Photon Detection Events Research Articles

Superconducting nanowire detector jitter limited by detector geometry

Detection jitter quantifies variance introduced by the detector in the determination of photon arrival time. It is a crucial performance parameter for systems using superconducting nanowire single photon detectors (SNSPDs). In this work, we have demonstrated that the detection timing jitter is limited in part by the spatial variation of the photon detection events along the length of the wire. We define this jitter source as a geometric jitter since it is related to the length and area of the SNSPD. To characterize the geometric jitter, we have constructed a differential cryogenic readout with less than 7 ps of an electronic jitter that can amplify the pulses generated from the two ends of an SNSPD. By differencing the measured arrival times of the two electrical pulses, we were able to partially cancel out the difference of the propagation times and thus reduce the uncertainty of the photon arrival time. We proved that the variation of the differential propagation time was a few ps for a 3 μm × 3 μm device, while it increased up to 50 ps for a 20 μm × 20 μm device. In a 20 μm × 20 μm large SNSPD, we achieved a 20% reduction in the overall detection timing jitter for detecting the telecom-wavelength photons by using the differential cryogenic readout.

Quantum Logic with Cavity Photons From Single Atoms.

We demonstrate quantum logic using narrow linewidth photons that are produced with an apriori nonprobabilistic scheme from a single ^{87}Rb atom strongly coupled to a high-finesse cavity. We use a controlled-not gate integrated into a photonic chip to entangle these photons, and we observe nonclassical correlations between photon detection events separated by periods exceeding the travel time across the chip by 3 orders of magnitude. This enables quantum technology that will use the properties of both narrow-band single photon sources and integrated quantum photonics.

Generation of heralded entanglement between distant hole spins

Quantum entanglement emerges naturally in interacting quantum systems and plays a central role in quantum information processing. Remarkably, it is possible to generate entanglement even in the absence of direct interactions: provided that which path information is erased, weak spin-state dependent light scattering can be used to project two distant spins onto a maximally entangled state upon detection of a single photon. Even though this approach is necessarily probabilistic, successful generation of entanglement is heralded by the photon detection event. Here, we demonstrate heralded quantum entanglement of two quantum dot heavy-hole spins separated by 5 meters using single-photon interference. Thanks to the long coherence time of hole spins and the efficient spin-photon interface provided by self-assembled quantum dots embedded in leaky microcavity structures, we generate 2300 entangled spin pairs per second, which represents an improvement approaching three orders of magnitude as compared to prior experiments. Delayed two-photon interference scheme we developed allows for efficient verification of quantum correlations. Our results lay the groundwork for the realization of quantum networks in semiconductor nanostructures. Combined with schemes for transferring quantum information to a long-lived memory qubit, fast entanglement generation we demonstrate could also impact quantum repeater architectures.

A Bias-Free Quantum Random Number Generation Using Photon Arrival Time Selectively

We present a high-quality, bias-free quantum random number generator (QRNG) using photon arrival time selectively in accordance with the number of photon detection events within a sampling time interval in attenuated light. It is well showed in both theoretical analysis and experiments verification that this random number production method eliminates both bias and correlation perfectly without more post processing and the random number can clearly pass the standard randomness tests. We fulfill theoretical analysis and experimental verification of the method whose rate can reach up to 45Mbps.

Effect of charging on CdSe/CdS dot-in-rods single-photon emission

The photon statistics of CdSe/CdS dot-in-rods nanocrystals is studied with a method involving postselection of the photon detection events based on the photoluminescence count rate. We show that flickering between two states needs to be taken into account to interpret the single-photon emission properties. With postselection we are able to identify two emitting states: the exciton and the charged exciton (trion), characterized by different lifetimes and different second-order correlation functions. Measurements of the second-order autocorrelation function at zero delay with postselection shows a degradation of the single-photon emission for CdSe/CdS dot-in-rods in a charged state that we explain by deriving the neutral and charged biexciton quantum yields.

