## Quantum-enhanced Markov chain Monte Carlo

The not-so-mathematical journal Nature recently published a paper proposing a new Markov chain Monte Carlo method:

• 2023 – Layden, Mazzola, Mishmash, Motta, Wocjan, Kim, and Sheldon – Quantum-enhanced Markov chain Monte Carlo.

Appearing earlier as this preprint, the paper’s publication in such a journal is a rare event indeed. This post notes this, as well as the fact that we can already simulate perfectly1For small instances of the model, we can do this directly. For large instances, we can use coupling from the past proposed by Propp and Wilson. the paper’s test model, the Ising or Potts model.2Wilhelm Lenz asked his PhD student Ernst Ising to study the one-dimensional version of the model. Renfrey Potts studied the generalization and presented it in his PhD. But this is a quantum algorithm, which is exciting and explains how it can end up in that journal.

##### The algorithm

The paper’s proposed algorithm adds a quantum mechanical edge or enhancement to the classic Metropolis-Hastings algorithm.3More accurately, it should be called the Metropolis-Rosenbluth-Rosenbluth-Teller-Teller-Hastings algorithm. As I covered in a recent post, the original algorithm uses a Markov chain defined on some mathematical space. Running it on a traditional or classical computer, at each time step, the algorithm consists of proposing a random jump and then accepting the proposed jump or not. Owing to the magic of Markov chains, in the long run, the algorithm simulates a desired probability distribution.

The new quantum version of the algorithm uses a quantum computer to propose the jump, while still using a classical computer to accept the proposal or not.4In my Metropolis-Hastings post, the classical jumper process, a discrete-time Markov chain, is replaced with a quantum mechanical variant. The quantum jump proposals are driven by a time-independent Hamiltonian, which is a central object in quantum and, in fact, all physics. This leads to a Boltzmann (or Gibbs) probability distribution for the jumping process.

Then, running the quantum part on a quantum computer, the algorithm will hopefully outperform its classical counterpart. The paper nurtures this hope by giving empirical evidence of the algorithm’s convergence speed. The researchers performed the numerical experiments on a 27-qubit quantum processor at IBM using the platform Qiskit.

##### Quantum is so hot right now

In recent years researchers have been focusing on such algorithms that exploit the strangeness and spookiness of quantum mechanics. You will see more and more quantum versions of algorithms that appear in statistics, machine learning, and related fields, as suggested by this survey paper, which also appeared in Nature.

##### Quantum lite

Sometimes quantum mechanics only loosely inspires algorithms and models. In this setting, some of my machine learning work uses determinantal point processes. This kernel-based random model draws direct inspiration from the wave function, a standard object in quantum mechanics. Under suitable simplifying conditions, the model describes the locations of particles known as fermions such as electrons and protons. Still, it’s fascinating that a quantum physics model inspired an interesting random object that has found applications in spatial statistics and machine learning.

## Creating a reversible Markov chain using acceptance(-rejection)

The study of Markov chains is generally the study of their long term behaviour, which, under certain conditions, is captured by them having a unique stationary distribution. Stationarity is an important property. It is, in a sense, a local property of a Markov chain.

For a Markov chain, a more global property is something called reversibility. Markov chains with this property must possess a stationary distribution, which, we see below, is an immediate consequence of reversibility. Reversible Markov chains (or processes) with discrete time1Asmussen writes in his book Applied Probability and Queues:

In this book, we use the terminology that a Markov chain has discrete time and a Markov process has continuous time (the state space may be discrete or general). However, one should note that it is equally common to let “chain” refer to a discrete state space and “process” to a general one (time may be discrete or continuous).
are the cornerstone of Markov chain Monte Carlo (MCMC) methods.

In this post we look at how a reversible Markov chain is constructed from a non-reversible (but irreducible) Markov chain by introducing an acceptance-rejection step. This post complements another post I wrote on the Metropolis-Hastings algorithm.

