New papers dance!

Two new papers were recently posted on the arXiv with my first two official PhD students since becoming a faculty member! The earlier paper is titled Efficient online quantum state estimation using a matrix-exponentiated gradient method by Akram Youssry and the more recent paper is Minimax quantum state estimation under Bregman divergence by Maria Quadeer. Both papers are co-authored by Marco Tomamichel and are on the topic of quantum tomography. If you want an expert’s summary of each, look no further than the abstracts. Here, I want to give a slightly more popular summary of the work.

Efficient online quantum state estimation using a matrix-exponentiated gradient method

This work is about a practical algorithm for online quantum tomography. Let’s unpack that. First, the term work. Akram did most of that. Algorithm can be understood to be synonymous with method or approach. It’s just a way, among many possibilities, to do a thing. The thing is called quantum tomography. It’s online because it works on-the-fly as opposed to after-the-fact.

Quantum tomography refers to the problem of assigning a description to physical system that is consistent with the laws of quantum physics. The context of the problem is one of data analysis. It is assumed that experiments on this to-be-determine physical system will be made and the results of measurements are all that will be available. From those measurement results, one needs to assign a mathematical object to the physical system, called the quantum state. So, another phrase for quantum tomography is quantum state estimation.

The laws of quantum physics are painfully abstract and tricky to deal with. Usually, then, quantum state estimation proceeds in two steps: first, get a crude idea of what’s going on, and then find something nearby which satisfies the quantum constraints. The new method we propose automatically satisfies the quantum constraints and is thus more efficient. Akram proved this and performed many simulations of the algorithm doing its thing.

Minimax quantum state estimation under Bregman divergence

This work is more theoretical. You might call it mathematical quantum statistics… quantum mathematical statistics? It doesn’t yet have a name. Anyway, it definitely has those three things in it. The topic is quantum tomography again, but the focus is different. Whereas for the above paper the problem was to devise an algorithm that works fast, the goal here was to understand what the best algorithm can achieve (independent of how fast it might be).

Work along these lines in the past considered a single figure of merit, the thing the defines what “best” means. In this work Maria looked at general figures of merit called Bregman divergences. She proved several theorems about the optimal algorithm and the optimal measurement strategy. For the smallest quantum system, a qubit, a complete answer was worked out in concrete detail.

Both Maria and Akram are presenting their work next week at AQIS 2018 in Nagoya, Japan.

Estimation… with quantum technology… using machine learning… on the blockchain

A snarky academic joke which might actually be interesting (but still a snarky joke).

Abstract

A device verification protocol using quantum technology, machine learning, and blockchain is outlined. The self-learning protocol, SKYNET, uses quantum resources to adaptively come to know itself. The data integrity is guaranteed with blockchain technology using the FelixBlochChain.

Introduction

You may have a problem. Maybe you’re interested in leveraging the new economy to maximize your B2B ROI in the mission-critical logistic sector. Maybe, like some of the administration at an unnamed university, you like to annoy your faculty with bullshit about innovation mindshare in the enterprise market. Or, maybe like me, you’d like to solve the problem of verifying the operation of a physical device. Whatever your problem, you know about the new tech hype: quantum, machine learning, and blockchain. Could one of these solve your problem? Could you really impress your boss by suggesting the use of one of these buzzwords? Yes. Yes, you can.

Here I will solve my problem using all the hype. This is the ultimate evolution of disruptive tech. Synergy of quantum and machine learning is already a hot topic1. But this is all in-the-box. Now maybe you thought I was going outside-the-box to quantum agent-based learning or quantum artificial intelligence—but, no! We go even deeper, looking into the box that was outside the box—the meta-box, as it were. This is where quantum self-learning sits. Self-learning is protocol wherein the quantum device itself comes to learn its own description. The protocol is called Self Knowing Yielding Nearly Extremal Targets (SKYNET). If that was hard to follow, it is depicted below.

hypebox
Inside the box is where the low hanging fruit lies—pip install tensorflow type stuff. Outside the box is true quantum learning, where a “quantum agent” lives. But even further outside-the-meta-box is this work, quantum self-learning—SKYNET.

