Nonclassical States Research Articles

Electronic and photonic counting statistics as probes of non-equilibrium quantum dynamics

We analyse the full counting statistics (FCS) of photons flowing in and out of a microwave cavity coupled to a voltage-biased Josephson junction. Tunnelling of Cooper pairs generates a coherent flow of photons into the cavity whilst at the same time photons can also leak out incoherently. We use a very general unitary transformation method to demonstrate that there is a simple connection between the FCS of the charges and the photons in the long time limit, revealing that all the cumulants of the coherent and the incoherent processes match in that limit. We also explore some of the interesting features in the counting statistics of the charges and photons which arise from the strongly nonlinear dynamics of the system. These include very narrow distributions associated with the emergence of coherent transport and regimes where counting of either an odd or an even number of photons leaving the cavity can result in strongly non-classical cat states within the cavity.

Heralded dissipative preparation of nonclassical states in a Kerr oscillator

We present a heralded state preparation scheme for driven nonlinear open quantum systems. The protocol is based on a continuous photon counting measurement of the system's decay channel. When no photons are detected for a period of time, the system has relaxed to a measurement-induced pseudo-steady state. We illustrate the protocol by the creation of states with a negative Wigner function in a Kerr oscillator, a system whose unconditional steady state is strictly positive.

Strong mechanical squeezing for a levitated particle by coherent scattering

Levitated particles are a promising platform for precision sensing of external perturbations and probing the boundary between quantum and classical worlds. A critical obstacle for these applications is the difficulty of generating nonclassical states of motion which have not been realized so far. Here, we show that strong squeezing of the motion of a levitated particle below the vacuum level is feasible with available experimental parameters. Using suitable modulation of the trapping potential (which is impossible with clamped mechanical resonators) and coherent scattering of trapping photons into a cavity mode, we explore several strategies to achieve strong phase-sensitive suppression of mechanical fluctuations. We analyze mechanical squeezing in both transient and steady-state regimes, and discuss conditions for preparing nonclassical mechanical squeezing. Our results pave the way to full, deterministic optomechanical control of levitated particles in the quantum regime.

Quantum Signature of a Squeezed Mechanical Oscillator.

Recent optomechanical experiments have observed nonclassical properties in macroscopic mechanical oscillators. A key indicator of such properties is the asymmetry in the strength of the motional sidebands produced in the probe electromagnetic field, which is originated by the noncommutativity between the oscillator ladder operators. Here we extend the analysis to a squeezed state of an oscillator embedded in an optical cavity, produced by the parametric effect originated by a suitable combination of optical fields. The motional sidebands assume a peculiar shape, related to the modified system dynamics, with asymmetric features revealing and quantifying the quantum component of the squeezed oscillator motion.

Research progress in non-classical microwave states preparation based on cavity optomechanical system

As a novel hybrid quantum system, cavity optomechanical system shows super strong coupling strength, extremely low noise level and considerable coherent time under superconducting condition. In this paper, we briefly introduce basic principles of cavity optomechanics and cavity optomechanical systems. Meanwhile, we also classify the widely studied cavity optomechanical systems as five categories in their materials and structures. Significant parameters of these optomechanical systems, such as quality factor, mass and vibrating frequency of mechanical oscillator, are listed in detail. Technical merits and defects of these optomechanical systems are summarized. Furthermore, we introduce the research progress of non-classical microwave quantum states preparation by utilizing generalized cavity optomechanical systems, and we also analyze the performance advancements and remaining problems of this preparation method. In the end, we summarize the application cases at present and look forward to the potential application scenarios in the future. Our summary may be helpful for researchers who are focusing on quantum applications in sensing, radar, navigation, and communication in microwave domain.

Experimental realization of high-efficiency blue light at 426 nm by external frequency doubling resonator

