Polarization and Orbital Angular Momentum Encoded Quantum Toffoli Gate Enabled by Diffractive Neural Networks.

The development of noisy intermediate-scale quantum (NISQ) devices has extended the scope of executable quantum circuits with high-fidelity single- and two-qubit gates. Equipping NISQ devices with three-qubit gates will enable the realization of more complex quantum algorithms and efficient quantum error correction protocols with reduced circuit depth. Several three-qubit gates have been implemented for superconducting qubits, but their use in gate synthesis has been limited due to their low fidelity. Here, using fixed-frequency superconducting qubits, we demonstrate a high-fidelity iToffoli gate based on two-qubit interactions, the so-called cross-resonance effect. As with the Toffoli gate, this three-qubit gate can be used to perform universal quantum computation. The iToffoli gate is implemented by simultaneously applying microwave pulses to a linear chain of three qubits, revealing a process fidelity as high as 98.26(2)%. Moreover, we numerically show that our gate scheme can produce additional three-qubit gates which provide more efficient gate synthesis than the Toffoli and iToffoli gates. Our work not only brings a high-fidelity iToffoli gate to current superconducting quantum processors but also opens a pathway for developing multi-qubit gates based on two-qubit interactions.

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.

Accurate detection of orbital angular momentum (OAM) is one of the critical techniques for OAM‐related communications. However, conventional OAM identification approaches are often complicated and costly. In this letter, a convenient method based on diffraction neural network (DNN) is presented for OAM states detection in the sub‐terahertz (sub‐THz) regime. The optimal phase distribution of the lens for OAM state detection is obtained from DNN training. Then, a prototype based on three‐dimensional printing is fabricated according to the training results. Numerical simulation and experiment are performed at 140 GHz to verify the proposed approach. The results show that the proposed method can accurately detect the OAM state with lower complexity. This convenient detection approach can provide a novel pathway for realizing low‐cost THz OAM receiving systems, promoting the development of future communication technology.

Unlike classical digital computers in which a bit can represent either 1 or 0 at any time, quantum computers use a two-level system, i.e., a qubit, to implement logical operations based on quantum mechanical laws, which can represent both values at once. Owing to the superposition property of qubits, quantum computers have natural parallel processing advantages and thus have potential to exceed the computational efficiency of classical computers for particular tasks. Quantum logic gates are the generalization of classical logic gates in computational networks. It has been proved that two-qubit quantum gates together with one-qubit quantum gates are adequate for constructing networks with any possible quantum computational property. Directional couplers are the most critical elementsfor constructing the quantum gates. In recent years, photonic quantum technologies have emerged as a promising experimental platform for quantum computing. Single photons have robust noise resistance, long coherence time, high transmission speed and great compatibility with other systems. They can be easily manipulated and encoded in any of several degrees of freedom, for example, polarization, path, spatial mode or time bin. Optical waveguide technology enables the realizing of complex optical schemes comprised of many elements with desired scalability, stability and miniaturization. Femtosecond laser direct writing of waveguide has been adopted as a powerful tool for integrated quantum photonics with characteristics of rapidness, cost-effectiveness, mask-less and single-step process. In particular, it has the ability to build arbitrary three-dimensional circuits directly inside bulk materials, which is impossible to achieve with conventional lithography. In this article we review the femtosecond laser writing and quantum characterization of directional coupler and important one-qubit and two-qubit optical quantum logic gates, such as Hadamard gate, Pauli-X gate, controlled-NOT gate, and controlled-Phase gate. The qubits in these gates are usually encoded through optical paths or polarizations of photons. The key to the realization of polarization-encoded one-qubit gates is to achieve flexible wave-plate operations, which is described in detail. Controlled-NOT gate and controlled-phase gate are the most crucial two-qubit gates in the linear optics computation and sometimes they can be converted into each other by adding some one-qubit gates or special superposition states. Many different kinds of waveguide circuits have been used to implement these two-qubit gates. The outlook and challenges for the femtosecond laser writing of three-qubit gates, such as Toffoli gate and Fredkin gate, are briefly introduced.

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.

Heat dissipation is one of the major problems in the construction of electronic devices. Reversible computing is one of the emerging computing paradigms to overcome heat dissipation problem. One of the nanoscale devices that has low power consumption and can be used to construct digital logic components is quantum-dot cellular automata (QCA). In this paper, the benefits of reversible computing of QCA will be used to propose QCA designs for two main reversible gates, the CNOT and the TOFFOLI gates. The proposed CNOT gate has been used to design QCA for double CNOT gate to show its reversibility, and QCA design for the SWAP circuit that contains 3 CNOT gates. The suggested TOFFOLI gate is utilized to construct QCA for double TOFFOLI gate and a reversible circuit which consists of several reversible gates. The suggested QCA designs for double CNOT, double TOFFOLI and reversible circuits such as SWAP circuit have been proposed by adding cells with certain clocks to the wires that connects the proposed gates. The proposed QCA designs show better cell count, area used in the construction, number of majority gate, wire crossover, and/or clock cycle delay when compared with relevant designs in literature.

We designed and fabricated a silicon photonic orbital angular momentum (OAM) multiplexer (MUX) chip that enables simultaneous wavelength-division multiplexing (WDM) and OAM mode-division multiplexing (MDM) in optical fiber communication systems. We successfully utilized the fabricated MUX chip to demonstrate simultaneous 3 OAM MDM transmission as well as OAM MDM/WDM transmission with an 800-m OAM ring-core fiber. In both cases, 10 Gbit/s OOK signals were utilized and BERs below the 7% FEC limit were obtained.

