Statistical Analysis of Quantum Entanglement

(This section is an overview of material which is available on the arXiv.)

Quantum mechanics is an inherently probabilistic theory of the physical world. This is perhaps the fundamental weirdness of the theory, which made it so hard to swallow for established giants of the field, such as Albert Einstein.¹ Another aspect of quantum mechanics that made many physicists uncomfortable is the phenomena of entanglement, in which actions taken upon one particle in an entangled pair (including something as seemingly innocuous as measuring the position of the particle) can effect the entangled partner instantly, regardless of the separation between the two.²

Measuring Entanglement

For the first decades of quantum mechanics, the theory enjoyed immense success in this quantitatively explaining phenomena which made no sense under the old-fashioned “classical” theory. However, the so-called “action at a distance” of entangled particles was not thought to be directly observable, and so it was relegated to philosophical discussions. But then in the early 1960s, a physicist named John Bell theorized a property of two entangled particles that one could measure which would have two different values depending on whether the particles were exhibiting action at a distance or not. A full explanation of Bell’s theorem is far beyond the scope of this article, but suffice to say that entanglement was now measurable in principle.

In the 1970s, experimentalists began conducting tests based upon Bell’s theorem, which are generally referred to as Bell tests. Refining these tests continues to be an active area of research; the centrality of the issue being tested means that it is important that all theoretical loopholes be closed.

Uncertainty in Measurement

Scientific experiments do not give a simple yes or no answer to the question at hand; the uncertainty inherent in experimental design and implementation means that we can gather evidence, sometimes overwhelming evidence, for a hypothesis, but we can never be perfectly sure that it is correct. With an issue as central as that being examined by Bell tests, the scientific community wishes to have an extreme degree of certainty regarding the outcome of any given experiment. The classic \(p\)-value bound of 0.05 is insufficient; physicists often look for \(p\)-values of \(10^{-6}\) and below in Bell tests. A recent experiment even measured a \(p\)-value on the order of \(10^{-16}\)!

Clearly, when making such drastic statistical claims, it is essential that the statistical methodology involved in the calculation must be sound. Unfortunately, many phsyicists employ statistical techniques which careful mathematics prove to be invalid. In particular, suppose we are measuring a quantity \(S\), and we have a null hypothesis that \(S\leq 2\) which we intend to disprove. A scientists might measure many samples of \(S\), and then calculate a sample mean and standard deviation, and measure how many sample deviations the mean is away from 2. If we measure \(S = 10 \pm 1\), then we are 8 standard deviations away from the null hypothesis, which seems to be strong evidence against the null hypothesis.

But how strong is this evidence? In introductory statistics courses, one often learns \(p\)-values associated with a given number of standard deviations: one standard deviation is a \(p\)-value of about 0.32, 2 standard deviations is about \(p=0.05\), and so on. This is implicitly assuming that the underlying distribution is Gaussian, an assumption seemingly supported by the famous central limit theorem. However, a mathematical subtlety in the CLT renders this assumption invalid when one measures increasingly low \(p\)-values.³

Building New Methods

This problem was the focus of my time working as a research assistant at the National Institute of Standards and Technology. The group I joined had previously developed a tool that provided a statistically valid method for calculating \(p\)-values in Bell tests, and had proven some results regarding the method’s efficiency. The method is based on a statistical tool called test supermartingales, and so I will use this term to refer to the method as a whole from now on. I won’t get in to an explanation of the method here; it’s quite technical, although those with an interest in probability and statistics may find its simplicity quite surprising. A thorough description can be found on the pre-print of our paper, which is available on the arXiv.

In my year or so working with the group, we wrote a paper in which we perform a thorough analysis of the efficacy of test supermartingales when applied to a very simple situation, in which one is trying to determine the bias of an unfair coin by observing a sequence of flips. The advantage of this setting is that the probabilistic setting is simple enough that many of our quantities of interest can be calculated directly; furthermore, for a given sequence of flips, we know the exact \(p\)-value, and so can compare the test margtingale \(p\)-value to this “optimal” method.

What we learn gives us optimism that our approach will be experimentally useful; the method is shown to be very near optimal, but enjoys many advantages over traditional methods. In particular, for most staitstical methods one must decide beforehand how long to perform the experiment, and cannot simply wait until the \(p\)-value is as low as desired. However, the use of test martingales allows for so-called “arbitrary stopping,” so that \(p\)-value monitoring does not invalidate the calculated \(p\)-value.

Moving forward, we hope to see test martingales employed in the myriad new Bell tests being performed each year. We also hope to continue to prove results on the performance of the method in more complicated settings, so that we can further bridge the gap between the tidy scenarios used for theoretical analysis, and the messy, complicated situations in which these methods are actually employed.

1. Einstein famously declared that “God does not play dice with the universe,” which is to say that no physical theory can be at its base probabilistic. ↩

2. This seems to violate the laws of general relativity, which prohibit faster-than-light travel. This paradox is subtle, but at a gross level the resolution is that the random nature of the effects prevents one from sending any sort of information via the entangled pair. ↩

3. Essentially, the issue is that convergence is guaranteed if one fixes the fraction of the distribution we are measuring. This type of convergence is called convergence in distribution. In Bell tests, however, the \(p\)-value gets lower as the experiment continues, which is to say we’re moving further out into the tail of the Gaussian distribution, and so no such convergence guarantee exists. In fact, it can be shown that in many cases a Gaussian assumption gives wildly incorrect results. ↩