Second harmonic generation (SHG) is used to get continuous wave laser with a lot of applications, it is a major way to provide pump power for generating nonclassical states, especially for squeezed states and entanglement states. High-efficiency SHG resonant on atoms lines also provides laser sources for atomic entanglement generation, light-atom interaction and high-speed quantum memory. For the frequency-doubling process at 426 nm, the major challenge of increasing the conversion efficiency is the thermal effect caused by the absorption in crystal. The degradation of mode-match efficiency induced by the severely thermal effect limits the conversion efficiency of the second harmonic generator. Furthermore, the blue light induced infrared absorption (BLIIRA) in the nonlinear crystal intensifies the thermal effect, it makes the conversion efficiency of the frequency-doubling cavity and the stability of the output blue laser worse, and it is more serious at high input power. Based on the theoretical analysis of thermal lens, we find that the thermal lens should not be placed at the center of the crystal, the location of the equivalently thermals lens has a deviation from the center of the crystal. Follow the theoretical analysis of thermal lens, we design a ring cavity with a 10 mm-long periodically poled potassium titanyle phosphate (PPKTP) crystal to reduce the thermal lens effect induced mode-mismatch. The location of nonlinear crystal is adjusted precisely to reduce the mode-mismatch caused by the thermal lens under our theoretical analysis. Finally, we realized a high conversion efficiency blue laser at 426 nm with the conversion efficiency up to 83.1% with an output power of 428 mW after the adjustment of the crystal location, corresponding to our theoretical analysis well. The measured beam quality factors (<i>M</i><sup>2</sup> value) of the generated blue laser are <inline-formula><tex-math id="Z-20200217024354-1">\begin{document}$ M^2(x) = 1.05 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-1.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="Z-20200217024354-2">\begin{document}$ M^2(y) = 1.02 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-2.png"/></alternatives></inline-formula>, respectively. The measured power stability of Generated Blue laser in 15 mins is 1.25%. The output power of the SHG is strong enough to provide pump power for the generation of the continuous variable squeezed vacuum state at 852 nm and the long-term stability of the output blue laser is also measured to be fine. To the best of our knowledge, the conversion efficiency is the highest-reported one at this wavelength. We believe that such high-performance frequency doubling system is a fundamental building block for quantum information science based non-classical states.

Exploiting Silicon photomultipliers for measuring nonclassical optical states

The search for optimal receivers endowed with photon-number-resolving capability has led us to consider Silicon photomultipliers for Quantum Optics experiments. By adopting an optimization procedure in order to reduce the spurious effects characterizing these detectors, we succeeded in detecting nonclassical correlations between the two parties of mesoscopic twin-beam states and in obtaining subPoissonian conditional states. These results put SiPMs forward for the implementation of Quantum Information and Communication protocols.

Generation of non-classical states of photons from a metal-dielectric interface: a novel architecture for quantum information processing.

We show the possibility to generate photons in a certain class of non-classical states from a metal-dielectric interface using dipole emitters on the interface. The photons emitted into the surface plasmon mode from the initially excited emitters radiate out in free space in a cone-shaped geometry. When detected at two detectors, these photons exhibit coalescence, a clear signature of non-classicality. Such a system can also be employed as a building block for a distributed quantum network. We further show that it is indeed feasible to implement our model using available technology.

Nonclassical States for Non-Hermitian Hamiltonians with the Oscillator Spectrum

In this paper, we show that the standard techniques that are utilized to study the classical-like properties of the pure states for Hermitian systems can be adjusted to investigate the classicality of pure states for non-Hermitian systems. The method is applied to the states of complex-valued potentials that are generated by Darboux transformations and can model both non- P T -symmetric and P T -symmetric oscillators exhibiting real spectra.

Coherent Diffusive Photon Gun for Generating Nonclassical States

We suggest and discuss a concept of deterministic integrated source of non-classical light based on the coherent diffusive photonics, a coherent light flow in a system of dissipatively coupled waveguides. We show how this practical quantum device can be realized with a system of single-mode waveguides laser-inscribed in nonlinear glass. We describe a hierarchy of models, from the complete multi-mode model of the waveguide network to the single mode coupled to a bath, analyze the conditions for validity of the simplest single-mode model and demonstrate feasibility of the generation of bright sub-Poissonian light states merely from a coherent input. Notably, the generation of non-classical states occurs at the initial stages of the dynamics, and can be accounted for in the linear model that allows us to circumvent the prohibiting computational complexity of the exact full quantum representation.

Enhancing phase sensitivity with number state filtered coherent states

Number state filtered coherent states are a class of nonclassical states obtained by removing one or more number states from a coherent state. Phase sensitivity of an interferometer is enhanced if these nonclassical states are used as input states. The optimal phase sensitivity, which is related to the quantum Cramer-Rao bound (QCRB) for the input state, improves beyond the standard quantum limit. It is argued that removal of more than one suitable number state leads to better phase sensitivity. As an important limiting case in this context, the even and odd coherent states, where the odd and even number states are filtered from coherent state respectively, are considered. The optimal phase sensitivity for these limiting cases equals that of the squeezed vacuum. It is observed that the improvement in phase sensitivity is not in direct proportion to the nonclassicality of the input states.

