Using a photon-counting quantum light spectroscopy that probes photosynthetic light harvesting with a single photon at a time, we experimentally demonstrated that photosynthesis begins and proceeds with a single quantum of energy. We report the observation of individual single-photon absorption and emission events in spatially distinct regions in photosynthetic systems. The experiments were carried out on an ensemble of pigment-protein complexes light-harvesting 2 from purple bacteria Rhodobacter sphaeroides under ambient conditions in vitro.

Flow cytometry is one of the most widespread optical methods used in the diagnosis of health disorders and disease monitoring. The ultimate goal is achieving a single biomarker sensitivity. Here we experimentally characterize the nonclassical light from the flow cytometer and observe g(2)(0) = 0.4(2). Thus, we demonstrate a single emitter sensitivity and determine that the signal due to one biomarker is at least 6 times brighter than the background noise. This result potentially enables detecting rare single biomarker occurrences with high throughput.

Photosynthesis is generally assumed to be initiated by a single photon(1-3) from the Sun, which, as a weak light source, delivers at most a few tens of photons per nanometre squared per second within a chlorophyll absorption band(1). Yet much experimental and theoretical work over the past 40 years has explored the events during photosynthesis subsequent to absorption of light from intense, ultrashort laser pulses(2-15). Here, we use single photons to excite under ambient conditions the light-harvesting 2 (LH2) complex of the purple bacterium Rhodobacter sphaeroides, comprising B800 and B850 rings that contain 9 and 18 bacteriochlorophyll molecules, respectively. Excitation of the B800 ring leads to electronic energy transfer to the B850 ring in approximately 0.7 ps, followed by rapid B850-to-B850 energy transfer on an approximately 100-fs timescale and light emission at 850-875 nm (refs. (16-19)). Using a heralded single-photon source(20,21) along with coincidence counting, we establish time correlation functions for B800 excitation and B850 fluorescence emission and demonstrate that both events involve single photons. We also find that the probability distribution of the number of heralds per detected fluorescence photon supports the view that a single photon can upon absorption drive the subsequent energy transfer and fluorescence emission and hence, by extension, the primary charge separation of photosynthesis. An analytical stochastic model and a Monte Carlo numerical model capture the data, further confirming that absorption of single photons is correlated with emission of single photons in a natural light-harvesting complex.

Flow cytometry is the most common technique used to probe heterogeneous populations of cells in a high throughput manner. We are interested in the limit when (1) the occurrence of a specific target biomarker is rare and (2) biomarker concentration is low. Here we discuss a framework for in-situ characterizing signal noise levels in those systems, and the use of quantum measurement to optimize detection. Our method enables us to measure biomarker concentration and unambiguously identify the presence of a single emitter from first principles: i.e. without the need for auxiliary calibration.

Fluorescent biomarkers are used to detect target molecules within inhomogeneous populations of cells. When these biomarkers are found in trace amounts it becomes extremely challenging to detect their presence in a flow cytometer. Here, we present a framework to draw a detection baseline for single emitters and enable absolute calibration of a flow cytometer based on quantum measurements. We used single-photon detection and found the second-order autocorrelation function of fluorescent light. We computed the success of rare-event detection for different signal-to-noise ratios (SNR). We showed high-accuracy identification of the events with occurrence rates below 10−5 even at modest SNR levels, enabling early disease diagnostics and post-disease monitoring.

We use a spatial light modulator and a genetic algorithm to manipulate the spatial profile of the pump beam of a down-conversion source in order to improve its Klyshko efficiency.

We demonstrate single quantum emitter sensitivity in a flow cytometer by measuring the second order correlation function to be g®( 0)= 0.5(1).

The utility of transmission measurement has made it a target for quantum enhanced measurement strategies. Here we find if the length of an absorbing object is a controllable variable, then via the Beer-Lambert law, classical strategies can be optimised to reach within 83% of the absolute quantum limit. Our analysis includes experimental losses, detector noise, and input states with arbitrary photon statistics. We derive optimal operating conditions for both classical and quantum sources, and observe experimental agreement with theory using Fock and thermal states.

We theoretically and experimentally demonstrate that when absorbance estimation in the Beer-Lambert law is considered, the enhancement offered by optical quantum sensing strategies is drastically reduced by optimisation over the length of absorbing material.

By exploiting the quantised nature of light, we demonstrate a sub-shot-noise scanning optical transmittance microscope. Our microscope demonstrates, with micron scale resolution, a factor of improvement in precision of 1.76(9) in transmittance estimation gained per probe photon relative to an optimal classical version at the same optical power. This would allow us to observe photosensitive samples with nearly twice the precision,without sacrificing image resolution or increasing optical power to improve signal-to-noise ratio. Our setup uses correlated twin-beams produced by parametric down-conversion, and a hybrid detection scheme comprising photon-counting-based feed-forward and a highly efficient CCD camera.

By exploiting the quantised nature of light, we demonstrate a sub-shot-noise scanning optical transmittance microscope. Our microscope demonstrates, with micron scale resolution, a factor of improvement in precision of 1.76(9) in transmittance estimation gained per probe photon relative to the theoretical model, a shot-noise-limited source of light, in an equivalent single-pass classical version of the same experiment using the same number of photons detected with a 90% efficient detector. This would allow us to observe photosensitive samples with nearly twice the precision, without sacrificing image resolution or increasing optical power to improve signal-to-noise ratio. Our setup uses correlated twin-beams produced by parametric down-conversion, and a hybrid detection scheme comprising photon-counting-based feed-forward and a highly efficient CCD camera.

