We demonstrate that a quantum receiver can monitor the conditions of a communications channel simultaneously with data transmission, with no additional measurements required. The sensing scheme uses the confidence vector estimated by the state identification algorithm to monitor changes in power and phase noise and is demonstrated experimentally. To measure confidence, the output of the quantum receiver is processed using the Bayesian formula for each individual measurement. By optimizing the statistical analysis of confidence vector data we achieve 0.5% sensitivity to power fluctuations with no interruption of data transmission.
We build a network source of indistinguishable photons, synchronized to an external clock, that could be used as scalable modular unit in an extended quantum network infrastructure. We characterize the indistinguishability and synchronization of this source. The Hong-Ou-Mandel dip shows near unity indistinguishability. Allan deviation analysis shows sub-picosecond jitter when locked to an external clock. The jitter figure is more than 10 times smaller than the pulse duration of 30ps. The source is compatible with many clock recovery systems, including the White Rabbit Precision Time Protocol (WR-PTP). This source enables scalable quantum protocols over multi-node, long-distance optical networks.
We build a test setup for metrological characterization of the qCMOS photon number-resolving camera. We evaluate the quality of the photon number distributions measured with the camera for coherent and pseudo-thermal sources and perform quantum tomography of the camera. Further, we make measurements of multiple spatially overlapping pseudo-thermal states to perform full statistical mode reconstruction and calculate the spatial distribution of the high-order correlation functions $g^{(n)}(0)$. We use the obtained results to demonstrate spatio-angular super-resolution with a photon number-resolving camera. The methods reported here can be used as a toolkit for quantum super-resolving microscopy with photon number-resolving cameras.
Quantum enabled optical receivers can achieve a symbol error rate below the shot noise limit and have been extensively studied at visible wavelengths. Yet, the practical use of a quantum receiver requires the implementation at telecom wavelengths. Here we report a successful implementation of a telecom-enabled, versatile time resolving quantum receiver testbed that enables a range of modulation schemes and supports long communication alphabets. We achieved the record sensitivity for a legacy 4-symbol modulation and implemented other modulations and longer alphabets for the first time by combining a highly efficient single photon detector with the time-resolved receiver design.
Practical quantum networks require transform-limited single photon sources that can be synchronized with high timing accuracy over long distances. We build a source of heralded single photons at telecom C-band using a Ti-Sapphire mode locked laser that can be synchronized to an external frequency reference. We target 10 ps single photon pulses so that the independent sources should be carrier frequency matched to within 10 GHz and experience ~3 ps shot-to-shot jitter to independently produce indistinguishable photons. In this progress report, we demonstrate the picosecond single-photon source and discuss high-accuracy precision time protocol (HA-PTP) and pulsed laser-based network synchronization options.
We investigate the coexistence of clock synchronization protocols with quantum signals propagating in a common optical single-mode fiber channel. By measuring optical forward and backward scattering noise in a range from 1510 nm to 1620 nm we demonstrate a potential for up to 100 quantum channels each with 100 GHz bandwidth coexisting with the classical synchronization signals using off-the-shelf optical components. Both “White Rabbit'' and a pulsed laser-based synchronization were characterized and compared.
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.
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.
We investigate how accuracy information available in quantum measurement can be used to enhance error correction codes using experimentally obtained state identification data. We test conventional and novel error correction protocols and find a significant reduction of errors in user data transmission. The error reduction and the energy use of error correcting codes compare favorably to that of quantum receivers without error correction. Ordinarily, quantum receivers identify the maximally likely input state out of M possible states. We take advantage of the full vector of Bayesian probabilities available for each act of measurement that supplements the conventional state discrimination.
The exponential growth of the internet is currently fulfilled by optical communication, but photonic communication channels are on the verge of a capacity crunch. We systematically study the use of bandwidth and energy of quantum-enhanced communication channels for classical information with different practical encodings. We introduce hybrid encoding protocols with a simultaneous coherent frequency and phase modulation and optimize them for the most advantageous quantum measurement at the receiver. We experimentally map the energy and bandwidth use for the optimization of practical resource-limited channels using a tabletop platform. We also report the initial progress on developing an integrated telecom testbed.
We show that 1D array of nonlinear evanescently-coupled waveguides could be used as a quasi 2D lattice via a synthetic frequency dimension induced by nonlinear coupling. We demonstrate the analogy of this platform to a multi-level atom interacting with light for classical and quantum photonic states. Using this framework, we adapt well-known coherent processes from atomic optics, such as electromagnetically induced transparency and stimulated Raman adiabatic passage to design novel photonic devices. Owing to demonstrated ultra-low noise of sum-frequency generation in lithium niobate, our devices are particularly useful for quantum information applications: quantum memory and quantum transduction.
