Source Of Entangled Photons Research Articles

Independent control of exciton and biexciton energies in single quantum dots via electroelastic fields

We investigate the effect of large in-plane strain and vertical electric fields on the binding energies of excitonic complexes confined in single InGaAs/GaAs quantum dots (QDs) and we find that the two independently tunable perturbations modify the interaction energies among electrons and holes in a different manner. By taking advantage of this difference, we frequency-lock the QD fundamental excitation (the neutral exciton) at a predefined value, while the biexciton transition is actively tuned from a binding to an antibinding configuration. Our electrically controlled dual-knob device demonstrates unprecedented control over the electronic properties of the few-particle states in a QD and may be applied to create novel energy-tunable sources of entangled photons using the time-reordering or the time-bin scheme.

Optical Spin Squeezing: Bright Beams as High-Flux Entangled Photon Sources

In analogy with the spin-squeezing inequality of Wang and Sanders [Phys. Rev. A 68, 012101 (2003)], we find inequalities describing macroscopic polarization correlations that are obeyed by all classical fields and whose violation implies entanglement of the photons that make up the optical beam. We consider a realistic and exactly solvable experimental scenario employing polarization-squeezed light from an optical parametric oscillator (OPO) and find polarization entanglement for postselected photon pairs separated by less than the OPO coherence time. The entanglement is robust against losses and extremely bright: efficiency can exceed that of existing "ultrabright" pair sources by at least an order of magnitude. This translation of spin-squeezing inequalities to the optical domain will enable direct tests of discrete-variable entanglement in a squeezed state.

Long distance measurement-device-independent quantum key distribution with entangled photon sources

We present a feasible method that can make quantum key distribution (QKD), both ultra-long-distance and immune, to all attacks in the detection system. This method is called measurement-device-independent QKD (MDI-QKD) with entangled photon sources in the middle. By proposing a model and simulating a QKD experiment, we find that MDI-QKD with one entangled photon source can tolerate 77 dB loss (367 km standard fiber) in the asymptotic limit and 60 dB loss (286 km standard fiber) in the finite-key case with state-of-the-art detectors. Our general model can also be applied to other non-QKD experiments involving entanglement and Bell state measurements.

Flexible source of nondegenerate entangled photons based on a two-crystal Sagnac interferometer

Sources of entangled photon pairs are a key component in both fundamental tests of quantum theory and practical applications such as quantum key distribution and quantum computing. In this work, we describe and characterize a source of polarization entangled photon pairs based on two spontaneous parametric down-conversion (SPDC) crystals in a Sagnac interferometer. Our source is compact and produces high-quality entangled states in a very flexible manner. The wavelengths of the photon pairs, around 810 and 1550 nm, the phase between orthogonal components of the entangled state, and the tangle of the state are all independently adjustable. In addition to presenting basic characterization data, we present experimental violations of CHSH and Leggett inequalities, as well as an instance of the "beautiful" Bell inequality, which has not previously been tested experimentally.

Guided Modes of Positive and Negative Group Index in the Honeycomb Photonic Crystal Waveguides

A guided mode of positive and negative group index structure is essential to quantum optics for design and development of high efficiency optical devices which are useful to security optical communication system and to diverse applications such as Optical Time-Division-Multiplexing, Optical Switch, Laser, LED, Entangled Photon Source and Single Photon Source. Thus, we proposed to develop an efficient photon emission along line defect of two-dimensional honeycomb photonic crystal waveguide in the silicon slab. The honeycomb lattice of circular air holes on a silicon plate is simulated to obtain two nearest guided modes between positive and negative group index regimes. This significant property shows the potential applied guided modes of photonic crystal waveguide enhancing spontaneous emission for controlling photon emission between two resonance frequencies. Significantly, this work is oriented to produce the novel optical devices for control photon emission in the optical communication system. In addition to the honeycomb lattice, it can easily be made on a Si on insulator (SOI) wafer.

Analytical description of photonic waveguides with multilayer claddings: Towards on-chip generation of entangled photons and Bell states

We present a theoretical description of slab and ridge waveguides with multilayer claddings, such as Bragg reflection waveguides (BRWs), using analytical expressions based on Fresnel and Airy formulas. This approach simplifies the waveguide calculations in comparison with more traditional techniques such as the transfer matrix method or direct stitching of plane-wave solutions at layer interfaces. Approximate but simple and straightforward analytic relations describing the effective indices and group velocities of guided modes in arbitrary 1D BRWs are derived. The formalism is then extended to 2D ridge waveguides using the effective index method. The approach is employed to engineer BRWs where several types of phase matching are present simultaneously at the same wavelength, as well as BRWs where photons generated by spontaneous parametric downconversion have the maximal degree of polarization entanglement. These results promote the use of BRWs as on-chip entangled photon sources, and facilitate on-chip generation of multiple optical Bell states. The designs are based on the AlGaAs fabrication platform and are within reach of experimental realization.

