Polarization-entangled States Research Articles

Fiber loop quantum buffer for photonic qubits

We report a fiber loop quantum buffer based on a low-loss 2 × 2 switch and a unit delay made of a fiber delay line. We characterize the device by using a two-photon polarization entangled state in which one photon of the entangled photon pair is stored and retrieved at a repetition rate up to 78 kHz. The device, which enables integer multiples of a unit delay, can store the qubit state in a unit of fiber delay line up to 5.4 km and the number of loop round-trips up to 3. Furthermore, we configure the device with other active elements to realize integer multiplier and divider of a unit delay of a qubit. The quantum state tomography is performed on the retrieved photon and its entangled photon. We obtain a state fidelity > 94 % with a maximum storage time of 52 s with an insertion loss of 5.56 dB. To further characterize the storing and retrieving processes of the device, we perform entanglement-assisted quantum process tomography on the buffered qubit state. The process fidelity of the device is > 0.98. Our result implies that the device preserves the superposition and entanglement of a qubit state from a two-photon polarization-entangled state. This is a significant step towards facilitating applications in optical asynchronous transfer mode based quantum networks.

Compact polarization-entangled photon source based on coexisting noncritically birefringent and quasi phase matching in a nonlinear crystal

Polarization-entangled photons are indispensable to numerous quantum technologies and fundamental studies. In this paper, we propose and demonstrate what we believe to be a novel source that generates collinear polarization-entangled photons by simultaneously achieving two distinct types of phase-matching conditions (noncritically birefringent and quasi phase matching) in a periodically poled nonlinear crystal with a large poling period of 2 mm. The photon pairs are generated in a polarization-entangled state with a fidelity and concurrence of 0.998 and 0.935, respectively, and violate the Clauser-Horne-Shimony-Holt inequality by 84 standard deviations. The compact source does not require interferometer, delicate domain structures, or post selection, and is advantageous for scalable quantum computing and communication, where many replicas or chip-scale devices are needed.

Polarization entanglement generation in silicon nitride waveguide-coupled dual microring resonators

Polarization-entangled photon pair sources exhibiting nonlocal quantum correlations are crucial to developments of quantum computing, quantum communications, quantum cryptography, and quantum sensing technologies. On-chip polarization entanglement generation thus constitutes one enabling component for integrated quantum photonic circuits. Here, we present to our knowledge the first polarization-entangled photon pair sources in a silicon nitride platform for integrated quantum photonic circuits. We demonstrate the generation of a polarization-entangled state by adopting a configuration comprising dual microring resonators, with nearly degenerate transverse electric and transverse magnetic polarized cavity resonances for the two resonators coupled in series to a common bus waveguide. We measure two-photon interference and quantum state tomography to characterize the polarization entanglement of the generated state and to reconstruct the density matrix. Our experiments reveal a visibility of 96.4% ± 3.1% and of 86.7% ± 3.2% with the |H⟩ and |V⟩ bases, respectively (and a visibility of 89.4% ± 6.6% and 81.3% ± 7.3% with the |D⟩ and |A⟩ bases), and a fidelity of ∼75.7% from the tomographic reconstructed density matrix.

Generation and characterization of polarization-entangled states using quantum dot single-photon sources

Single-photon sources based on semiconductor quantum dots find several applications in quantum information processing due to their high single-photon indistinguishability, on-demand generation, and low multiphoton emission. In this context, the generation of entangled photons represents a challenging task with a possible solution relying on the interference in probabilistic gates of identical photons emitted at different pulses from the same source. In this work, we implement this approach via a simple and compact design that generates entangled photon pairs in the polarization degree of freedom. We operate the proposed platform with single photons produced through two different pumping schemes, the resonant excited one and the longitudinal-acoustic phonon-assisted configuration. We then characterize the produced entangled two-photon states by developing a complete model taking into account relevant experimental parameters, such as the second-order correlation function, Hong–Ou–Mandel visibility, multiphoton emission and pump laser filtering. Our source shows long-term stability and high quality of the generated entangled states, thus constituting a reliable building block for optical quantum technologies.

Multifunction integrated lithium niobate photonic chip for photon pairs generation and manipulation.

