Quantum Optical Measurements Research Articles

Quantum-fluctuation asymmetry in multiphoton Jaynes–Cummings resonances

We explore the statistical behavior of the light emanating from a coherently driven Jaynes–Cummings (JC) oscillator operating in the regime of multiphoton blockade with two monitored output channels causing the loss of coherence at equal rates. We do so by adopting an operational approach that draws the particle and wave aspects of the forward-scattered radiation together, building upon the relationship between quantum optical correlation functions and conditional measurements. We first derive an analytical expression of the intensity cross-correlation function at the peak of the two-photon JC resonance to demonstrate the breakdown of detailed balance. The application of the quantum trajectory theory in parallel with the quantum regression formula subsequently uncovers various aspects of temporal asymmetry in the quantum fluctuations characterizing the cascaded process through which a multiphoton resonance is established and read out. We find that monitoring different quadratures of the cavity field in conditional homodyne detection affects the times waited between successive photon counter “clicks,” which in turn trigger the sampling of the homodyne current. Despite the fact that the steady-state cavity occupation is of the order of a photon, monitoring of the developing bimodality also impacts the ratio between the emissions directed along the two decoherence channels.

New fluorene-based bipolar charge transporting materials.

Air-stable and solution-processable fluorene-based bipolar charge transporting materials (CTMs) were designed, synthesized, and analyzed. These CTMs feature anthraquinone, 9-fluorenone, and 9-dicyanofluorenylidine groups and exhibit good film formation properties for solvent processing. Quantum chemistry simulations and optical absorption measurements proved that several stable conformers and charge transfer complexes form inside the molecules. Hole mobilities in CTMs were around 10-4 to 10-5 cm2 V-1 s-1, while electron mobility in compounds with anthraquinone and 9-dicyanofluorenylidine groups was approximately one order of magnitude lower.

Quantum Optics Measurement Scheme for Quantum Geometry and Topological Invariants.

We show how a quantum optical measurement scheme based on heterodyne detection can be used to explore geometrical and topological properties of condensed matter systems. Considering a 2D material placed in a cavity with a coupling to the environment, we compute correlation functions of the photons exiting the cavity and relate them to the hybrid light-matter state within the cavity. Different polarizations of the intracavity field give access to all components of the quantum geometric tensor on contours in the Brillouin zone defined by the transition energy. Combining recent results based on the metric-curvature correspondence with the measured quantum metric allows us to characterize the topological phase of the material. Moreover, in systems where S_{z} is a good quantum number, the procedure also allows us to extract the spin Chern number. As an interesting application, we consider a minimal model for twisted bilayer graphene at the magic angle, and discuss the feasibility of extracting the Euler number.

Non-Gaussian Mechanical Motion via Single and Multiphonon Subtraction from a Thermal State.

Quantum optical measurement techniques offer a rich avenue for quantum control of mechanical oscillators via cavity optomechanics. In particular, a powerful yet little explored combination utilizes optical measurements to perform heralded non-Gaussian mechanical state preparation followed by tomography to determine the mechanical phase-space distribution. Here, we experimentally perform heralded single-phonon and multiphonon subtraction via photon counting to a laser-cooled mechanical thermal state with a Brillouin optomechanical system at room temperature and use optical heterodyne detection to measure the s-parametrized Wigner distribution of the non-Gaussian mechanical states generated. The techniques developed here advance the state of the art for optics-based tomography of mechanical states and will be useful for a broad range of applied and fundamental studies that utilize mechanical quantum-state engineering and tomography.

