Generation of continuous-variable high-dimensional entanglement with three degrees of freedom and multiplexing quantum dense coding

Due to the infinite range of possibly achievable degrees of freedom, orbital angular momentum (OAM) can tremendously increase the capacity of communication system. Here, we propose a scheme to generate OAM entanglement by using interaction-free measurement (IFM). As the superposition state of the quantum absorption object is not changed after IFM, our scheme can be extended to multiparty easily. The numerical analysis results show that the fidelity of generated OAM entanglement can be arbitrarily close to unity. Besides, the implementation issues are also discussed to evaluate the feasibility in experiment. This OAM entanglement with multiple degrees of freedom will play a key role in distributed entanglement computing and efficient quantum communication.

Orbital angular momentum (OAM) entangled photon pairs with narrow bandwidths play a crucial role in the interaction of light and quantum states of matter. In this article, we demonstrate an approach for generating OAM entangled photon pairs with a narrow bandwidth by using a single driving beam in a 85Rb atomic vapor cell. This single driving beam is able to simultaneously couple two atomic transitions and directly generate OAM entangled biphotons by leveraging the OAM conservation law through the spontaneous four-wave mixing (SFWM) process. The photon pairs exhibit a maximum cross-correlation function value of 27.7 and a linewidth of 4 MHz. The OAM entanglement is confirmed through quantum state tomography, revealing a fidelity of 95.7% and a concurrence of 0.926 when compared to the maximally entangled state. Our scheme is notably simpler than previously proposed schemes and represents the first demonstration of generating subnatural-linewidth entangled photon pairs in hot atomic systems.

High-dimensional entanglement provides valuable resources for quantum technologies, including quantum communication, quantum optical coherence tomography, and quantum computing. Obtaining a high brightness and dimensional entanglement source has significant value. Here we utilize a tunable asymmetric Mach–Zehnder interferometer coupled silicon microring resonator with 100 GHz free spectral range to achieve this goal. With the strategy of the tunable coupler, the dynamical and extensive tuning range of quality factors of the microring can be obtained, and then the biphoton pair generation rate can be optimized. By selecting and characterizing 28 pairs from a more than 30-pair modes biphoton frequency comb, we obtain a Schmidt number of at least 23.4 and on-chip pair generation rate of 19.9 MHz/mW2 under a low on-chip pump power, which corresponds to 547 dimensions Hilbert space in frequency freedom. These results will prompt the wide applications of quantum frequency comb and boost the further large density and scalable on-chip quantum information processing.

The orbital angular momentum (OAM) of photons provides a pivotal resource for carrying out high-dimensional quantum Pauli-X operations due to its unique discrete infinite orthogonality, which opens enticing perspectives for high-capacity quantum communication and multi-parallel quantum computation. However, the state-of-the-art Pauli-X gates that require cyclic transformation of OAM modes are mostly limited to four modes, face scalability challenges owing to the system redundancy and fidelity degradation induced by increased processing modes. Here, we propose a general strategy to design scalable quantum Pauli-X gates in high-dimensional OAM space by taking full advantage of the order-correlated spatial divergence property of OAM modes. Leveraging the mechanism of balance mode filtering, partitioned phase is embedded between two structurally symmetric Q-plate elements that enable bidirectional mode shifting to filter out the highest-order OAM mode for spatially selective phase modulation. Derived from radial matching between modes and partitioned phase profiles, this design of parallel mode transformation underpins the extension to higher dimensions by merely adjusting filter parameters. We demonstrate the five-mode cyclic transformation with mode purity and diffraction efficiency higher than 92% and 92.7%, respectively. By incorporating polarization-entangled photon sources, we further realize five-dimensional quantum Pauli-X gates encoded on OAM space and experimentally verify the entanglement of output mode states. Low modulation loss and efficient quantum state tomography (QST) techniques endow the reconstructed density matrices with fidelities exceeding 92.3% ± 0.1%, indicating the well-performed entanglement preservation of output states. These results confirm that our approach achieves scalable quantum Pauli-X gate operations through a simple balanced architecture with high coherence, consolidating a theoretical and experimental foundation for large-scale quantum computation encoded on high-dimensional OAM space.