We report on a unique photonic quantum source chip highly integrating four-stage photonic elements in a lithium niobate (LN) waveguide circuit platform, where an aperiodically poled LN (APPLN) electro-optic (EO) polarization mode converter (PMC) is sandwiched between two identical type-0 PPLN spontaneous parametric down-converters (SPDCs), followed by an EO phase controller (PC). These core nonlinear optic and EO building blocks on the chip are systematically characterized stage by stage to show its high performance as an integrated quantum source. The APPLN EO PMC, optimally constructed by a genetic algorithm, is characterized to have a broad bandwidth (>13 nm), benefiting an efficient control of broadband type-0 SPDC photon pairs featuring a short correlation time. We demonstrate an efficient conversion of the |VV› photon-pair state generated from the first PPLN SPDC stage to the |HH› state through the APPLN EO PMC stage over its operating bandwidth, a broadband or broadly tunable polarization-entangled state can thus be possibly produced via the superposition of the |VV› state generated from the other PPLN SPDC on the third stage of the chip. Such a state can be further manipulated into two of the Bell states if the relative phases between the two polarization states can be properly modulated through the EO PC on the fourth stage of the chip. Such a multifunction integrated quantum photonic source chip can be of high value to developing a compact, efficient, and high-speed quantum information processor.

Transformation of Bell states using linear optics

Bell states form a complete set of four maximally polarization entangled two-qubit quantum state. Being a key ingredient of many quantum applications such as entanglement based quantum key distribution protocols, superdense coding, quantum teleportation, entanglement swapping etc, Bell states have to be prepared and measured. Spontaneous parametric down-conversion (SPDC) is the easiest way of preparing Bell states and a desired Bell state can be prepared from any entangled photon pair through single-qubit logic gates. In this paper, we present the generation of complete set of Bell states, only by applying unitary transformations of half-wave plate (HWP) on the initial Bell state. The initial Bell state is prepared using a combination of a nonlinear crystal and a beam-splitter (BS) and the rest of the Bell states are created by applying single-qubit logic gates on the entangled photon pairs using HWPs. Our results can be useful in many quantum applications, especially in superdense coding where control over basis of maximally entangled state is required.

Highly-enhanced active beam-wander-correction for free-space quantum communications.

In practical applications to free-space quantum communications, the utilization of active beam coupling and stabilization techniques offers notable advantages, particularly when dealing with limited detecting areas or coupling into single-mode fibers(SMFs) to mitigate background noise. In this work, we introduce highly-enhanced active beam-wander-correction technique, specifically tailored to efficiently couple and stabilize beams into SMFs, particularly in scenarios where initial optical alignment with the SMF is misaligned. To achieve this objective, we implement a SMF auto-coupling algorithm and a decoupled stabilization method, effectively and reliably correcting beam wander caused by atmospheric turbulence effects. The performance of the proposed technique is thoroughly validated through quantitative measurements of the temporal variation in coupling efficiency(coincidence counts) of a laser beam(entangled photons). The results show significant improvements in both mean values and standard deviations of the coupling efficiency, even in the presence of 2.6 km atmospheric turbulence effects. When utilizing a laser source, the coupling efficiency demonstrates a remarkable mean value increase of over 50 %, accompanied by a substantial 4.4-fold improvement in the standard deviation. For the entangled photon source, a fine mean value increase of 14 % and an approximate 2-fold improvement in the standard deviation are observed. Furthermore,the proposed technique successfully restores the fidelity of the polarization-entangled state, which has been compromised by atmospheric effects in the free-space channel, to a level close to the fidelity measured directly from the source. Our work will be helpful in designing spatial light-fiber coupling system not only for free-space quantum communications but also for high-speed laser communications.

Polarization Engineering of Entangled Photons from a Lithium Niobate Nonlinear Metasurface.