Unconventional cavity optomechanics: Nonlinear control of phonons in the acoustic quantum vacuum

We study unconventional cavity optomechanics and the acoustic analogue of radiation pressure to show the possibility of nonlinear coherent control of phonons in the acoustic quantum vacuum. Specifically, we study systems where a quantized optical field effectively modifies the frequency of an acoustic resonator. We present a general method to enhance such a nonlinear interaction by employing an intermediate qubit. Compared with conventional optomechanical systems, the roles of mechanical and optical resonators are interchanged, and the boundary condition of the phonon resonator can be modulated with an ultrahigh optical frequency. These differences allow to test some quantum effects with parameters which are far beyond the reach of conventional cavity optomechanics. Based on this novel interaction form, we show that various nonclassical quantum effects can be realized. Examples include an effective method for modulating the resonance frequency of a phonon resonator (e.g., a surface-acoustic-wave resonator), demonstrating mechanical parametric amplification, and the dynamical Casimir effect of phonons originating from the acoustic quantum vacuum. Our results demonstrate that unconventional optomechanics offers a versatile hybrid platform for quantum engineering of nonclassical phonon states in quantum acoustodynamics.

Numerical study of Wigner negativity in one-dimensional steady-state resonance fluorescence

In a numerical study, we investigate the steady-state generation of nonclassical states of light from a coherently driven two-level atom in a one-dimensional waveguide. Specifically, we look for states with a negative Wigner function, since such nonclassical states are a resource for quantum information processing applications, including quantum computing. We find that a waveguide terminated by a mirror at the position of the atom can provide Wigner-negative states, while an infinite waveguide yields strictly positive Wigner functions. Moreover, our investigation reveals a connection between the purity of a quantum state and its Wigner negativity. We also analyze the effects of decoherence on the negativity of a state.

On the Cerenkov effect and non-classical states of electromagnetic vacuum: from classical pattern to quantum approach

In this paper, using the quantum approach, we consider the Cerenkov radiation in the case in which an electron moves in the background of the non-classical state of the electromagnetic field, such as the squeezed state. In the first part we briefly describe: the standard results by Frank and Tamm for the Cerenkov effect based on the principles of classical electrodynamics; Fermi's results based on the radiative attenuation of the charged particle; the results of quantum-mechanical calculations. The quantum approach gives an important conclusion on the essence of the Cerenkov effect: in contrast to the classical approach, where the Cerenkov effect explains as a result of interaction of the charged particle with a medium, in the quantum case this effect can be explained as the result of interaction of the charged particle with the electromagnetic vacuum changed by a medium. Within the quantum approach, we obtained also the explicit analytic expression for the Cerenkov radiation in the case when an electron moves in the background of the squeezed state of electromagnetic vacuum (only quantum approach can be applied in this case due to a non-classical essence of the squeezed electromagnetic vacuum). Also, we obtained the explicit analytic expression for the Cerenkov radiation in an external laser field. In this case, the Cerenkov radiation can be interpreted as a stimulated process in contrast to the “classical” case, where the Cerenkov radiation has a spontaneous essence.

Manipulating Nonclassicality via Quantum State Engineering Processes: Vacuum Filtration and Single Photon Addition

AbstractThe effect of two quantum state engineering processes that can be used to burn a hole at vacuum in the photon number distribution of quantum states of radiation field is compared using various witnesses of lower‐ and higher‐order nonclassicality as well as a measure of nonclassicality. Specifically, the modification in nonclassical properties due to vacuum state filtration and a single photon addition on an even coherent state, binomial state, and Kerr state are investigated using the criteria of lower‐ and higher‐order antibunching, squeezing, and sub‐Poissonian photon statistics. Further, the amount of nonclassicality present in these engineered quantum states having enormous applications in continuous variable quantum communication is quantified and analyzed by using an linear entropy‐based entanglement potential. It is observed that all the quantum states studied here are highly nonclassical, and the hole‐burning processes can introduce/enhance nonclassical features. However, it is not true in general. A hole at vacuum implies a maximally nonclassical state (as far as Lee's nonclassical depth is concerned), but a particular process of hole burning at vacuum does not ensure the existence of any particular nonclassical feature. Specifically, lower‐ and higher‐order squeezing are not observed for photon‐added and vacuum filtered even coherent states.

