Single-photon Polarization Research Articles

Tunable Linear Polarization-State Generator of Single Photons on a Lithium Niobate Chip

Controlling the polarization state of light at a fast speed is important in both classical and quantum regimes. Here, we present an easily integrated compact tunable linear polarization-state generator of single photons in the telecom wavelength band on a periodically poled lithium niobate on insulator (PPLNOI) chip. The linear polarization of single photons can be continuously rotated by applying a transverse electric field utilizing the electro-optic (EO) effect. With strong light confinement and the large EO coefficient in the PPLNOI ridge waveguide, dramatically reduced driving voltage and fast-speed modulation is realized. Moreover, quantum entanglement is well maintained during the switching of polarization states, as proved in the time-energy entanglement measurement. The scheme holds promise for realizing integrated quantum optics.

Topological quantum walks in cavity-based quantum networks

We present a protocol to implement discrete-time quantum walks and simulate topological insulator phases in cavity-based quantum networks, where the single photon is the quantum walker and the cavity input-output process is employed to realize the state-dependent translation operation. Different topological phases can be simulated through tuning the single-photon polarization rotation angles. We show that both the topological boundary states and topological phase transitions can be directly observed via measuring the final photonic density distribution. Moreover, we also demonstrate that these topological signatures are quite robust to practical imperfections. Our work opens a new prospect using cavity-based quantum networks as quantum simulators to study discrete-time quantum walks and mimic condensed matter physics.

New method of second quantization of the strained-graphene Kerr and Faraday rotations.

We illustrate Kerr and Faraday rotation in the strained-graphene by applying the second quantization method as an alternative approach. We consider the right- and left-going photon fields coupling with strained graphene. In other words, we have a new stationary state solution describing this phenomenon. A single-photon polarization in the provided state is considered in cases of a non-magnetic field, and uniform strained graphene. We show that the optical l properties of Faraday rotation, reflectance, and transmittance depend on the spinor phase and the energy level of an electron in strained graphene. These values can be controlled by variation of a strain parameter and strain types. Then, it is possible to have an alternative measurement of the pseudo-spin state and electronic structure in the 2-D layer by observing the optical properties of the single-photon in the provided state.

High-Efficiency Three-Party Quantum Key Agreement Protocol with Quantum Dense Coding and Bell States

We propose a high-efficiency three-party quantum key agreement protocol, by utilizing two-photon polarization-entangled Bell states and a few single-photon polarization states as the information carriers, and we use the quantum dense coding method to improve its efficiency. In this protocol, each participant performs one of four unitary operations to encode their sub-secret key on the passing photons which contain two parts, the first quantum qubits of Bell states and a small number of single-photon states. At the end of this protocol, based on very little information announced by other, all participants involved can deduce the same final shared key simultaneously. We analyze the security and the efficiency of this protocol, showing that it has a high efficiency and can resist both outside attacks and inside attacks. As a consequence, our protocol is a secure and efficient three-party quantum key agreement protocol.

Non-Markovianity through quantum coherence in an all-optical setup.

We propose an all-optical experiment to quantify non-Markovianity in an open quantum system through quantum coherence of a single quantum bit. We use an amplitude damping channel implemented by an optical setup with an intense laser beam simulating a single-photon polarization. The optimization over initial states required to quantify non-Markovianity is analytically evaluated. The experimental results are in very good agreement with the theoretical predictions.

Polarization Conversion of Single Photon via Scattering by a Λ System in a Semi-Infinite Waveguide**Supported by the Anhui Provincial Natural Science Foundation under Grant Nos 1608085MA05 and 1608085MA09, and the National Natural Science Foundation of China under Grant Nos 11774262 and 11474003.

We theoretically investigate single-photon polarization conversion via scattering by an atom with Λ configuration coupled to a semi-infinite waveguide and discuss the two cases in which the Λ system is non-degenerated and degenerated. By applying the hard-wall boundary condition of the semi-infinite waveguide, it is found that single-photon polarization conversion can be realized with unit probability for both cases under the ideal condition. Together with the polarization conversion, the frequency conversion of a single photon can also be realized with unit probability in the ideal case if the Λ system is not degenerated.

Efficient quantum memory for single-photon polarization qubits

A quantum memory, for storing and retrieving flying photonic quantum states, is a key interface for realizing long-distance quantum communication and large-scale quantum computation. While many experimental schemes of high storage-retrieval efficiency have been performed with weak coherent light pulses, all quantum memories for true single photons achieved so far have efficiencies far below 50%, a threshold value for practical applications. Here, we report the demonstration of a quantum memory for single-photon polarization qubits with an efficiency of >85% and a fidelity of >99 %, basing on balanced two-channel electromagnetically induced transparency in laser-cooled rubidium atoms. For the single-channel quantum memory, the optimized efficiency for storing and retrieving single-photon temporal waveforms can be as high as 90.6 %. Our result pushes the photonic quantum memory closer to its practical applications in quantum information processing.