Stabilizing Electro-Optic Modulation Depth With Carrier to Sideband Ratio by Intermediate Optical Beatings

Electro-optic modulation with a constant modulation depth plays important roles in quantum metrology, microwave photonics, and optical precision measurement and sensing, etc. To compensate the continuous drift of the half-wave voltage and maintain the modulation depth, we incidentally generated a reference laser to make intermedia optical beatings with the carrier and +1st sideband, and tightly locked the carrier to sideband ratio by extracting the relative microwave power of those intermedia beat signals using a home-made electronic control module. Experiments of the short-term test for locking bandwidth, the anti-jamming capability, and the long-term stability were demonstrated. As is verified that, the locking bandwidth was estimated up to tens of kHz, and the anti-jamming capability was confirmed despite of a strong hair-dryer agitation. The long-term stability for the modulation depth stabilization showed that the peak-to-peak relative stability for 150-minute was 5.67 × 10−6, and the Allan deviation reached a minimum of 2.19 × 10−7 at an averaging time of 400 s, which is improved by more than four orders of magnitude compared with the open-loop regime. We believe the reported method as well as the designed module is versatile and potential to precision measurement and sensing applications badly requiring a stable modulation depth, such as atom interferometry, optical interferometry and lidar detection.

Quantum nanophotonic and nanoplasmonic sensing: towards quantum optical bioscience laboratories on chip

Abstract Quantum-enhanced sensing and metrology pave the way for promising routes to fulfil the present day fundamental and technological demands for integrated chips which surpass the classical functional and measurement limits. The most precise measurements of optical properties such as phase or intensity require quantum optical measurement schemes. These non-classical measurements exploit phenomena such as entanglement and squeezing of optical probe states. They are also subject to lower detection limits as compared to classical photodetection schemes. Biosensing with non-classical light sources of entangled photons or squeezed light holds the key for realizing quantum optical bioscience laboratories which could be integrated on chip. Single-molecule sensing with such non-classical sources of light would be a forerunner to attaining the smallest uncertainty and the highest information per photon number. This demands an integrated non-classical sensing approach which would combine the subtle non-deterministic measurement techniques of quantum optics with the device-level integration capabilities attained through nanophotonics as well as nanoplasmonics. In this back drop, we review the underlining principles in quantum sensing, the quantum optical probes and protocols as well as state-of-the-art building blocks in quantum optical sensing. We further explore the recent developments in quantum photonic/plasmonic sensing and imaging together with the potential of combining them with burgeoning field of coupled cavity integrated optoplasmonic biosensing platforms.

Experimental quantum probing measurements with no knowledge of the system-probe interaction

In any natural science, measurements are the essential link between theory and observable reality. Is it possible to obtain accurate and relevant information via measurement whose action on the probed system is unknown? In other words, can one be convinced to know something about the nature without knowing in detail how the information was obtained? In this paper, we show that the answer is surprisingly, yes. We construct and experimentally implement a quantum optical probing measurement where measurements on the probes, the photons' polarization states, are used to extract information on the systems, the frequency spectra of the same photons. Unlike the pre-existing probing protocols, our measurement does not require any knowledge of the interaction between the probe and the system.

Spiral spectrum analysis and application ofcoherent synthetic vortex beams

The vortex beam is a ring-shaped beam whose center intensity or axial intensity is zero in the propagation direction and whose phase has a spiral rising or falling gradient distribution, which is also called a dark hollow beam. Vortex beams have important applications in free-space optical communication, optical micromanipulation, quantum information processing, optical measurement, super-resolution imaging, laser processing, and material processing. In recent years, with the in-depth research on vortex beams, the application requirements for high-power vortex beams also increase. High-power and high-quality vortex beam can be obtained by coherent combining technology. However, the spiral spectrum characteristics of the vortex beam generated by coherent combining technology need further exploring. In this paper, based on the theory of spectral analysis, we derive the position and magnitude of the spiral phase spectral component of the coherent synthetic vortex beam. The numerical results verify the correctness of the theoretical derivation. Based on the above spectral analysis theory, the mode purity of the target synthesis topology charge can be used as the evaluation function to evaluate quality and optimize the parameters for the coherent synthetic vortex beam, and then to quantitatively guide the coherent synthesis of the vortex beam. The results show that with the increase of the number of sub-beams and the radius of the beam waist of the source plane, the reduction of the radius of the bundle ring and the mode purity of the target synthesis topology charge can be improved, and then we can obtain the high-quality vortex beam. This is consistent with the conclusion obtained by using traditional evaluation functions such as power in the bucket. The spiral spectrum analysis of the coherent synthetic vortex beam not only makes up for the lack of evaluation of the spiral phase synthesis effect by the traditional evaluation function, but also has certain reference significance for understanding the nature of the coherent synthesis technique.

