, 243601 (2015)10.1103/PhysRevLett.115.243601 and Nature569, 692 (2019)10.1038/s41586-019-1196-1), instead of the free space's one, have been demonstrated for the sensitive quantum parameter estimations. Here, we propose a waveguide Mach-Zehnder interferometer (MZI) to further enhance the sensitivity of the relevant parameter estimations. The configuration is formed by two one-dimensional waveguides coupled sequentially to two atomic mirrors, which are served as the beam splitters of the waveguide photons to control the probabilities of the photons being transferred from one waveguide to another. Due to the quantum interference of the waveguide photons, the acquired phase of the photons when they pass through a phase shifter can be sensitively estimated by measuring either the transmitted or reflected probabilities of the transporting photons. Interestingly, we show that, with the proposed waveguide MZI the sensitivity of the quantum parameter estimation could be further optimized, compared with the waveguide FPI, in the same condition. The feasibility of the proposal, with the current atom-waveguide integrated technique, is also discussed.

Reconfigurable nonreciprocity with low insertion loss using a simple two-level system

Nonreciprocal light propagation is essential to control the direction of the light flow. Here, we report the realization of magnetic-free optical nonreciprocity using a simple two-level system driven by a pump field in warm atoms. In our experiment, we not only demonstrate less than 0.5 dB of insertion loss and up to 20 dB of isolation but also provide flexible and reconfigurable operations of the isolation bandwidth, frequency, and direction. Nonreciprocal scheme with these characteristics may find important applications in photonic devices.

Controllable unidirectional transport and light trapping using a one-dimensional lattice with non-Hermitian coupling

We propose a one-dimensional tight-binding lattice with special non-Hermitian coupling, the imaginary part of which is modulated by an effective Peierls phase arising from the synthetic magnetic field. Such a non-Hermitian lattice supports robust unidirectional transport that is reflectionless and immune to defects; it thus can serve as a frequency-selectable light filter. To achieve more applications, we further construct two well-designed structures involving this lattice, namely a heterostructure and a sandwich structure. An optical diode can be realized using the heterostructure, while tunable light trapping and reversal can be realized through phase modulations on the sandwich structure. The results in this paper may not only open up a new path for unconventional light transport but also have potential applications for optical communication.

Realization of Nonlinear Optical Nonreciprocity on a Few-Photon Level Based on Atoms Strongly Coupled to an Asymmetric Cavity

Optical nonreciprocity is important in photonic information processing to route the optical signal or prevent the reverse flow of noise. By adopting the strong nonlinearity associated with a few atoms in a strongly coupled cavity QED system and an asymmetric cavity configuration, we experimentally demonstrate the nonreciprocal transmission between two counterpropagating light fields with extremely low power. The transmission of 18% is achieved for the forward light field, and the maximum blocking ratio for the reverse light is 30 dB. Though the transmission of the forward light can be maximized by optimizing the impedance matching of the cavity, it is ultimately limited by the inherent loss of the scheme. This nonreciprocity can even occur on a few-photon level due to the high optical nonlinearity of the system. The working power can be flexibly tuned by changing the effective number of atoms strongly coupled to the cavity. The idea and result can be applied to optical chips as optical diodes by using fiber-based cavity QED systems. Our work opens up new perspectives for realizing optical nonreciprocity on a few-photon level based on the nonlinearities of atoms strongly coupled to an optical cavity.

Nonreciprocity Realized with Quantum Nonlinearity

Nonreciprocal devices are a key element for signal routing and noise isolation. Rapid development of quantum technologies has boosted the demand for a new generation of miniaturized and low-loss nonreciprocal components. Here, we use a pair of tunable superconducting artificial atoms in a 1D waveguide to experimentally realize a minimal passive nonreciprocal device. Taking advantage of the quantum nonlinear behavior of artificial atoms, we achieve nonreciprocal transmission through the waveguide in a wide range of powers. Our results are consistent with theoretical modeling showing that nonreciprocity is associated with the population of the two-qubit nonlocal entangled quasidark state, which responds asymmetrically to incident fields from opposing directions. Our experiment highlights the role of quantum correlations in enabling nonreciprocal behavior and opens a path to building passive quantum nonreciprocal devices without magnetic fields.