Deterministic quantum state transfer between remote atoms with photon-number superposition states

We propose a protocol for quantum networking based on deterministic quantum state transfer between distant memory nodes using photon-number superposition states (PNSS). In the suggested scheme, the quantum nodes are single atoms confined in high-finesse optical cavities linked by photonic channels. The quantum information written in a superposition of atomic Zeeman states of sending system is faithfully mapped through cavity-assisted Raman scattering onto PNSS of linearly polarized cavity photons. The photons travel to the receiving cavity, where they are coherently absorbed with unit probability creating the same superposition state of the second atom, thus ensuring high-fidelity transfer between distant nodes. We develop this approach at first for photonic qubit and show that this superposition state is no less reliably protected against the propagation losses compared to the single-photon polarization states, whereas the limitation associated with the delivery of more than one photon does not affect the process fidelity. Then, by preserving the advantages of qubits, we extend the developed technique to the case of state transfer by photonic qutrit, which evidently possesses more information capacity. This reliable and efficient scheme promises also a successful distribution of entanglement over long distances in quantum networks.

Single photon polarization conversion via scattering by a pair of atoms.

The single photon scattering in one-dimensional waveguide coupled to two separated atoms is investigated. The first atom is considered as a Λ system and the second one is taken as V -type configuration. The analytical expressions of the single photon scattering spectra are obtained. The calculated results show that the polarization conversion of single photon can be realized by controlling the distance between the two atoms due to the interference effects. The conversion efficiency can reach unit in the ideal case. Furthermore, the polarization conversion of the single photon also depends on the initial state of the Λ system. The influences of dissipations on the single photon polarization conversion are also shown.

Proof of principle implementation of phase-flip error rejection quantum key distribution

Improving the tolerance of channel noise is an important task for devising and implementing quantum key distribution (QKD) protocols. Quantum phase-flip error rejection (QPFER) code [Phys. Rev. Lett.92, 077902 (2004)PRLTAO0031-900710.1103/PhysRevLett.92.077902] has been introduced by Wang to increase the tolerable phase-flip noise of QKD implementations. However, an experiment that demonstrates its advantages is still missing. Here, we experimentally verify the QPFER code with the assistance of two photon quantum states generated by spontaneous parametric downconversion. The methods of parity check and postselection are introduced to the protocol for achieving the phase-flipping rejection. Comparing with the standard realization of the single photon polarization encoding BB84 scheme, the quantum error rate after decoding is obviously reduced when the probability of channel noise is less than 25%. The experiment results also show that QPFER protocol can reduce error rate, obtain a higher key rate, and be robust in the noisy channel when the noise level is in a proper region.

Single electron-photon pair creation from a single polarization-entangled photon pair

Quantum entanglement between different forms of qubits is an indication of the universality of quantum mechanics. Entanglement transfer between light and matter, especially photon and spin, has long been studied as the central concept, but it remains technically challenging for single photons and spins. In this paper, we show paired generation of a single electron in a GaAs quantum dot and a single photon from a single polarization-entangled photon pair. We measure temporal coincidence between the single photo-electron detection and the single photon detection. Considering a single photon polarization is converted to an electron spin via an optical selection rule, the present result indicates the capability of photon to spin entanglement transfer. This may be useful to explore the physics of entanglement transfer and also for applications to quantum teleportation based quantum communication.

Ultralow-Noise Room-Temperature Quantum Memory for Polarization Qubits

Here we show an ultra-low noise regime of operation in a simple quantum memory in warm Rb atomic vapor. By modelling the quantum dynamics of four-level room temperature atoms, we achieve fidelities >90% for single-photon level polarization qubits, clearly surpassing any classical strategy exploiting the non-unitary memory efficiency. This is the first time such important threshold has been crossed with a room temperature device. Additionally we also show novel experimental techniques capable of producing fidelities close to unity. Our results demonstrate the potential of simple, resource-moderate experimental room temperature quantum devices.

Cut-and-paste restoration of entanglement transmission

The distribution of entangled quantum systems among two or more nodes of a network is a key task at the basis of quantum communication, quantum computation and quantum cryptography. Unfortunately the transmission lines used in this procedure can introduce so much perturbations and noise in the transmitted signal that prevent the possibility of restoring quantum correlations in the received messages either by means of encoding optimization or by exploiting local operations and classical communication. In this work we present a procedure which allows one to improve the performance of some of these channels. The mechanism underpinning this result is a protocol which we dub cut-and-paste, as it consists in extracting and reshuffling the sub-components of these communication lines, which finally succeed in "correcting each other". The proof of this counterin- tuitive phenomenon, while improving our theoretical understanding of quantum entanglement, has also a direct application in the realization of quantum information networks based on imperfect and highly noisy communication lines. A quantum optics experiment, based on the transmission of single-photon polarization states, is also presented which provides a proof-of-principle test of the proposed protocol.