High-capacity quantum Fibonacci coding for key distribution

Quantum cryptography and quantum key distribution (QKD) have been the most successful applications of quantum information processing, highlighting the unique capability of quantum mechanics, through the no-cloning theorem, to securely share encryption keys between two parties. Here, we present an approach to high-capacity, high-efficiency QKD by exploiting cross-disciplinary ideas from quantum information theory and the theory of light scattering of aperiodic photonic media. We propose a unique type of entangled-photon source, as well as a physical mechanism for efficiently sharing keys. The key-sharing protocol combines entanglement with the mathematical properties of a recursive sequence to allow a realization of the physical conditions necessary for implementation of the no-cloning principle for QKD, while the source produces entangled photons whose orbital angular momenta (OAM) are in a superposition of Fibonacci numbers. The source is used to implement a particular physical realization of the protocol by randomly encoding the Fibonacci sequence onto entangled OAM states, allowing secure generation of long keys from few photons. Unlike in polarization-based protocols, reference frame alignment is unnecessary, while the required experimental setup is simpler than other OAM-based protocols capable of achieving the same capacity and its complexity grows less rapidly with increasing range of OAM used.

Near-infrared Hong-Ou-Mandel interference on a silicon quantum photonic chip

Near-infrared Hong-Ou-Mandel quantum interference is observed in silicon nanophotonic directional couplers with raw visibilities on-chip at 90.5%. Spectrally-bright 1557-nm two-photon states are generated in a periodically-poled KTiOPO₄ waveguide chip, serving as the entangled photon source and pumped with a self-injection locked laser, for the photon statistical measurements. Efficient four-port coupling in the communications C-band and in the high-index-contrast silicon photonics platform is demonstrated, with matching theoretical predictions of the quantum interference visibility. Constituents for the residual quantum visibility imperfection are examined, supported with theoretical analysis of the sequentially-triggered multipair biphoton, towards scalable high-bitrate quantum information processing and communications. The on-chip HOM interference is useful towards scalable high-bitrate quantum information processing and communications.

Ghost Imaging of Space Objects

The term "ghost imaging" was coined in 1995 when an optical correlation measurement in combination with an entangled photon-pair source was used to image a mask placed in one optical channel by raster-scanning a detector in the other, empty, optical channel. Later, it was shown that the entangled photon source could be replaced with thermal sources of light, which are abundantly available as natural illumination sources. It was also shown that the bucket detector could be replaced with a remote point-like detector, opening the possibility to remote-sensing imaging applications. In this paper, we discuss the application of ghost-imaging-like techniques to astronomy, with the objective of detecting intensity-correlation signatures resulting from space objects of interest, such as exo-planets, gas clouds, and gravitational lenses. An important aspect of being able to utilize ghost imaging in astronomy, is the recognition that in interstellar imaging geometries the object of interest can act as an effective beam splitter, yielding detectable variations in the intensity-correlation signature.

Multimode entanglement in coupled cavity arrays

We study a driven-dissipative array of coupled nonlinear optical resonators by numerically solving the von Neumann equation for the density matrix. We demonstrate that quantum correlated states of many photons can also be generated in the limit where the nonlinearity is much smaller than the losses, contrary to common expectations. Quantum correlations in this case arise from the interference between different pathways that the system can follow in the Hilbert space to reach its steady state under the effect of coherent driving fields. We characterize, in particular, two systems: a linear chain of three coupled cavities and an array of eight coupled cavities. We demonstrate the existence of a parameter range where the system emits photons with continuous-variable bipartite and quadripartite entanglement, in the case of the first and the second system, respectively. This entanglement is shown to survive realistic rates of pure dephasing and opens up a new perspective for the realization of quantum simulators or entangled photon sources without the challenging requirement of strong optical nonlinearities.

