VQAs performance

Comparison table

Better results than other methods
small number of variational parameters
large number of measurements required to perform the optimization process
Even if one is able to train, the final cost value will be corrupted by noise. For VQE problems, this is important, since one is ultimately interested in an accurate estimation of the energy
QCC (Qubit Coupled Clusters) and qubit-ADAPT-VQE work directly in the qubit space and are considered more “hardware-efficient” because they are generally more compact and easier to run on NISQ hardware. However, they require additional work to ensure the preservation of all physical symmetries
UCC-based (Unitary Coupled Cluster) methods and ansatz are constructed from fermionic operators and therefore avoid the problems with symmetry preservation. However, the number of multi-qubit gates remains high in this method, which requires the error rates on quantum hardware to significantly improve before such methods can be used
QSE-based (Quantum Subspace Expansion) approaches exhibit higher accuracy from ansatzes prepared with relatively low circuit depth and require fewer numerical parameters to be optimized, but they need extra measurements for Hamiltonian powers, overlap matrix, and/or energy gradient, and may suffer from the linearly dependent many-body basis.
Compared with Quantum Phase Estimation (QPE) algorithm
- Large scale controlled unitaries, required for QPE, cannot be reliably implemented on NISQ devices
-The VQE trades off the depth and number of qubits required under QPE with a higher number of measurements and repetitions of the circuit, as well as the constraints of an approximate ansatz for the state.
- VQE and QPE are likely to provide the most benefit when combined as complementary approaches, offering algorithmic flexibility that can be adjusted depending on the progress of quantum hardware
VQE can be configured with arbitrary ansatzes
Chemistry-inspired ansatzes have a goal of a more careful choice of excitations to include in the ansatz, while hardware-efficient approaches focus on the reduction of the number of multi-qubit gates and using various techniques to choose compact blocks of entangling gates with single-qubit rotational gates more efficiently
When solving the ground state energy problem with a VQE on an application-relevant scale, Natural Gradient appears to be the optimizer of choice for the classical processing step. This holds for both investigated spin chain models and, given the asymptotically vanishing cost overhead of NatGrad for Hamiltonians with many non commuting terms, probably even more so for quantum chemical systems.
Finding optimal values γ and β is a hard problem since the optimization landscape in QAOA is non-convex with many local optima.

One can classically compute the cost after sampling ⇒ corruption of final cost by noise is not an important issue.
Provable non-trivial performance guarantees for p= 1
In general even p= 1 QAOA ansatz cannot be efficiently simulated on any classical device

Limited performance on some problems on bounded-degree graphs when the depth is shallow. This limitation may result from the fact that the algorithm cannot “see” the entire graph at low depth. ⇒ one may need the depth to grow with the system size (e.g., p ≥ logN) in order to outperform the best classical algorithms.
As p increases, performance can only improve. Even at p = 1, worst case performance guarantees have been established. These beat random guessing but not the best classical algorithms
MaxCut is a widely studied problem, and there is a polynomial-time algorithm due to Goemans and Williamson which guarantees a certain approximation ratio for all graphs, and it is an open question whether QAOA can efficiently achieve this or beat it
It is believed that classical algorithms like Montanari’s will fail to find near-optimal solutions. It would be interesting to see how the QAOA performs for those problems, and whether it will outperform the classical algorithms
To reduce the problem’s dimensionality, an ansatz with a single layer of Ry gate was attempted. This approach does indeed reduce optimization complexity and lead to lower iteration count. However, the quality of the solution deteriorates, showing a shallow (hardware efficient) ansatz is not capable of achieving satisfactory results for this test case and leads to errors hardly acceptable in typical fluid dynamics applications
Gradual ansatz parameters optimization was attempted but resulted in no improvement in overall results, indicating high interconnection between ansatz parameters does not allow independent optimization
While an efficient matrix decomposition was found, ansatz optimization remains one of the main hurdles preventing successful implementation. If no problem-specific ansatz can be devised, optimization complexity and circuit depth increase exponentially, making a quantum advantage unlikely
Improvement was detected when using a non normalized cost function with respect to a normalized one. Although this does not guarantee a solution when the cost is null, this can be easily verified and re-attempted until a solution is found
A linear combination of non-normalized and normalized cost functions did not bring any advantage compared to the normalized case
VQLS can solve simple Finite Element problems for n = 2, but fails to scale to larger problems successfully
The total number of iteration was far larger compared to best classical methods

Compared to previous optimization-based approaches, the optimization algorithm in QVECTOR is, in principle, scalable
The impact of physical errors on the simulator’s performance is far lower than in an (optimised) Trotterization protocol (quantum simulation is to approximate the dynamics of a given target Hamiltonian via a sequence of quantum gate) & does not worsen with the duration of the simulation.
Because of the assistance of the classical computer ⇒ can be implemented with quantum circuits of much less depth compared with the canonical Trotterization algorithm.
Can automatically correct some errors induced by the noise in the quantum computer.
Can be parallelized easily. (ability to encode multiple computational results into a quantum state in a single quantum computational step)
Ansatz employed for the autoencoder unitary impacts the degree of compression achievable with the autoencoder model
Generally similar performance to their classical limit where the transverse field of the Quantum Boltzmann Machine (QBM) (neural network whose model parameters are inspired by the Transverse Field Ising Model) is absent.
Scales with the number of qubits ⇒ exponentially fewer learnable parameters than what traditional methods would use

For the VQOCC (~one class classifier)
- The classification performance of VQOCC is comparable to that of OC-SVM (one-class support vector machine) and PCA, although the number of model parameters grows only logarithmically with the data size.
- The quantum algorithm outperformed DCAE (Deep Convolutional AutoEncoder) in most cases under similar training conditions
The required gradients can be calculated using existing techniques for evaluating gradients of variational functions
We can train the generator with both classical and quantum discriminators ⇒ benefit the implementation of other HQC algorithms, such as VQE
Hybrid quantum-classical architecture proposed in [4] -consisting of a quantum encoding circuit, variational circuit, and measurement decoding - has been shown to be able to approximate non-linear functions
It is an open question whether it can offer a practical advantage with respect to purely classical models for generative learning
There are no rows in this table

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Johnson, P. D., Romero, J., Olson, J., Cao, Y., & Aspuru-Guzik, A. (2017). QVECTOR: an algorithm for device-tailored quantum error correction. In arXiv [quant-ph].
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Khoshaman, A., Vinci, W., Denis, B., Andriyash, E., Sadeghi, H., & Amin, M. H. (2018). Quantum Variational Autoencoder. In arXiv [quant-ph].

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