Decoherence-Free Interaction between Giant Atoms in Waveguide Quantum Electrodynamics

In quantum-optics experiments with both natural and artificial atoms, the atoms are usually small enough that they can be approximated as pointlike compared to the wavelength of the electromagnetic radiation with which they interact. However, superconducting qubits coupled to a meandering transmission line, or to surface acoustic waves, can realize "giant artificial atoms" that couple to a bosonic field at several points which are wavelengths apart. Here, we study setups with multiple giant atoms coupled at multiple points to a one-dimensional (1D) waveguide. We show that the giant atoms can be protected from decohering through the waveguide, but still have exchange interactions mediated by the waveguide. Unlike in decoherence-free subspaces, here the entire multiatom Hilbert space (2^{N} states for N atoms) is protected from decoherence. This is not possible with "small" atoms. We further show how this decoherence-free interaction can be designed in setups with multiple atoms to implement, e.g., a 1D chain of atoms with nearest-neighbor couplings or a collection of atoms with all-to-all connectivity. This may have important applications in quantum simulation and quantum computing.

Measurement of deep-subwavelength emitter separation in a waveguide-QED system

In the waveguide quantum electrodynamics (QED) system, emitter separation plays an important role for its functionality. Here, we present a method to measure the deep-subwavelength emitter separation in a waveguide-QED system. In this method, we can also determine the number of emitters within one diffraction-limited spot. In addition, we also show that ultrasmall emitter separation change can be detected in this system which may then be used as a waveguide-QED-based sensor to measure tiny local temperature/strain variation.

Smallest quantum thermal machine: The effect of strong coupling and distributed thermal tasks

The functions of the smallest self-contained thermal machine consisting of a single qutrit are studied when the weak internal coupling assumption is relaxed. It is shown that in the presence of one target to be cooled the strong coupling is not beneficial to the refrigeration. The reason is explained by examining the effect of the strong coupling on the contributions of all eigenstates transitions to the heat current of the related thermal reservoir. When acting simultaneously on two targets, the machine can be manipulated to implement distributed tasks on them, such as cooling one target and meanwhile heating another one, by adjusting the coupling strengths between the machine with the two targets. In particular, we show that the machine can realize temperature reversal for the two qubits, namely, the qubit that is coupled to the high temperature reservoir is refrigerated to a temperature below that of the qubit contacting with the low temperature reservoir.

Dynamic Stark shift induced by a single photon packet

The dynamic Stark shift results from the interaction of an atom with the electromagnetic field. We show how a propagating single-photon wave packet can induce a time-dependent dynamical Stark shift on a two-level system (TLS). A non-perturbative fully quantum treatment is employed, where the quantum dynamics of both the field and the TLS are analyzed. We also provide the means to experimentally access such time-dependent frequency by measuring the interference pattern in the electromagnetic field inside a 1D waveguide. The effect we evidence here may find applications in the autonomous quantum control of quantum systems without classical external fields, which can be useful for quantum information processing as well as for quantum thermodynamical tasks.

Quantum thermal diode based on two interacting spinlike systems under different excitations

We demonstrate that two interacting spinlike systems characterized by different excitation frequencies and coupled to a thermal bath each, can be used as a quantum thermal diode capable of efficiently rectifying the heat current. This is done by deriving analytical expressions for both the heat current and rectification factor of the diode, based on the solution of a master equation for the density matrix. Higher rectification factors are obtained for lower heat currents, whose magnitude takes their maximum values for a given interaction coupling proportional to the temperature of the hotter thermal bath. It is shown that the rectification ability of the diode increases with the excitation frequencies difference, which drives the asymmetry of the heat current, when the temperatures of the thermal baths are inverted. Furthermore, explicit conditions for the optimization of the rectification factor and heat current are explicitly found.