Single‐Molecule Studies on the Label Number Distribution of Fluorescent Markers

Over the past decade, a vast variety of different fluorescent labeling systems have emerged for use in fluorescence microscopy and fluorescence-based analytical techniques. A difficulty frequently arising when quantifying fluorescently labeled samples is that the number of labels per protein is neither well defined, for example, due to multiple functional groups that can undergo covalent coupling with activated dyes, nor well known, for example, due to limited methods mostly estimating ensemble averages. Herein, we use a recently established method that evaluates the statistics of multiple photon detection events to measure the label number distribution of different fluorescent marker molecules at the single-molecule level. We tested five different far-red dyes frequently used for fluorescence labeling and found all of them suitable for our counting method. We used two dyes, ATTO647N and Alexa647, to investigate the label number distribution of fluorescently labeled proteins. In the experiments, we found that the label number distribution of antibodies and streptavidin has a significant fraction of molecules labeled with two, three, or more fluorophores. In contrast, the distribution of label numbers for nanobodies resembles the one acquired for SNAP-tag, which can have a maximum of one label per protein. This is also reflected in the ensemble degree of labeling, which is in good agreement for the latter samples, whereas stronger deviations were observed for antibodies and streptavidin. Our single-molecule studies enable full characterization of the label number distribution for various fluorescent markers. This work puts quantitative studies on the stoichiometry of fluorescently tagged oligomers and protein aggregates into perspective.

Correlation between photon-emission intervals in blinking luminescence of single CdSe/CdS nanocrystals

The statistics of luminescence from single CdSe/CdS core/shell semiconductor nanocrystals under CW laser excitation at room temperature is experimentally investigated by recording sequences of absolute arrival times of the emitted photons. It is shown that the correlation coefficient for consecutive intervals between the photon-arrival times differs from zero. The correlation persists for photon-arrival intervals separated by two or more photon-detection events, until the time between the two intervals becomes, on average, as long as 180 ms, which corresponds to about 103 detected photons. A simulation of the luminescence process supports the conclusion that this correlation is linked to the blinking character of the quantum-dot luminescence.

Metal-driven Operation of the Human Large-conductance Voltage- and Ca2+-dependent Potassium Channel (BK) Gating Ring Apparatus

Large-conductance voltage- and Ca(2+)-dependent K(+) (BK, also known as MaxiK) channels are homo-tetrameric proteins with a broad expression pattern that potently regulate cellular excitability and Ca(2+) homeostasis. Their activation results from the complex synergy between the transmembrane voltage sensors and a large (>300 kDa) C-terminal, cytoplasmic complex (the "gating ring"), which confers sensitivity to intracellular Ca(2+) and other ligands. However, the molecular and biophysical operation of the gating ring remains unclear. We have used spectroscopic and particle-scale optical approaches to probe the metal-sensing properties of the human BK gating ring under physiologically relevant conditions. This functional molecular sensor undergoes Ca(2+)- and Mg(2+)-dependent conformational changes at physiologically relevant concentrations, detected by time-resolved and steady-state fluorescence spectroscopy. The lack of detectable Ba(2+)-evoked structural changes defined the metal selectivity of the gating ring. Neutralization of a high-affinity Ca(2+)-binding site (the "calcium bowl") reduced the Ca(2+) and abolished the Mg(2+) dependence of structural rearrangements. In congruence with electrophysiological investigations, these findings provide biochemical evidence that the gating ring possesses an additional high-affinity Ca(2+)-binding site and that Mg(2+) can bind to the calcium bowl with less affinity than Ca(2+). Dynamic light scattering analysis revealed a reversible Ca(2+)-dependent decrease of the hydrodynamic radius of the gating ring, consistent with a more compact overall shape. These structural changes, resolved under physiologically relevant conditions, likely represent the molecular transitions that initiate the ligand-induced activation of the human BK channel.

Unified quantum jump superoperator for optical fields from the weak- to the strong-coupling limit

We derive a generalized quantum jump superoperator that can be used in the quantum trajectory description of single photon detectors, light-emitting diodes (LEDs), and lasers. Our model describes an optical single-mode cavity field coupled to a reservoir through a two-state quantum system and includes three physical parameters: the coupling of the field to the two-state system, the coupling of the two-state system to the reservoir, and the cavity loss rate. In this setup, the two-state system can act as a photodetector or as an energy-adding mechanism. In the first case, we assume that the reservoir acts as a damping mechanism for an ideal cavity and derive reduced field operators describing the photon detection events. Our model coincides with the commonly known quantum trajectory based photon counting models at the weak- and strong-coupling limits and is, furthermore, also applicable between their validity regimes. In the second case, we assume that the reservoir injects energy into a lossy cavity through the two-state system. Again we derive the reduced field operators describing photon creation events into the lossy cavity. We show that this setup can act as an LED or as a laser depending on the strength of the injection. We also investigatemore » how the setup operates at the close proximity of the lasing threshold.« less

