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Squeezed states of the electromagnetic field are generated by degenerate parametric down conversion in an optical cavity. Noise reductions greater than 50% relative to the vacuum noise level are observed in a balanced homodyne detector. A quantitative comparison with theory suggests that the observed squeezing results from a field that in the absence of linear attenuation would be squeezed by greater then tenfold.

Content uploaded by Ling-An Wu

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All content in this area was uploaded by Ling-An Wu on Jul 19, 2016

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... Note that for practical use, the generation of squeezed cat codes may be more feasible than binomial states with recent technologies. For instance, the generation of squeezed states was realized decades ago using parametric down conversion [51], whereas the generation of binomial codes remains challenging, although the schemes to generate binomial states have been proposed theoretically [52,53]. As such, in the near term, squeezed cat codes are predicted to be more feasible for quantum communication. ...

Quantum error correction codes based on continuous variables play an important role for the implementation of quantum communication systems. A natural application of such codes occurs within quantum repeater systems which are used to combat severe channel losses and local gate errors. In particular, channel loss drastically reduces the distance of communication between remote users. Here we consider a cavity-QED based repeater scheme to address the losses in the quantum channel. This repeater scheme relies on the transmission of a specific class of rotationally invariant error-correcting codes. We compare several rotation-symmetric bosonic codes (RSBCs) being used to encode the initial states of two remote users connected by a quantum repeater network against the convention of the cat codes and we quantify the performance of the system using the secret key rate. In particular, we determine the number of stations required to exchange a secret key over a fixed distance and establish the resource overhead.

... While, Yang et al. 30 have proposed a scheme to generate robust tripartite optomechanical entanglement with a single-cavity optomechanical system driven by a single input laser field. To this aim, some interesting phenomena will occur when an optical parametric amplifier is introduced into an optomechanical cavity, such as the generation of entangled and squeezed states of light [31][32][33][34] , enhance mechanical cooling 35 , generate strong mechanical squeezing 36 , and enhance the degree of precision of optomechanical position detection 37 . For instance, Huang et al. 38 have analyzed the ground state cooling of a macroscopic mechanical oscillator for the quantum manipulation of the mirror by degenerate optical parametric amplifier. ...

In this paper, we investigated the quantum correlation of nano-electro-optomechanical system enhanced by an optical parametric amplifier (OPA) and Coulomb-type interaction. In particular, we consider a hybrid system consisting of a cavity and two charged mechanical oscillators with an OPA, where the optical cavity mode is coupled with a charged mechanical oscillator via radiation pressure, and the two charged mechanical oscillators are coupled through a Coulomb interaction. We use logarithmic negativity to quantify quantum entanglement, and quantum discord to measure the quantumness correlation between the two mechanical oscillators. We characterize quantum steering using the steerability between the two mechanical oscillators. Our results show that the presence of OPA and strong Coulomb coupling enhances the quantum correlations between the two mechanical oscillators. In addition, Coulomb interactions are more prominent in quantum correlations. Besides, in the presence of OPA, the maximum amount of quantum entanglement, quantum steering, and quantum discord were achieved between the two mechanical oscillators is greater than in the absence of OPA. Moreover, a proper phase choice of the optical field driving the OPA enhances quantum correlations under suitable conditions. We obtain quantum entanglement confines quantum steering and quantum discord beyond entanglement. Furthermore, quantum entanglement, quantum steering, and quantum discord decrease rapidly with increasing temperature as a result of decoherence. In addition, quantum discord persists at higher temperature values, although the quantum entanglement between the systems also vanishes completely. Our proposed scheme enhances quantum correlation and proves robust against fluctuations in the bath environment. We believe that the present scheme of quantum correlation provides a promising platform for the realization of continuous variable quantum information processing.

... Spontaneous parametric down-conversion (SPDC) under ambient conditions is a well-established technique for the generation of quantum light, such as heralded single photons [1], entangled photon pairs [2], and squeezed states [3]. Integrated SPDC in particular benefits from high conversion efficiencies and the propagation of photon pairs into well-defined waveguide modes [4,5]. ...

We demonstrate the generation of degenerate photon pairs from spontaneous parametric down-conversion in titanium-indiffused waveguides in lithium niobate at cryogenic temperatures. Since the phase matching cannot be temperature tuned inside a cryostat, we rely on a precise empirical model of the refractive indices when fabricating a fixed poling period. We design the phase-matching properties of our periodic poling to enable signal and idler photons at 1559.3(6) nm and characterize the indistinguishability of our photons by performing a Hong-Ou-Mandel interference measurement. Despite the effects of photorefraction and pyroelectricity, which can locally alter the phase matching, we achieve cryogenic indistinguishable photons within 1.5 nm of our design wavelength. Our results verify sufficient understanding and control of the cryogenic nonlinear process, which has wider implications when combining quasi-phase-matched nonlinear optical processes with other cryogenic photonic quantum technologies, such as superconducting detectors.

