Quantum States Of Light Research Articles

Theory of four-wave mixing of cylindrical vector beams in optical fibers

Cylindrical vector (CV) beams are a set of transverse spatial modes that exhibit a cylindrically symmetric intensity profile and a variable polarization about the beam axis. They are composed of a non-separable superposition of orbital and spin angular momenta. Critically, CV beams are also the eigenmodes of optical fiber and, as such, are of widespread practical importance in photonics and have the potential to increase communications bandwidth through spatial multiplexing. Here, we derive the coupled amplitude equations that describe the four-wave mixing (FWM) of CV beams in optical fibers. These equations allow us to determine the selection rules that govern the interconversion of CV modes in FWM processes. With these selection rules, we show that FWM conserves the total angular momentum, the sum of orbital and spin angular momenta, in the conversion of two input photons to two output photons. When applied to spontaneous FWM, the selection rules show that photon pairs can be generated in CV modes directly and can be entangled in those modes. Such quantum states of light in CV modes could benefit technologies such as quantum key distribution with satellites.

Quantum Light and Coherent States in Conducting Media

We present a simple description of classical and quantum light propagating through homogeneous conducting linear media. With the choice of Coulomb gauge, we demonstrate that this description can be performed in terms of a damped harmonic oscillator which is governed by the Caldirola-Kanai Hamiltonian. By using the dynamical invariant method and the Fock states representation we solve the time-dependent Schrodinger equation associated with this Hamiltonian and write its solutions in terms of a special solution of the Milne-Pinney equation. We also construct coherent states for the quantized light and show that they are equivalent to the well-known squeezed states. Finally, we evaluate some important properties of the quantized light such as expectation values of the amplitude and momentum of each mode, their variances and the respective uncertainty principle.

Entropic nonclassicality and quantum non-Gaussianity tests via beam splitting

We propose entropic nonclassicality criteria for quantum states of light that can be readily tested using homodyne detection with beam splitting operation. Our method draws on the fact that the entropy of quadrature distributions for a classical state is non-increasing under an arbitrary loss channel. We show that our test is strictly stronger than the variance-based squeezing condition and that it can also be extended to detect quantum non-Gaussianity in conjunction with phase randomization. Furthermore, we address how our criteria can be used to identify single-mode resource states to generate two-mode states demonstrating EPR paradox, i.e., quantum steering, via beam-splitter setting.

Entropy- and purity-tailored broadband entanglement from vectorial four-wave mixing: Insights from pulse modes and classical-field dynamics

We present a time-domain Schmidt-mode analysis of a broadband continuous-variable entanglement of photon pairs generated via a vectorial four-wave mixing (FWM) of ultrashort laser pulses in a highly nonlinear birefringent optical fiber. We demonstrate that the time-domain eigenmodes of high-purity two-photon states generated through vectorial FWM can be steered, by varying the pump wavelength and FWM polarization geometry, from a high-purity entangled ket to a high-entropy entangled state in a space of a very high dimensionality. Moreover, this pulse-mode analysis is shown to provide a clear physical perspective on how the entanglement structure of two-photon states builds up as a result of short-pulse FWM dynamics. This insight reveals a correspondence-type relation between the quantum and classical pictures of photon-pair generation. With an eye on practical applications, a clear understanding of the temporal profile of pulse modes representing high-purity two-photon states is central to a meaningful shaping of ultrashort photon-packet waveforms for super-resolving microscopy and multiphoton spectroscopy using quantum states of light.

Quantum hacking of free-space continuous-variable quantum key distribution by using a machine-learning technique

The transmission coefficient of the free-space channel is fluctuating due to the occurrence of random fluctuations of the refractive index and noise. The Gaussian-modulated quantum states of light may degrade into a non-Gaussian mixture at a certain probability after being transmitted through the free-space channel. In such a situation, Eve can apply an entanglement-distillation attack to steal secret information without it being revealed if Alice and Bob regard the practical fluctuation losses as constant. However, the entanglement-distillation attack is available only when the transmitting states are a non-Gaussian mixture; otherwise, Eve will expose herself. Exploiting this loophole and considering the limitations of the attack, we enhance Eve's eavesdropping ability by integrating a machine-learning technique with the entanglement-distillation attack. With the help of convolutional neural network (CNN, one of the most popular machine leaning tools), Eve can choose the best opportunity to launch the entanglement-distillation attack. Therefore, Eve has a chance to bias the estimation of the parameters by improving the entanglement resource of the mixed states, which allows Eve to render the final keys shared between the legitimate parties insecure. Our proposed CNN-based entanglement-distillation attack does not require additional resources and can be implemented using simple means based on the present technologies.