Quantum jumps and photon statistics in fluorescent systems coupled to classically fluctuating reservoirs

In this paper, we develop a quantum-jump approach for describing the photon emission process of single fluorophore systems coupled to complex classically fluctuating reservoirs. The formalism relies on an open quantum system approach where the dynamics of the system and the reservoir fluctuations are described through a density matrix whose evolution is defined by a Lindblad rate equation. For each realization of the photon-measurement processes it is possible to define a conditional system state (stochastic density matrix) whose evolution depends on both the photon detection events and the fluctuations between the configurational states of the reservoir. In contrast to standard fluorescent systems the photon-to-photon emission process is not a renewal one, being defined by a (stochastic) waiting time distribution that in each recording event parametrically depends on the conditional state. The formalism allows calculating experimental observables such as the full hierarchy of joint probabilities associated with the time intervals between consecutive photon recording events. These results provide a powerful basis for characterizing different situations arising in single-molecule spectroscopy, such as spectral fluctuations, lifetime fluctuations and light-assisted processes.

Simultaneous, coincident 2-D ACAR and DBAR using segmented HPGe detectors incorporating sub-pixel interpolation

A three-dimensional Positron Annihilation Spectrometry System (3D PASS) for determination of 3D electron-positron (e--e+) momentum densities by measuring coincident annihilation photons was designed, constructed and characterized. 3D PASS collects a single data set including correlated photon energies and coincident photon positions which are typically collected separately by two-dimensional angular correlation of annihilation radiation (2D ACAR) and two-detector coincident Doppler broadening of annihilation radiation (CDBAR) spectrometry. 3D PASS is composed of two position-sensitive, high-purity germanium (HPGe) double-sided strip detectors (DSSD(s)) linked together by a 32-channel, 50 MHz digital electronics suite. The DSSDs data were analyzed to determine location of photon detection events using an interpolation method to achieve a spatial resolution less than the 5-mm width of the DSSDs' charge collection strips. The interpolation method relies on measuring a figure-of-merit proportional to the area of the transient charges observed on both strips directly adjacent to the charge collection strip detecting the full charge deposited by the annihilation photon. This sub-pixel resolution, corresponding to the error associated with event location within a sub-pixel was measured for both DSSDs using the approach outlined in Williams et al [1] and was on the order of ± 0.20 mm (± one-standard deviation). As a result of the sub-pixel resolution, the distance between the DSSDs and material sample was reduced by a factor of five compared to what is typically required in 2D ACAR systems was necessary to achieve 0.5-mrad angular resolution. This reduction in the system's footprint decreases attenuation of the annihilation photons in the air between the material sample and the DSSDs and increases the solid angle between the sample and the DSSDs, ultimately resulting in higher system detection efficiency. 3D PASS was characterized in the same manner comparable to state-of-the-art 2D ACAR and CDBAR spectrometers. 3D PASS spectra were collected and analyzed for single-crystal copper (Cu) and silicon carbide (6H SiC) and compared with the results in the literature.

Overcoming non-Markovian dephasing in single-photon sources through postselection

We study the effects of realistic dephasing environments on a pair of solid-state single-photon sources in the context of the Hong-Ou-Mandel dip. By means of solutions for the Markovian or exact non-Markovian dephasing dynamics of the sources, we show that the resulting loss of visibility depends crucially on the timing of photon detection events. Our results demonstrate that the effective visibility can be improved via temporal postselection, and also that time-resolved interference can be a useful probe of the interaction between the emitter and its host environment.

Quantum jumps in non-Markovian single fluorescent systems

In this paper, we formulate a quantum jump approach associated with single fluorescent systems whose radiative decay is defined by a Lindblad-like equation with a memory kernel. We base our results in a generalized Born–Markov approximation, which allows us to get our results from a full microscopic description. In the weak-laser-excitation regime, the photon-emission process can be described through a delay function, which in a natural way provides the basis for unravelling the dynamics in terms of a stochastic density matrix. As in the Markovian case, the stochastic dynamics consists in periods where the evolution is smooth and non-unitary, interrupted at random times where the photon-detection events produce a sudden change in the system state. The phenomenon of non-Poissonian intermittent fluorescence induced by complex environments (Budini 2006 Phys. Rev. A 73 061802) is analysed in the context of this approach.

