Quantum integrated photonics requires large-scale linear optical circuitry, and for many applications, it is desirable to have a universally programmable circuit, able to implement an arbitrary unitary transformation on a number of modes. This has been achieved using the Reck scheme, consisting of a network of Mach–Zehnder interferometers containing a variable phase shifter in one path as well as an external phase shifter after each Mach–Zehnder. It subsequently became apparent that with symmetric Mach–Zehnders containing a phase shifter in both paths, the external phase shifters are redundant, resulting in a more compact circuit. The rectangular Clements scheme improves on the Reck scheme in terms of circuit depth, but it has been thought that an external phase-shifter was necessary after each Mach–Zehnder. Here, we show that the Clements scheme can be realized using symmetric Mach–Zehnders, requiring only a small number of external phase-shifters that do not contribute to the depth of the circuit. This will result in a significant saving in the length of these devices, allowing more complex circuits to fit onto a photonic chip, and reducing the propagation losses associated with these circuits. We also discuss how similar savings can be made to alternative schemes, which have robustness to imbalanced beam-splitters.

中文翻译：

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进一步紧凑化线性光学单元

量子集成光子学需要大规模的线性光学电路，对于许多应用，希望有一个通用可编程电路，能够在多种模式上实现任意单一变换。这是使用 Reck 方案实现的，该方案由一个马赫-曾德干涉仪网络组成，该网络包含一个路径中的可变移相器以及每个马赫-曾德之后的外部移相器。随后很明显，对称的 Mach-Zehnders 在两条路径中都包含一个移相器，外部移相器是多余的，从而使电路更加紧凑。矩形 Clements 方案在电路深度方面改进了 Reck 方案，但人们认为在每个 Mach-Zehnder 之后都需要一个外部移相器。这里，我们展示了 Clements 方案可以使用对称 Mach-Zehnders 实现，只需要少量对电路深度没有贡献的外部移相器。这将显着节省这些设备的长度，允许将更复杂的电路安装到光子芯片上，并减少与这些电路相关的传播损耗。我们还讨论了如何对替代方案进行类似的节省，这些方案对不平衡的分束器具有鲁棒性。并减少与这些电路相关的传播损耗。我们还讨论了如何对替代方案进行类似的节省，这些方案对不平衡的分束器具有鲁棒性。并减少与这些电路相关的传播损耗。我们还讨论了如何对替代方案进行类似的节省，这些方案对不平衡的分束器具有鲁棒性。