Frequency comb single-photon interferometry

Due to their frequency scaling and long-term coherence, frequency combs at the single-photon level can provide a fascinating platform for developments in quantum technology. Here we demonstrate frequency comb single-photon interferometry in an unheralded manner. We are able to induce coherence by erasing the which-way information of path-entangled photon pairs. The photon pairs are prepared using a dual parametric down-conversion pumped by a highly stable frequency comb laser and an ultra-narrow seed laser. This is conducted at the extremely low-conversion efficiency regime. The unique feature of our quantum interferometer is that the induced one-photon interference of the path-encoded single photons (signal), with multiple frequency components, is observed with a unit visibility without heralding conjugate photons (idler). We demonstrate that quantum information and frequency comb technology can be combined to realize quantum information platforms. We expect this will contribute to the application of quantum information and optical measurements beyond the classical limit.

Label-Free Optical Single-Molecule Micro- and Nanosensors.

Label-free optical sensor systems have emerged that exhibit extraordinary sensitivity for detecting physical, chemical, and biological entities at the micro/nanoscale. Particularly exciting is the detection and analysis of molecules, on miniature optical devices that have many possible applications in health, environment, and security. These micro- and nanosensors have now reached a sensitivity level that allows for the detection and analysis of even single molecules. Their small size enables an exceedingly high sensitivity, and the application of quantum optical measurement techniques can allow the classical limits of detection to be approached or surpassed. The new class of label-free micro- and nanosensors allows dynamic processes at the single-molecule level to be observed directly with light. By virtue of their small interaction length, these micro- and nanosensors probe light-matter interactions over a dynamic range often inaccessible by other optical techniques. For researchers entering this rapidly advancing field of single-molecule micro- and nanosensors, there is an urgent need for a timely review that covers the most recent developments and that identifies the most exciting opportunities. The focus here is to provide a summary of the recent techniques that have either demonstrated label-free single-molecule detection or claim single-molecule sensitivity.

Quantum optical measurement with tripartite entangled photons generated by triple parametric down-conversion.

Parametric down-conversion is a second-order nonlinear optical process annihilating a pump photon and creating a pair of photons in the signal and idler modes. Then, by using two parametric down-converters and introducing a path indistinguishability for the two generated idler modes, a quantum coherence between two conjugate signal beams can be induced. Such a double spontaneous or stimulated parametric down-conversion scheme has been used to demonstrate quantum spectroscopy and imaging with undetected idler photons via measuring one-photon interference between their correlated signal beams. Recently, we considered another quantum optical measurement scheme utilizing W-type tripartite entangled signal photons that can be generated by employing three spontaneous parametric down-conversion crystals and by inducing coherences or path-indistinguishabilities between their correlated idler beams and between quantum vacuum fields. Here, we consider an extended triple stimulated parametric down-conversion scheme for quantum optical measurement of sample properties with undetected idler and photons. Noting the real effect of vacuum field indistinguishability on the fringe visibility as well as the role of zero-point field energy in the interferometry, we show that this scheme is an ideal and efficient way to create a coherent state of W-type entangled signal photons. We anticipate that this scheme would be of critical use in further developing quantum optical measurements in spectroscopy and microscopy with undetected photons.

Simultaneous, Full Characterization of a Single-Photon State

As single-photon sources become more mature and are used more often in quantum information, communications and measurement applications, their characterization becomes more important. Single-photon-like light is often characterized by its brightness, and two quantum properties: the single-photon composition and the photon indistinguishability. While it is desirable to obtain these quantities from a single measurement, currently two or more measurements are required. Here, we simultaneously determine the brightness, the single photon purity, the indistinguishability, and the statistical distribution of Fock states to third order for a quantum light source. The measurement uses a pair of two-photon (n = 2) number-resolving detectors. n > 2 number-resolving detectors provide no additional advantage in the single-photon characterization. The new method extracts more information per experimental trial than a conventional measurement for all input states, and is particularly more e cient for statistical mixtures of photon states. Thus, using this n=2, number- resolving detector scheme will provide advantages in a variety of quantum optics measurements and systems.