Optical quantum sensing strategies that utilise features of quantum states of light have implications for precision measurement in areas as wide ranging as such as gravitational wave sensing [1] and biological imaging [2]. Optical absorption estimation is the task of estimating the transmission parameter η which is defined by the ratio of input, I in , to output, I out , intensity of light from a sample of interest (I out = ηI ιη ). The optimal quantum strategy provides a quantum advantage (per incident photon) of 1/(1-η) over the best classical strategy [3]. This technique has been demonstrated experimentally in single parameter estimation and imaging scenarios [4,5].

We demonstrate sub-Poissonian intensity correlations of twin beams at short wavelengths: 442nm and 665nm. The beams are generated via four-wave mixing in photonic crystal fiber, measured with a CCD camera.

We demonstrate a scanning microscopy system for high resolution, sub-shot-noise absorption imaging. A genuine quantum advantage of factor 1.66 in information gained per probe photon, relative to an ideal classical measurement, is demonstrated.

Engineering apparatus that harness quantum theory promises to offer practical advantages over current technology. A fundamentally more powerful prospect is that such quantum technologies could out-perform any future iteration of their classical counterparts, no matter how well the attributes of those classical strategies can be improved. Here, for optical direct absorption measurement, we experimentally demonstrate such an instance of an absolute advantage per photon probe that is exposed to the absorbative sample. We use correlated intensity measurements of spontaneous parametric downconversion using a commercially available air-cooled CCD, a new estimator for data analysis and a high heralding efficiency photon-pair source. We show this enables improvement in the precision of measurement, per photon probe, beyond what is achievable with an ideal coherent state (a perfect laser) detected with 100% efficient and noiseless detection. We see this absolute improvement for up to 50% absorption, with a maximum observed factor of improvement of 1.46. This equates to around 32% reduction in the total number of photons traversing an optical sample, compared to any future direct optical absorption measurement using classical light.

Quantum metrology enables estimation of optical phase shifts with precision beyond the shot-noise limit. One way to exceed this limit is to use squeezed states, where the quantum noise of one observable is reduced at the expense of increased quantum noise for its complementary partner. Because shot-noise limits the phase sensitivity of all classical states, reduced noise in the average value for the observable being measured allows for improved phase sensitivity. However, additional phase sensitivity can be achieved using phase estimation strategies that account for the full distribution of measurement outcomes. Here we experimentally investigate a model of optical spin-squeezing, which uses post-selection and photon subtraction from the state generated using a parametric downconversion photon source, and we investigate the phase sensitivity of this model. The Fisher information for all photon-number outcomes shows it is possible to obtain a quantum advantage of 1.58 compared to the shot-noise value for five-photon events, even though due to experimental imperfection, the average noise for the relevant spin-observable does not achieve sub-shot-noise precision. Our demonstration implies improved performance of spin squeezing for applications to quantum metrology.

Received 5 October 2017DOI:https://doi.org/10.1103/PhysRevApplied.8.049902Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasOptical quantum information processingPhotonicsQuantum fluctuations & noiseQuantum measurementsQuantum metrologyQuantum opticsSingle photon sourcesAtomic, Molecular & Optical

Harnessing the unique properties of quantum mechanics offers the possibility of delivering alternative technologies that can fundamentally outperform their classical counterparts. These technologies deliver advantages only when components operate with performance beyond specific thresholds. For optical quantum metrology, the biggest challenge that impacts on performance thresholds is optical loss. Here, we demonstrate how including an optical delay and an optical switch in a feed-forward configuration with a stable and efficient correlated photon-pair source reduces the detector efficiency required to enable quantum-enhanced sensing down to the detection level of single photons and without postselection. When the switch is active, we observe a factor of improvement in precision of 1.27 for transmission measurement on a per-input-photon basis compared to the performance of a laser emitting an ideal coherent state and measured with the same detection efficiency as our setup. When the switch is inoperative, we observe no quantum advantage.

Quantummetrology enables estimation of optical phase shifts with precision beyond the shot-noise limit. Oneway to exceed this limit is to use squeezed states, where the quantumnoise of one observable is reduced at the expense of increased quantumnoise for its complementary partner. Because shotnoise limits the phase sensitivity of all classical states, reduced noise in the average value for the observable beingmeasured allows for improved phase sensitivity. However, additional phase sensitivity can be achieved using phase estimation strategies that account for the full distribution of measurement outcomes.Herewe experimentally investigate amodel of optical spin-squeezing, which uses post-selection and photon subtraction from the state generated using a parametric downconversion photon source, andwe investigate the phase sensitivity of thismodel. The Fisher information for all photon-number outcomes shows it is possible to obtain a quantum advantage of 1.58 compared to the shot-noise value for five-photon events, even though due to experimental imperfection, the average noise for the relevant spin-observable does not achieve sub-shot-noise precision. Our demonstration implies improved performance of spin squeezing for applications to quantummetrology.