... Displacement gates can be achieved by utilizing an ancillary qumode [60]. However, generating squeezing is challenging as it requires second-order nonlinearity, which can be accomplished using an optical parametric oscillator [61]. Optical parametric oscillators can also generate multimode squeezing and entanglement [62]. ...

We propose a hybrid quantum-classical approximate optimization algorithm for photonic quantum computing, specifically tailored for addressing continuous-variable optimization problems. Inspired by counterdiabatic protocols, our algorithm significantly reduces the required quantum operations for optimization as compared to adiabatic protocols. This reduction enables us to tackle non-convex continuous optimization and countably infinite integer programming within the near-term era of quantum computing. Through comprehensive benchmarking, we demonstrate that our approach outperforms existing state-of-the-art hybrid adiabatic quantum algorithms in terms of convergence and implementability. Remarkably, our algorithm offers a practical and accessible experimental realization, bypassing the need for high-order operations and overcoming experimental constraints. We conduct proof-of-principle experiments on an eight-mode nanophotonic quantum chip, successfully showcasing the feasibility and potential impact of the algorithm.

We experimentally reconstruct Wigner's current of quantum phase-space dynamics. We reveal the “push-and-pull” associated with damping and diffusion due to the coupling of a squeezed vacuum state to its environment. In contrast to classical dynamics, where (at zero temperature) dissipation only “pulls” the system toward the origin of phase space, we also observe an outward “push” because our system has to obey Heisenberg's uncertainty relations. With squeezed vacuum states generated by an optical parametric oscillator at variable pumping levels, we identify the pure squeezing dynamics and its central stagnation point with a topological charge of “−1”. This work demonstrates high resolving power and establishes an experimental paradigm for measuring the quantumness and nonclassicality of the dynamics of open quantum systems.

In this paper, we investigate how to generate coherent squeezed like light using a nonlinear photonic crystal. Because the photonic crystal reduces the group velocity of the incident light, if it is composed of a material with a second-order nonlinear optical susceptibility $\chi^{(2)}$, the interaction between the nonlinear material and the light passing through it strengthens and the quantum state of the emitted light is largely squeezed. Thus, we can generate a coherent squeezed like light with a resonating cavity in which the nonlinear photonic crystal is placed. This coherent squeezed like state is defined with the Lie--Trotter product formula and its mathematical expression is different from those of conventional squeezed coherent states. We show that we can obtain this coherent squeezed like state with a squeezing level $15.9$ dB practically by adjusting physical parameters for our proposed method. Feeding the squeezed light whose average number of photons is given by one or two into a beam splitter and splitting the flow of the squeezed light into a pair of entangled light beams, we estimate their entanglement quantitatively.
This paper is a sequel to H.~Azuma, J. Phys. D: Appl. Phys. \textbf{55}, 315106 (2022).

To benefit high-power interferometry and the creation of low-noise light sources, we develop a simple lead-compensated photodetector enabling quantum-limited readout from 0.3 mW to 10 mW and 10 k$\Omega$ transimpedance gain from 85 Hz - 35 MHz. Feeding the detector output back to an intensity modulator, we squash the classical amplitude noise of a commercial 1550 nm fiber laser to the shot noise limit over a bandwidth of 700 Hz - 200 kHz, observing no degradation to its (nominally ~100 Hz) linewidth.

Integrated optical systems have evolved into suitable platforms in the field of photonic quantum technologies. New technologies open up new possibilities for multimode quantum operations. Here we study how circularly coupled waveguide arrays generate bipartite and tripartite continuous-variable (CV) entanglement. We focus on the single-mode squeezed state as input to the circular array of the waveguide system. Our findings suggest that the circularly coupled arrays can be used to generate entangled sources in CV quantum technologies. So the generation of entanglement makes the circular arrays more critical for further investigation and in the applications of photonic CV quantum-information processing.

Quantum squeezing is an important resource in modern quantum technologies, such as quantum precision measurement and continuous-variable quantum information processing. The generation of squeezed states of mechanical modes is a significant task in cavity optomechanics. Motivated by recent interest in multimode optomechanics, it becomes an interesting topic to create quadrature squeezing in multiple mechanical resonators. However, in the multiple-degenerate-mechanical-mode optomechanical systems, the dark-mode effect strongly suppresses the quantum effects in mechanical modes. Here we study the generation of mechanical squeezing in a two-mechanical-mode optomechanical system by breaking the dark-mode effect with the synthetic-gauge-field method. We find that, when the mechanical modes work at a finite temperature, the mechanical squeezing is weak or even disappears due to the dark-mode effect, while the strong mechanical squeezing can be generated once the dark-mode effect is broken. In particular, the thermal-phonon-occupation tolerance of the mechanical squeezing is approximately three orders of magnitude larger than that without breaking the dark-mode effect. We also generalize this method to break the dark modes and to create the mechanical squeezing in multiple-mechanical-mode optomechanical systems. Our results describe a general physical mechanism and pave the way towards the generation of noise-resistant quantum resources.