## Reversibility

A Markov process on state space $$\mathbb{X}$$ with kernel $$K$$ is (time) reversible with respect to the distribution $$\mu$$ if the following holds

$$\mu(x)K(x,y) = \mu (y) K(y,x)\quad x,y\in\mathbb{X}\,.$$

This reversibility condition is also called the detailed balance equation. If this condition is met, then the Markov process will have a stationary distribution $$\mu$$. By summing over $$x$$, we can verify this because we obtain

$$\sum_{x\in\mathbb{X}}\mu(x)K(x,y) =\mu(y)\sum_{x\in\mathbb{X}} K(y,x)=\mu(y)\,.$$

This is just the balance equation, often written as $$\mu=K\mu$$, which says that the transition kernel $$K$$ has a stationary distribution $$\mu$$.

## First Markov chain

We consider a Markov chain with kernel $$J$$ defined on a finite state space $$\mathbb{X}$$. If the Markov chain is at state $$x\in \mathbb{X}$$, it visits another state $$y\in \mathbb{X}$$ with the probability $$J(x,y)$$. This is a simple time-homogeneous finite Markov chain.

### Irreducibility

For our Markov chain, we assume that every state $$x$$ in $$\mathbb{X}$$ where $$\pi(x)>0$$ is reachable with positive probability in a single step. This implies the easy-to-achieve condition $$J(x,y)>0$$ where $$\pi(x)>0$$ for all points $$x,y \in \mathbb{X}$$. This requirement is a stronger form of irreducibility.

## Creating a new Markov chain with acceptance

We create a new Markov chain by introducing an acceptance step. For the Markov chain with kernel $$J$$, we assume that each time step, after choosing the jump direction but before jumping, a biased coin is flipped . The success probability $$\alpha(x,y)$$ depends on the current position $$x\in \mathbb{X}$$ and the (potential) next position $$y\in \mathbb{X}$$.

### Transition kernel

For our new Markov chain, we can quickly reason the transition kernel $$M$$. We first look at the off-diagonal elements of the kernel (matrix) $$M$$. To go from state $$x$$ and to another state $$y\neq x$$, the probability is simply

$$M(x,y) = \alpha(x,y) J(x,y), \quad x\neq y\,.$$

The transition matrix $$M$$ needs to be stochastic, so the rows sum to one, so $$\sum_{y\in\mathbb{X}}M(x,y)=1$$. That gives us the diagonal elements of $$M$$, although their exact form is not needed to show reversibility.

### $$\alpha(x,y)$$ needs a symmetric function $$s(x,y)$$

For reversibility, we just need to swap rows and columns. Clearly we only need to look at the off-diagonal entries, which implies the requirement

$$\pi(x)M(x,y) = \pi (y) M(y,x)\quad x\neq y\,.$$

Both sides are symmetric in $$x$$ and $$y$$, meaning they are equal to some non-negative symmetric $$s(x,y)=s(y,x)$$. Looking at the right-hand side, we get

\begin{aligned}\pi(y) M(y,x)&=\pi(y)J(y,x) \alpha(y,x)\\ &= s(y,x)\,.\end{aligned}

This implies that the function $$\alpha$$ is a non-negative function such that $$\alpha\leq 1$$, to ensure it’s a probability, with the form

$$\alpha(x,y)=\frac{s(x,y)}{\pi(x)J(x,y)}\,.$$

The only task remaining now is to choose a reasonable symmetric function $$s$$ such that $$\alpha\leq 1$$, ensuring $$\alpha$$ is a probability. Of course, our choice for the symmetric function $$s$$ should also be a function of the stationary distribution $$\pi$$ and the underlying kernel $$J$$.

## Examples

I’ll give two principal examples of the symmetric function $$s(x,y)$$. Working in reverse chronological order, I’ll give the simpler of the two examples first.

### Barker

A somewhat natural example is

$$s(x,y) = \frac{\pi(x)J(x,y)\pi(y)J(y,x)}{\pi(x)J(x,y)+\pi(y)J(y,x)}\,.$$

This is clearly a symmetric function, which only has the terms $$\pi$$ and $$J$$. The acceptance probability becomes

$$\alpha(x,y) = \frac{\pi(y)J(y,x)}{\pi(x)J(x,y)+\pi(y)J(y,x)}\,.$$

A.A. Barker proposed this function in a 1965 paper as part of his PhD work in mathematical physics at the University of Adelaide. Barker had been inspired by a previous 1953 paper, which brings us to the next example.