Complex polarization states of photon pairs are indispensable in various quantum technologies. Conventional methods for preparing desired two-photon polarization states are realized through bulky nonlinear crystals, which can restrict the versatility and tunability of the generated quantum states due to the fixed crystal nonlinear susceptibility. Here we present a solution using a nonlinear metasurface incorporating multiplexed silica metagratings on a lithium niobate film of 300 nm thickness. We fabricate two orthogonal metagratings on a single substrate with an identical resonant wavelength, thereby enabling the spectral indistinguishability of the emitted photons, and we demonstrate in experiments that the two-photon polarization states can be shaped by the metagrating orientation. Leveraging this essential property, we formulate a theoretical approach for generating arbitrary polarization-entangled qutrit states by combining three metagratings on a single metasurface, allowing the encoding of the desired quantum states or information. Our findings enable miniaturized optically controlled quantum devices by using ultrathin metasurfaces as polarization-entangled photon sources.

High-Dimensional Entanglement-Enabled Holography.

As an important imaging technique, holography has been realized with different physical dimensions of light, including polarization, wavelength, and time. Recently, quantum holography has been demonstrated by utilizing polarization entangled state with the advantages of high robustness and enhanced spatial resolution, comparing with classical holography. However, the polarization is only a two-dimensional degree of freedom, which greatly limits the capacity of quantum holography. Here, we propose a method to realize high-dimensional quantum holography by using high-dimensional orbital angular momentum (OAM) entanglement. A high-capacity OAM-encoded quantum holographic system can be obtained by multiplexing a wide range of OAM-dependent holographic images. Proof-of-principle experiments with four- and six-dimensional OAM entangled states have been implemented and verify the feasibility of our idea. Our experimental results also demonstrate that the high-dimensional quantum holography shows a high robustness to classical noise. What is more, the level of security of the holographic imaging encryption system can be greatly improved in our high-dimensional quantum holography.

Polarization‐Entangled Photon Pairs from Warm Atomic Ensemble with Magnetic Background Noise

AbstractAtomic ensembles are important quantum resources for the generation, manipulation, and quantum memory of entangled photons. In photonic quantum information based on atom–photon interactions, high‐quality entangled‐photon‐pair sources are essential for realizing quantum information networks consisting of channels to connect the nodes through atomic ensembles. Here, a proof‐of‐concept for controlling polarization‐entangled photon‐pair sources from atomic ensembles by an external magnetic field under a magnetic noise environment is demonstrated. In the unshielded magnetic field, the polarization entangled state of the photon pair could be optimized to the target state by adjusting the magnetic field in an atomic vapor cell. The polarization‐interference fringe, Bell's inequality value, quantum state tomography, and Hong–Ou–Mandel interference of the polarization entangled photon pairs from the cascade‐type 5S1/2–5P3/2–5D5/2 transition of 87Rb according to the direction of the external magnetic field. Accordingly, a magnetic field is found to be a promising means for controlling entangled two‐qubit states based on atom–photon.

Implement quantum tomography of polarization-entangled states via nondiffractive metasurfaces

Traditional optical elements, such as waveplates and polarization beam splitters, are essential for quantum state tomography (QST). Yet, their bulky size and heavy weight are prejudicial for miniaturizing quantum information systems. Here, we introduce nondiffractive silicon metasurfaces with high transmission efficiency to replace the traditional optical elements for QST of polarization-entangled states. Two identical silicon metasurfaces are employed, and each metasurface comprises four independent districts on a micrometer scale. The unit cell of each district consists of two silicon nanopillars with different geometrical sizes and orientation angles, and the interference of the scattered waves from the nanopillars leads to a single output beam from the district with a specific polarization state with a transmission efficiency above 92%. When the two-photon polarization-entangled state shines on different districts of two metasurfaces, each photon of the photon pair interacts with the local nanopillars within the district, and the two-photon state is projected onto 16 polarization bases for state reconstruction. We experimentally demonstrate the reconstruction of four input Bell states with high fidelities. This approach significantly reduces the number of conventional optical components in the QST process and is inspiring for advancing quantum information technology.

