Applications In Quantum Information Processing Research Articles

Bipartite and tripartite continuous variable entanglement in a circular array of coupled optical waveguides

Integrated optical systems have evolved into suitable platforms in the field of photonic quantum technologies. New technologies open up new possibilities for multimode quantum operations. Here we study how circularly coupled waveguide arrays generate bipartite and tripartite continuous-variable (CV) entanglement. We focus on the single-mode squeezed state as input to the circular array of the waveguide system. Our findings suggest that the circularly coupled arrays can be used to generate entangled sources in CV quantum technologies. So the generation of entanglement makes the circular arrays more critical for further investigation and in the applications of photonic CV quantum-information processing.

Development of Flux-Tuneable Inductive Nanobridge SQUIDs for Quantum Technology Applications

Niobium nanobridge SQUIDs have shown exceptional noise performance with potential applications in quantum information processing, weak signal detection and single spin detection where the nanobridge geometry should enable efficient electromagnetic coupling to implanted spins. Combining such devices with dispersive microwave readout circuitry allows the spin sensitivity to be further improved by overcoming the standard thermal limit. Here we report on the fabrication and dispersive microwave readout of an array of niobium nanobridge rf SQUIDs incorporated into a superconducting resonator, including the optimization of the nanobridge fabrication process by electron beam lithography. We show the measured flux-tuneability of the resonance is in good agreement with theory, and we also discuss how the nonlinearity of the weak-link in the resonator structure allows for the mediation of parametric effects to enhance performance.

Magnon-photon cross-correlations via optical nonlinearity in cavity magnonical system.

We propose an alternative scheme to achieve the cross-correlations between magnon and photon in a hybrid nonlinear system including two microwave cavities and one yttrium iron garnet (YIG) sphere, where two cavities nonlinearly interact and meanwhile one of cavities couples to magnon representing the collective excitation in YIG sphere via magnetic dipole interaction. Based on dispersive couplings between two cavities and between one cavity and magnon with the larger detunings, the nonlinear interaction occurs between the other cavity and magnon, which plays a crucial role in generating quantum correlations. By analyzing the second-order correlation functions via numerical simulations and analytical calculations, the remarkable nonclassical correlations are existent in such a system, where the magnon blockade and photon antibunching could be obtainable on demand. The scheme we present is focused on the magnon-photon cross-correlations in the weak coupling regime and relaxes the requirements of experimental conditions, which may have potential applications in quantum information processing in the hybrid system.

Fisher Information as General Metrics of Quantum Synchronization.

Quantum synchronization has emerged as a crucial phenomenon in quantum nonlinear dynamics with potential applications in quantum information processing. Multiple measures for quantifying quantum synchronization exist. However, there is currently no widely agreed metric that is universally adopted. In this paper, we propose using classical and quantum Fisher information (FI) as alternative metrics to detect and measure quantum synchronization. We establish the connection between FI and quantum synchronization, demonstrating that both classical and quantum FI can be deployed as more general indicators of quantum phase synchronization in some regimes where all other existing measures fail to provide reliable results. We show advantages in FI-based measures, especially in 2-to-1 synchronization. Furthermore, we analyze the impact of noise on the synchronization measures, revealing the robustness and susceptibility of each method in the presence of dissipation and decoherence. Our results open up new avenues for understanding and exploiting quantum synchronization.

General Classification of Qubit Encodings in Ultracold Diatomic Molecules.

Owing to their rich internal structure and significant long-range interactions, ultracold molecules have been widely explored as carriers of quantum information. Several different schemes for encoding qubits into molecular states, both bare and field-dressed, have been proposed. At the same time, the rich internal structure of molecules leaves many unexplored possibilities for qubit encodings. We show that all molecular qubit encodings can be classified into four classes by the type of the effective interaction between the qubits. In the case of polar molecules, the four classes are determined by the relative magnitudes of matrix elements of the dipole moment operator in the single-molecule basis. We exemplify our classification scheme by considering the encoding of the effective spin-1/2 system into nonadjacent rotational states (e.g., N = 0 and 2) of polar and nonpolar molecules with the same nuclear spin projection. Our classification scheme is designed to inform the optimal choice of molecular qubit encoding for quantum information storage and processing applications, as well as for dynamical generation of many-body entangled states and for quantum annealing.