Resonance Fluorescence of an InGaAs Quantum Dot in a Planar Cavity Using Orthogonal Excitation and Detection.

The ability to perform simultaneous resonant excitation and fluorescence detection is important for quantum optical measurements of quantum dots (QDs). Resonant excitation without fluorescence detection - for example, a differential transmission measurement - can determine some properties of the emitting system, but does not allow applications or measurements based on the emitted photons. For example, the measurement of photon correlations, observation of the Mollow triplet, and realization of single photon sources all require collection of the fluorescence. Incoherent excitation with fluorescence detection - for example, above band-gap excitation - can be used to create single photon sources, but the disturbance of the environment due to the excitation reduces the indistinguishability of the photons. Single photon sources based on QDs will have to be resonantly excited to have high photon indistinguishability, and simultaneous collection of the photons will be necessary to make use of them. We demonstrate a method to resonantly excite a single QD embedded in a planar cavity by coupling the excitation beam into this cavity from the cleaved face of the sample while collecting the fluorescence along the sample's surface normal direction. By carefully matching the excitation beam to the waveguide mode of the cavity, the excitation light can couple into the cavity and interact with the QD. The scattered photons can couple to the Fabry-Perot mode of the cavity and escape in the surface normal direction. This method allows complete freedom in the detection polarization, but the excitation polarization is restricted by the propagation direction of the excitation beam. The fluorescence from the wetting layer provides a guide to align the collection path with respect to the excitation beam. The orthogonality of the excitation and detection modes enables resonant excitation of a single QD with negligible laser scattering background.

Notes on a Continuous-Variable Quantum Key Distribution Scheme

We develop a physical model to describe the signal transmission for a continuous-variable quantum key distribution scheme and investigate its security against a couple of eavesdropping attacks assuming that the eavesdropper’s power is partly restricted owing to today’s technological limitations. We consider an eavesdropper performing quantum optical homodyne measurement on the signal obtained by a type of beamsplitting attack. We also consider the case in which the eavesdropper Eve is unable to access a quantum memory and she performs heterodyne measurement on her signal without performing a delayed measurement. Our formulation includes a model in which the receiver’s loss and noise are unaccessible by the eavesdropper. This setup enables us to investigate the condition that Eve uses a practical fiber differently from the usual beamsplitting attack where she can deploy a lossless transmission channel. The secret key rates are calculated in both the direct and reverse reconciliation scenarios.

Quantum optical measurements with undetected photons through vacuum field indistinguishability

Quantum spectroscopy and imaging with undetected idler photons have been demonstrated by measuring one-photon interference between the corresponding entangled signal fields from two spontaneous parametric down conversion (SPDC) crystals. In this Report, we present a new quantum optical measurement scheme utilizing three SPDC crystals in a cascading arrangement; here, neither the detection of the idler photons which interact with materials of interest nor their conjugate signal photons which do not interact with the sample is required. The coherence of signal beams in a single photon W-type path-entangled state is induced and modulated by indistinguishabilities of the idler beams and crucially the quantum vacuum fields. As a result, the optical properties of materials or objects interacting with the idler beam from the first SPDC crystal can be measured by detecting second-order interference between the signal beams generated by the other two SPDC crystals further down the set-up. This gedankenexperiment illustrates the fundamental importance of vacuum fields in generating an optical tripartite entangled state and thus its crucial role in quantum optical measurements.

Benchtop Fluorination of Fluorescent Nanodiamonds on a Preparative Scale: Toward Unusually Hydrophilic Bright Particles

