pec-5-theory.md

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What is the theory behind PEC?

Probabilistic error cancellation (PEC) {cite}Temme_2017_PRL, Sun_2021_PRAppl, Shuaining_2020_NatComm is a noise-aware error mitigation technique which is based on two main ideas:

• The first idea is to express ideal gates as linear combinations of implementable noisy gates. These linear combinations are called quasi-probability representations {cite}Pashayan_2015_PRL;

• The second idea is to probabilistically sample from the previous quasi-probability representations to approximate quantum expectation values via a Monte Carlo average.

Note: In this section we follow the same notation of {cite}Mari_2021_PRA.

Quasi-probability representations

In PEC, each ideal gate $\mathcal G_i$ of a circuit of interest $\mathcal U = {\mathcal G}_t \circ \dots \circ {\mathcal G}_2 \circ {\mathcal G}1 $ is represented as a linear combination of noisy implementable operations ${\mathcal O{i, \alpha}}$ (i.e., operations that can be directly applied with a noisy backend):

$$ \mathcal G_i = \sum_\alpha \eta_{i, \alpha} \mathcal O_{i, \alpha}, \quad \eta_{i, \alpha} \in \mathbb R, $$

where the calligraphic symbols ($\mathcal U$, $\mathcal G_i$, $\mathcal O_{i, \alpha}$) stand for super-operators acting on the density matrix of the qubits as linear quantum channels.

The real coefficients ${\eta_{i,\alpha}}$ form a quasi-probability distribution {cite}Pashayan_2015_PRL with respect to the index $\alpha$. Their sum is normalized but, differently from standard probabilities, they can take negative values:

$$ \sum_\alpha \eta_{i,\alpha}=1, \qquad \gamma_i = \sum_\alpha |\eta_{i, \alpha}| \ge 1.$$

The constant $\gamma_i$ quantifies the negativity of the quasi-probability distribution which is directly related to the error mitigation cost associated to the gate $\mathcal G_i$.

Note: In principle, the gate index "$i$" in the noisy operations $\mathcal O_{i, \alpha}$ could be dropped. However, we keep it to explicitly define gate-dependent basis of implementable operations, consistently with the structure of the OperationRepresentation class discussed in What additional options are available in PEC?.

Error cancellation

The aim of PEC is estimating the ideal expectation value of some observable $A=A^\dagger$ with respect to the quantum state prepared by an ideal circuit of interest $\mathcal U$ acting on some initial reference state $\rho_0$ (typically $\rho_0= |0\dots 0 \rangle \langle 0 \dots 0 |$).

Replacing each gate $\mathcal G_i$ with its noisy representation, we can express the ideal expectation value as a linear combination of noisy expectation values:

$$ \langle A \rangle_{\rm ideal}= {\rm tr}[A \mathcal U (\rho_0)] = \sum_{\vec{\alpha}} \eta_{\vec{\alpha}} \langle A_{\vec{\alpha}}\rangle_{\rm noisy} $$

where we introduced the multi-index $\vec{\alpha}=(\alpha_1, \alpha_2, \dots ,\alpha_t)$ and

$$ \eta_{\vec{\alpha}} := \prod_{i=1}^t \eta_{i, \alpha_i}, \quad \langle A_{\vec{\alpha}}\rangle_{\rm noisy} := {\rm tr}[A \Phi_{\vec{\alpha}}(\rho_0)], \quad \Phi_{\vec{\alpha}} := \mathcal O_{t, \alpha_t} \circ \dots \circ \mathcal O_{2, \alpha_2} \circ \mathcal O_{1, \alpha_1}. $$

The coefficients ${ \eta_{\vec{\alpha}} }$ form a quasi-probability distribution for the global circuit over the noisy circuits. Indeed it is easy to check that, at the level of super-operators, we have:

$$ \mathcal U = \sum_{\vec{\alpha}} \eta_{\vec{\alpha}} \Phi_{\vec{\alpha}}. $$

The one-norm $\gamma$ of the global quasi-probability distribution is the product of those of the gates:

$$ \sum_{\vec \alpha} \eta_{\vec{\alpha}}=1, \qquad \gamma = \sum_{\vec{\alpha}} |\eta_{\vec \alpha}| = \prod_{i=1}^{t} \gamma_i. $$

All the noisy expectation values $\langle A_{\vec{\alpha}}\rangle_{\rm noisy}$ can be directly measured with a noisy backend since they only require circuits composed of implementable noisy operations. In principle, by combining all the noisy expectation values, one could compute the ideal result $\langle A \rangle_{\rm ideal}$. Unfortunately this approach requires executing a number of circuits which grows exponentially with the circuit depth and which is typically unfeasible.

An important fact at the basis of PEC is that, for weak noise, only a small number of noisy expectation values actually contribute to the linear combination because many of the coefficients $\eta_{\vec \alpha}$ are negligible. For this reason, it is more efficient to estimate $\langle A \rangle_{\rm ideal}$ using an importance-sampling Monte Carlo approach as described in the next section.

Monte Carlo estimation

To apply a Monte Carlo estimation, we need to replace quasi-probabilities with positive probabilities. This can be achieved as follows:

$$ \mathcal{G_i} = \sum_{\alpha} \eta_{i, \alpha} \mathcal{O}{i, \alpha} = \gamma_i \sum{\alpha} p_i(\alpha) , {\rm sgn}(\eta_{i, \alpha}), \mathcal{O}_{i, \alpha},$$

where $p_{i}(\alpha)=|\eta_{i, \alpha}|/\gamma_i$ is a valid probability distribution with respect to $\alpha$.

If for each gate $\mathcal G_i$ of the circuit we sample a value of $\alpha$ from $p_{i}(\alpha)$ and we apply the corresponding noisy operation $\mathcal O_{i, \alpha}$, we are effectively sampling a noisy circuit $\Phi_{\vec{\alpha}}$ from the global probability distribution $p(\vec{\alpha})= |\eta_{\vec{\alpha}}| / \gamma$.

Therefore, at the level of quantum channels, we have:

$$ \mathcal U = \gamma \mathbb E \left{ {\rm sgn}(\eta_{i, \vec{\alpha}}) \Phi_{\vec{\alpha}} \right},$$

where $\mathbb E$ is the sample average over many repetitions of the previous probabilistic procedure and ${\rm sgn}(\eta_{\vec{ \alpha}}) = \prod_i {\rm sgn}(\eta_{i, \alpha})$. As a direct consequence, we can express the ideal expectation value as follows:

$$\langle A \rangle_{\text{ideal}} = \gamma, \mathbb E \left{ {\rm sgn}(\eta_{\vec{\alpha}}) \langle A_{\vec{\alpha}}\rangle_{\rm noisy} \right}.$$

By averaging a finite number $N$ of samples we obtain an unbiased estimate of $\langle A \rangle_{\text{ideal}}$. Assuming a bounded observable $|A|\le 1$, the number of samples $N$ necessary to approximate $\langle A\rangle_{\text{ideal}}$ within an absolute error $\delta$, scales as {cite}Takagi_2020_PRR:

$$ N \propto \frac{\gamma^2}{\delta^2}. $$

The term $\delta^2$ in the denominator is due to the stochastic nature of quantum measurements and is present even when directly estimating an expectation value without error mitigation. The $\gamma^2$ factor instead represents the sampling overhead associated to PEC. For weak noise and short circuits, $\gamma$ is typically small and PEC is applicable with a reasonable cost. On the contrary, if a circuit is too noisy or too deep, the value of $\gamma$ can be so large that PEC becomes unfeasible.