Thin film aluminum nitride surface acoustic wave resonators for quantum acoustodynamics

Quantum excitations of macroscopic surface acoustic waves (SAWs) have been tailored to control, communicate, and transduce stationary and flying quantum states. However, the limited lifetime of these hybrid quantum systems remains critical obstacles to extend their applications in quantum information processing. Here, we present potentials of thin film aluminum nitride to on-chip integrated phonons with superconducting qubits over previous bulk piezoelectric substrates. We have reported high-quality thin film GHz-SAW resonators with the highest internal quality factor Qi of 4.92×104 in the quantum regime. The internal losses of SAW resonators are systematically investigated by tuning the parameters of sample layout, power, and temperature. Our results manifest that SAWs on piezoelectric films are readily integrated with standard fabrication of Josephson junction quantum circuits and offer excellent acoustic platforms for high-coherence quantum acoustodynamics architectures.

Fast Wide‐Field Quantum Sensor Based on Solid‐State Spins Integrated with a SPAD Array

AbstractAchieving fast, sensitive, and parallel measurement of a large number of quantum particles is an essential task in building large‐scale quantum platforms for different quantum information processing applications such as sensing, computation, simulation, and communication. Current quantum platforms in experimental atomic and optical physics based on CMOS sensors and charged coupled device cameras are limited by either low sensitivity or slow operational speed. Here an array of single‐photon avalanche diodes is integrated with solid‐state spin defects in diamond to build a fast wide‐field quantum sensor, achieving a frame rate up to 100 kHz. The design of the experimental setup to perform spatially resolved imaging of quantum systems is presented. A few exemplary applications, including sensing DC and AC magnetic fields, temperature, strain, local spin density, and charge dynamics, are experimentally demonstrated using a nitrogen‐vacancy ensemble diamond sample. The developed photon detection array is broadly applicable to other platforms such as atom arrays trapped in optical tweezers, optical lattices, donors in silicon, and rare earth ions in solids.

Top-down integration of an hBN quantum emitter in a monolithic photonic waveguide

Integrated quantum photonics, with potential applications in quantum information processing, relies on the integration of quantum emitters into on-chip photonic circuits. Hexagonal boron nitride (hBN) is recognized as a material that is compatible with such implementations, owing to its relatively high refractive index and low losses in the visible range, together with advantageous fabrication techniques. Here, we combine hBN waveguide nanofabrication with the recently demonstrated local generation of quantum emitters using electron irradiation to realize a fully top-down elementary quantum photonic circuit in this material, operating at room temperature. This proof of principle constitutes a first step toward deterministic quantum photonic circuits in hBN.

Continuous heralding control of vortex beams using quantum metasurface

Metasurfaces utilize engineered nanostructures to achieve control on all possible dimensions of light, encouraging versatile applications, including beam steering, multifunctional lensing, and multiplexed holograms. Towards the quantum optical regime for metasurfaces, although significant efforts have been put into generating and analyzing specific quantum states, control schemes to further manipulate these quantum states or information are still limited. Here, based on a metasurface, we propose and experimentally demonstrate a continuous heralding scheme to remotely control a vortex beam with high robustness to noise using polarization-entangled photon pairs. Our metasurface entangles polarization and orbital angular momentum (OAM) and the polarization selection on heralding photon erases the which-OAM information on signal photon. It induces an interference of two different OAM states remotely, manifesting a continuous orbital rotation. Our results show that metasurfaces have potential applications in quantum communication and information processing in entangling information with increasing complexity in the content.

Generating optical cat states via quantum interference of multi-path free-electron–photons interactions

The novel quantum effects induced by the free-electron–photons interaction have attracted increasing interest due to their potential applications in ultrafast quantum information processing. Here, we propose a scheme to generate optical cat states based on the quantum interference of multi-path free-electron–photons interactions that take place simultaneously with strong coupling strength. By performing a projection measurement on the electron, the state of light changes significantly from a coherent state into a non-Gaussian state with either Wigner negativity or squeezing property, both possess metrological power to achieve quantum advantage. More importantly, we show that the Wigner negativity oscillates with the coupling strength, and the optical cat states are successfully generated with high fidelity at all the oscillation peaks. This oscillation reveals the quantum interference effect of the multiple quantum pathways in the interaction of the electron with photons, by that various nonclassical states of light are promising to be fast prepared and manipulated. These findings inspire further exploration of emergent quantum phenomena and advanced quantum technologies with free electrons.