### Metropolis(-Rosenbluth-Rosenbluth-Teller-Teller)-Hastings

The now most important example is

$$s(x,y) = \min[\pi(x)J(x,y),\pi(y)J(y,x)]\,.$$

We can see that this is a symmetric function. The acceptance probability becomes

$$\alpha(x,y) = \min[1,\frac{\pi(y)J(y,x)}{\pi(x)J(x,y)}]\,.$$

This example is very famous in the world of Markov chain Monte Carlo methods. It is the main part of the so-called Metropolis-Hastings algorithm, which comes from a 1953 paper by Nicholas Metropolis, Arianna W. Rosenbluth, Marshall Rosenbluth, Augusta H. Teller, and Edward Teller (two husband-wife pairs), who looked at a special case, and a 1970 paper by W.K. Hastings, who generalized the method.

### Articles

##### Historical

The two important historical papers that gave the two examples for $$s(x,y)$$ are:

• 1965 – Barker – Monte Carlo calculations of the radial distribution functions for a proton-electron plasma;
• 1953 – Metropolis, Rosenbluth, Rosenbluth, Teller, Teller – Equation of state calculations by fast computing machines.

This paper is also important:

• 1970 – Hastings – Monte Carlo sampling methods using Markov chains and their applications.
##### Introductory

A good explanatory article about the Metropolis-Hastings algorithm is:

• 1995 – Chib and Greenberg – Understanding the Metropolis-Hastings Algorithm.

I also recommend:

• 2015 – Minh and Minh – Understanding the Hastings Algorithm;
• 2016 – Robert – The Metropolis-Hastings algorithm;
• 2017 – Gonçalves, Łatuszyński, Roberts – Barker’s algorithm for Bayesian inference with intractable likelihoods.
##### History

The history of this and related methods is described in the following article:

• 2011 – Robert and Casella – A Short History of Markov Chain Monte Carlo: Subjective Recollections from Incomplete Data.

Incidentally, the first author, Christian P. Robert, writes on his blog about Markov chain Monte Carlo methods, such as the Metropolis-Hastings algorithm, as well as many other topics.

### Books

Modern books on stochastic simulations and Monte Carlo methods will often detail this method. For example, see the Handbook of Monte Carlo Methods (Section 6.1) by Kroese, Taimre and Botev. The book Stochastic Simulation: Algorithms and Analysis by Asmussen and Glynn also covers the method in Chapter XIII, Section 3. (For my remark on reversibility, see Remark 3.2 in Asmussen and Glynn.) There is also the book Monte Carlo Strategies in Scientific Computing by Liu; see Chapter 5.

### Websites

The web abounds with pages dedicated to explaining Markov chain Monte Carlo methods with the focus typically on the Metropolis-Hastings algorithm. I would try this following one because it gives a detailed explanation on how the Metropolis-Hastings algorithm works:

• https://gregorygundersen.com/blog/2019/11/02/metropolis-hastings/

## Markov goes to Monte Carlo

Numerical and scientific fields such as statistics, computational physics, and optimization methods heavily rely upon Markov chain Monte Carlo methods. These simulation techniques use the power of Markov chains to sample general probability distributions, which in turn give Monte Carlo methods for estimating integrals and optimal solutions.

This is the third part of a multi-part series of posts. The first part covers Markov chains. The second part covers the basics of Monte Carlo methods. This post combines the ideas from the first two parts. Overall, the posts sketch the mechanics of Markov chain Monte Carlo (MCMC) methods, whose importance and applications I detailed in a previous post.

We will only examine Markov chains with countable state spaces. In this setting, we only need standard probability and matrix knowledge. But the results extend to general state spaces, such as Euclidean space.

By the way, I doubt that Andrey A. Markov ever went to Monte Carlo in the small country of Monaco.1But Herbie did.

