Entangled two-photon spectroscopy is expected to provide advantages compared with classical protocols. It is achieved by coherently controlling the spectral properties of energy-entangled photons. We present here an experimental setup that allows the spectral shaping of entangled photons with high resolution. We evaluate its performances by detecting sum frequency generation in a non-linear crystal. The efficiency of the process is compared when performed with classical or entangled light.
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.
Traditionally, spectroscopy is performed by examining the position of absorption lines. However, at frequencies near the transition frequency, additional information can be obtained from the phase shift. In this work we consider the information about the transition frequency obtained from both the absorption and the phase shift, as quantified by the Fisher information in an interferometric measurement. We examine the use of multiple single-photon states, NOON states, and numerically optimized states that are entangled and have multiple photons. We find the optimized states that improve over th…
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…
We introduce the concept of initial-phase spectroscopy as a control of the dynamics of entangled states encoded into a two-atom system interacting with a broadband squeezed vacuum field. We illustrate our considerations by examining the transient spectrum of the field emitted by two systems, the small sample (Dicke) and the spatially extended (non-Dicke) models. It is found that the shape of the spectral components depends crucially on the relative phase between the initial entangled state and the squeezed field. We follow the temporal evolution of the spectrum and show that depending on the r…
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）
In virtual-state spectroscopy, information about the energy-level structure of an arbitrary sample is retrieved by Fourier transforming sets of measured two-photon absorption probabilities of entangled photon pairs where the degree of entanglement and the delay time between the photons have been varied. This works well for simple systems but quickly becomes rather difficult when many intermediate states are involved. We propose and discuss an extension of entangled two-photon absorption spectroscopy that solves this problem by means of repeated measurements at different pump wavelengths. Speci…
We numerically analyze the use of intense entangled twin beams for ultra-sensitive spectroscopic measurements in chemical and biological systems. The examined scheme makes use of intense frequency-modulated (chirped) entangled beams to successfully extract information about the intermediate material states that contribute to the two-photon excitation of an absorbing medium. Robustness of the presented method is examined with respect to the applied intervals of the frequency chirp.
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…
In this contribution we analyze virtual-state spectroscopy --- a unique tool for extracting information about the virtual states that contribute to the two-photon excitation of an absorbing medium --- as implemented by means of intense entangled beams with tunable spectral correlations. We provide a thorough description of all contributing terms (classical and quantum) in the two-photon absorption signal, as well as the limits imposed by the power of the pump that produces the entangled beams on the observability of the spectral lines of the virtual transitions. We find that virtual-state spec…
The use of entangled states was shown to improve the fundamental limits of spectroscopy to beyond the standard-quantum limit. In these Heisenberg-limited protocols the phase between two states in an entangled superposition evolves N-fold faster than in the uncorrelated case, where N for example can be the number of entangled atoms in a Greenberger-Horne-Zeilinger (GHZ) state. Here we propose and demonstrate the use of correlated spin-Hamiltonians for the realization of Heisenberg-limited Rabi-type spectroscopy. Rather than probing the free evolution of the phase of an entangled state with resp…
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 propose a novel femtosecond stimulated Raman spectroscopy (FSRS) technique that combines entangled photons with interference detection to select matter pathways and enhance the resolution. Following photo excitation by an actinic pump, the measurement uses a pair of broadband entangled photons, one (signal) interacts with the molecule together with a third narrowband pulse induces the Raman process. The other (idler) photon provides a reference for the coincidence measurement. This interferometric photon-coincidence counting detection allows to separately measure Raman gain and loss signals…
Entangled photons exhibit non-classical light-matter interactions that create new opportunities in materials and molecular science. For example, in entangled two-photon absorption, the intensity-dependence scales linearly as if only one photon was present. The entangled two-photon absorption cross section approaches but does not match the one-photon absorption cross section. The entangled two photon cross section also does not follow classical two photon molecular design motifs. Questions such as these seed the rich but nascent field of entangled light-matter interactions. In this perspective,…
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…
We prove, using a new method based on map-state duality, lower bounds on entanglement resources needed to deterministically implement a bipartite unitary using separable (SEP) operations, which include LOCC (local operations and classical communication) as a particular case. It is known that the Schmidt rank of an entangled pure state resource cannot be less than the Schmidt rank of the unitary. We prove that if these ranks are equal the resource must be uniformly (maximally) entangled: equal nonzero Schmidt coefficients. Higher rank resources can have less entanglement: we have found numerica…
