In the process of spontaneous parametric downconversion, photons from a pump field are converted to signal and idler photon pairs in a nonlinear crystal. The reversed process, or upconversion of these pairs back to single photons in a second crystal, is also possible. Here, we present experimental measurements of the upconversion rate with a controlled time delay introduced between the signal and idler photons. As a function of delay, this rate presents a full width at half maximum of 27.9 fs under our experimental conditions, and we further demonstrate that group delay dispersion of the photon pairs broadens this width. These observations are in close agreement with our calculations, thus demonstrating an ultrafast, nonclassical correlation between the signal and idler waves.
Pairs of photons entangled in their time-frequency degree of freedom are of great interest in quantum optics research and applications, due to their relative ease of generation and their high capacity for encoding information. Here we analyze, both theoretically and experimentally, the behavior of phase-insensitive spectrally-resolved interferences arising from two pairs of time-frequency entangled photons. At its core, this is a multimode entanglement swapping experiment, whereby a spectrally resolved joint measurement on the idler photons from both pairs results in projecting the signal phot…
We study broadband polarization entangled states generated within the line width of type-II Spontaneous Parametric Down Conversion (SPDC). Applying a complete quantum polarization tomography protocol at arbitrary combination of frequency and angular sideband modes we reveal a complex structure of non-degenerate non-collinear polarization entangled states generated within the SPDC line width. It is demonstrated that simultaneous compensation of longitudinal and transverse walk off leads to homogenizing the structure of polarization entanglement.
We describe a simple and efficient setup to generate and characterize femtosecond quadrature-entangled pulses. Quantum correlations equivalent to about 2.5 dB squeezing are efficiently and easily reached using the non-degenerate parametric amplification of femtosecond pulses through a single-pass in a thin (0.1 mm) potassium niobate crystal. The entangled pulses are then individually sampled to characterize the non-separability and the entropy of formation of the states. The complete experiment is analysed in the time-domain, from the pulsed source of quadrature entanglement to the time-resolv…
We demonstrate a new technique for characterizing two-photon quantum states based on joint temporal correlation measurements using time resolved single photon detection by femtosecond upconversion. We measure for the first time the joint temporal density of a two-photon entangled state, showing clearly the time anti-correlation of the coincident-frequency entangled photon pair generated by ultrafast spontaneous parametric down-conversion under extended phase-matching conditions. The new technique enables us to manipulate the frequency entanglement by varying the down-conversion pump bandwidth …
Frequency-Resolved Simulations of Highly Entangled Biphoton States: Beyond the Single-Pair Approximation. II. Application to Entanglement-based Quantum Key Distribution
Entanglement, describing the inseparability of a quantum multiparty system, is one of the most intriguing features of quantum mechanics. Violation of Bell inequality, for ruling out the possibility of local hidden variable theories, is commonly used as a strong witness for quantum entanglement. In previous Bell test experiments with photonic entanglement based on two-photon coincidence measurement, the photon temporal wave packets are absorbed completely by the detectors. That is, the photon coherence time is much shorter than the detection time window. Here we demonstrate generation of freque…
We develop an ultrafast frequency-resolved Raman spectroscopy with entangled photons for polyatomic molecules in condensed phases, to probe the electronic and vibrational coherences. Using quantum correlation between the photons, the signal shows the capability of both temporal and spectral resolutions that are not accessible by either classical pulses or the fields without entanglement. We develop a microscopic theory for this Raman spectroscopy, revealing the electronic coherence dynamics which often shows a rapid decay within $\sim$50fs. The heterodyne-detected Raman signal is further devel…
The double-slit experiment strikingly demonstrates the wave-particle duality of quantum objects. In this famous experiment, particles pass one-by-one through a pair of slits and are detected on a distant screen. A distinct wave-like pattern emerges after many discrete particle impacts as if each particle is passing through both slits and interfering with itself. While the direct event-by-event buildup of this interference pattern has been observed for massive particles such as electrons, neutrons, atoms and molecules, it has not yet been measured for massless particles like photons. Here we pr…
In this article, we introduce a framework for entanglement characterization by time-resolved single-photon counting with measurement operators defined in the time domain. For a quantum system with unitary dynamics, we generate time-continuous measurements by shifting from the Schrodinger picture to the Heisenberg representation. In particular, we discuss this approach in reference to photonic tomography. To make the measurement scheme realistic, we impose timing uncertainty on photon counts along with the Poisson noise. Then, the framework is tested numerically on quantum tomography of qubits.…
Quantum spectroscopy was performed using the frequency-entangled broadband photon pairs generated by spontaneous parametric down-conversion. An absorptive sample was placed in front of the idler photon detector, and the frequency of signal photons was resolved by a diffraction grating. The absorption spectrum of the sample was measured by counting the coincidences, and the result is in agreement with the one measured by a conventional spectrophotometer with a classical light source.
