Heat-bath algorithmic cooling (HBAC) has been proven to be a powerful and effective method for obtaining high polarization of the target system. Its cooling upper bound has been recently found using a specific algorithm, the partner pairing algorithm (PPA-HBAC). It has been shown that by including cross-relaxation, it is possible to surpass the cooling bounds. Herein, by combining cross-relaxation and decoherence-free subspace, we present a two-qubit reset sequence and then generate a new algorithmic cooling (AC) technique using irreversible polarization compression to further surpass the bound. The proposed two-qubit reset sequence can prepare one of the two qubits to four times the polarization of a single-qubit reset operation in PPA-HBAC for low polarization. When the qubit number is large, the cooling limit of the proposed AC is approximately five times as high as the PPA-HBAC. The results reveal that cross-relaxation and decoherence-free subspace are promising resources to create new AC for higher polarization.

中文翻译：

---

基于交叉松弛和无相干子空间的算法冷却

热浴算法冷却（HBAC）已被证明是获得目标系统高度极化的有效方法。最近，使用特定算法（伙伴配对算法（PPA-HBAC））发现了其冷却上限。已经表明，通过包括交叉松弛，可以超过冷却界限。在本文中，通过组合交叉松弛和无退相干子空间，我们提出了一个两个量子比特的重置序列，然后使用不可逆极化压缩生成新的算法冷却（AC）技术，以进一步超越界限。所提出的两个qubit复位序列可以准备两个qubit中的一个，使其极化强度达到PPA-HBAC中单个qubit复位操作的四倍，从而实现低极化。当量子位数很大时 建议的AC的冷却极限大约是PPA-HBAC的五倍。结果表明，交叉松弛和无退相干子空间是有希望的资源，可用于创建更高极化的新AC。