Abstract To achieve a high degree of quantum noise squeezing, nonlinear optical interaction process is often employed. Here, we propose to utilize quasi-bound states in the continuum (quasi-BICs) and Fano resonances to enhance optical squeezing without nonlinearity. The theory of quantization for electromagnetic fields in the periodic nanostructure with dispersion and absorption has been developed by means of the Green’s function technique with the plane wave expansion method. The quasi-BICs and Fano resonances of radiation modes are realized by designing the photonic crystal slab structure. Based on these quasi-BICs and Fano resonances, we demonstrate that strong squeezed states can be realized by using the balanced homodyne detection scheme. The squeezing degree can be improved by more than 14 times when a weak squeezed states passes through the structure with quasi-BICs and Fano resonances. The advantage of this method is that it is not only efficient but also easy to implement because the nonlinear optical processes are not employed, which is very beneficial for the quantum information processing and precision metrology.
Abstract To achieve a high degree of quantum noise squeezing, nonlinear optical interaction process is often employed. Here, we propose to utilize quasi-bound states in the continuum (quasi-BICs) and Fano resonances to enhance optical squeezing without nonlinearity. The theory of quantization for electromagnetic fields in the periodic nanostructure with dispersion and absorption has been developed by means of the Green’s function technique with the plane wave expansion method. The quasi-BICs and Fano resonances of radiation modes are realized by designing the photonic crystal slab structure. Based on these quasi-BICs and Fano resonances, we demonstrate that strong squeezed states can be realized by using the balanced homodyne detection scheme. The squeezing degree can be improved by more than 14 times when a weak squeezed states passes through the structure with quasi-BICs and Fano resonances. The advantage of this method is that it is not only efficient but also easy to implement because the nonlinear optical processes are not employed, which is very beneficial for the quantum information processing and precision metrology.
We propose an optical approach to realizing Fano-type spectra of quadrature squeezing in a double-cavity-waveguide structure based on photonic crystals (PhCs). In this scheme, a partially transmitting element (PTE) in the waveguide creates the transmission and reflection light, which interferes with the outflow from the intracavity field and subsequently gives rise to Fano-type interference. Meanwhile, a degenerate parametric amplifier (DPA) embedded into the cavity is expected to yield quantum squeezed states in the interference process. After verifying the existence of the Fano resonance, we report that increasing the nonlinear gain of the DPA not only amplifies the transmitted intensity of the output field, but also improves its quadrature squeezing degree. More importantly, we illustrate that, when maintaining the high performance of quadrature squeezing, the linewidths and frequencies of the asymmetrical spectra can be modulated by adjusting the double-cavity coupling strength. This combination of Fano-type spectra and quadrature squeezing is beneficial for optimizing optical communications and signal processing with a low noise level.
The quantization of the electromagnetic field leads directly to the existence of quantum mechanical states, called Fock states, with an exact integer number of photons. Despite these fundamental states being long-understood, and despite their many potential applications, generating them is largely an open problem. For example, at optical frequencies, it is challenging to deterministically generate Fock states of order two and beyond. Here, we predict the existence of an effect in nonlinear optics, which enables the deterministic generation of large Fock states at arbitrary frequencies. The effect, which we call an n -photon bound state in the continuum, is one in which a photonic resonance (such as a cavity mode) becomes lossless when a precise number of photons n is inside the resonance. Based on analytical theory and numerical simulations, we show that these bound states enable a remarkable phenomenon in which a coherent state of light, when injected into a system supporting this bound state, can spontaneously evolve into a Fock state of a controllable photon number. This effect is also directly applicable for creating (highly) squeezed states of light, whose photon number fluctuations are (far) below the value expected from classical physics (i.e., shot noise). We suggest several examples of systems to experimentally realize the effects predicted here in nonlinear nanophotonic systems, showing examples of generating both optical Fock states with large n ( n > 10), as well as more macroscopic photonic states with very large squeezing, with over 90% less noise (10 dB) than the classical value associated with shot noise.
Nonlinear optical phenomena associated with high-order soliton breakup in photonic crystal fibres and squeezed state generation in three dimensional photonic crystal microcavities are investigated. In both cases, the properties of periodically patterned, high-index contrast dielectric structures are engineered to control the dispersion and local field enhancements of the electromagnetic field. Ultra-short pulse propagation in a polarization-maintaining microstructured fibre (with 1 um core diameter and 1.1 m length) is investigated experimentally and theoretically. For an 80 MHz train of 130 fs pulses with average propagating powers in the fibre up to 13.8 mW, the output spectra consist of multiple discrete solitons that shift continuously to lower energies as they propagate in the lowest transverse mode of the fibre. The number of solitons and the amount that they shift both increase with the launched power. All of the data is quantitatively consistent with solutions of the nonlinear Schrodinger equation, but only when the Raman nonlinearity is treated without approximation, and self-steepening is included. The feasibility of using a parametric down-conversion process to generate squeezed electromagnetic states in 3D photonic crystal microcavity structures is investigated for the first time. The spectrum of the squeezed light is theoretically calculated by using an open cavity quantum mechanical formalism. The cavity communicates with two main channels, which model vertical radiation losses and coupling into a single-mode waveguide respectively. The amount of squeezing is determined by the correlation functions relating the field quadratures of light coupled into the waveguide. All of the relevant model parameters are realistically estimated using 3D finite-difference time-domain (FDTD) simulations. Squeezing up to ~20% below the shot noise level is predicted for reasonable optical excitation levels. To preserve the squeezed nature of the light generated in the microcavity, a unidirectional coupling geometry from the microcavity to a ridge waveguide in a slab photonic crystal structure is studied. The structure was successfully fabricated in a silicon membrane, and experimental measurements of the efficiency for the signal coupled out of the structure are in good agreement with the result of FDTD simulations. The coupling efficiency of the cavity mode to the output channel is ~60%.