Quantum repeaters using continuous-variable teleportation

Quantum optical states are fragile and can become corrupted when passed through a lossy communication channel. Unlike for classical signals, optical amplifiers cannot be used to recover quantum signals. Quantum repeaters have been proposed as a way of reducing errors and hence increasing the range of quantum communications. Current protocols target specific discrete encodings, for example quantum bits encoded on the polarization of single photons. We introduce a more general approach that can reduce the effect of loss on any quantum optical encoding, including those based on continuous variables such as the field amplitudes. We show that in principle the protocol incurs a resource cost that scales polynomially with distance. We analyse the simplest implementation and find that whilst its range is limited it can still achieve useful improvements in the distance over which quantum entanglement of field amplitudes can be distributed.

Single Photon in Hierarchical Architecture for Physical Decision Making: Photon Intelligence

Understanding and using photonic processes for intelligent functionalities, referred to as photonic intelligence, has recently attracted interest from a variety of fields, including postsilicon computing for artificial intelligence and decision making in the behavioral sciences. In a past study, we successfully used the wave–particle duality of single photons to solve the two-armed bandit problem, which constitutes one of the important foundations of decision making and reinforcement learning. In this paper, we propose and confirm a hierarchical architecture for single-photon-based decision making that verifies the scalability of the principle. Specifically, the four-armed bandit problem is solved given zero prior knowledge in a two-layer hierarchical architecture, where the polarization of single photons is autonomously adapted in order to effect adequate decision making. In the hierarchical structure, the notion of layer-dependent decisions emerges. The optimal solutions in the coarse layer and the fine...

Experimental investigation of a four-qubit linear-optical quantum logic circuit.

We experimentally demonstrate and characterize a four-qubit linear-optical quantum logic circuit. Our robust and versatile scheme exploits encoding of two qubits into polarization and path degrees of single photons and involves two crossed inherently stable interferometers. This approach allows us to design a complex quantum logic circuit that combines a genuine four-qubit C3Z gate and several two-qubit and single-qubit gates. The C3Z gate introduces a sign flip if and only if all four qubits are in the computational state |1〉. We verify high-fidelity performance of this central four-qubit gate using Hofmann bounds on quantum gate fidelity and Monte Carlo fidelity sampling. We also experimentally demonstrate that the quantum logic circuit can generate genuine multipartite entanglement and we certify the entanglement with the use of suitably tailored entanglement witnesses.

Incompatible measurements on quantum causal networks

The existence of incompatible measurements, epitomized by Heisenberg's uncertainty principle, is one of the distinctive features of quantum theory. So far, quantum incompatibility has been studied for measurements that test the preparation of physical systems. Here we extend the notion to measurements that test dynamical processes, possibly consisting of multiple time steps. Such measurements are known as testers and are implemented by interacting with the tested process through a sequence of state preparations, interactions, and measurements. Our first result is a characterization of the incompatibility of quantum testers, for which we provide necessary and sufficient conditions. Then, we propose a quantitative measure of incompatibility. We call this measure the robustness of incompatibility and define it as the minimum amount of noise that has to be added to a set of testers in order to make them compatible. We show that (i) the robustness is lower bounded by the distinguishability of the sequence of interactions used by the tester and (ii) maximum robustness is attained when the interactions are perfectly distinguishable. The general results are illustrated in the concrete example of binary testers probing the time-evolution of a single-photon polarization.

Elliptical quantum dots as on-demand single photons sources with deterministic polarization states

In quantum information, control of the single photon's polarization is essential. Here, we demonstrate single photon generation in a pre-programmed and deterministic polarization state, on a chip-scale platform, utilizing site-controlled elliptical quantum dots (QDs) synthesized by a top-down approach. The polarization from the QD emission is found to be linear with a high degree of linear polarization and parallel to the long axis of the ellipse. Single photon emission with orthogonal polarizations is achieved, and the dependence of the degree of linear polarization on the QD geometry is analyzed.

Broadband enhancement of single photon emission and polarization dependent coupling in silicon nitride waveguides.

Single-photon (SP) sources are important for a number of optical quantum information processing applications. We study the possibility to integrate triggered solid-state SP emitters directly on a photonic chip. A major challenge consists in efficiently extracting their emission into a single guided mode. Using 3D finite-difference time-domain simulations, we investigate the SP emission from dipole-like nanometer-sized inclusions embedded into different silicon nitride (SiNx) photonic nanowire waveguide designs. We elucidate the effect of the geometry on the emission lifetime and the polarization of the emitted SP. The results show that highly efficient and polarized SP sources can be realized using suspended SiNx slot-waveguides. Combining this with the well-established CMOS-compatible processing technology, fully integrated and complex optical circuits for quantum optics experiments can be developed.

Experimental realization of generalized qubit measurements based on quantum walks

We report an experimental implementation of a single-qubit generalized measurement scenario, the positive-operator valued measure (POVM), based on a quantum walk model. The qubit is encoded in a single-photon polarization. The photon performs a quantum walk on an array of optical elements, where the polarization-dependent translation is performed via birefringent beam displacers and a change of the polarization is implemented with the help of wave plates. We implement: (i) trine POVM, i.e., the POVM elements uniformly distributed on an equatorial plane of the Bloch sphere; (ii) symmetric-informationally-complete (SIC) POVM; and (iii) unambiguous discrimination of two nonorthogonal qubit states.