Optimal Polarization-Entanglement Source: Frequency-Converted SPDC with Degeneracy, Indistinguishability, and Ultrahigh Purity That is Configurable Over a Large Spectral Range

Modeling and simulations of entanglement-based quantum-networking protocols commonly assume perfect entangled states. Some investigations have been performed, which show how imperfections cause the efficiency of the protocols to rapidly deteriorate. For polarization-entangled states created by spontaneous-parametric down-conversion (SPDC), the fundamentals of phase matching lead to a trade-off problem for the optimal properties of the state. We present a SPDC method, which circumvents the trade-off problem and allows one to obtain degeneracy, indistinguishability, and heralded-single-photon spectral purity greater than $99\mathrm{%}$ for any target SPDC wavelength in the visible and near-infrared spectrum. Therefore, our method can, in principle, generate polarization-entangled states that are optimal for polarization-entanglement-based quantum-networking protocols.

Waveguide-based swappable hyperentangled photon states in a multi-period, postselection-free device

We explore the generation of four postselection-free, photonic hyperentangled states with the help of a multi-period waveguide device based on potassium titanyl phosphate. The output photons are emitted in the much-in-demand telecommunication wavelengths of 1550-nm and 1310-nm. The device geometry and design enables hyperentanglement in the spatial mode and polarization bases, through multi-period quasi-phase-matching involved in different spontaneous parametric downconversion processes. The spatially entangled part of these hyperentangled states are rendered swappable with the help of two embedded electro-optic phase modulators. A two-path design provides the flexibility of such a device in the choice of polarization-entangled states which is important in relevant experiments. Through the biphoton joint spectral amplitude, we decipher the parameters that make frequency correlation and further entanglement of the photon pairs feasible. Such maximally-hyperentangled biphoton states are useful in quantum information applications.

Quantum entanglement in the lab

The recent and rapid progress in the field of quantum technologies stimulates developments of specific courses and experimental training for future engineers and scientists. We describe below the experimental setup developed at Institut d’Optique Graduate School for engineering and Master students. During a labwork session afternoon, students can study a source of polarization entangled state pairs of photons and perform an experimental violation of Bell’s inequalities. This emblematic experiment is one of the experiments dedicated to quantum photonics. It was built in 2005 in the LEnsE (the Laboratoire d’Enseignement Experimental) of the Institut d’Optique Graduate School and has been perfectly working for almost eighteen years already.

Polarization-entangled biphoton states: a comparison of biperiod waveguides in KTP and LN

Polarization-entangled biphoton states: a comparison of biperiod waveguides in KTP and LN

Quantum interference in anti-parity-time symmetric coupled waveguide system.

We theoretically demonstrate quantum interference in an anti-parity-time (anti-PT) symmetric system based on coupled waveguides. We calculate the coincidence probability in an input polarization-entangled two-photon state, which can be used to simulate different statistical particles. When the birefringence of the waveguides is negligible, our results indicate that the coincidence probabilities of the bosons and fermions decrease exponentially with the propagation distance in both the unbroken and broken anti-PT symmetry regions owing to the dissipation. Particularly, loss-induced transparency is observed for the bosons. Simultaneously, the statistical rule valid in the Hermitian system is violated and the antibunching of bosons is observed. When the birefringence of the waveguides is considered, the coincidence probability of the bosons and fermions is equalized at the exceptional point (EP), whereas that of the bosons is less(greater) than that of the fermions in the broken(unbroken) anti-PT symmetry region. Additionally, we observe the Hong-Ou-Mandel dip for bosons in the broken anti-PT phase. Our research provides a complementary technique for the manipulation of quantum interference compared with the PT symmetric system and may be applied in building quantum devices with anti-PT symmetric quantum mechanics.

Measurement of entropy and quantum coherence properties of two type-I entangled photonic qubits

AbstractUsing the type-I SPDC process in BBO nonlinear crystal, we generate a polarization-entangled state near to the maximally-entangled Bell-state with high-visibility (high-brightness) 98.50 ± 1.33% (87.71 ± 4.45%) for HV (DA) basis. We calculate the CHSH version of the Bell inequality, as a nonlocal realism test, and find a strong violation from the classical physics or any hidden variable theory, S = 2.71 ± 0.10. Via measuring the coincidence count rate in the SPDC process, we obtain the quantum efficiency of single-photon detectors around (25.5 ± 3.4)%, which is in good agreement to their manufacturer company. As expected, we verify the linear dependency of the CC rate vs. pump power of input CW-laser, which may yield to find the effective second-order susceptibility crystal. Using the theory of the measurement of qubits, includes a tomographic reconstruction of quantum states due to the linear set of 16 polarization-measurement, together with a maximum-likelihood-technique, which is based on the numerical optimization, we calculate the physical non-negative definite density matrices, which implies on the non-separability and entanglement of prepared state. By having the maximum likelihood density operator, we calculate precisely the entanglement measures such as Concurrence, entanglement of formation, tangle, logarithmic negativity, and different entanglement entropies such as linear entropy, Von-Neumann entropy, and Renyi 2-entropy. Finally, this high-brightness and low-rate entangled photons source can be used for short-range quantum measurements in the Lab.