A comprehensive design and performance analysis of low Earth orbit satellite quantum communication

Optical quantum communication utilizing satellite platforms has the potential to extend the reach of quantum key distribution (QKD) from terrestrial limits of ∼200 km to global scales. We have developed a thorough numerical simulation using realistic simulated orbits and incorporating the effects of pointing error, diffraction, atmosphere and telescope design, to obtain estimates of the loss and background noise which a satellite-based system would experience. Combining with quantum optics simulations of sources and detection, we determine the length of secure key for QKD, as well as entanglement visibility and achievable distances for fundamental experiments. We analyse the performance of a low Earth orbit satellite for downlink and uplink scenarios of the quantum optical signals. We argue that the advantages of locating the quantum source on the ground justify a greater scientific interest in an uplink as compared to a downlink. An uplink with a ground transmitter of at least 25 cm diameter and a 30 cm receiver telescope on the satellite could be used to successfully perform QKD multiple times per week with either an entangled photon source or with a weak coherent pulse source, as well as perform long-distance Bell tests and quantum teleportation. Our model helps to resolve important design considerations such as operating wavelength, type and specifications of sources and detectors, telescope designs, specific orbits and ground station locations, in view of anticipated overall system performance.

Polarization-Entanglement Purification for Ideal Sources Using Weak Cross-Kerr Nonlinearity

We propose an entanglement purification protocol (EPP) for ideal entangled photon sources resorting to weak cross-Kerr nonlinearities. The key element of this EPP is the quantum nondemolition detector (QND) which uses weak cross-Kerr nonlinearity effect to generate phase shifts on the coherent states conditionally. By exploiting the double cross-phase modulation (XPM) method in QND, we present an efficient EPP protocol, which reduces the requirement for coupling strength of the cross-Kerr nonlinearity medium. The proposed protocol may open up promising possibilities for the practicability of quantum information processing using weak cross-Kerr nonlinearities.

Observation of quantum Talbot effect from a domain-engineered nonlinear photonic crystal

The quantum Talbot effect is observed from a domain-engineered nonlinear photonic crystal dispensing with a real grating. We deduce and experimentally verify the quantum self-imaging formula which is related to the crystal's structure parameter and working wavelengths. A two-photon Talbot carpet is captured to characterize the Fresnel diffraction dynamics of entangled photons wherein the quantum fractional Talbot effect is specified. The compact and stable quantum Talbot effect can be considered as the contactless diagnosis of domain's homogeneity and developed for new types of entangled photon source and quantum technologies such as quantum lithography with improved performance.

Introduction to the Issue on Quantum and Nanoscale Photonics

The purpose of this issue is to highlight the recent progress and trends in the development of novel nano- and quantum photonic technologies. The articles published in this issue cover a broad range of areas, including: 1) novel nanophotonic light sources-both classical (lasers) and nonclassical (single- and entangled photon sources); 2) nonlinear optics in nanophotonic structures; 3) quantum optics and quantum information processing based on nanophotonics platform; 4) efficient devices for optical interconnects and optical information processing based on novel nanophotonic structures. This issue contains 25 papers, including eight invited, authored by well-established research groups and promising scientists from all over the world. The invited articles include extended reviews on entangled photon generation using silicon nanowire waveguides; cavity quantum electrodynamics and lasing oscillation in single quantum dot photonic crystal nanocavity systems; quantum plasmonic circuits; theoretical analysis of few-photon single-atom cavity quantum electrodynamics; semiconductor quantum dot micropillar microcavities for quantum optics in solid state; nanofiber-based optical trapping of cold neutral atoms; switching and counting with atomic vapors in photonic-crystal fibers; and production of multiple diamondbased single-photon sources. The contributed articles cover a broad variety of key nanophotonics and quantum photonics research areas, including recently obtained results on bright single-photon emission from a quantum dot in a circular Bragg grating microcavity, supercontinuum generation in nanostructured silicon waveguides, electrically driven photonic crystal nanocavity devices, and silicon quantum dots.

Universal Recovery of the Energy-Level Degeneracy of Bright Excitons in InGaAs Quantum Dots without a Structure Symmetry

The lack of structural symmetry which usually characterizes semiconductor quantum dots lifts the energetic degeneracy of the bright excitonic states and hampers severely their use as high-fidelity sources of entangled photons. We demonstrate experimentally and theoretically that it is always possible to restore the excitonic degeneracy by the simultaneous application of large strain and electric fields. This is achieved by using one external perturbation to align the polarization of the exciton emission along the axis of the second perturbation, which then erases completely the energy splitting of the states. This result, which holds for any quantum dot structure, highlights the potential of combining complementary external fields to create artificial atoms meeting the stringent requirements posed by scalable semiconductor-based quantum technology.