Near-Unity Light Collection Efficiency from Quantum Emitters in Boron Nitride by Coupling to Metallo-Dielectric Antennas.

The bright and stable single-photon emission under room temperature conditions from color centers in hexagonal boron nitride (hBN) is considered as one of the most promising quantum light sources for quantum cryptography as well as spin-based qubits, similar to recent advances in nitrogen-vacancy centers in diamond. To this end, integration with cavity or waveguide modes is required to enable ideally lossless transduction of quantum light states. Here, we demonstrate a scheme to embed hBN quantum emitters into on-chip arrays of metallo-dielectric antennas that provides near unity light collection efficiencies with experimental values up to 98%, i.e. a 7-fold enhancement compared to bare quantum emitters. Room-temperature quantum light emission in the 700 nm band is characterized with single-photon emission rates into the first lens up to 44 MHz under continuous excitation and up to 10 MHz under 80 MHz pulsed excitation (0.13 photons per trigger pulse) into a narrow output cone (±15°) that facilitates fiber butt-coupling. We furthermore provide here a direct measurement of the quantum yield under pulsed excitation with values of 6-12% for hBN nanoflakes. Our demonstrated scheme could enable low loss spin-photon interfaces on a chip.

Enhancing continuous variable quantum key distribution with a heralded hybrid linear amplifier

We propose a method to improve transmission distance of continuous-variable quantum key distribution via a heralded hybrid linear amplifier (HLA), involving an ideal deterministic linear amplifier and a probabilistic noiseless linear amplifier. The phase-insensitive amplification of quantum light states may experience a degradation of signal-to-noise ratio (SNR) of 3dB which can be surmounted by the probabilistic amplification process. The new hybrid amplifier allows one to tune between the regimes of high gain or high noise reduction and control the trade-off between the amplification coefficient and a finite heralding probability flexibly. Our equipment could clone unknown coherent states with fidelity surpassing the no-cloning limit by abandoning determinism and using probabilistic methods. The setup has a better compatibility with continuous variable protocols as it only uses Gaussian elements and a post-selection algorithm. The hybrid linear amplifier enhances the SNR and hence the transmission distance of the CVQKD protocol is extended. Both the asymptotic secret key rate and the secret key rate in a finite-size scenario are analyzed in this paper.

Quantum sensing for dynamical tracking of chemical processes

Quantum photonics has demonstrated its potential for enhanced sensing. Current sources of quantum light states tailored to measuring, allow to monitor phenomena evolving on time scales of the order of the second. These are characteristic of product accumulation in chemical reactions of technologically interest, in particular those involving chiral compounds. Here we adopt a quantum multiparameter approach to investigate the dynamic process of sucrose acid hydrolysis as a test bed for such applications. The estimation is made robust by monitoring different parameters at once.

Reinforcement Learning with Deep Quantum Neural Networks

The advantage of quantum computers over classical computers fuels the recent trend of developing machine learning algorithms on quantum computers, which can potentially lead to breakthroughs and new learning models in this area. The aim of our study is to explore deep quantum reinforcement learning (RL) on photonic quantum computers, which can process information stored in the quantum states of light. These quantum computers can naturally represent continuous variables, making them an ideal platform to create quantum versions of neural networks. Using quantum photonic circuits, we implement Q learning and actor-critic algorithms with multilayer quantum neural networks and test them in the grid world environment. Our experiments show that 1) these quantum algorithms can solve the RL problem and 2) compared to one layer, using three layer quantum networks improves the learning of both algorithms in terms of rewards collected. In summary, our findings suggest that having more layers in deep quantum RL can enhance the learning outcome.