Entangled photons can be used to make measurements with an accuracy beyond that possible with classical light. While most implementations of quantum metrology have used states made up of a single colour of photons, we show that entangled states of two colours can show supersensitivity to optical phase and path-length by using a photonic crystal fibre source of photon pairs inside an interferometer. This setup is relatively simple and robust to experimental imperfections. We demonstrate sensitivity beyond the standard quantum limit and show super-resolved interference fringes using entangled st…
Interaction of light with media often occurs with a femtosecond response time. Its measurement by conventional techniques requires the use of femtosecond lasers and sophisticated time-gated optical detection1-3. Here we demonstrate that by exploiting quantum interference of entangled photons it is possible to measure the phase relaxation time of a media on the femtosecond time scale (down to 100 fs) using accessible continuous wave laser and single-photon counting. We insert the sample in the Hong-Ou-Mandel interferometer4 and infer the phase relaxation time from the modification of the two-ph…
An experimental study of the dispersion cancellation occurring in frequency-entangled photon pairs is presented. The approach uses time-resolved up conversion of the pairs, which has temporal resolution at the fs level, and group-delay dispersion sensitivity of $\approx \ 20 \, \mathrm{fs}^2$ under experimental conditions. The cancellation is demonstrated with dispersion stronger than $\pm 10^3 \, \mathrm{fs}^2$ in the signal $(-)$ and idler $(+)$ modes. The observations represent the generation, compression, and characterization of ultrashort biphotons with correlation width as small as 6.8 t…
Recent advances in techniques for generating quantum light have stimulated research on novel spectroscopic measurements using quantum entangled photons. One such spectroscopy technique utilizes non-classical correlations among entangled photons to enable measurements with enhanced sensitivity and selectivity. Here, we investigate spectroscopic measurement utilizing entangled three photons. In this measurement, time-resolved entangled photon spectroscopy with monochromatic pumping [J. Chem. Phys. 153, 051102 (2020).] is integrated with the frequency-dispersed two-photon counting technique, whic…
Polarization-resolved single dot spectroscopy performed on (211)B InAs/GaAs quantum dots reveals that the fine structure splitting of the excitonic levels in these dots is much lower compared to the usual (100)-grown InAs dots. Time-resolved measurements confirm the high oscillator strength of these dots, and thus their good quantum efficiency at 4 K, comparable with that of (100) InAs/GaAs dots. Last, photon correlation measurements demonstrate single photon emission out of the excitonic optical transition of these dots. All these features make this novel dot system very promising for impleme…
Quantum light is a key resource for promoting quantum technology. One such class of technology aims to improve the precision of optical measurements using engineered quantum states of light. In this study, we investigate transmission measurement of frequency-entangled broadband photon pairs generated via parametric down-conversion with a monochromatic laser. It is observed that state-to-state dynamics in the system under study are temporally resolved by adjusting the path difference between the entangled twin beams when the entanglement time is sufficiently short. The non-classical photon corr…
We perform full time resolved tomographic measurements of the polarization state of pairs of photons emitted during the radiative cascade of the confined biexciton in a semiconductor quantum dot. The biexciton was deterministically initiated using a $π$-area pulse into the biexciton two-photon absorption resonance. Our measurements demonstrate that the polarization states of the emitted photon pair are maximally entangled. We show that the measured degree of entanglement depends solely on the temporal resolution by which the time difference between the emissions of the photon pair is determine…
We develop closed expressions for a time-resolved photon counting signal induced by an entangled photon pair in an interferometric spectroscopy setup. Superoperator expressions in Liouville-space are derived that can account for relaxation and dephasing induced by coupling to a bath. Interferometric setups mix matter and light variables non-trivially, which complicates their interpretation. We provide an intuitive modular framework for this setup that simplifies its description. Based on separation between the detection stage and the light-matter interaction processes. We show that the pair en…