Simultaneous nonreciprocal conventional photon blockades of two independent optical modes by a two-level system

We propose a scheme to achieve nonreciprocal conventional photon blockades simultaneously in two independent optical modes, which are connected by a two-level system. In the case that only one optical mode is weakly driven, we find that strong nonreciprocal photon blockades of both optical modes can be observed. We show that, for both optical modes, the single-photon blockades happens by driving the nonlinear device from one side, while photon-induced tunneling appears when driving the system from the other side, which is attributed to the anharmonic eigenenergy spectrum constructed by resonantly coupling to a two-level system. According to photon resonance transition processes under different driving directions, the four optimal Fizeau-Sagnac shifts can be obtained to generate perfect nonreciprocal conventional photon blockades of both optical modes. Our study opens an avenue to simultaneously manipulate multiple nonreciprocal single-photon devices and may have potential applications in chiral quantum information processing.

Deterministic all-versus-nothing proofs of Bell nonlocality based on non-stabilizer states

The all-versus-nothing proof of Bell nonlocality is a prominent demonstration of Bell’s theorem without inequalities. There are two kinds of such proofs: the deterministic all-versus-nothing proof and the probabilistic all-versus-nothing proof, which have received extensive research attention. Traditionally, all previous deterministic all-versus-nothing proofs are constructed based on stabilizer states. However, this work presents new deterministic proofs derived from non-stabilizer states, thereby breaking away from this conventional approach. These novel results not only significantly broaden the range of demonstrations of Bell nonlocality without inequalities but also offer valuable resources for certain quantum information processing applications.

Discrete frequency-bin entanglement generation via cascaded second-order nonlinear processes in Sagnac interferometer.

Discrete frequency-bin entanglement is an essential resource for applications in quantum information processing. In this Letter, we propose and demonstrate a scheme to generate discrete frequency-bin entanglement with a single piece of periodically poled lithium niobate waveguide in a modified Sagnac interferometer. Correlated two-photon states in both directions of the Sagnac interferometer are generated through cascaded second-order optical nonlinear processes. A relative phase difference between the two states is introduced by changing the polarization state of pump light, thus manipulating the two-photon state at the output of the Sagnac interferometer. The generated two-photon state is sent into a fiber polarization splitter, and then a pure discrete frequency-bin entangled two-photon state is obtained by setting the pump light. The frequency entanglement property is measured by a spatial quantum beating with a visibility of 96.0±6.1%. The density matrix is further obtained with a fidelity of 98.0±3.0% to the ideal state. Our demonstration provides a promising method for the generation of pure discrete frequency-bin entanglement at the telecom band, which is desired in quantum photonics.

Electrically controlled nonreciprocity in a hybrid opto-electromechanical system

The nonreciprocity between two signal fields is regarded as a key function in future quantum networks and modern communication technologies. Here, we theoretically propose a scheme of nonreciprocal devices between signal fields in two different arbitrarily frequency domains in a hybrid cavity opto-electromechanical system. The model consists of a microwave cavity and an optical cavity, respectively, coupled with two different mechanical oscillators, which are coupled together by the tunable Coulomb interaction and driven by the external electrical fields. We study the nonreciprocal response between two different frequency fields. Nonreciprocal transmission is based on multichannel quantum interference to break time-reversal symmetry. The perfect nonreciprocity is shown in the certain conditions. By adjusting the Coulomb interaction, the phase differences, and strength of the electrically driven fields on the mechanical oscillators, we find that nonreciprocity can be modulated and even transformed into perfect nonreciprocity and reciprocity. These results provide a new insight into the design of nonreciprocal devices and present the potential applications in quantum information processing and quantum networks.

Triple Fano resonance-induced slow light in multiple-mode coupling nanomechanical resonators

We propose a multiple-mechanical-mode-coupling nanomechanical system comprising three nanomechanical resonators (NRs) with different frequencies and each two NRs able to interact with each other. By weaving different NRs and controlling the coupling of NRs, the nanomechanical system can be series-, parallel-, or network-coupled nanoresonators network. Using two-tone fields to drive quantum dot implanted in one NR, we investigate the coherence optical responses and absorption spectrum manifested as a triple Fano resonance. Furthermore, each Fano resonance peak of the triple Fano resonance that induced the coherent optical propagation were demonstrated. The numerical results indicate that the slow light and the slow-to-fast light transition can be obtained by manipulating the interaction of NRs. These findings imply that the multiple-mechanical-mode coupling system can be a promising integration platform to achieve chip-scale applications in quantum information processing.

Experimental 61-partite entanglement on a three-dimensional photonic chip.