Reduction of Multiplexed Quantum Ghost Images

Ghost imaging allows extracting information about an object from the spatial correlation function between the photons that interacted with an object and the photons in the reference arm(s). The reference beam does not interact with the object; however, it is measured by a CCD matrix. The mathematical method of measurement reduction allows using both the mutual correlations of multiplexed ghost images and prior information about how the image of the research object can look in order to improve the quality of the reconstructed image. The information about the object is the sparsity of its transparency distribution under a given transform. Computer modelling of image reconstruction using multiplexed ghost images formed by four-mode entangled quantum light states confirms the efficiency of the proposed algorithm even with a small number of photons that illuminate the research object.

Comparison of coherence-area measurement techniques for bright entangled twin beams

Quantum states of light with multiple spatial modes are fundamental for quantum imaging and parallel quantum information processing. Thus, their characterization, which can be achieved through measurements of the coherence area, is an important area of research. We present a comparative study between two different measurement techniques for the coherence area of bright entangled twin beams of light generated with a four-wave mixing process in a hot rubidium vapor cell. The first one provides a direct characterization of the size of the coherence area and is based on correlation measurements between spatial intensity fluctuations of the twin beams with an electron-multiplying charge-coupled-device camera. The second one provides an indirect measure and is based on a noise analysis of different spatial regions of the twin beams in the time domain with a single photodiode. We show that the indirect technique, which can be implemented with a significantly less complicated measurement scheme, gives an estimate of the size of the coherence area consistent with the direct measurement technique performed in the spatial domain.

A quantum enhancement for plasmonic sensors

Quantum Sensing Laser light can induce electronic excitations on metal surfaces. These excitations, or plasmons, are sensitive to the surface environment and are now routinely used in chemical and biological sensing applications. Dowran et al. show that the sensitivity of these plasmonic sensors can be enhanced by using quantum states of light. They replace the laser light (used in the classical configuration) with quantum light, in this case a twin beam of entangled photons; one beam is used as a reference and the other to excite the surface plasmons. They find an appreciable sensitivity enhancement in the quantum configuration. Moreover, the reduced light intensity could also prove useful for the detection of light-sensitive chemical or biological samples. Optica 5 , 628 (2018).

(Invited) Interplay of Spectral Diffusion and Phonon Broadening in Carbon Nanotubes: Implications for Quantum Optics

We performed a thorough and quantitative investigation of spectral diffusion at low temperature in carbon nanotubes wrapped in PFO and embedded in a polystyren matrix, a system of interest for applications for quantum light states generation. We show that an intrinsic line shape typical of exciton-acoustic phonon coupling can be traced back down to the µs time-scale, while spectral diffusion yields a feature-less spectral broadening of the zero-phonon line. By tuning physical parameters such as temperature and pump power we investigate the line-shape deformation at several integration time-scales. Finally, we discuss the implications of these optical properties as regards the efficiency of the static and dynamical coupling of carbon nanotube to optical micro-cavities for single photon generation.

Entangled Photonic-Nuclear Molecular Dynamics of LiF in Quantum Optical Cavities.

The quantum photodynamics of a simple diatomic molecule with a permanent dipole immersed within an optical cavity containing a quantized radiation field is studied in detail. The chosen molecule under study, lithium fluoride (LiF), is characterized by the presence of an avoided crossing between the two lowest 1Σ potential energy curves (covalent-ionic diabatic crossing). Without field, after prompt excitation from the ground state 1 1Σ, the excited nuclear wave packet moves back and forth in the upper 2 1Σ state, but in the proximity of the avoided crossing, the nonadiabatic coupling transfers part of the nuclear wave packet to the lower 1 1Σ state, which eventually leads to dissociation. The quantized field of a cavity also induces an additional light crossing in the modified dressed potential energy curves with similar transfer properties. To understand the entangled photonic-nuclear dynamics, we solve the time-dependent Schrödinger equation by using the multiconfigurational time-dependent Hartree method (MCTDH). The single mode quantized field of the cavity is represented in the coordinate space instead of in the Fock space, which allows us to deal with the field as an additional vibrational mode within the MCTDH procedure on equal footing. We prepare the cavity with different quantum states of light, namely, Fock states, coherent states, and squeezed coherent states. Our results reveal pure quantum light effects on the molecular photodynamics and the dissociation yields of LiF, which are quite different from the light-undressed case and which cannot be described in general by a semiclassical approach using classical electromagnetic fields.