Quantum information science is driving progress in a vast number of scientific and technological areas that cover molecular spectroscopy and matter-light interactions in general. In these fields, the ability to generate quantum mechanically-entangled photons is opening avenues to explore the interaction of molecules with quantum light. This work considers an alternative way of correlating photons by including energy superpositions. We study how the multichromatic quantum superposition, or color superposition of photon-pair states, influences the optical properties of organic chromophores. This…
Optical interferometry has been a long-standing setup for characterization of quantum states of light. Both the linear and the nonlinear interferences can provide information about the light statistics an underlying detail of the light-matter interactions. Here we demonstrate how interferometric detection of nonlinear spectroscopic signals may be used to improve the measurement accuracy of matter susceptibilities. Light-matter interactions change the photon statistics of quantum light, which are encoded in the field correlation functions. Application is made to the Hong-Ou-Mandel two-photon in…
Entangled two-photon absorption spectroscopy (TPA) has been widely recognized as a powerful tool for revealing relevant information about the structure of complex molecular systems. However, to date, the experimental implementation of this technique has remained elusive, mainly because of two major difficulties. First, the need to perform multiple experiments with two-photon states bearing different temporal correlations, which translates in the necessity to have at the experimenter's disposal tens, if not hundreds, of sources of entangled photons. Second, the need to have \emph{a priori} know…
Multidimensional optical signals are commonly recorded by varying the delays between time ordered pulses. These control the evolution of the density matrix and are described by ladder diagrams. We propose a new non-time-ordered protocol based on following the time evolution of the wavefunction and described by loop diagrams. The time variables in this protocol allow to observe different types of resonances and reveal information about intraband dephasing not readily available by time ordered techniques. The time variables involved in this protocol become coupled when using entangled light, whi…
Correlated photons inspire abundance of metrology-related platforms, which benefit from quantum (anti-) correlations and outperform their classical-light counterparts. While such demonstrations mainly focus on entanglement, the role of photon exchange-phase and degree of distinguishability have not been widely utilized in quantum-enhanced applications. Using an interferometric setup we show that even at low degree entanglement, when a two-photon wave-function is coupled to matter, it is encoded with a reliable "which pathway?" information. An interferometric exchange-phase-cycling protocol is …
The authors demonstrate a form of two-photon-counting interferometry by measuring the coincidence counts between single-photon-counting detectors at an output port of a Mach-Zehnder Interferometer (MZI) following injection of broad-band time-frequency-entangled photon pairs (EPP) generated from collinear spontaneous parametric down conversion into a single input port. Spectroscopy and refractometry are performed on a sample inserted in one internal path of the MZI by scanning the other path in length, which acquires phase and amplitude information about the samples linear response. Phase modul…
Optical signals obtained by the material response to classical laser fields are given by nonlinear response functions which can be expressed by sums over various quantum pathways of matter. We show that some pathways can be selected by using nonclassical fields, through the entanglement of photon and material pathways, which results in a different-power law dependence on the incoming field intensity. Spectrally overlapping stimulated Raman scattering (SRS) and two-photon-absorption (TPA) pathways in a pump probe experiment are separated by controlling the degree of entanglement of pairs of inc…
The generation and long-haul transmission of highly entangled photon pairs is a cornerstone of emerging photonic quantum technologies, with key applications such as quantum key distribution and distributed quantum computing. However, a natural limit for the maximum transmission distance is inevitably set by attenuation in the medium. A network of quantum repeaters containing multiple sources of entangled photons would allow to overcome this limit. For this purpose, the requirements on the source's brightness and the photon pairs' degree of entanglement and indistinguishability are stringent. D…
While two-photon absorption (TPA) and other forms of nonlinear interactions of molecules with isolated time-frequency-entangled photon pairs (EPP) have been predicted to display a variety of fascinating effects, their potential use in practical quantum-enhanced molecular spectroscopy requires close examination. This paper presents a detailed theoretical study of quantum-enhanced TPA by both photon-number correlations and spectral correlations, including an account of the deleterious effects of dispersion. While such correlations in EPP created by spontaneous parametric down conversion can incr…
In this paper, we propose two protocols for generating super-resolving \textit{single-photon} path-entangled states from general maximally path-entangled N00N states. We also show that both protocols generate the desired state with different probabilities depending on the type of detectors being used. Such super-resolving single-photon path-entangled states preserve high resolving power but lack the requirement of a multi-photon absorbing resist, which makes this state a perfect candidate for quantum lithography.