## Monte Carlo conditions

### Sum

In the first post we covered Markov chains with countable state spaces, because the mathematics is more transparent. Consequently, we can use such Markov chains to estimate the sum of a function $$f$$ over a countable (possibly infinite) set $$\mathbb{S}$$. We can write the sum as

\begin{align}S(f)&=\sum_{x\in \mathbb{S}} f(x) \\&= \sum_{x\in\mathbb{S}} \frac{f(x)q(x)}{q(x)}\\& = \mathbb{E}_{q}[ \frac{f(Y)}{q(Y)}]\\& = \mathbb{E}_{q}[ g(Y)]\,,\end{align}

where $$g=f/q$$ and $$Y$$ is a suitable random variable that has a probability mass function $$q$$ with support $$\mathbb{S}$$.

Sampling the random variable (or point) $$Y$$, gives the samples $$y_1,\dots,y_n$$, which yields an unbiased Monte Carlo estimate of the sum, namely

$$S_n (f)=\frac{1}{n}\sum_{i=1}^n \frac{f(y_i)}{q(y_i)}.$$

The closer the function $$q$$ is to $$|f|$$, the better the estimate.

### Integral

We can write an integral over some region $$\mathbb{S}$$ as

\begin{align}\int_{\mathbb{S}} f(x) dx &= \int_{\mathbb{S}} \frac{f(x)}{p(x)}p(x)dx\\ & =\mathbb{E}_p[\frac{f(Y)}{p(Y)}]\,,\end{align}

where $$p$$ is the probability density of a random variable (or point) $$Y$$ with support $$A$$. Then by sampling $$Y$$, the resulting samples $$y_1, \dots, y_n\in [0,1]$$ give the unbiased Monte Carlo estimate

$$I_n (f)=\frac{1}{n}\sum_{i=1}^n \frac{f(y_i)}{p(y_i)}\,.$$

### Ergodic sequence

To estimate the sum or integral, we’ll need a sequence of ergodic random variables $$\{Y_i\}_{i\in\mathbb{N}}$$, which we can achieve with an ergodic Markov chain with a state space $$\mathbb{S}$$.

## Markov chain conditions

In the context of Markov chains and, more generally, stochastic processes, we can think of ergodicity as convergence and a slightly more general version of the law of large numbers. A Markov chain with a countable state space needs some conditions to ensure ergodicity.

Regularity conditions for countable a Markov chain

1. A stationary distribution $$\pi$$
2. Aperiodicity
3. Irreducibility
4. Postive recurrence

In general, most of the needed structure for the Markov chain comes from irreducibility and a stationary distribution existing. Sometimes the above conditions are redundant. For example, an aperiodic, irreducible Markov chain with a finite state space is always positive recurrent. We look at ways on how to satisfy these conditions in the Monte Carlo setting when the underlying state space is countable.

### Stationary distribution $$\pi$$

A stationary distribution $$\pi$$ satisfies equation

$$\pi=P\pi.$$

We know exactly what our stationary distribution $$\pi$$ needs to be. It is the one used in the Monte Carlo method, namely the distribution of $$Y$$. The challenge is constructing a Markov chain with such a stationary distribution. This will of course depend on the Markov (transition) kernel $$P$$ of the Markov chain.

We’ll look at the other conditions on the Markov kernel $$P$$.

### Aperiodicity

We recall that the period $$d_x$$ of a state $$x\in \mathbb{X}$$ is the greatest common divisor of all $$n$$ values such that $$P(x,x)^n>0$$. We need every state of the countable Markov chain to be aperiodic, meaning $$d_x=1$$ for all $$x\in\mathbb{X}$$.

One sufficient way to achieve this requirement is to first have the countable Markov chain to be irreducible, which we’ll cover in the next section. Then if there exists at least one state $$x\in \mathbb{x}$$ such that $$P(x,x)>0$$, then the Markov chain is aperiodic.2Lemma 1.8.2 in Markov Chains by Norris or Remark 6.2.5 in Probability and Stochastic Processes by Brémaud.

Clearly the last condition is not difficult to satisfy, putting the focus now on the irreducibility requirement.

### Irreducibility

The irreducibility property says that it is possible for the Marko chain to get from any point to another point in a finite number of steps. More precisely, for an irreducible Markov chain with countable state space, there must exist for all $$x,y\in\mathbb{X}$$ a natural number $$s$$ (possibly depending $$x$$ and $$y$$) such that $$P(x,y)^s>0$$.

