量子气体中的自旋向列压缩真空
Spin-Nematic Squeezed Vacuum in a Quantum Gas
摘要：
利用压缩态，可以通过牺牲另一个互补性质来降低一个性质的测量不确定性，从而超过测量不确定性的标准量子极限。压缩态首先在光场中得到证实，后来又在赝自旋1/2原子系综中利用非线性原子-光相互作用得到证实。最近，超冷原子气体中的碰撞相互作用被用来在两组分玻色凝聚体中产生较大程度的正交自旋压缩。对于赝自旋为1/2的系统，其互补性质是总自旋矢量的不同分量<S>，它们充分表征了SU（2）Bloch球上的态。在这里，我们测量了自旋为1的玻色凝聚体（SU（3）系统）中的压缩，这需要测量秩为2的四极张量或四极张量<Q_ij>来充分表征状态。在通过铁磁到铁磁量子相变的淬火之后，在低于标准量子极限的高达-8.3（-0.7 +0.6）dB（针对检测噪声校正的-10.3（-0.9 +0.7）dB）的正交方差中观察到压缩。这种自旋压缩被观察到的压缩模式的占用可以忽略不计，类似于光学双模真空压缩。这项工作在连续变量量子信息和量子增强磁测量中具有潜在的应用价值。
英文摘要：
Using squeezed states it is possible to surpass the standard quantum limit of measurement uncertainty by reducing the measurement uncertainty of one property at the expense of another complementary property. Squeezed states were first demonstrated in optical fields and later with ensembles of pseudo spin-1/2 atoms using non-linear atom-light interactions. Recently, collisional interactions in ultracold atomic gases have been used to generate a large degree of quadrature spin squeezing in two-component Bose condensates. For pseudo spin-1/2 systems, the complementary properties are the different components of the total spin vector <S>, which fully characterize the state on an SU(2) Bloch sphere. Here, we measure squeezing in a spin-1 Bose condensate, an SU(3) system, which requires measurement of the rank-2 nematic or quadrupole tensor <Q_ij> as well to fully characterize the state. Following a quench through a nematic to ferromagnetic quantum phase transition, squeezing is observed in the variance of the quadratures up to -8.3(-0.7 +0.6) dB (-10.3(-0.9 +0.7) dB corrected for detection noise) below the standard quantum limit. This spin-nematic squeezing is observed for negligible occupation of the squeezed modes and is analogous to optical two-mode vacuum squeezing. This work has potential applications to continuous variable quantum information and quantum-enhanced magnetometry.
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