Experimental lower bounds to the classical capacity of quantum channels

We show an experimental procedure to certify the classical capacity for noisy qubit channels. The method makes use of a fixed bipartite entangled state, where the system qubit is sent to the channel input and the set of local measurements, ${\ensuremath{\sigma}}_{x}\ensuremath{\bigotimes}{\ensuremath{\sigma}}_{x}, {\ensuremath{\sigma}}_{y}\ensuremath{\bigotimes}{\ensuremath{\sigma}}_{y}$, and ${\ensuremath{\sigma}}_{z}\ensuremath{\bigotimes}{\ensuremath{\sigma}}_{z}$, is performed at the channel output and the ancilla qubit, thus without resorting to full quantum process tomography. The witness to the classical capacity is then achieved by reconstructing sets of conditional probabilities, noise deconvolution, and classical optimization of the pertaining mutual information. The performance of the method to provide lower bounds to the classical capacity is tested by a two-photon polarization entangled state in Pauli channels and amplitude damping channels. The measured lower bounds to the channels are in high agreement with the simulated data, which take into account both the experimental entanglement fidelity $F=0.979\ifmmode\pm\else\textpm\fi{}0.011$ of the input state and the systematic experimental imperfections.

Nonlocal realism tests and quantum state tomography in Sagnac-based type-II polarization-entanglement SPDC-source

We have experimentally created a robust, ultrabright and phase-stable polarization-entangled state close to maximally entangled Bell-state with %98-fidelity using the type-II spontaneous parametric down-conversion (SPDC) process in periodically-poled KTiOPO4 (PPKTP) collinear crystal inside a Sagnac interferometer (SI). Bell inequality measurement, Freedman's test, as the different versions of CHSH inequality, and also visibility test which all can be seen as the nonlocal realism tests, imply that our created entangled state shows a strong violation from the classical physics or any hidden-variable theory. We have obtained very reliable and very strong Bell violation as S=2.78±0.01 with high brightness VHV=%(99.969±0.003) and VDA=%(96.751±0.002) and very strong violation due to Freedman test as δF=0.01715±0.00001. Furthermore, using the tomographic reconstruction of quantum states together a maximum-likelihood-technique (MLT) as the numerical optimization, we obtain the physical non-negative definite density operator which shows the nonseparability and entanglement of our prepared state. By having the maximum likelihood density operator, we calculate some important entanglement-measures and entanglement entropies. The Sagnac configuration provides bidirectional crystal pumping yields to high-rate entanglement source which is very applicable in quantum communication, sensing and metrology as well as quantum information protocols, and has potential to be used in quantum illumination-based LIDAR and free-space quantum key distribution (QKD).

Massively-multiplexed generation of Bell-type entanglement using a quantum memory

High-rate generation of hybrid photon-matter entanglement remains a fundamental building block of quantum network architectures enabling protocols such as quantum secure communication or quantum distributed computing. While a tremendous effort has been made to overcome technological constraints limiting the efficiency and coherence times of current systems, an important complementary approach is to employ parallel and multiplexed architectures. Here we follow this approach experimentally demonstrating the generation of bipartite polarization-entangled photonic states across more than 500 modes, with a programmable delay for the second photon enabled by qubit storage in a wavevector-multiplexed cold-atomic quantum memory. We demonstrate Clauser, Horne, Shimony, Holt inequality violation by over 3 standard deviations, lasting for at least 45 μs storage time for half of the modes. The ability to shape hybrid entanglement between the polarization and wavevector degrees of freedom provides not only multiplexing capabilities but also brings prospects for novel protocols.