The high designability of metamaterials has made them an attractive platform for devising novel optoelectronic devices. The demonstration of nonlinear metamaterials further indicates their potential in developing quantum applications. Here, we investigate designing nonlinear metamaterials consisting of the 3-fold (C3) rotationally symmetrical nanoantennas for generating and modulating entangled photons in the spatial degrees of freedom. Through tailoring the geometry and orientation of the nanoantennas, the parametric down conversion process inside the metamaterials can be locally engineered to generate entangled states with desired spatial properties. As the orbital angular momentum (OAM) states are valuable for enhancing the data capacity of quantum information systems, the photonic OAM entanglement is practically considered. With suitable nanostructure design, the generation of OAM entangled states is shown to be effectively realized in the discussed nonlinear metamaterial system. The nonlinear metamaterials present a perspective to provide a flexible platform for quantum photonic applications.

We report the realization of a classically phase-locked source of quantum twin beams. Theoretical work by Reid and Drummond, and more recently by van Loock and Braunstein, predicts that such a source is adequate for creating bipartite and multipartite continuous-variable entanglement. The source is a type-II optical parametric oscillator (OPO) above threshold. Its exceptional frequency and intensity stability is derived from careful design and from three servo loops. The OPO can stably emit on the frequency-degenerate mode, pumped a few percent above the threshold. We observe a preliminary number-difference squeezing of 5.5 dB (6.4 dB inferred) at 200 kHz. In order to enable future EPR measurements, the signal and idler fields must also be classically phase-locked: we measure a signal-idler-frequency-difference linewidth smaller than 300 Hz for our OPO.

Quantum information can be encoded in a scalable manner over the continuous variables (CV) that are the canonically conjugate quantum amplitudes of the electromagnetic field, which are mathematical equivalents of the position and momentum of the quantum harmonic oscillator. Previous results in our group in Charlottesville and the groups of Akira Furusawa in Tokyo, of Nicolas Treps in Paris, and of Ping-Koy Lam in Canberra, have shown that CV quantum information is massively scalable. In addition, Nicolas Menicucci in Melbourne has shown that there exists a fault tolerance threshold for CV quantum computing with the Gottesman-Kitaev-Preskill qubit error encoding. Here, we report on our exploration of the extension of scalable CV encoding by way of intramode, rather than intermode, squeezing and entanglement. As is well known, single-mode squeezing can only be considered along with the quasi-mode approximation which assimilates the modes of the optical parametric oscillator cavity used in the experiment to delta functions. As Hans Bachor and Tim Ralph noted, when one ambles beyond this approximation, single-mode squeezing can be described as intramode two-mode squeezing. Here, we aim at generalizing this situation to the case of multipartite entangled cluster states.

Summary form only given. With the rapid development of spatially-resolving single-photon detectors, spatially structured multidimensional entangled states start to play a key role in modern quantum science. In particular, they find extensive applications in emerging fields such as quantum imaging, holography, computation or quantum-enhanced metrology. Moreover, spatially-multiplexed schemes hold a promise to increase the capacity of quantum channels, essential for quantum key distribution. An appealing perspective is to apply these ideas to light-atom interfaces, that are capable of storing and processing continuous-variable (CV) entanglement, critical for novel quantum cryptography, computation and imaging schemes.Here we generate a hybrid bipartite entangled state of a photon and an atomic spin-wave excitation [1], exhibiting Einstein-Podolsky-Rosen (EPR) correlations in real space of continuous position-momentum variables, as in the original EPR proposal [2]. We verify the entanglement by measuring the coincidence patterns corresponding to modulus-squared spatial wavefunctions of the state. By studying temporal evolution and decoherence we find that the EPR entanglement may be stored for several microseconds. Our approach turns out to be by far more robust than the only hitherto performed experiment, based on quadratures of squeezed light and a slow-light medium, where time-delayed EPR correlations were demonstrated [3]. We achieve two orders of magnitude longer delay time using quantum memory setup and significantly stronger violation of EPR inequality [4], with product of variances 3 times below the EPR bound for the full two-dimensional coincidence distribution.

We present the measurement of entanglement between twin beams generated with a doubly resonant optical parameter oscillator (OPO) based on four-wave mixing in hot ^{85}Rb vapor above threshold. This is the first measurement of entanglement in an OPO with a χ^{(3)} media above threshold. We reconstruct the covariance matrix for several configurations and based on a full picture of the four side band mode state, we study entanglement between all possible bipartitions. We show a robust generation of entanglement with stronger generation for a specific pair of modes. For this system, we show that atomic density is a determinant factor for the generation and loss of quantum correlations. The generation of entangled fields with an atomic OPO operating close to atomic resonance of alkali atoms enables a natural integration into quantum networks.