Green-Horne-Zeilinger states are a typical type of multipartite entangled states, which plays a central role in quantum information processing. For the generation of multipartite entangled states, the single-step method is more preferable as the needed time will not increase with the increasing of the qubit number. However, this scenario has a strict requirement that all the two-qubit interaction strengths should be the same, or the generated state will be of low quality. Here, we propose a scheme for generating multipartite entangled states of superconducting qubits, from a coupled circuit ca…
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…
Entangled photons offer two advantages for two-photon absorption spectroscopy. One of them, the linear scaling of two-photon absorption rate with the input photon flux, is only valid at very low photon fluxes and is therefore impractical. The other is the overcoming of the classical constraints for simultaneous resolution in time-frequency and in space-wavevector. Here we consider bright squeezed vacuum (BSV) as an alternative to entangled photons. The efficiency increase it offers in comparison with coherent light is modest, but it does not depend on the photon flux. Moreover, and this is wha…
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…
We propose a potential scheme to generate entangled photons by manipulating trapped ions embedded in two-mode microcavities, respectively, assisted by a magnetic field gradient. By means of the spin-spin coupling due to the magnetic field gradient and the Coulomb repulsion between the ions, we show how to efficiently generate entangled photons by detecting the internal states of the trapped ions. We emphasize that our scheme is advantageous to create complete sets of entangled multi-photon states. The requirement and the experimental feasibility of our proposal are discussed in detail.
We propose a feasible experimental scheme, employing methods of population spectroscopy with two-level atoms, for a test of Bell's inequality for massive particles. The correlation function measured in this scheme is the joint atomic $Q$ function. An inequality imposed by local realism is violated by any entangled state of a pair of atoms.
We discuss techniques that allow for long coherence times in laser spectroscopy experiments with two trapped ions. We show that for this purpose not only entangled ions prepared in decoherence-free subspaces can be used but also a pair of ions that are not entangled but subject to the same kind of phase noise. We apply this technique to a measurement of the electric quadrupole moment of the 3d D5/2 state of 40Ca+ and to a measurement of the linewidth of an ultrastable laser exciting a pair of 40Ca+ ions.
Conventional nonlinear spectroscopy uses classical light to detect matter properties through the variation of its response with frequencies or time delays. Quantum light opens up new avenues for spectroscopy by utilizing parameters of the quantum state of light as novel control knobs and through the variation of photon statistics by coupling to matter. We present an intuitive diagrammatic approach for calculating ultrafast spectroscopy signals induced by quantum light, focusing on applications involving entangled photons with nonclassical bandwidth properties - known as "time-energy entangleme…
We propose a review of recent developments on entanglement and non-classical effects in collective two-atom systems and present a uniform physical picture of the many predicted phenomena. The collective effects have brought into sharp focus some of the most basic features of quantum theory, such as nonclassical states of light and entangled states of multiatom systems. The entangled states are linear superpositions of the internal states of the system which cannot be separated into product states of the individual atoms. This property is recognized as entirely quantum-mechanical effect and hav…
We describe two-photon absorption processes excited by entangled pairs, but not by non-entangled pairs of the same energy and polarization. Photon states are selected for destructive interference in the non-entangled process between different sequences of absorption via multiple intermediate states. A non-zero entangled absorption cross section is obtained by varying the entanglement time and pair delay parameters. Detailed energy and polarization requirements are derived for Rb 5S1/2 --> 5D3/2 transitions. Applications to Quantum Steganography and Quantum Key Distribution are discussed.