Blockchain is the technology behind bitcoin2 and many internet scams. The core protocol was quickly realised to be applicable beyond digital currency and has been suggested to solve problems in health, logistics, bananas, and more. Here I introduce FelixBlochChain—a data ledger which stores runs of experimental outcomes (transactions) in blocks. The data chain is an immutable database and can easily be delocalised. As a way to solve the data integrity problem, this could be one of the few legitimate, non-scammy uses of blockchain. So, if you want to give me money for that, consider this the whitepaper.

Problem

 

99probs
Above: the conceptual problem. Below: the problem cast in its purest form using the formalism of quantum mechanics.

The problem is succinctly described above. Naively, it seems we desire a description of an unknown process. A complete description of such a process using traditional means is known as quantum process tomography in the physics community3. However, by applying some higher-order thinking, the envelope can be pushed and a quantum solution can be sought. Quantum process tomography is data-intensive and not scalable afterall.

The solution proposed is shown below. The paradigm shift is a reverse-datafication which breaks through the clutter of the data-overloaded quantum process tomography.

fuckyeahquantum
The proposed quantum-centric approach, called self-learning, wherein the device itself learns to know itself. Whoa. 

It might seem like performing a measurement of \{|\psi\rangle\!\langle \psi|, \mathbb I - |\psi\rangle\!\langle \psi|\} is the correct choice since this would certainly produce a deterministic outcome when V = U. However, there are many other unitaries which would do the same for a fixed choice of |\psi\rangle. One solution is to turn to repeating the experiment many times with a complete set of input states. However, this gets us nearly back to quantum process tomography—killing any advantage that might have been had with our quantum resource.

Solution

quantumintensifies
Schematic of the self-learning protocol, SKYNET. Notice me, Senpai!

This is addressed by drawing inspiration from ancilla-assisted quantum process tomography4. This is depicted above. Now the naive looking measurement, \{|\mathbb I\rangle\!\langle\mathbb I |, \mathbb I - |\mathbb I\rangle\!\langle \mathbb I|\}, is a viable choice as

|\langle\mathbb I |V^\dagger U \otimes \mathbb I |\mathbb I\rangle|^2 = |\langle V | U\rangle|^2,

where |U\rangle = U\otimes \mathbb I |\mathbb I\rangle. This is exactly the entanglement fidelity or channel fidelity5. Now, we have |\langle V | U\rangle| = 1 \Leftrightarrow U = V, and we’re in business.

Though |\langle V | U\rangle| is not accessible directly, it can be approximated with the estimator P(V) = \frac{n}{N}, where N is the number of trials and n is the number of successes. Clearly, \mathbb E[P(V)] = |\langle V | U\rangle|^2.

Thus, we are left with the following optimisation problem:
\min_{V} \mathbb E[P(V)] \label{eq:opt},

subject to V^\dagger V= \mathbb I. This is exactly the type of problem suitable for the gradient-free cousin of stochastic gradient ascent (of deep learning fame), called simultaneous perturbation stochastic approximation6. I’ll skip to the conclusion and give you the protocol. Each epoch consists of two experiments and a update rule:

V_{k+1} = V_{k} + \frac12\alpha_k \beta_k^{-1} (P(V+\beta_k \triangle_k) - P(V-\beta_k \triangle_k))\triangle_k.

Here V_0 is some arbitrary starting unitary (I chose \mathbb I). The gain sequences \alpha_k, \beta_k are chosen as prescribed by Spall6. The main advantage of this protocol is \triangle_k, which is a random direction in unitary-space. Each epoch, a random direction is chosen which guarantees an unbiased estimation of the gradient and avoids all the measurements necessary to estimation the exact gradient. As applied to the estimation of quantum gates, this can be seen as a generalisation of Self-guided quantum tomography7 beyond pure quantum states.

To ensure integrity of the data—to make sure I’m not lying, fudging the data, p-hacking, or post-selecting—a blochchain-based solution is implemented. In analogy with the original bitcoin proposal, each experimental datum is a transaction. After a set number of epochs, a block is added to the datachain. Since this is not implemented in a peer-to-peer network, I have the datachain—called FelixBlochChain—tweet the block hashes at @FelixBlochChain. This provides a timestamp and validation that the data taken was that used to produce the final result.

Results

results
SKYNET finds a description of its own process. Each N is a different number of bits per epoch. The shaded region is the interquartile range over 100 trials using a randomly selected “true” gate. The solid black lines are fits which suggest the expected 1/\sqrt{N} performance.