We demonstrate the methods for increasing the observed squeezing level in the generation of squeezed states of light at 1064 nm with periodically poled KTiOPO4 (PPKTP) crystal. We analyze the technical limits to the reduction of noise in the squeezed quadrature theoretically, including the intra-cavity loss of the optical parametric amplifier (OPA) cavity, the normalized amplitude, the detection frequency, and the mode matching on the balanced homodyning stage. To observe a high degree of squeezing, we improve the quality of mode matching into the OPA cavity and the mode cleaning (MC) cavity experimentally. By optimizing mode matching of the light into cavities, the finesse of the cavities would be higher in practice and the desirable spatial mode can be realized. As the intensity noise of the laser light reach the shot noise limit above the frequency of 1 MHz, which is sufficient for our experiment, so the MC cavity is incorporated to filter higher-order transverse modes of the local oscillator (LO) beam for the balanced homodyne detection. The experimental and theoretical results show that it is essential to optimize the mode matching efficiency to generate and detect high degree of squeezing otherwise an inefficiency mode matching will throw away the squeezing and transform the squeezed field into a vacuum field.
Squeezed states are important sources in quantum physics, which have potential applications in fields such as quantum teleportation, quantum information networks, quantum memory, and quantum metrology and precise measurements. For our interest, the squeezed vacuum will be used in the quantum-enhanced optical atomic magnetometers, filling the vacuum port of the probe beam to improve measurement sensitivity. Based on the sub-threshold optical parametric oscillator (OPO) with PPKTP crystal, the squeezed vacuum at rubidium D1 line of 795 nm is obtained. In our work, we investigate the noise sources in an OPO system. By carefully controlling the classical noise source, the squeezing band extends to the analysis frequency of 2.6 kHz. The flat squeezing trace is 2.8 dB below the shot noise limit. In our work, we focus on the difference between the squeezing results at the analysis frequency of kilohertz regime at two different wavelengths, 1064 nm and 795 nm. The difference mainly comes from the absorption of 795 nm laser and its second harmonic at 397.5 nm in crystal (397.5 nm laser is at the edge of transparent window of PPKTP crystal that has an absorption index much higher than at other wavelength). The absorption induced nonlinear loss and thermal instability greatly affect the squeezing results, which is discussed in our work. Squeezing level at 795 nm is worse than at 1064 nm due to the above-mentioned factors. Noise coupling to the detection system limits the squeezing band. In the audio frequency band, squeezing is easily submerged in roll-up noises and the measured squeezing level is limited. Two factors limit the obtained squeezing:the technical noise induced in the detection and the squeezing degradation by the noise coupling of the control beams. In experiment, we carefully control the classical noise at analytical frequency of kilohertz by means of a vacuum-injected OPO, a counter-propagating cavity locking beam with orthogonal polarization, low noise homodyne detector, stable experimental system and quantum noise locking method for squeezing phase locking. Firstly, to preclude the classical noise from coupling the laser source, we use the vacuum injected OPO. A signal beam helps optimize the parametric gain and is blocked in the squeezing measurement process. In order to maintain the OPO, a counter-propagating beam with orthogonal polarization is used for locking the cavity. Then, a low noise balanced homodyne detector with a common-mode rejection ratio of 45 dB helps improve the audio frequency detection. Finally, the quantum noise locking provides a method to lock the relative phase between the coherent beam and the squeezed vacuum field. With the combination of these technical improvements, a squeezed vacuum of 2.8 dB is obtained at the analysis frequency of 2.6-100 kHz. The obtained squeezing level is mainly limited by the relatively large loss in OPO, which is induced by ultra-violet absorption in PPKTP crystal. The generated squeezed field is used to reduce the polarization noise of probe beam in an optical magnetometer, in order to increase detection sensitivity.
Summary form only given. In photonic crystals with spatially periodic dielectric constants, a variety of novel optical and quantum optical phenomena are expected to be realized. One of their peculiar features is the presence of those electromagnetic eigenmodes with extremely small group velocities (group-velocity anomaly) that leads to the enhancement of various optical processes. The phase-matching condition is relaxed in photonic crystals compared with uniform materials. Therefore nonlinear optics of the photonic crystals is a promising field for future experimental studies. In this paper, we believe we will report the first theoretical prediction of the enhancement of quadrature-phase squeezing due to optical parametric amplification by means of a perturbation theory based on a Green's function formalism.