Multipartite entanglements are essential resources for proceeding tasks in quantum information science and technology. However, generating and verifying them present significant challenges, such as the stringent requirements for manipulations and the need for a huge number of building-blocks as the systems scale up. Here, we propose and experimentally demonstrate the heralded multipartite entanglements on a three-dimensional photonic chip. Integrated photonics provide a physically scalable way to achieve an extensive and adjustable architecture. Through sophisticated Hamiltonian engineering, we are able to control the coherent evolution of shared single photon in the multiple spatial modes, dynamically tuning the induced high-order W-states of different orders in a single photonic chip. Using an effective witness, we successfully observe and verify 61-partite quantum entanglements in a 121-site photonic lattice. Our results, together with the single-site-addressable platform, offer new insights into the accessible size of quantum entanglements and may facilitate the developments of large-scale quantum information processing applications.

Photoionisation detection of a single Er3+ ion with sub-100-ns time resolution.

Efficient detection of single optical centres in solids is essential for quantum information processing, sensing and single-photon generation applications. In this work, we use radio-frequency (RF) reflectometry to electrically detect the photoionisation induced by a single Er3+ ion in Si. The high bandwidth and sensitivity of the RF reflectometry provide sub-100-ns time resolution for the photoionisation detection. With this technique, the optically excited state lifetime of a single Er3+ ion in a Si nano-transistor is measured for the first time to be [Formula: see text]s. Our results demonstrate an efficient approach for detecting a charge state change induced by Er excitation and relaxation. This approach could be used for fast readout of other single optical centres in solids and is attractive for large-scale integrated optical quantum systems thanks to the multi-channel RF reflectometry demonstrated with frequency multiplexing techniques.

Quantum Nondemolition Measurement of the Spin Precession of Laser-Trapped 171Yb Atoms

Quantum nondemolition (QND) measurement enhances the detection efficiency and measurement fidelity, and is highly desired for its applications in precision measurements and quantum information processing. We propose and demonstrate a QND measurement scheme for the spin states of laser-trapped atoms. On ${}^{171}\mathrm{Yb}$ atoms held in an optical dipole trap, a transition that is simultaneously cycling, spin-selective, and spin-preserving is created by introducing a circularly polarized beam of a control laser to optically dress the spin states in the excited level, while leaving the spin states in the ground level unperturbed. We measure the phase of spin precession of $5\ifmmode\times\else\texttimes\fi{}{10}^{4}$ atoms in a bias magnetic field of 20 mG. This QND approach reduces the optical absorption detection noise by $\ensuremath{\sim}19\phantom{\rule{0.2em}{0ex}}\mathrm{dB}$, to an inferred level of 2.3 dB below the atomic quantum projection noise. In addition to providing a general approach for efficient spin-state readout, this all-optical technique allows quick switching and real-time programming for quantum sensing and quantum information processing.

100 GHz bandwidth, 1 volt integrated electro-optic Mach–Zehnder modulator at near-IR wavelengths

Integrated photonics at near-IR (NIR) wavelengths currently lacks high bandwidth and low-voltage modulators, which add electro-optic functionality to passive circuits. Here, integrated hybrid thin-film lithium niobate (TFLN) electro-optic Mach–Zehnder modulators (MZM) are shown, using TFLN bonded to planarized silicon nitride waveguides. The design does not require TFLN etching or patterning. The push–pull MZM achieves a half-wave voltage length product (V π L) of 0.8 V.cm at 784 nm. MZM devices with 0.4 cm and 0.8 cm modulation length show a broadband electro-optic response with a 3 dB bandwidth beyond 100 GHz, with the latter showing a record bandwidth to half-wave voltage ratio of 100 GHz/V and a high extinction ratio exceeding 30 dB. Such fully integrated high-performance NIR electro-optic devices may benefit data communications, analog signal processing, test and measurement instrumentation, quantum information processing and other applications.

Schrödinger cat states of a 16-microgram mechanical oscillator

According to quantum mechanics, a physical system can be in any linear superposition of its possible states. Although the validity of this principle is routinely validated for microscopic systems, it is still unclear why we do not observe macroscopic objects to be in superpositions of states that can be distinguished by some classical property. Here we demonstrate the preparation of a mechanical resonator in Schrödinger cat states of motion, where the ∼1017 constituent atoms are in a superposition of two opposite-phase oscillations. We control the size and phase of the superpositions and investigate their decoherence dynamics. Our results offer the possibility of exploring the boundary between the quantum and classical worlds and may find applications in continuous-variable quantum information processing and metrology with mechanical resonators.