Speaking of final result, it seems SKYNET works quite well, as shown above. There is still much to do—but now that SKYNET is online, maybe that’s the least of our worries. In any case, go download the source8 and have fun!

Acknowledgements

The author thanks the quantum technology start-up community for inspiring this work. I probably shouldn’t say this was financially supported by ARC DE170100421.


  1. V. Dunjko and H. J. Briegel, Machine learning and artificial intelligence in the quantum domain, arXiv:1709.02779 (2017)
  2. N. Satoshi, Bitcoin: A peer-to-peer electronic cash system, (2008), bitcoin.org. 
  3. I. L. Chuang and M. A. Nielsen, Prescription for experimental determination of the dynamics of a quantum black box, Journal of Modern Optics 44, 2455 (1997)
  4. J. B. Altepeter, D. Branning, E. Jerey, T. C. Wei, P. G. Kwiat, R. T. Thew, J. L. O’Brien, M. A. Nielsen, and A. G. White, Ancilla-assisted quantum process tomography, Phys. Rev. Lett. 90, 193601 (2003)
  5. B. Schumacher, Sending quantum entanglement through noisy channels, arXiv:quant-ph/9604023 (1996)
  6. J. C. Spall, Multivariate stochastic approximation using a simultaneous perturbation gradient approximation, IEEE Transactions on Automatic Control 37, 332 (1992)
  7. C. Ferrie, Self-guided quantum tomography, Physical Review Letters 113, 190404 (2014)
  8. The source code for this work is available at https://gist.github.com/csferrie/1414515793de359744712c07584c6990

David Wolfe doesn’t want you to share these answers debunking quantum avocados

Everyone knows you need to microwave your avocados to release their quantum memory effects.

Recently, I joined Byrne and Wade on Scigasm Podcast to talk about misconceptions of quantum physics. Apparently, people are wrong about quantum physics on the internet! Now, since the vast majority of people don’t listen to Scigasm Podcast [burn emoji], I thought I’d expand a bit on dispelling some of the mysticism surrounding the quantum.

Would it be fair to say quantum physics is a new field in the applied sciences, though it has been around for a while in the theoretical world?

No. That couldn’t be further from the truth. There are two ways to answer this question.

The super pedantic way: all is quantum. And so all technology is based on quantum physics. Electricity is the flow of electrons. Electrons are fundamental quantum particles. However, you could rightfully say that knowledge of quantum physics was not necessary to develop the technology.

In reality, though, all the technology around us today would not exist without understanding quantum physics. Obvious examples are lasers, MRI and atomic clocks. Then there are technologies such as GPS, for example, that rely on the precision timing afforded by atomic clocks. Probably most importantly is the develop of the modern transistor, which required the understanding of semiconductors. Transistors exist, and are necessary, for the probably of electronic devices surrounding you right now.

However, all of that is based on an understanding of bulk quantum properties—lots of quantum systems behaving the same way. You could say this is quantum technology 1.0.

Today, we are developing quantum technology 2.0. This is built on the ability to control individual quantum systems and get them to interact with each other. Different properties emerge with this capability.

Does the human brain operate using properties of the quantum world?

There are two things this could mean. One is legit and other is not. There is a real field of study called quantum biology. This is basically material physics, where the material is biological. People want to know if we need more than classical physics to explain, say, energy transfer in ever more microscopic biochemical interactions.

The other thing is called quantum consciousness, or something equally grandiose. Now, some well-known physicists have written about this. I’ll note that this is usually long after tenure. These are mostly metaphysical musings, at best.

In either case, and this is true for anything scientific, it all depends on what you mean by properties of the quantum world. Of course, everything is quantum—we are all made of fundamental particles. So one has to be clear what is meant by the “true” quantum effects.

Then… there are the crackpots. There the flawed logic is as follows: consciousness is mysterious, quantum is mysterious, therefore consciousness is quantum. This is like saying: dogs have four legs, this chair has four legs, therefore this chair is a dog. It’s a logical fallacy.

Quantum healing is the idea that quantum phenomena are responsible for our health. Can we blame quantum mechanics for cancer? Or can we heal cancer with the power of thought alone?