Efficiency loss in HPGe detectors due to beta and gamma sum coincidence

Among the secondary detection effects in gamma-ray spectroscopy with HPGe detectors that reduce the peak area and must be properly taken into account in accurate measurements, the less well known is counting loss by beta and gamma-ray sum coincidence. The fraction of lost counts was estimated assuming that a photon detection event can sum either with the coincident beta-ray or its bremsstrahlung in the detector capsule or surrounding materials. The beta-ray was tracked through all the materials between source and detector. The result was calculated as a sequence of integral expressions evaluated numerically, without resorting to simulation. It is shown that the counting losses due to this effect are negligible for most calibration sources in typical detector arrangements.

A photon accurate model of the human eye

A photon accurate model of individual cones in the human eye perceiving images on digital display devices is presented. Playback of streams of pixel video data is modeled as individual photon emission events from within the physical substructure of each display pixel. The thus generated electromagnetic wavefronts are refracted through a four surface model of the human cornea and lens, and diffracted at the pupil. The position, size, shape, and orientation of each of the five million photoreceptor cones in the retina are individually modeled by a new synthetic retina model. Photon absorption events map the collapsing wavefront to photon detection events in a particular cone, resulting in images of the photon counts in the retinal cone array. The custom rendering systems used to generate sequences of these images takes a number of optical and physical properties of the image formation into account, including wavelength dependent absorption in the tissues of the eye, and the motion blur caused by slight movement of the eye during a frame of viewing. The creation of this new model is part of a larger framework for understanding how changes to computer graphics rendering algorithms and changes in image display devices are related to artifacts visible to human viewers.

Using fluorescence lifetime for discriminating detector afterpulsing in fluorescence-correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) has become an important and widely used technique for many applications in physics, chemistry, and biology. Usually, FCS is measured with sensitive light detectors working in the photon-counting Geiger mode. A common property of such detectors is afterpulsing: the generation of spurious photon detection events after a genuine photon detection. Such afterpulsing causes a significant deviation of the measured autocorrelation function from its true value on a short time scale and can seriously influence derived parameters for fast processes such as triplet-state photophysics. Here, we discuss the impact of afterpulsing on FCS in detail. A new method is developed to eliminate afterpulsing effects by using time-correlated single-photon counting for separating the true fluorescence signal from afterpulsing events.

Effect of Background Noise on the Photon Statistics ofTriggered Single Molecules

We theoretically derive exact expressions for Mandel's Qparameter of the triggered single molecular source, which is inferredfrom the probabilities PRS(n) using the record ofeach photon detection event based on Hanbury Brown and Twiss detection.The real triggered source is recognized as an ideal single photonsource with a Poissonian statistics background. How to decrease thebackground and to increase the efficiency are discussed. It isestablished that the sub-Poissonian statistics formation can bedetermined by comparing the measured QRS of the realsingle triggered molecular with QC of the Poissoniansource containing the same mean photons. By this method, we also givean efficient way to measure signal-to-background ratios of triggeredsingle photons.

A time- and energy-resolving, multichannel event logger

An analog/digital circuit is described that can record the time of occurence plus additional eight-bit information on events in several (currently 5, expandable to 15) detectors. The additional eight bits can be used to represent photon energy or a location on a position sensitive detector. It was developed for a photon correlation experiment and can be used to record all photon detection events in two or more detectors that pass very relaxed rejection criteria, leaving the more restrictive data filtering and the determination of correlation for later off-line analysis.

Time-correlated photon counting technique robust against multiple photon events using a multianode photomultiplier tube

A time-correlated photon counting technique which is robust against multiple photon events has been developed. Utilizing a 16 channel multianode photomultiplier tube, both the time interval between the excitation and the first detection of a fluorescence photon and the number of detected fluorescence photons are measured simultaneously. Based on the time and the number data fluorescence decay parameters can be estimated by maximizing the log likelihood. Results of computer simulations and experiments show that this technique can be successfully applicable at high detection rates of >0.9 fluorescence photon detection event per excitation on average.

Irrelevance of photon events distinguishability in a class of Bell experiments

We show that the possibility of distinguishing between single and two photon detection events, usually not met in the actual experiments, is not a necessary requirement for the proof that the experiments of Alley and Shih [Phys. Rev. Lett. 61, 2921 (1988)], and Ou and Mandel [Phys. Rev. Lett. 61, 50 (1988)], are modulo fair sampling assumption, valid tests of local realism. We also give the critical parameters for the experiments to be unconditional tests of local realism, and show that some other interesting phenomena (involving bosonic type particle indistinguishability) can be observed during such tests.