In the same manner that free-space propagation and curved glass lenses are used to shape the spatial properties of light, a combination of chromatic dispersion and devices known as time lenses may be used to reshape its temporal properties. These techniques have found extensive application in classical optical signal processing based on nonlinear optics. A new set of challenges presents itself when processing quantum signals, including noise suppression and high fidelity requirements. In this work, we construct a single-photon time lens based on dispersion and nonlinear sum-frequency generatio…
We report on an experiment demonstrating entanglement swapping of time-frequency entangled photons. We perform a frequency-resolved Bell-state measurement on the idler photons from two independent entangled photon pairs, which projects the signal photons onto a two-color Bell state. We verify entanglement in this heralded state using two-photon interference and observing quantum beating without the use of filters, indicating the presence of two-color entanglement. Our method could lend itself to use as a highly-tunable source of frequency-bin entangled single photons.
Using spontaneous parametric down-conversion, photon pairs entangled in frequency and polarization were generated. After frequency resolving the photon pairs, the polarization correlations were measured on several polarization basis, and it was confirmed that the frequency resolved photon pairs were entangled in polarization, indicating the photon pairs can be used as a source of wavelength division multiplexing quantum key distribution.
The Einstein-Podolsky-Rosen paradox is resolved by reconsidering what entangled state is actually prepared, what physical quantities are conserved and the character of the correlation measurements employed.
We present a method for measuring quantum states encoded in the temporal modes of photons. The basis for the multilevel quantum states is defined by the use of modes propagating in a dispersive medium, which is a fiber in this case. The propagation and time-resolved single photon detection allow us to define a positive-operator valued measure (POVM). The POVM depends on the amount of dispersion and the characteristics of a detector. This framework is numerically tested by performing quantum state tomography on a large number of states for a set of realistic experimental settings. Finally, the …
Entangling independent photons is not only of fundamental interest but also of crucial importance for quantum information science. Two-photon interference is a major method to entangle independent identical photons. If two photons are color-different, perfect two-photon coalescence cannot happen anymore, which makes the entangling of color-different photons difficult to realize. In this letter by exploring and developing time-resolved measurement and active feed-forward, we have entangled two independent photons of different colors for the first time. We find that entanglement with a varying f…
We propose a versatile and efficient method to generate a broad class of complex entangled states of many atoms via the detection of a single photon. For an atomic ensemble contained in a strongly coupled optical cavity illuminated by weak single- or multi-frequency light, the atom-light interaction entangles the frequency spectrum of a transmitted photon with the collective spin of the atomic ensemble. Simple time-resolved detection of the transmitted photon then projects the atomic ensemble into a desired pure entangled state. This method can be implemented with existing technology, yields h…
Entangled photon pair excitation and time-frequency-filtered multidimensional photon correlation spectroscopy as a probe for dissipative exciton kinetics
Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY（自由电子激光科学中心，德国电子同步加速器）；University of California, Irvine（加州大学尔湾分校）
Multiphoton quantum interference underpins fundamental tests of quantum mechanics and quantum technologies. Consequently, the detrimental effect of photon distinguishability in multiphoton interference experiments can be catastrophic. Here, we employ correlation measurements in the photonic inner modes, time or frequency, to restore quantum interference between photons differing in their colors or injection times in arbitrary linear optical networks, without the need for additional filtering or post selection. Interestingly, we demonstrate how harnessing the multiphoton inner-mode quantum info…