Sure, you can blame physics for cancer. The universe wants to kill us after all. I mean, on the whole, it is pretty inhospitable to life. We are fighting it back. I guess scientists are like jujitsu masters—we use the universe against itself for our benefit.

But, there is a sense in which diseases are cured by thought. It is the collective thoughts and intentional actions of scientists which cure disease. The thoughts of an individual alone are useless without a community.

Is it true that subatomic particles such as electrons can be in multiple places at once?

If you think of the particles has tiny billiard balls, then no, almost by definition. A thing, that is defined by its singular location, cannot be two places at once. That’s like asking if you can make a square circle. The question doesn’t even make sense.

Metaphors and analogies always have their limitations. It is useful to think this way about particles sometimes. For example, think of a laser. You likely are not going too far astray if you think of the light in a laser as a huge number of little balls flying straight at the speed of light. I mean that is how we draw it for students. But a physicist could quickly drum up a situation under which that picture would lead to wrong conclusions even microscopically.

Does quantum mechanics only apply to the subatomic?

Not quite. If you believe that quantum mechanics applies to fundamental particles and that fundamental particles make up you and me, then quantum mechanics also applies to you and me.

This is mostly true, but building a description of each of my particles and the way they interact using the rules of quantum mechanics would be impossible. Besides, Newtonian mechanics works perfectly fine for large objects and is much simpler. So we don’t use quantum mechanics to describe large objects.

Not yet, anyway. The idea of quantum engineering is to carefully design and build a large arrangement of atoms that behaves in fundamentally new ways. There is nothing in the rules of quantum mechanics that forbids it, just like there was nothing in the rules of Newtonian mechanics that forbade going to the moon. It’s just a hard problem that will take a lot of hard work.

Do quantum computers really assess every possible outcome at once?

No. If it could, it would be able to solve every possible problem instantaneously. In fact, we have found only a few classes of problems that we think a quantum computer could speed up. These are problems that have a mathematical structure that looks similar to quantum mechanics. So, we exploit that similarity to come up with easier solutions. There is nothing magical going on.

Can we use entanglement to send information at speeds faster than the speed of light?

No. Using entanglement to send information faster than light is like a perpetual motion machine. Each proposal looks detailed and intricate. But some non-physical thing is always hidden under the rug.

Could I use tachyons to become The Flash? And if so, where do I get tachyons?

This is described in my books. Go buy them.

Quantum Physics for Babies

This talk was given at the University of Sydney School of Physics Colloquium 19 June 2017.

It’s​ great to be back here. That feels a bit awkward to say since it’s only been 6 months since I left and I’m only 10 minutes away. But King and Broadway might as well be the Pacific Ocean for academics. I’m Chris Ferrie. I’m just down the road at the Centre for Quantum Software and Information. It’s an awesome new Centre. We’re on Twitter. You should check us out.

Now, though the title of the talk doesn’t make it obvious, I am a serious, well… maybe not serious, but I am an academic. But I also have a hobby… tennis. No, I write children’s books. Yes, it is a real book. And, yes, I wrote it and self published it several years ago when I was a postdoc. Why, and how, and for what purpose, well… that is the purpose of this talk.

measure-1509707_1920Measure twice, cut once. So the old proverb goes. It certainly it makes sense if you only have enough material to build a thing. However, and I see this all too often in otherwise very smart people, too much measuring leads to over optimisation and inaction, not enough cutting. Whereas, I like cut several times, toss things out, try new cutting instruments, and so on. I almost never measure. Ultimately, this is the story of Quantum Physics for Babies. I just did it. It wasn’t carefully planned, nor was there a spark or ah-ha moment which spawned the idea. I started, I failed, I started again.

And, for better or worse, the book became popular. Journalists starting asking me, “why did you write this book?” and, more seriously, “why teach quantum physics to babies, why is that important?”

brain-2062057_1920So, I started to rationalize. Why did I write this book? And, is it important? In particular, is it important for all children, not just my own? (because it is always important to find a way to discuss your passion with your own kids.) I think the answer to “is it important?” is yes. In this talk I’ll walk you through the various levels of rationalisation I’ve went through. Each has an element of truth to it, both for myself personally and what the experts on the topic of early childhood education espouse.