The Orbital Angular Momentum (OAM) carried by light beams with helical phasefront (vortex beams) has been widely employed in many applications such as optical tweezers, optical drives of micro-machines, atom trapping, and optical communication. OAM provides an additional dimension (diversity) to multiplexing techniques, which can be utilized in addition to conventional multiplexing methods to achieve higher data rates in wireless communication. OAM beams have been thoroughly studied and used in the optical regime but in the mm-wave and THz-wave region, they are still under investigation. In these frequency bands, there are difficulties associated with beam-splitting and beam-combining processes as well as with the use of spiral phase plates and other methods for OAM generation, since the wavelength is much larger compared to those at optical frequencies, leading to higher diffraction losses. The present paper describes the natural generation of high-power OAM modes by gyro-type vacuum electron devices with cylindrical interaction circuit and axial output of the generated rotating higher-order transverse electric mode TEm,n, where m > 1 and n are the azimuthal and radial mode index, respectively. The ratio between the total angular momentum (TAM) JN and total energy WN of N photons is given by m/ω, where ω is the angular frequency of the operating mode, which in a gyrotron oscillator is close to the TEm,n-mode cutoff frequency in the cavity. Therefore, m/ω = Rc/c, where Rc is the caustic radius and c the velocity of light in vacuum. This means that the OAM is proportional to the caustic radius and at a given frequency the same for all modes with the same azimuthal index m. Right-hand rotation (co-rotation with the electrons) corresponds to a positive value of m and left-hand rotation to negative m. The corresponding OAM mode number (topological charge) is l = m – 1. Circularly polarized TE1n modes only possess a Spin Angular Momentum (SAM: s = ±1). TE0n modes have neither SAM nor OAM. This is the result of the photonic (quasi-optical) approach to derive the TAM of modes generated in gyrotrons. The same result follows from the electromagnetic (EM) wave approach for the TAM within a given waveguide volume per total energy of the EM wave in the same volume. Such high-power output beams with very pure higher-order OAM, generated by gyrotron oscillators or amplifiers (broadband) could be used for multiplexing in long-range wireless communications. The corresponding mode and helical wavefront sensitive detectors for selective OAM-mode sorting are available and described in the present paper.

Shaping the transverse spatial domain of light has been experiencing recent growth in attention for its applications in optical tweezing, microscopy, communications, and quantum information sciences. The orbital angular momentum (OAM) of light is a transverse physical property that functions as a viable basis for many of such applications. While fields containing a single OAM mode order have extensively been used, fields containing a broad, discrete, and equally spaced OAM spectrum—an OAM comb—has largely been unexplored due to a lack of viable creation methods. Much like how frequency combs enabled myriad applications, it is conceivable that OAM combs would likewise enable further opportunities or expand upon single-OAM applications. For instance, an OAM comb may pave the way for more sophisticated particle manipulation, object detection, and pump shaping for high-dimensional spatial-mode entanglement. Here, we create four OAM combs with rectangular, sinusoidal, sinc, and Gaussian OAM spectral distributions using multiplane light conversion, which only involves phase modulations (hence is inherently lossless) and allows for improved tailoring of the spatial shape. The verification of such combs is done qualitatively by analyzing their propagation evolutions and quantitatively by employing a mode sorting technique—both exhibiting good agreement with user specifications and simulations. The scattering losses of all four combs were measured to be minimal, ranging from 0.66 to 1.04 dB.

In this keynote invited paper, to address key challenges for beyond 5G wireless technologies in a simultaneous manner, we propose an orbital angular momentum (OAM) based, energy-efficient multidimensional coded modulation. The key idea is to employ all available degrees of freedom (DOFs) to convey the information over the wireless links including amplitude, phase, polarization state, and spatial-domain DOFs. In particular, the OAM is associated with azimuthal phase dependence of the wavefront, and represents an underutilized DOF. Given that OAM eigenstates are orthogonal, an arbitrary number of bits per symbol can be transmitted. To implement OAM multiplexer and demultiplexer in the RF domain, we propose to use a properly designed antenna arrays. Monte Carlo simulations demonstrate high tolerance to fading effects of LDPC-coded multidimensional signaling schemes compared to conventional LDPC-coded QAM.