Impact of photon addition and subtraction on nonclassical and phase properties of a displaced Fock state

Various nonclassical and quantum phase properties of photon added then subtracted displaced Fock state have been examined systematically and rigorously. Higher-order moments of the relevant bosonic operators are computed to test the nonclassicality of the state of interest, which reduces to various quantum states (having applications in quantum optics, metrology and information processing) in different limits ranging from the coherent (classical) state to the Fock (most nonclassical) states. The nonclassical features are discussed using Klyshko’s, Vogel’s, and Agarwal–Tara’s criteria as well as the criteria of lower- and higher-order antibunching, sub-Poissonian photon statistics and squeezing. In addition, phase distribution function and quantum phase fluctuation have been studied. These properties are examined for various combinations of number of photon addition and/or subtraction and Fock parameter. The examination has revealed that photon addition generally improves nonclassicality, and this advantage enhances for the large (small) values of displacement parameter using photon subtraction (Fock parameter). The higher-order sub-Poissonian photon statistics is only observed for the odd orders. In general, higher-order nonclassicality criteria are found to detect nonclassicality even in the cases when corresponding lower-order criteria failed to do so. Photon subtraction is observed to induce squeezing, but only large number of photon addition can be used to probe squeezing for large values of displacement parameter. Further, photon subtraction is found to alter the phase properties more than photon addition, while Fock parameter has an opposite effect of the photon addition/subtraction. Finally, nonclassicality and non-Gaussianity is also established using Q function.

Nonclassical Properties in Optomechanical System Controlled by Single-photon Catalysis

A scheme is proposed to investigate the influence of single-photon catalysis operation on the dynamics of optomechanical system. The nonclassicalities of the catalyzed states are studied by analyzing mean photon(phonon) number, photon(phonon) number distribution, Mandel Q parameter, second-order correlation functions and Wigner function. According to the fidelity between the states before and after operation, it is found that the quantum properties of optomechanical states are significantly affected via the optimum catalysis. The optomechanical entanglement can be enhanced in the large catalysis parameter region with the small-amplitude coherent state. In particular, the Wigner function of the mechanical state can be represented by superposition of Wigner function of displaced number-like states in which the weights depend on the value of catalysis parameter. The interesting results show that tuning the catalysis parameter can manipulate efficiently the nonclassical properties of cavity mode and mechanical mode. This work might provide a new pathway to control the nonclassicalities of the optomechanical states, especially the nonclassical mechanical states.

Mechanical qubit-light entanglers in hybrid nonlinear qubit optomechanics

Interfacing between matter qubits and light is a crucial provision for numerous quantum technological applications. However, a generic qubit may not directly interact with a relevant optical field mode, and hence, one could necessitate to adjust frequencies to match resonance conditions between parties. In this work, we show how a parametric coupling of the qubit with a mechanical oscillator, in conjunction with the trilinear radiation pressure coupling of the same object with light, can induce maximal qubit-light entanglement at an optimal time. We also show how our method enables conditional nonclassical state preparation of an optical field state via qubit measurements in the weak optomechanical coupling regime, whereas nonclassical states of the same can dynamically be achieved in the moderate-to-strong single photon coupling limit. Our scheme benefits from not requiring any cooling of the mechanical component, and not needing an adjusting of the detunings and transition frequencies to have resonance between any pairs of quantum systems.

Quantum fluctuations and particle production in the oscillatory phase of a thermal inflaton in a FRW universe

By the use of coherent and squeezed thermal states formalism, we analyzed the phenomena of particle creation when coherently oscillating a homogeneous scalar field (Inflaton) in semi-classical gravity. We also obtained an estimated solution for the semi-classical Einstein equation in non-classical thermal states formalism perturbatively, which is similar to the power-law expansion of classical Einstein equation. This solution, apart from a particular condition, shows oscillatory behavior in nature. We also analyzed a coherently oscillating homogeneous scalar field, in a thermal vacuum, coherent thermal, squeezed thermal vacuum and coherent squeezed thermal states, suffering from particle creation, and created particles are showing oscillatory behavior. Particle production can be raised due to thermal and quantum effects. We also studied quantum fluctuations of a homogeneous scalar field in the above-mentioned non-classical thermal states.

Classical Behavior of Nonclassical Quantum States

We use a new formulation of nonclassicality in weak measurements based on the probabilistic behavior of “weak quasimoments” to show that nonclassical quantum states in the Glauber–Sudarshan representation can behave classically in weak measurements with post-selection. We directly calculate marginal quasidistributions related to the “weak quasimoments” and show that they are nonnegative.