But let me start at the same place I start most things, with a joke. Someone that has known me for only a short time probably wouldn’t be too surprised that I was voted “class clown” in high school. Humor plays a crucial part of almost every aspect of my life. I laugh with my partner, I laugh with my children, I laugh with my friends, and I laugh with other scientists. (Einstein didn’t think it was very funny—but, then again, he never liked quantum physics.) Happiness is the difference between your reality and your expectations. Humor often defies expectation and happiness ensues. So, hopefully you didn’t come to this talk with too many expectations and you’ll leave a little happier then when you came in. At least there’s cake.

There is no denying that I saw the irony as good for a laugh the first time the title popped into my head. Of course, the level of humour I’m talking about is not at all for the advertised audience. I’ve never seen a child laugh at the title of the book. Adults, on the other hand, love the juxtaposition of quantum physics with “for babies”. So I knew that at least a few people would buy it as a gag gift for a nerdy friend having a baby. What I didn’t expect was this nerdy friend getting a copy.

I’ve joked with various people about making other goofy “for babies” books. Why not “contract law for babies” or “geopolitical policy for babies”? Though, the only person in the world that needs to read such a book is too busy tweeting insults at women. But quantum physics—yeah—people seem to agree that is worth being more than a joke, and hopefully I knew something about it.

In the end, I put real thought and effort into the content. The goal became clear enough: how to fill a baby book out with short sentences, no jargon and a coherent description of quantum physics. It was a challenge and there is still probably room for improvement. But I’ve already had people say, “we all had a good laugh, then I started to read it and there was real quantum physics inside.” Many adults even claimed they learned something. But were the children learning?

The unanimous advice for new parents is to read to your newborn. Most say it doesn’t even matter what it is, just read. But, let’s play a little game here. Suppose a parent does read to their child and has no time to add a new book to the rotation. Then, Quantum Physics for Babies needs to replace a book. What book should it be? First, I don’t think it should replace fiction. Fiction and fairy tales serve many purposes and, besides, variety is the spice of life. So we are left with nonfiction, which for baby books is limited solely to only a few types of reference material.

Quantum Physics for Babies - Sydney Uni 19 June 2017 (1)A huge fraction of any newborn’s library will begin with the word “first”: “First Words”, “First book of numbers”, “First alphabet book”, and so on. One quickly gets the impression that these are essential reference books for the early learner. But beyond the obvious things—letters, numbers, shapes, three letter words—are a myriad of books about animals, and mostly farm animals.

Now, learning is tricky concept to define even for adults. There are numerous models of early childhood cognitive development, and so it is hard to say conclusively what is being “learned” and at what level, but something is clearly happening since every 3 year in the world knows what sound a cow makes. Do you? I think I do. But I have never heard one myself. Maybe there was a time when that was important, or at least relevant, but I don’t think that time is today.

Quantum Physics for Babies - Sydney Uni 19 June 2017 (2)Here is another example. Do you know what these birds are? My children seem to know and can identify the difference between a penguin and a puffin. Why? Why are there more books about puffins than there are puffins and no books about transistors when you are probably sitting on a billion of them right now. In your phone lives a few billion transistors making up, by the standards of only decade ago anyway, a supercomputer. A child today will probably spend their entire life closer to computer than they will an animal of comparable size. I’m not suggesting than all books on animals be replaced by physics for babies books, but we could maybe replace a few.

I won’t claim my children understand quantum physics, but they certainly understand it at the same level they understand anything else gotten from a book. They will tell you that everything in the world is made of atoms and atoms are made of neutrons, protons and electrons and electrons have energy. I think that is about the same level of understanding as being able to identify a puffin, or should I say Fratercula corniculata for the baby ornithologists in the crowd.

So it seems then that Quantum Physics for Babies is here to stay. But we’re all scientists here and we love nothing more than free cake and to categorize things. So where does Quantum Physics fit? In what aisle of the bookshop does it sit on the shelf? Well, it turns out that it has been shoehorned into the new educational buzzword de jour: STEM.

Quantum Physics for Babies - Sydney Uni 19 June 2017 (3).png

STEM (Science Technology Engineer and Mathematics) started out as an initiative to focus on its namesake topics with the goal of training a workforce ready for the careers that were assumed new technologies would create. Interestingly, the first press mention of the acronym seems to go back to 2008 when The Bill and Melinda Gates Foundation donated $12 million dollars to the Ohio STEM Learning Network, which is still going strong today. Most never looked back. [By the way, much backlash ensued over leaving out the Arts, for example. So you might see STEAM or even STREAM (Reading) out there.]

