We show that the joint behaviour of an arbitrary pair of quantum observablescan be described by quasi-probabilities, which are extensions of the standardprobabilities used for describing the behaviour of a single observable. Thephysical situations that require these quasi-probabilities arise when oneconsiders quantum measurement of an observable conditioned by some othervariable, with the notable example being the weak measurement employed toobtain Aharonov's weak value. Specifically, we present a general prescriptionfor the construction of quasi-joint-probability (QJP) distributions associatedwith a given pair of observables. These QJP distributions are introduced in twocomplementary approaches: one from a bottom-up, strictly operationalconstruction realised by examining the mathematical framework of theconditioned measurement scheme, and the other from a top-down viewpointrealised by applying the results of spectral theorem for normal operators andits Fourier transforms. It is then revealed that, for a pair of simultaneouslymeasurable observables, the QJP distribution reduces to their unique standardjoint-probability distribution, whereas for a non-commuting pair there existsan inherent indefiniteness in the choice, admitting a multitude of candidatesthat may equally be used for describing their joint behaviour. In the course ofour argument, we find that the QJP distributions furnish the space of operatorswith their characteristic geometric structures such that the orthogonalprojections and inner products of observables can, respectively, be givenstatistical interpretations as `conditionings' and `correlations'. The weakvalue $A_{w}$ for an observable $A$ is then given a geometric/statisticalinterpretation as either the orthogonal projection of $A$ onto the subspacegenerated by another observable $B$, or equivalently, as the conditioning of$A$ given $B$.
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机译:我们表明,可以通过准概率来描述任意一对量子可观察物的联合行为,准概率是准概率的扩展,用于描述单个可观察物的行为。当人们考虑以某个其他变量为条件的可观察物的量子测量时,就会出现需要这些准概率的物理情况,其中最显着的例子是用于获得阿哈罗诺夫的弱值的弱测量。具体来说,我们为构造与给定可观测量对相关的准联合概率(QJP)分布提供了一般处方。这些QJP分布以两种互补的方式引入:一种是通过自下而上的严格操作构造,通过检查条件测量方案的数学框架来实现,另一种是通过自上而下的观点通过将谱定理的结果应用于正常算子及其傅里叶来实现的。转换。然后揭示出,对于一对同时可测量的可观测物,QJP分布减小到其唯一的标准联合概率分布,而对于非上下班对,则选择中存在固有的不确定性,从而允许大量可同等地用于描述他们的共同行为。在我们的论证过程中,我们发现QJP分布为其算子的空间提供了其特征性的几何结构,从而可以分别将可观测量的正交投影和内积作为“条件”和“相关性”进行统计解释。然后,将可观察的$ A $的弱值$ A_ {w} $进行几何/统计解释,或者是$ A $在另一个可观察的$ B $生成的子空间上的正交投影,或者等效地,作为$ A $的条件给定$ B $。
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