image: a Conceptual illustration of the D2NN quantum gate’s abstract architecture, showing how the neural networks map quantum states in high-dimensional Hilbert space to other states in that space. The specific three-dimensional X gate scheme is shown in the lower part as an example, including input states, multilayer D2NN, and output states. The mapping between inputs and outputs is indicated on the side of the D2NN, with the currently active mapping highlighted in yellow. b Experimental setup of a 4-layers spatial mode quantum gate. c A heralded single photon source is used. FPC: fiber polarization controller; Col.: collimator; SLM: spatial light modulator; L1-L6: lenses; BS: beam splitter; Cam.: camera; SPD1&2: single photon detector; C.C.: coincidence counting; PPLN1&2: periodically poled lithium niobate; SHG: second harmonic generation; SPDC: spontaneous parametric down-conversion; BPF: bandpass filter. While the heralding photons of the photon pairs are sent to the SPD1, the heralded photons are prepared into input states by complex modulation using SLM1, followed by D2NN-generated phase layers loaded onto SLM2 to perform the desired operation. Output states are analyzed using SLM3, a fiber coupling system, and the SPD2. The coincidence counting of SPD1&2 indicates the single photon passing the entire system. d Second-order correlation function g2(τ) of the heralded single photon source determined using the Hanbury Brown and Twiss setup. The g2(0) =0.024(2) implies the remarkable performance of the source.
Credit: by Qianke Wang, Jun Liu, Dawei Lyu, Jian Wang
Quantum computing holds the promise of transforming our information processing methodologies, and at its core, reliable quantum logic gates play an essential role in quantum information processing. While various types of quantum gates have been demonstrated, photonic quantum gates are naturally compatible with quantum communications and have attracted considerable interest in the field of quantum information. The intrinsic infinity of orthogonal bases in the spatial modes of photons offers an extensive coding alphabet, encouraging creativity in high-dimensional quantum information processing. However, achieving high-dimensional unitary transformations in an accurate, scalable and compact way with ultrahigh fidelities remains a significant challenge.
In a new paper published in Light Science & Application, a team of scientists, led by Professor Jian Wang from Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology, China, Optics Valley Laboratory, China, and co-workers have demonstrated the use of deep diffractive neural networks (D2NNs) to construct a series of high-dimensional quantum gates, which are encoded by the spatial modes of photons. They implemented all three-dimensional X gates and Hadamard gates encoded by three Laguerre-Gaussian modes. The gates exhibit ultrahigh fidelities up to 99.4(3)%, as characterized through quantum process tomography. They also adopt a unique coding method to encode two bits of information, utilizing four orbital angular momentum (OAM) modes of a single photon. With this method, they achieved the interchange of OAM’s wave-front rotation direction (the sign of the mode) according to their mode orders. The reconstructed process matrix of this controlled-NOT gate has a fidelity of 99.6(2)%, and this high-fidelity gate enables reliable quantum computations.
They also demonstrated the applicability of this approach by successfully implementing the Deutsch algorithm, which involves performing the whole 2-qubit quantum circuit based on their experimental configuration. This demonstration validates the potential of performing complex operations or even quantum circuits.
The experimentally demonstrations of all previously mentioned gates display the advantages of small footprint, great scalability and robustness to different mode bases. Moreover, based on the reconfigurable phase modulation device, this implementation is conducive to intelligent deployment, which shows extraordinary talents in performing automatic protocols to realize desired operations or to optimize the experimental performance.
To provide guidelines for experiments, they analyzed the relationship between quantum gate performance and various parameters, including loss and characteristics of the spatial light modulator. Additionally, they conducted a comparative analysis of the D2NN gate's performance to the traditional wave-front matching approach, leading to the conclusion that our approach significantly improves visibility at a small cost of energy loss.