Now governments all over the world currently have numerous initiatives at all levels of the curriculum to enhance what they called STEM-based learning. This is vaguely and variably defined and can mean anything from simply having access to more technology in the classroom to the design and building of simple machines to solve practical problems. But the motivation and directives that follow are often based on decade-old studies suggesting rises in STEM-related jobs. One recent state-level education​ department cited a study with data collected prior to the release of the first iPhone (that was only 10 years ago, by the way). The often cited report of the Chief Scientist of Australia contained recommendations citing data accumulated from 1964-2005. Policy is good, but it cannot keep up with the pace of technology.

Disruption! The fear today—fueled by startups, makers, and ever younger entrepreneurs—is that we just have no idea what jobs will look like in the future. And so STEM, at least for the trailblazers, is now a movement with the audacious goal of graduating creators and innovators. We no longer want graduates who simply have more and integrated technical skills.

What does this look like? Let me give you an example. Here is Taj Parabi, now 17, CEO of his own business which ships DIY tablets. His company, fiftysix, also visits schools and puts on extracurricular workshops for students on technology and… entrepreneuring! That’s right. Your children are competing with 8-year-olds trained to be CEOs of their own companies!

On a topic near and dear to my own heart, a now veteran effort from the Institute for Quantum Computing is the Quantum Cryptography School for Young Students (QCSYS)Quantum Physics for Babies - Sydney Uni 19 June 2017 (4), which invites international high-school students for a week of intensive training on quantum technology. Indeed, many of these students eventually become PhD students in Quantum Information Theory. Other efforts include school incursions and the new QUANTUM: The Exhibition which is an all-ages, hands-on exploratory exhibit.

Quantum Physics for Babies - Sydney Uni 19 June 2017 (5)On the other side of the border (remember: the wall is on the souther border), IBM has recently released the “Quantum Experience”, an app that lets you program a quantum computer, a real quantum computer. You create an algorithm and then jump in the queue for it to run on real device housed in IBM’s labs. Here they are video conferencing with a school in South Africa and hosting local students.

So that is the tiniest snapshot of STEM education today. Is Quantum Physics for Babies on par with these efforts? Are the children learning the skills necessary to be quantum engineering start-up entrepreneurs? Of course not. Quantum Physics for Babies, at least as far as reading to actual babies is concerned, is about the parents.

20 years from now, your child might be sitting in an interview for the job of Quantum Communication Analyst or Quantum Software Engineer. How long will it be before such topics feature in the report of the Chief Scientist on the curriculum? How long before it is mainstream in public schools? I’m not holding my breath.

The problem today is that it’s impossible to keep up. Pilot studies, kids maker studios, programming toys and apps, … These are all beautiful, but the growth of STEM education has now outpaced even the technology. The curriculum cannot keep up, and so the onus of STEM education, however you want to define it, is largely on the parents.

Again, the efforts of STEM education researchers are impressive, but a parent cannot assume that their child will happen to be in the school that benefits from these one-off pilot studies or incursions. The education system in most developed countries has been too long taken for granted and is now depleted from underfunding. No doubt there are many great principals and great teachers out there. Two days from now, I’m going to go speak with a dozen principals and teachers about STEM education. But there are almost 1500 primary schools in Sydney alone (over 3000 in New South Wales). There is much that needs to be done at the larger scale—but even if I said that was being done, it is little comfort for parents today.

So—in the end—this is what I both want and expect from the book: the elimination of doubt and fear. I want quantum physics, indeed all physics and math and science, to be normal for a child to take interest in. When your child asks about going to Canada for a summer school on quantum cryptography, that should be seen as normal request. When she asks to help her set up an account for a quantum cloud computing service, you should be like, no worries I already have one.

Today, when 1 in 3 Americans would rather clean a toilet then do a math problem, when a search for “quantum physics” brings up Deepak Chopra instead of Stephen Hawking, and when the facts pointing to climate change are seen as equally compelling as a celebrity’s argument for a flat earth, we need all the help we can get. And we need to start that conversation as early as possible.