We discuss how to build some partially entangled states of $n$ two-state quantum systems (qubits). The optimal partially entangled state with a high degree of symmetry is considered to be useful for overcoming a shot noise limit of Ramsey spectroscopy under some decoherence. This state is invariant under permutation of any two qubits and inversion between the ground state $|0\ket$ and an excited state $|1\ket$ for each qubit. We show that using selective phase shifts in certain basis vectors and Grover's inversion about average operations, we can construct this high symmetric entangled state b…
Efficient heralded generation of entanglement together with its manipulation is of great importance for quantum communications. In addition, states generated with bandwidths naturally compatible with atomic transitions allow a more efficient mapping of light into matter which is an essential requirement for long distance quantum communications. Here we propose a scheme where the indistinguishability between two spontaneous four-wave mixing processes is engineered to herald generation of single-photon frequency-bin entangled states, i.e., single-photons shared by two distinct frequency modes. W…
In this paper we present a compact source of narrow-band energy-time entangled photon pairs in the telecom regime based on a Ti-indiffused Periodically Poled Lithium Niobate (PPLN) waveguide resonator, i.e. a waveguide with end-face dielectric multi-layer mirrors. This is a monolithic doubly resonant Optical Parametric Oscillator (OPO) far below threshold, which generates photon pairs by Spontaneous Parametric Down Conversion (SPDC) at around 1560nm with a 117MHz (0.91 pm)- bandwidth. A coherence time of 2.7 ns is estimated by a time correlation measurement and a high quality of the entangled …
Nonlinear spectroscopy and microscopy techniques are ubiquitous in a wide range of applications across physics and biology. However, these usually rely on high-powered pulsed laser systems. A promising alternative is to exploit entangled two-photon absorption (ETPA), which can lead to tens of orders of magnitude lower incident flux rates than in conventional two-photon absorption (TPA) schemes. However, the role of different entangled degrees of freedom in ETPA was unclear following recent experimental studies, when compared to earlier theoretical works. Here, we first demonstrate a linear dep…
Quantum teleportation faces increasingly demanding requirements for transmitting large or even entangled systems. However, knowledge of the state to be transmitted eases its reconstruction, resulting in a protocol known as remote state preparation. A number of experimental demonstrations to date have been restricted to single-qubit systems. We report the remote preparation of two-qubit "hybrid" entangled states, including a family of vector-polarization beams. Our single-photon states are encoded in the photon spin and orbital angular momentum. We reconstruct the states by spin-orbit state tom…
Multiphoton entanglement, as a quantum resource, plays an essential role in linear optical quantum information processing. Krenn et al. (Phys. Rev. Lett. 118, 080401 2017) proposed an innovative scheme that generating entanglement by path identity, in which two-photon interference (called Hong-Ou-Mandel effect) is not necessary in experiment. However, the experiments in this scheme have strict requirements in stability and scalability, which is difficult to be realized in bulk optics. To solve this problem, in this paper we first propose an on-chip scheme to generate multi-photon polarization …
Quantum interference, like Hong-Ou-Mandel interference, has played an important role to test fundamental concepts in quantum physics. We experimentally show that the multiple quantum interference effects enable the generation of high-performance polarization entangled photons. These photons have a high-emission rate, are degenerate, have a broadband distribution, and are postselection free. A quantum interferometric scheme, based on a round-trip configuration of a double-pass polarization Sagnac interferometer, makes it possible to use the large generation efficiency of polarization entangled …
Recent investigations suggest that the use of non-classical states of light, such as entangled photon pairs, may open new and exciting avenues in experimental two-photon absorption spectroscopy. Despite several experimental studies of entangled two-photon absorption (eTPA), there is still a heated debate on whether eTPA has truly been observed. This interesting debate has arisen, mainly because it has been recently argued that single-photon-loss mechanisms, such as scattering or hot-band absorption may mimic the expected entangled-photon linear absorption behavior. In this work, we focus on tr…