Observation of three-photon bound states in a quantum nonlinear medium

Bound states of massive particles, such as nuclei, atoms, or molecules, constitute the bulk of the visible world around us. By contrast, photons typically only interact weakly. We report the observation of traveling three-photon bound states in a quantum nonlinear medium where the interactions between photons are mediated by atomic Rydberg states. Photon correlation and conditional phase measurements reveal the distinct bunching and phase features associated with three-photon and two-photon bound states. Such photonic trimers and dimers possess shape-preserving wave functions that depend on the constituent photon number. The observed bunching and strongly nonlinear optical phase are described by an effective field theory of Rydberg-induced photon-photon interactions. These observations demonstrate the ability to realize and control strongly interacting quantum many-body states of light.

Forming photonic bound states

Quantum Optics Photons do not naturally interact with each other and must be coaxed into doing so. Liang et al. show that a gas of Rydberg atoms—a cloud of rubidium atoms excited by a sequence of laser pulses—can induce strong interactions between propagating photons. The authors could tune the strength of the interaction to make the photons form dimer and trimer bound states. This approach should prove useful for producing novel quantum states of light and quantum entanglement on demand. Science , this issue p. [783][1] [1]: /lookup/doi/10.1126/science.aao7293

Generation of photonic entanglement in green fluorescent proteins

Recent development of spectroscopic techniques based on quantum states of light can precipitate many breakthroughs in observing and controlling light-matter interactions in biological materials on a fundamental quantum level. For this reason, the generation of entangled light in biologically produced fluorescent proteins would be promising because of their biocompatibility. Here we demonstrate the generation of polarization-entangled two-photon state through spontaneous four-wave mixing in enhanced green fluorescent proteins. The reconstructed density matrix indicates that the entangled state is subject to decoherence originating from two-photon absorption. However, the prepared state is less sensitive to environmental decoherence because of the protective β-barrel structure that encapsulates the fluorophore in the protein. We further explore the quantumness, including classical and quantum correlations, of the state in the decoherence environment. Our method for photonic entanglement generation may have potential for developing quantum spectroscopic techniques and quantum-enhanced measurements in biological materials.

Quantum light and topological surface states

We demonstrate theoretically that, when quantum light interacts with massless Dirac fermions on the surface of a three-dimensional topological insulator, the elementary excitation spectrum depends on the polarizations of quantum light. Linear-polarized light cannot open a gap and leads to an anisotropic Dirac cone, but circular-polarized light can induce a mass term and the sign of mass is determined by the helicity of light. The effects due to quantum fluctuations are also discussed.

Comment on “Generating a perfect quantum optical vortex”

In a recent article, Banerji et al. introduced a novel quantum state of light, coined as the perfect quantum optical vortex state [Phys. Rev. A 94, 053838 (2016)] due to its mathematical similarity with the classical perfect vortex beam. This state is obtained by means of the Fourier transform of a Bessel-Gaussian vortex state, and the authors claim that this can be accomplished by means of a simple lens. Here, we will show that this statement is wrong since a lens cannot modify the quantum noise distribution related to the input optical quantum state and this has to be exchanged by an "effective lens".

Theory of noise suppression inΛ-type quantum memories by means of a cavity

Quantum memories, capable of storing single photons or other quantum states of light, to be retrieved on-demand, offer a route to large-scale quantum information processing with light. A promising class of memories is based on far-off-resonant Raman absorption in ensembles of $\Lambda$-type atoms. However at room temperature these systems exhibit unwanted four-wave mixing, which is prohibitive for applications at the single-photon level. Here we show how this noise can be suppressed by placing the storage medium inside a moderate-finesse optical cavity, thereby removing the main roadblock hindering this approach to quantum memory.