We can achieve this requirement by introducing a stronger requirement, which is easier to verify. A countable Markov chain with a transition $$P$$ will be irreducible if for all $$x,y\in\mathbb{X}$$ the condition $$P(x,y)>0$$ holds. We have simply required that the above number $$s=1$$ for all $$x,y\in\mathbb{X}$$.

Furthermore, clearly $$P(x,x)>0$$ for all $$x\in\mathbb{X}$$, then our irreducible countable Markov chain is also aperiodic.

### Positive recurrence

For a point or state $$x\in\mathbb{X}$$, we recall its first return time being defined as

$$T_x^+=\min\{ t\geq 1: X_t=x\} \,.$$

A state $$x$$ is called positive recurrent if the expected value of its first return time is finite, meaning $$\mathbb{E}_x(T_x^+)<\infty$$. For a countable Markov chain, if all the states in the state space are positive recurrent, then we say the Markov chain is positive recurrent.

A countable irreducible Markov chain is positive recurrent if (and only if) it has a stationary distribution $$\pi$$. Furthermore, that stationary distribution will be unique and nonzero, meaning $$\pi(x)>0$$ for all $$x\in\mathbb{X}$$.3Theorem 1.7.7 in Markov Chains by Norris or Theorem 6.3.14 in Probability and Stochastic Processes by Brémaud.

In other words, for positive recurrence, we need to prove that the Markov chain has a stationary distribution.

## Ergodicity

A countable Markov chain that is aperiodic and irreducible is ergodic. In other words, an ergodic Markov process $$X$$ with stationary distribution $$\pi$$ is a random sequence $$X_0, X_1,\dots$$ such that the following holds almost surely

$$\frac{1}{t}\sum_{i=0}^{t-1} g(X_i) \rightarrow \sum_{x\in\mathbb{X}}g(x)\pi(x)$$

as $$t\rightarrow\infty$$ for all bounded, nonnegative functions $$g$$. Consequently, we can use such an ergodic Markov chain to make statistical estimates.

## Sampling with a Markov chain

For the Monte Carlo sample $$y_1,\dots,y_n$$, we simply use the Markov chain samples $$x_0,x_1,\dots,x_{n-1}$$, giving the Monte Carlo estimate. In the discrete case, this is simply

$$S_n (f)=\frac{1}{n}\sum_{i=1}^n g(x_{i-1}).$$

which, due to ergodicity, converges to $$S(f)=\mathbb{E}_{Y}[g(Y)]$$.

## Summary

In summary, a sufficient way to have a countable ergodic Markov chain is for the transition kernel to be such that $$P(x,y)>0$$ for all $$x\in\mathbb{X}$$ and there exists a (stationary) distribution $$\pi=P\pi$$. For the first requirement, one imagines using a non-negative function such as $$e^{-(x-y)^2}/C$$, where $$C>0$$ is a suitable normalization constant.

The requirements seem possible, but to bring them all together is still a great challenge for the uninitiated. Fortunately, some great minds almost seven decades ago proposed the first Markov chain that meets all the requirements for a specific stationary distribution, which was used for a Monte Carlo estimate.

The first such Markov chain Monte Carlo method appeared in a paper written by five scientists, including two husband-wife pairs. It became known as the Metropolis or Metropolis-Hastings algorithm, and it will be the subject of a future post.

### Books

##### Markov chains

Any stochastic process book will include a couple of chapters on Markov chains. For more details, there are many books dedicated entirely to the subject of Markov chains. For example, introductory books include:

• Brémaud – Markov Chains, Gibbs Fields, Monte Carlo Simulation and Queues;
• Levin, Peres, and Wilmer – Markov Chains and Mixing Times;
• Norris – Markov Chains.

Those books cover Markov chains with countable state spaces. If you want to read about discrete-time Markov chains with general state spaces, try the book:

• Meyn, Tweedie – Markov chains and stochastic stability

All the above books have a section detailing a Markov chain Monte Carlo method, such as the Metropolis-Hastings algorithm and the Gibbs sampler.

##### Monte Carlo methods

Books dedicated to Monte Carlo approaches include:

For a statistics context, there’s also the good book:

• Casella and Robert – Monte Carlo Statistical Methods.