Generation of positively-momentum-correlated biphotons from spontaneous parametric down-conversion

We propose to generate a counter-Einstein-Podolsky-Rosen (CEPR) state which possesses a positively-correlated momentum and anticorrelated position directly from spontaneous parametric down-conversion. The general CEPR state is given under a tightly focused Gaussian pump based on type II noncritical phase matching. We analytically and numerically study how a positively-correlated momentum and anticorrelated position of a biphoton can be achieved by choosing the proper focal parameter of the pump. To verify the degree of entanglement of the CEPR state, we calculate its entanglement entropy and it shows a unique dependence on the pump's focal parameter. Suggestions for practical engineering of the state are also given. Furthermore, based on an ideal CEPR state, we deduce a type of erect quantum ghost imaging which can be applied as the entanglement criterion. This work gives applicable references to the spatial control of an entangled photon source in experiments and may stimulate the development of new types of quantum technologies.

Bragg reflection waveguides as integrated sources of entangled photon pairs

We explore the potential of versatile and efficient entangled photon pair generation by spontaneous parametric downconversion in Bragg reflection waveguides. By employing a quantum treatment of modes in channel waveguides, and by accounting for group velocity dispersion in the modes, the quantum state of the generated biphotons is realistically calculated. The pair production rate is predicted to reach 4×108 pairs/s/nm/mW of pump light in a 2 mm-long structure, on par with or exceeding the performance of previously reported designs. This is attributable to an enhanced nonlinear interaction through tight mode confinement in the waveguide. Strategies for device performance optimization and phase matching wavelength tunability are outlined and numerically demonstrated. The proposed design platform is versatile and allows photon pair generation with controllable flux, bandwidth, Schmidt number, and degree of polarization entanglement. The possibility of monolithic integration with a diode laser pump offers a way to design an electrically pumped entangled photon source.

Piezoelectric InAs/GaAs quantum dots with reduced fine-structure splitting for the generation of entangled photons

Polarization-resolved single-dot spectroscopy reveals that the exciton fine-structure splitting in piezoelectric (211)B InAs/GaAs quantum dots is smaller than 10 \ensuremath{\mu}eV in the vast majority of examined dots. These values are significantly reduced compared to as-grown (100)-oriented InAs dots. Time-resolved measurements confirm the high oscillator strength of these dots, in spite of the internal piezoelectric field, suggesting good quantum efficiency at 4 K, comparable with that of (100) InAs/GaAs dots. Lastly, photon correlation measurements demonstrate single-photon emission from exciton levels of these dots. All these features make this intriguing dot system promising for implementing solid-state entangled photon sources.

Scalable fiber integrated source for higher-dimensional path-entangled photonic quNits

Integrated photonic circuits offer the possibility for complex quantum optical experiments in higher-dimensional photonic systems. However, the advantages of integration and scalability can only be fully utilized with the availability of a source for higher-dimensional entangled photons. Here, a novel fiber integrated source for path-entangled photons in the telecom band at 1.55\mum using only standard fiber technology is presented. Due to the special design the source shows good scalability towards higher-dimensional entangled photonic states (quNits), while path entanglement offers direct compatibility with on-chip path encoding. We present an experimental realization of a path-entangled two-qubit source. A very high quality of entanglement is verified by various measurements, i.a. a tomographic state reconstruction is performed leading to a background corrected fidelity of (99.45+-0.06)%. Moreover, we describe an easy method for extending our source to arbitrarily high dimensions.

THE GENERATION OF POLARIZATION-ENTANGLED PHOTON PAIRS USING GRATING STRUCTURES OF Ti-DIFFUSED LITHIUM NIOBATE WAVEGUIDES IN INTEGRATED OPTICS

Entangled photon source is an essential tool for quantum information experiments, such as tests of Bell inequalities, quantum teleportation, and entanglement swapping. We report the generation of polarization-entangled photon pairs using grating structures of nonlinear materials such as Ti -diffused Lithium Niobate. This approach is useful for quantum integrated optics because the size of this element is less than 325 micron. Polarization-entangled photon pairs are created using the nonlinear optical process of type-II spontaneous parametric down-conversion. The pump laser at 800 nm was doubled to a wavelength of 400 nm, which was used to produce SPDC in the grating structure of Lithium Niobate. The entangled photon pairs were generated at wavelengths 692 nm and 950 nm. The used grating structure consists of two sections. Grating period and length of the first section are 162 nm and 129.6 micron, respectively. Grating period of the second section is 193 nm and its length is equal to 193 micron.