Quantum Physics for Babies was just the beginning…IMG_20170606_175007

How quantum mechanics turned me into a Bayesian

A few elder-statesmen of quantum theory gathered together while a handful of students listened in eagerly. Paraphrasing, one of them—quite seriously—said, “I don’t think any of the interpretations are logically consistent… but there is this ‘transactional interpretation’, where influences come from the future, that might be the only consistent one.” The students nodded their heads in agreement—I walked away.

Bayesianism is (some would say) a radical alternative philosophy and practice for both understanding probability and performing statistical analysis. So, like all young contrarian students of science, I was intrigued when I first found Bayesian probability. But where is the fun in blaming this on my own faults? I’m going to blame someone else—I’m going to blame it on Chris Fuchs.

On two occasions the Perimeter Institute for Theoretical Physics (PI) hosted lectures on the Foundations of Quantum Mechanics. I was lucky enough to be a graduate student in Waterloo at the time. The first, in 2007, was my first taste of the field. It was exciting to hear the experts at the forefront speaking about deep implications for physics andindeed—even the philosophy of science itself. I knew then that this was the area I wanted to work in.

However, I quickly became disillusioned. The literature was plagued by lazy physicists posing as armchair philosophers. There was no interest in real problems—only the pandering of borderline pseudoscience. It’s no wonder—why bother doing hard work and difficult mathematics when peddling quantum mysticism is what gets you press?

I stayed, though, because there were several researchers at PI who seemed interested in solving real, technical problemsand, they were doing so using techniques from another field I had already worked in: Quantum Information Theory. I learned an immense amount from Robin Blume-Kohout, Rob Spekkens and Lucien Hardy while there, but the one who left a lasting impression was Chris Fuchs.

Before we get to Fuchs, though, let’s back up for a momentjust what is this Quantum Foundations thing, and what has it got to do with Bayesianism? As you know, quantum theory dictates that the world is uncertain. That is, as a scientific theory, it makes only probabilistic predictions. Many of the philosophical problems and misunderstandings of quantum theory can be traced back to this fact. Thus, if one really wants to understand quantum theory, one ought to understand probability first. Easy, right?

Nope. As it turns out, more people argue about how to interpret the seemingly simple and everyday concept of probability than do our most sophisticated and complex physical theory. Generally speaking, there are two camps in the interpretations of probability: frequentists and Bayesians. As noted, every student begins as one of the former. It was in 2010 when my conversion to the latter was complete.

foundations_table.png
Summary of the interpretations of quantum theory.

In 2010, PI hosted its second course on the foundations of quantum theory. This time around I had a few years of experience under my belt and my bullshit detectors were on high alert. My final assignment was to summarize the course, as shown above. The only lectures that didn’t leave me disappointed where Chris Fuchs’. Because I had been reading up on Bayesian probability anyway, his “Quantum Bayesian” interpretation of quantum theory just clicked.

And it wasn’t just about philosophy. Concurrently, I was taking a great course on Stochastic Processes from Matt Scott. Most of this field takes an objective (frequentist) view of probability. Matt was patient with my constant questions on how to phrase the concepts in terms of the subjective Bayesian view. I was starting to feel a bit overwhelmed with the burden of translating everything to the new framework… then it happened.

jh75tdv

The assignment question was as follows: Show that \int_\infty^\infty p(x_2,x_1;t_2-t_1) d_{x_1} = p(x_2;t_2), the simplest case of the Chapman-Kolmogorov equation. What you were supposed to do is use the Markov property, p(x_2,x_1;t_2-t_1) = p(x_2|x_1;t_2-t_1)p(x_1|t_1), and integrate. It was a straightforward, but tedious, calculation. Here is what I wrote: first  p(x_2,x_1;t_2-t_1) = p(x_1|x_2;t_1-t_2)p(x_2|t_2). Since p(x_1|x_2;t_1-t_2) is a probability, it integrates to 1. Done.

This was seen as unphysical because t_1-t_2 is a negative time. So what?I thoughtprobabilities are not physical, they are subjective inferences. If I want to consider negative time to help me do my calculation, so be it. After all, I considered negative money to get me into university. But what I couldn’t believe is how difficult it was to convince others the solution was correct. It was at that moment I realized how powerful a slight change of view can be. I was a Bayesian.