Entangled photon pairs are predicted to linearize and increase the efficiency of two-photon absorption, allowing continuous wave laser diodes to drive ultrafast time-resolved spectroscopy and nonlinear processes. Despite a range of theoretical studies and experimental measurements, inconsistencies persist about the value of the entanglement enhanced interaction cross section. A spectrometer is constructed that can temporally and spectrally characterize the entangled photon state before, during, and after any potential two-photon excitation event. For the molecule Rhodamine 6G, which has a virt…
We propose and analyze a multi-functional setup consisting of high finesse optical cavities, beam splitters, and phase shifters. The basic scheme projects arbitrary photonic two-mode input states onto the subspace spanned by the product of Fock states |n>|n> with n=0,1,2,.... This protocol does not only provide the possibility to conditionally generate highly entangled photon number states as resource for quantum information protocols but also allows one to test and hence purify this type of quantum states in a communication scenario, which is of great practical importance. The scheme is espec…
Entanglement is a unique property of quantum systems and an essential resource for many quantum technologies. The ability to transfer or swap entanglement between systems is an important protocol in quantum information science. Entanglement between photons forms the basis of distributed quantum networks and the demonstration of photonic entanglement swapping is essential for their realization. Here an experiment demonstrating entanglement swapping from two independent multimode time-frequency entangled sources is presented, resulting in multiple heralded temporal-mode Bell states. Entanglement…
Pixelation occurs in many imaging systems and limits the spatial resolution of the acquired images. This effect is notably present in quantum imaging experiments with correlated photons in which the number of pixels used to detect coincidences is often limited by the sensor technology or the acquisition speed. Here, we introduce a pixel super-resolution technique based on measuring the full spatially-resolved joint probability distribution (JPD) of spatially-entangled photons. Without shifting optical elements or using prior information, our technique increases the pixel resolution of the imag…
Precision measurements of optical phases have many applications in science and technology. Entangled multi-photon states have been suggested for performing such measurements with precision that significantly surpasses the shot-noise limit. Until recently, such states have been generated mainly using spontaneous parametric down-conversion -- a process which is intrinsically probabilistic, counteracting the advantages that the entangled photon states might have. Here, we use a semiconductor quantum dot to generate entangled multi-photon states in a deterministic manner, using periodic timed exci…
Matrix Product States (MPS) and Projected Entangled Pair States (PEPS) are powerful analytical and numerical tools to assess quantum many-body systems in one and higher dimensions, respectively. While MPS are comprehensively understood, in PEPS fundamental questions, relevant analytically as well as numerically, remain open, such as how to encode symmetries in full generality, or how to stabilize numerical methods using canonical forms. Here, we show that these key problems, as well as a number of related questions, are algorithmically undecidable, that is, they cannot be fully resolved in a s…
We present a 1 GHz-clocked, maximally entangled and on-demand photon pair source based on droplet etched GaAs quantum dots using two-photon excitation. By employing these GaP microlensenhanced devices in conjunction with their substantial brightness, raw entanglement fidelities of up to $0.95 \pm 0.01$ and post-selected photon indistinguishabilities of up to $0.93 \pm 0.01$, the suitability for quantum repeater based long range quantum entanglement distribution schemes is shown. Comprehensive investigations of a complete set of polarization selective two-photon correlations as well as time res…
The interplay of nonclassical light and surface plasmons has attracted considerable attention due to fundamental interests and potential applications. To gain more insight into the quantum nature of the photon-surface-plasmon coupling, time-resolved detection of the interaction is invaluable. Here we demonstrate the time-resolved detection of photon-surface-plasmon coupling by exploiting single and entangled photons with long coherence time to excite single optical plasmons. We examine the nonclassical correlation between the single photons and single optical plasmons in such systems using the…