We generated orbital angular momentum in atoms with a spin degree of freedom and coherently transferred it to a light field, which resulted in the generation of a Laguerre-Gaussian (LG) beam. In this method, atoms obtain orbital angular momentum through Larmor precession around the quadrupole field. Successive operations of quadrupole and homogeneous magnetic fields lead to a change of sign in the LG beam or even generation of superposition states of the LG beam with different indexes in real time. Possible applications to control orbital angular momentum of arbitrary photons are also discussed.

Orbital Angular Momentum (OAM) of photons are ubiquitously used for numerous applications. However, there is a fundamental question whether photonic OAM operators satisfy standard quantum mechanical commutation relationship or not; this also poses a serious concern on the interpretation of an optical vortex as a fundamental quantum degree of freedom. Here, we examined canonical angular momentum operators defined in cylindrical coordinates, and applied them to Laguerre-Gauss (LG) modes in a graded index (GRIN) fibre. We confirmed the validity of commutation relationship for the LG modes and found that ladder operators also work properly with the increment or the decrement in units of the Dirac constant (ℏ). With those operators, we calculated the quantum-mechanical expectation value of the magnitude of angular momentum, which includes contributions from both intrinsic and extrinsic OAM. The obtained results suggest that OAM characterised by the LG modes exhibits a well-defined quantum degree of freedom.

In the context of orbital angular momentum (OAM) associated optical chirality, the involvement of photonic spin angular momentum and higher-order transition moments has long been regarded as essential. To date, the existence of helical dichroism that is exclusively determined by OAM remains elusive. Through the excitation of plasmonic surface lattice resonances (SLRs), we report a polarization-independent helical dichroism without the engagement of electric quadrupole moments, a scenario previously deemed implausible. Using a modified dipole sum under rotational symmetry, the theoretical framework of SLRs is successfully generalized to plasmonic helices. Our results reveal that the arbitrary plasmonic helix can be structurally optimized to exhibit both narrowband SLR peaks and polarization-independent helical dichroism. Notably, SLRs excitation can simultaneously generate tightly confined near-fields and collimated far-field radiation, which facilitate the OAM transfer in the chiral interaction and directional OAM projection, respectively. This work may clarify the intrinsic physics governing OAM-mediated chirality, potentially enabling advances in enantioselective detection and vortex-enhanced spectroscopy.

Inspired by topological insulators, topological photonics has attracted the attention of researchers since its discovery. Many phenomena like topological state bring numerous new opportunities to the entire photonics. Previous studies have proved that topological systems can protect photonic orbital angular momentum (OAM). In this work, two Su–Schrieffer–Heeger (SSH) waveguide chains are combined, and the interface waveguide is designed to generate next‐nearest‐neighbor coupling (NNC) between itself and the waveguides in the SSH chains. It is discovered that with NNC, more modes are topologically protected. Laguerre‐Gaussian (LG) modes are typical OAM light beams, which are composed of several Hermite‐Gaussian (HG) modes. With NNC and special design of the waveguide, which is located at the interface, it is proven that SSH chains can protect more OAM modes. This model is named the NNC‐SSH model. Thus, this study expands the field of topological photonics and presents a new idea of topological protection, which has a very high potential in many fields such as OAM multiplexing.

We propose an efficiently measurable lower bound on quantum process fidelity of N-qubit controlled-Z gates. This bound is determined by average output state fidelities for N partially conjugate product bases. A distinct advantage of our approach is that only fidelities with product states need to be measured while keeping the total number of measurements much smaller than what is necessary for full quantum process tomography. As an application, we use this method to experimentally estimate quantum process fidelity F of a three-qubit linear optical quantum Toffoli gate and we find that F≥0.83. We also demonstrate the entangling capability of the gate by preparing Greenberger-Horne-Zeilinger-type three-qubit entangled states from input product states.

Through scanned coincidence counting, we probe the quantum image produced by parametric down-conversion with a pump-beam carrying orbital angular momentum. Nonlocal spatial correlations are manifested through splitting of the coincidence spot into two.

High-dimensional entanglement is a critical foundation for the growing demand for information capacity to implement the high-capacity quantum task. Here, we report continuous-variable high-dimensional entanglement with three degrees of freedom (frequency, polarization, and orbital angular momentum) directly generated with a single type-II optical parametric oscillator (OPO) cavity. By compensating both for dispersion in frequency modes and astigmatism in higher-order transverse modes, the OPO is capable of oscillating simultaneously and outputting thousands of entanglement pairs. The three degrees of freedom high-dimensional entanglement are verified simultaneously possessing frequency comb, spin, and orbital angular momentum entanglement via 14 pairs of Hermite–Gaussian mode correlations measurement. Then, the “space-frequency” multiplexing quantum dense coding communication is also demonstrated by using the entanglement resource. It shows the great superiority of high-dimensional entanglement in implementing the high-capacity quantum task. Apart from an increased channel capacity, it is possible to conduct deterministic high-dimensional quantum protocols, quantum imaging, and especially quantum computing.

Photonic spin texture (PST), the spatial distribution of the spin angular momentum (SAM) of light, is connected to unique properties of light, such as optical skyrmions and topological optical N-invariants. There has been recent progress on the generation and manipulation of PST using various methodologies. However, a challenge remains for the sub-wavelength characterization of PST. Here, we demonstrate nitrogen-vacancy (NV) centers in diamond as nanoscale quantum sensors for imaging the PST of a beam with orbital angular momentum (OAM). Leveraging the coherent interaction between photon spin and NV center electron spin at cryogenic temperature (77 K), and using the Hahn-Echo magnetometry technique, we experimentally demonstrate the imprinting of the PST on the quantum phase shift of NV centers. Our work can lead to the development of a quantum imaging platform capable of characterization of the spin texture of light at sub-wavelength scales.