We show that all the minimum uncertainty packets are unitarily equivalent to the coherent states and that coherence is in fact stationary minimality.

In a previous paper we have shown that all minimum-uncertainty packets are unitarily equivalent to the coherent states and that coherence may be viewed as stationary minimality. In this note we give some additional information relating to the nature of the unitary-equivalence structure. We also give a new calculation of some matrix elements of the operator that implements the unitary equivalence which is not subject to the shortcoming inherent in the original calculation.

We describe a new and highly effective optical frequency discriminator and laser stabilization system based on signals reflected from a stable Fabry-Perot reference interferometer. High sensitivity for detection of resonance information is achieved by optical heterodyne detection with sidebands produced by rf phase modulation. Physical, optical, and electronic aspects of this discriminator/laser frequency stabilization system are considered in detail. We show that a high-speed domain exists in which the system responds to the phase (rather than frequency) change of the laser; thus with suitable design the servo loop bandwidth is not limited by the cavity response time. We report diagnostic experiments in which a dye laser and gas laser were independently locked to one stable cavity. Because of the precautions employed, the observed sub-100 Hz beat line width shows that the lasers were this stable. Applications of this system of laser stabilization include precision laser spectroscopy and interferometric gravity-wave detectors.

Quantum-mechanical calculations of the mean-square fluctuation spectra in optical homodyning and heterodyning are made for arbitrary input and local-oscillator quantum states. In addition to the unavoidable quantum fluctuations, it is shown that excess noise from the local oscillator always affects homodyning and, when it is broadband, also heterodyning. Both the quantum and the excess noise of the local oscillator can be eliminated by coherent subtraction of the two outputs of a 50-50 beam splitter. This result also demonstrates the fact that the basic quantum noise in homodyning and heterodyning is signal quantum fluctuation, not local-oscillator shot noise.

A general approach, within the framework of canonical quantization, is described for analyzing the quantum behavior of complicated electronic circuits. This approach is capable of dealing with electrical networks having nonlinear or dissipative elements. The techniques are applied to circuits capable of generating squeezed-state or two-photon coherent-state signals. Circuits capable of performing back-action-evading electrical measurements are also discussed.

The problem of detecting a coherent light beam in the presence of unwanted background radiation by the heterodyne method is examined. For a sufficiently strong local-oscillator field, the detectability of the signal is unaffected by the presence of the background radiation. It is shown that, in general, there exists an optimum receiver size that maximizes the signal-to-noise ratio. This result is illustrated by several examples. A procedure for the detection of a light signal of unknown direction is suggested.

The properties of a unique set of quantum states of the electromagnetic field are reviewed. These 'squeezed states' have less uncertainty in one quadrature than a coherent state. Proposed schemes for the generation and detection of squeezed states as well as potential applications are discussed.

It is pointed out that single-frequency emission from a Nd:YAG laser at
1.06 microns at power levels in excess of 1 W would be useful for the
investigation of dynamic processes in nonlinear optics. The emission
could also be important for applications related to high-resolution
nonlinear spectroscopy. However, due to thermal loading of the laser
rod, it is very difficult to obtain submegahertz frequency stability for
Nd:YAG lasers at high levels of lamp pumping power. The present
investigation is concerned with a single-frequency Nd:YAG laser with an
output power exceeding 1.1 W and a frequency stability of 120-kHz rms.
This performance is obtained in a ring cavity. This approach makes it
possible to eliminate problems associated with spatial hole burning. The
ring cavity is designed to minimize laser fluctuations due to noise in
the pumping and cooling processes.

Quantum-mechanical calculations of the mean-square fluctuation spectra in optical homodyning and heterodyning are made for arbitrary input and local-oscillator quantum states. In addition to the unavoidable quantum fluctuations, it is shown that excess noise from the local oscillator always affects homodyning and, when it is broadband, also heterodyning. Both the quantum and the excess noise of the local oscillator can be eliminated by coherent subtraction of the two outputs of a 50-50 beam splitter. This result also demonstrates the fact that the basic quantum noise in homodyning and heterodyning is signal quantum fluctuation, not local-oscillator shot noise.

The concept of a two-photon coherent state is introduced for applications in quantum optics. It is a simple generalization of the well-known minimum-uncertainty wave packets. The detailed properties of two-photon coherent states are developed and distinguished from ordinary coherent states. These two-photon coherent states are mathematically generated from coherent states through unitary operators associated with quadratic Hamiltonians. Physically they are the radiation states of ideal two-photon lasers operating far above threshold, according to the self-consistent-field approximation. The mean-square quantum noise behavior of these states, which is basically the same as those of minimum-uncertainty states, leads to applications not obtainable from coherent states or one-photon lasers. The essential behavior of two-photon coherent states is unchanged by small losses in the system. The counting rates or distributions these states generate in photocount experiments also reveal their difference from coherent states.