Single-photon light detection and ranging (LiDAR) is a key technology for depth imaging through complex environments. Despite recent advances, an open challenge is the ability to isolate the LiDAR signal from other spurious sources including background light and jamming signals. Here we show that a time-resolved coincidence scheme can address these challenges by exploiting spatiotemporal correlations between entangled photon pairs. We demonstrate that a photon-pair-based LiDAR can distill desired depth information in the presence of both synchronous and asynchronous spurious signals without pr…
Maximizing the information transmission rate through quantum channels is essential for practical implementation of quantum communication. Time-division multiplexing is an approach for which the ultimate rate requires the ability to manipulate and detect single photons on ultrafast timescales while preserving their quantum correlations. Here we demonstrate the demultiplexing of a train of pulsed single photons using time-to-frequency conversion while preserving their polarization entanglement with a partner photon. Our technique converts a pulse train with 2.69 ps spacing to a frequency comb wi…
Entanglement and spontaneous emission are fundamental quantum phenomena that drive many applications of quantum physics. During the spontaneous emission of light from an excited two-level atom, the atom briefly becomes entangled with the photonic field. Here, we show that this natural process can be used to produce photon-number entangled states of light distributed in time. By exciting a quantum dot -- an artificial two-level atom -- with two sequential $π$ pulses, we generate a photon-number Bell state. We characterise this state using time-resolved intensity and phase correlation measuremen…
We demonstrate quantum interference of three photons that are distinguishable in time, by resolving them in the conjugate parameter, frequency. We show that the multiphoton interference pattern in our setup can be manipulated by tuning the relative delays between the photons, without the need for reconfiguring the optical network. Furthermore, we observe that the symmetries of our optical network and the spectral amplitude of the input photons are manifested in the interference pattern. Moreover, we demonstrate time-reversed HOM-like interference in the spectral correlations using time-bin ent…
A quantum dot can be used as a source of one- and two-photon states and of polarisation entangled photon pairs. The emission of such states is investigated from the point of view of frequency-resolved two-photon correlations. These follow from a spectral filtering of the dot emission, which can be achieved either by using a cavity or by placing a number of interference filters before the detectors. The combination of these various options is used to iteratively refine the emission in a "distillation" process and arrive at highly correlated states with a high purity. So-called "leapfrog process…
We study the generation of single-photon pulses with the tailored temporal shape via nonlocal spectral filtering. A shaped photon is heralded from a time-energy entangled photon pair upon spectral filtering and time-resolved detection of its entangled counterpart. We show that the temporal shape of the heralded photon is defined by the time-inverted impulse response of the spectral filter and does not depend on the heralding instant. Thus one can avoid post-selection of particular heralding instants and achieve substantially higher heralding rate of shaped photons as compared to the generation…
We characterize correlations in polarization and time of photon pairs generated from a cold cloud of Rb-87 atoms via a four-wave mixing process in a cascade level scheme. Quantum state tomography reveals entangled polarization states of high purity for each of the decay paths through two different intermediate hyperfine levels. When allowing both decay paths, we observe quantum beats in time-resolved correlation measurements.
Picosecond-resolved entanglement distribution over an urban free-space channel
University of Vienna, Faculty of Physics（维也纳大学物理学院）；Christian Doppler Laboratory for Photonic Quantum Computer, University of Vienna, Faculty of Physics（维也纳大学物理学院光子量子计算机克莉丝汀·多普勒实验室）；Quantum Space Systems GmbH（量子空间系统有限公司）；Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz（林茨约翰内斯开普勒大学半导体与固态物理研究所）；Instituto de Fı́sica Gleb Wataghin, Universidade Estadual de Campinas (UNICAMP)（坎皮纳斯州立大学格列布·瓦塔金物理研究所）