Synchronously pumped optical parametric oscillators (OPOs) are highly efficient sources of long-wavelength pulses and nonclassical light, making them invaluable for applications in spectroscopy, metrology, multi-photon microscopy, and quantum computation. Typical systems based on free-space cavities either operate non-degenerately, which limits their efficiency, or use active feedback control to achieve degenerate operation, which limits these systems to dedicated low-noise environments. In this work, we demonstrate a femtosecond monolithically integrated OPO. In contrast with bulk OPOs, our monolithic 10 GHz cavity, based on reverse-proton-exchanged lithium niobate, operates stably without active locking. By detuning the repetition rate of the free-running pump laser from the cavity free spectral range, we control the intracavity pulse dynamics and observe many of the operating regimes previously encountered in free-space degenerate OPOs, such as box-pulsing and quadratic bright-dark solitons (simultons), in addition to non-degenerate operation. When operated in the simulton regime and pumped with 125 fs pulses at 1 µm, this monolithic OPO chip outputs broadband sech2 pulses (63 nm, 3 dB) with tens of milliwatts of average power.

The generation and control of quantum correlations in high-dimensional systems is a major challenge in the present landscape of quantum technologies. Achieving such non-classical high-dimensional resources will potentially unlock enhanced capabilities for quantum cryptography, communication and computation. We propose a protocol that is able to attain entangled states of d-dimensional systems through a quantum-walk (QW)-based transfer & accumulate mechanism involving coin and walker degrees of freedom. The choice of investigating QW is motivated by their generality and versatility, complemented by their successful implementation in several physical systems. Hence, given the cross-cutting role of QW across quantum information, our protocol potentially represents a versatile general tool to control high-dimensional entanglement generation in various experimental platforms. In particular, we illustrate a possible photonic implementation where the information is encoded in the orbital angular momentum and polarization degrees of freedom of single photons.

Tidal effects have important imprints on gravitational waves (GWs) emitted during the final stage of the coalescence of binaries that involve neutron stars (NSs). Dynamical tides can be significant when NS oscillations become resonant with orbital motion; understanding this process is important for accurately modeling GW emission from these binaries and for extracting NS information from GW data. In this paper, we use semianalytic methods to carry out a systematic study on the tidal excitation of fundamental modes ($f$-modes) of spinning NSs in coalescencing binaries, focusing on the case when the NS spin is antialigned with the orbital angular momentum---where the tidal resonance is most likely to take place. We first expand NS oscillations into stellar eigenmodes, and then obtain a Hamiltonian that governs the tidally coupled orbit-mode evolution. (Our treatment is at Newtonian order, including a gravitational radiation reaction at quadrupole order.) We then find a new approximation that can lead to analytic expressions of tidal excitations to a high accuracy, and are valid in all regimes of the binary evolution: adiabatic, resonant, and postresonance. Using the method of osculating orbits, we obtain semianalytic approximations to the orbital evolution and GW emission; their agreements with numerical results give us confidence in our understanding of the system's dynamics. In particular, we recover both the averaged postresonance evolution, which differs from the preresonance point-particle orbit by shifts in orbital energy and angular momentum, as well as instantaneous perturbations driven by the tidal motion. Finally, we use the Fisher matrix technique to study the effect of dynamical tides on parameter estimation. We find that, for a system with component masses of $(1.4,1.4)\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ at 100 Mpc, the constraints on the effective Love number of the (2,2) mode at Newtonian order can be improved by a factor of $3\ensuremath{\sim}4$ if spin frequency is as high as 500 Hz. The relative errors are $0.7\ensuremath{\sim}0.8$ in the Cosmic Explorer, and they might be further improved by post-Newtonian effects. The constraints on the $f$-mode frequency and the spin frequency are improved by factors of $5\ensuremath{\sim}6$ and $19\ensuremath{\sim}27$, respectively. In the Cosmic Explorer case, the relative errors are $0.2\ensuremath{\sim}0.4$ and $0.7\ensuremath{\sim}1.0$, respectively. Hence, the dynamical tides may potentially provide an additional channel to study the physics of NSs. The method presented in this paper is generic and not restricted to $f$-mode; it can also be applied to other types of tides.