Fluorination of diamonds modulates their optical and electromagnetic properties and creates surfaces with increased hydrophobicity. In addition, fluorination of diamonds and nanodiamonds has been recently shown to stabilize fluorescent nitrogen‐vacancy centers, which can serve as extremely sensitive single atomic defects in a vast range of sensing applications from quantum physics to high‐resolution biological imaging. Traditionally, fluorination of carbon nanomaterials has been achieved using harsh and complex experimental conditions, creating hydrophobic interfaces with difficult dispersibility in aqueous environments. Here, a mild benchtop approach to nanodiamond fluorination is described using selective Ag+‐catalyzed radical substitution of surface carboxyls for fluorine. In contrast to other approaches, this high‐yielding procedure does not etch diamond carbons and produces a highly hydrophilic interface with mixed C−F and C−OH termination. This dual functionalization of nanodiamonds suppresses detrimental hydrophobic interactions that would lead to colloidal destabilization of nanodiamonds. It is also demonstrated that even a relatively low surface density of fluorine contributes to stabilization of negatively charged nitrogen‐vacancy centers and boosts their fluorescence. The simultaneous control of the surface hydrophilicity and the fluorescence of nitrogen‐vacancy centers is an important issue enabling direct application of fluorescent nanodiamonds as nanosensors for quantum optical and magnetometry measurements operated in biological environment.

Uncertainty relations for characteristic functions

We present the uncertainty relation for the characteristic functions (ChUR) of the quantum mechanical position and momentum probability distributions. This inequality is more general than the Heisenberg Uncertainty Relation, and is saturated in two extremal cases for wavefunctions described by periodic Dirac combs. We further discuss a broad spectrum of applications of the ChUR, in particular, we constrain quantum optical measurements involving general detection apertures and provide the uncertainty relation that is relevant for Loop Quantum Cosmology. A method to measure the characteristic function directly using an auxiliary qubit is also briefly discussed.

Stacked bilayer phosphorene: strain-induced quantum spin Hall state and optical measurement.

Bilayer phosphorene attracted considerable interest, giving a potential application in nanoelectronics owing to its natural bandgap and high carrier mobility. However, very little is known regarding the possible usefulness in spintronics as a quantum spin Hall (QSH) state of material characterized by a bulk energy gap and gapless spin-filtered edge states. Here, we report a strain-induced topological phase transition from normal to QSH state in bilayer phosphorene, accompanied by band-inversion that changes number from 0 to 1, which is highly dependent on interlayer stacking. When the bottom layer is shifted by 1/2 unit-cell along zigzag/armchair direction with respect to the top layer, the maximum topological bandgap 92.5 meV is sufficiently large to realize QSH effect even at room-temperature. An optical measurement of QSH effect is therefore suggested in view of the wide optical absorption spectrum extending to far infra-red, making bilayer phosphorene a promising candidate for opto-spintronic devices.

Quantum optical arbitrary waveform manipulation and measurement in real time.

We describe a technique for dynamic quantum optical arbitrary-waveform generation and manipulation, which is capable of mode selectively operating on quantum signals without inducing significant loss or decoherence. It is built upon combining the developed tools of quantum frequency conversion and optical arbitrary waveform generation. Considering realistic parameters, we propose and analyze applications such as programmable reshaping of picosecond-scale temporal modes, selective frequency conversion of any one or superposition of those modes, and mode-resolved photon counting. We also report on experimental progress to distinguish two overlapping, orthogonal temporal modes, demonstrating over 8 dB extinction between picosecond-scale time-frequency modes, which agrees well with our theory. Our theoretical and experimental progress, as a whole, points to an enabling optical technique for various applications such as ultradense quantum coding, unity-efficiency cavity-atom quantum memories, and high-speed quantum computing.

Generation of optical-terahertz biphoton pairs via spontaneous parametric down-conversion

We examine the features of parametric down-conversion for generating non-classical optical-terahertz biphoton fields. The calculations were performed for a second-order correlation function and noise reduction factor (NRF), which were selected as measures of non-classicality of the field states. NRF measurement schemes were found to be less advantageous. Overall system temperature at which non-classical effects are still observable has been estimated as about 10 K. The priority of the main complicating factors for the quantum-optical measurements in the terahertz range has been determined. Thus, the most important parameter is the temperature of the system, after that come the absorption losses at terahertz frequencies, and the parametric conversion efficiency has the lowest impact. Increasing the pump radiation power has been found to be inefficient for "noise" suppression in this case. The calculations performed were based on lithium niobate dispersion properties due to its outstanding nonlinear optical properties and because it is frequently used for optical-to-terahertz frequency conversion.