Theory

POVMs

Let $\text{Her}(\mathcal{H}) \subset \mathcal{H}$ be the set of hermitian operators acting on a Hilbert space $\mathcal{H}$. Elements of $\text{Her}(\mathcal{H})$ may be written as double kets: $| F \rangle \! \rangle$. Such a space is also a real Hilbert space under the inner product

\[\langle \! \langle E | F \rangle \! \rangle= \text{Tr} E^\dagger F = \text{Tr} E F\]

The state of a quantum system is represented by an element $\rho$ of the set of positive semi-definite operators $\text{Pos}(\mathcal{H}) \subset \text{Her}(\mathcal{H})$ such that $\text{Tr} \rho = 1$. Observables are represented by elements $A \in \text{Her}(\mathcal{H})$ so that its expectation value is given by $\langle A \rangle = \langle \! \langle A | \rho \rangle \! \rangle$. A Positive Operator Valued Measure (POVM) is a set of observables $\{F_m\} \subset \text{Pos}(\mathcal{H})$ with the property that $\sum_m F_m = I$, where $I$ is the identity operator. These operators model the possible outcomes of an experiment: outcome $m$ happens with probability $p(m) = \langle \! \langle F_m | \rho \rangle \! \rangle$. The POVM conditions ensure that $p(m) \ge 0$ and $\sum_m p(m) = 1$.

Linear Inversion

The simplest method which solves the problem of quantum state tomography is linear inversion, which we describe in what follows. A POVM with $M$ elements induces a linear map $T: \text{Her}(\mathcal{H}) \to \mathbb{R}^M$ defined by the expression

\[T| \Omega \rangle \! \rangle = \left(\langle \! \langle F_1 | \Omega \rangle \! \rangle,\ldots,\langle \! \langle F_M | \Omega \rangle \! \rangle\right).\]

In order to be suitable for tomography, we want that the measurement probabilities $\mathbf{p} = T | \rho \rangle \! \rangle$ uniquely determine the state $| \rho \rangle \! \rangle$. This is assured if the transformation $T$ is injective. A POVM with this property is said to be informationally complete.

By choosing a basis $\{\Omega_n\} \subset \text{Her}(\mathcal{H})$, we can specify an arbitrary state $| \rho \rangle \! \rangle$ by a list of coefficients $\mathbf{x} = (x_1,\ldots,x_N)$ such that $| \rho \rangle \! \rangle = \sum_n x_n | \Omega_n \rangle \! \rangle$. Then, denoting by $\mathbb{T}$ the matrix of $T$ with respect to the canonical basis of $\mathbb{R}^M$ and $\{\Omega_m\}$, which has entries $\mathbb{T}_{mn} = \langle \! \langle F_m | \Omega_n \rangle \! \rangle$, we have $\mathbf{p} = \mathbb{T}\mathbf{x}$. When $\mathbb{T}$ is injective, $\mathbb{T}^\dagger\mathbb{T}$ is invertible, and then we can explicitly write the solution of the above equation as

\[\mathbf{x} = (\mathbb{T}^\dagger\mathbb{T})^{-1} \mathbb{T}^\dagger \mathbf{p},\]

which, in theory, solves the tomography problem.

Note that, to apply this method, one needs a reliable estimate of the probabilities $\mathbf{p}$. This can only be obtained by performing a large number of measurements.

Bayesian tomography

Bayesian tomography is the application of Bayesian inference to the problem of quantum state tomography ^[1]. The goal is to estimate the posterior distribution of the quantum state given the observed data. The posterior distribution is given by Bayes' theorem

\[P(\rho | \mathcal{M}) = \frac{P(\mathcal{M} | \rho) P(\rho)}{P(\mathcal{M})},\]

where $P(\mathcal{M} | \rho)$ is the likelihood of the observations $\mathcal{M}$ given the state, $P(\rho)$ is the prior distribution of the state, and $P(\mathcal{M})$ is the evidence. In the case of quantum state tomography, the likelihood is given by the Born rule

\[P(\mathcal{M} | \rho) = \prod_m p(m)^{n_m}, \ \ \ p(m) = \langle \! \langle F_m | \rho \rangle \! \rangle,\]

where $n_m$ is the number of times outcome $m$ was observed. The prior distribution is a probability distribution over the space of states, which encodes any prior knowledge about the state. The evidence is the normalization constant, given by $P(\mathcal{M}) = \int P(\mathcal{M} | \rho) P(\rho) d\rho$.

Bayesian inference does not provide a single estimate of the state, but a full distribution. The mean of the distribution is, in a sense, the best estimate of the state ^[1], and the variance gives an idea of the uncertainty of the estimate. Directly calculating the posterior distribution is infeasible, as it requires the computation of the evidence, which is a high-dimensional integral. The alternative is to sample the posterior distribution using Markov Chain Monte Carlo (MCMC) methods. The package provides an implementation of the Metropolis Adjusted Langevin Algorithm (MALA) ^{[2] [3]} to sample the posterior distribution.

As shown in the video above, MALA generates a random walk in the space of valid density operators (we reject all proposals falling outside this set) whose statistics are given by the desired posterior distribution.