Ordinarily, quantum-measurement based state discrimination methods use Bayesian inference to determine the maximally likely optical input state out of N possible inputs. Each practical single-shot measurement yields a unique vector of N likelihood values (one per each possible outcome) whose significance is not appreciated. Here we show theoretically and experimentally that measured Bayesian likelihood values are equal to observed probabilities of a successful state discrimination. Thus, the vector of Bayesian likelihoods provides significantly more information about the input state than is available in a typical state discrimination measurement.
We report on the first experimental implementation of a scalable M-ary quantum receiver with time-resolved photon detection and real-time adaptive displacement using off-the-shelf components. We implemented M=4, 6, 8, 12, and 16 coherent frequency shift keying communication links with optimal parameters for maximal energy sensitivity. Then, we compared the measured sensitivity with the best sensitivity that can be achieved with a perfect classical measurement (shot noise limit). We observe unconditional, below the shot-noise limit sensitivity and a favorable sensitivity scaling with the alphabet length M.
Exabytes of data are sent through the internet monthly, and the demand grows exponentially. Quantum measurement enables sensitivity beyond the shot noise, with a potential to minimize the energy and bandwidth required to transmit a bit. We developed a fully quantum-mechanical treatment of this measurement problem. We introduce new protocols of data exchange that are designed to take full advantage of quantum measurement, unlike legacy communication methods that seek to optimize classical measurements. We discuss the theoretical bounds of the new protocols and demonstrate proof of principle experiments.
KEYWORDS: Quantum communications, Receivers, Modulation, Energy efficiency, Quantum efficiency, Data communications, Signal to noise ratio, Frequency shift keying
In classical digital communications two main families of “M-ary” modulation schemes are generally distinguished: bandwidth limited and power limited. The canonical comparison of these modulation methods is based on normalized data rate (R/W) (bits per second per hertz of bandwidth) and the signal to noise rate per bit required to achieve a given error probability for different M. In a classical picture, the two families reside in two separate semi-plains R/W < 1 and
R/W > 1, i.e. energy efficiency and bandwidth efficiency cannot be optimized at the same time. However,
we find an alphabet family that can be paired with a quantum receiver to simultaneously optimize bandwidth and power efficiency of a communication channel. Particularly we found that coherent frequency shift keying (CFSK) gives rise to a family of communication protocols that are bandwidth limited in nature, but whose bandwidth usage can be optimized so that R/W>1 for a range of alphabet lengths M, while power sensitivity beats that of power-limited protocols. We will report our theoretical findings and experimental progress towards implementation of this protocol family.
The efficient use of communication channels motivates extensive research in novel communication protocols. Modern communication protocols use large alphabets contain up to a few thousand symbols, thus optimizing the use of power and available frequency space. To date, quantum receivers that discriminate up to approximately 20 symbols were theoretically investigated and receivers with as many as 4 nonorthogonal coherent states have been experimentally demonstrated. However, all heretofore explored quantum receivers suffer from the sensitivity degradation with the alphabet size. Particularly, their Helstrom Bound (HB) nearly reaches the classical standard quantum limit (SQL).
Here we introduce an M-ary quantum receiver based on a coherent frequency shift keying (CFSK) protocol with a record power sensitivity, free of the above deficiency. The CFSK not only provides better accuracy for longer alphabets but also allows discrimination with a practically attainable symbol error rate (SER) situated much below the HBs of other encodings for large alphabets. Our receiver operates with a classical transmitter, and with any communication channel, including the existing global fiber network. It can be used to increase the amplification-free range in a network and/or reduce power requirements on the transmitter by more than 1000 times. In addition, the quantum measurement advantage can significantly optimize the use of the frequency space in comparison to classical frequency keying protocols. This advantage can be used in deep-space telecom links to enhance the satellite power budget. In existing fiber network links, quantum CFSK receivers can improve the amplification-free range by approximately the factor of 2.
We characterize an efficient and nearly-noiseless parametric frequency upconverter. The ultra-low noise regime is reached by the wide spectral separation between the input and pump frequencies and the low pump frequency relative to the input photons. The background of only ≈100 photons per hour is demonstrated. We demonstrate phase preservation in a frequency upconversion process at the single-photon level. We summarize our efforts to measure this ultra-low noise level, and discuss both single-photon avalanche photodiode measurements and a photon-counting transition edge sensor (TES) measurements. To reach the required accuracy, we supplemented our TES with a dark count reduction algorithm. The preservation of the coherence was demonstrated by simultaneously upconverting the input of each arm of a Mach-Zehnder interferometer through high interference fringe contrast. We observe fringe visibilities of ≥0.97 with faint coherent input.
The non-stationary problem of electron-molecular ion scattering is solved analytically in the frame of the perturbation
theory on the scattering potential and without the plane wave approximation for the incident electron. The influence of
the parameters of the incident electron wave-packet on the observed diffraction images is studied. Effect of interference
between incident and scattered wave packets is analyzed in details and shown to result in dramatic changes of the
angular distributions.
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