Controlling heat flows among three reservoirs asymmetrically coupled to two two-level systems

We study heat flows among three thermal reservoirs via two two-level systems (TLSs). Two reservoirs are coupled to one TLS and the third reservoir to the second TLS. The two TLSs are also coupled to each other, thus bridging the third reservoir with the two other reservoirs. We show that the magnitudes and directions of the reservoirs' heat currents can be controlled by varying the various damping rates of the two TLSs due to coupling with the corresponding reservoirs. First, it is shown that by changing the damping rate due to one reservoir, magnitudes of heat currents of the other two reservoirs can behave in completely different manners, namely, although one may be enhanced, the other may instead be suppressed, and vice versa. Second, the sign of the heat current of one reservoir may change (i.e., crossover from heat absorption to heat release, or vice versa) if a damping rate or the coupling strength between the two TLSs is swept through a critical value, which depends on the temperature settings for the three reservoirs. Due to the asymmetric couplings of the two TLSs to the three reservoirs, the thermal rectification occurs without introducing any additional asymmetry to the systems.

Quantum scattering theory of a single-photon Fock state in three-dimensional spaces

A quantum scattering theory is developed for Fock states scattered by two-level systems in three-dimensional free space. It is built upon the one-dimensional scattering theory developed in waveguide quantum electrodynamics. The theory fully quantizes the incident light as Fock states and uses a non-perturbative method to calculate the scattering matrix.

Avoided resonance crossing and non-reciprocal nearly perfect absorption in plasmonic nanodisks with near-field and far-field couplings

Avoided resonance crossings (ARC) in plasmonic nanodisk structures due to near field or far field couplings were numerically demonstrated. Near field coupling in disk dimmer with both vertical or side-by-side arrangement leads to both energy and linewidth anti-crossing by varying one disk size across the other. Far field coupling in double layered disk arrays of extremely small gap size or gap size with Fabry Perot resonant condition close to the frequency selective surface (FSS) stopband center leads to non-reciprocal absorption spectrum as one disk size varying across the other. We observe linewidth anti-crossing but energy crossing of the absorption peak from different side illumination by varying either the size of one disk array or the gap in hetero disk arrays. The disappearing of Fabry-Perot resonant mode from one side illumination and the appearing of nonreciprocal nearly perfect absorption from the other side illumination are well explained by a FSS-Fabry-Perot model.

Non-Markovianity induced by a single-photon wave packet in a one-dimensional waveguide

The concept of non-Markovianity (NM) in quantum dynamics is still an open debate. Understanding how to generate and measure NM in specific models may aid in this quest. In quantum optics, an engineered electromagnetic environment coupled to a single atom can induce NM. The most common scenario of structured electromagnetic environment is an optical cavity, composed by a pair of mirrors. Here, we show how to generate and measure NM on a two-level system coupled to a one-dimensional waveguide with no mirrors required. The origin of the non-Markovian behavior lies in the initial state of the field, prepared as a single-photon packet. NM is shown to depend on two experimentally controllable parameters, namely, the linewidth of the packet and its central frequency. We relate the presence of NM to quantum interference. We also show how the two output channels of the waveguide provide distinct signatures of NM, both experimentally accessible.

Exact optimal control of photon blockade with weakly nonlinear coupled cavities

We propose a scheme of photon blockade in a system comprising of coupled cavities embedded in Kerr nonlinear material, where two cavities are driven and dissipated. We analytically derive the exact optimal conditions for strong photon antibunching, which are in good agreement with those obtained by numerical simulations. We find that conventional and unconventional photon blockades have controllable flexibilities by tuning the strength ratio and relative phase between two complex driving fields. Such unconventional photon-blockade effects are ascribed to the quantum interference effect to avoid two-photon excitation of the coupled cavities. We also discuss the statistical properties of the photons under given optimal conditions. Our results provide a promising platform for the coherent manipulation of photon blockade, which has potential applications for quantum information processing and quantum optical devices.