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SPIE Quantum Information Science, Sensing, and Computation

Comparative analysis of projector‐based compression and quantum autoencoders

Alexei Kaltchenko, Rahul Chakrabarti, Wrenen D'Cunha

Published May 30, 2025• 0 citations
Publisher

Abstract

Quantum compression is a cornerstone of quantum information science, promising efficient encoding of large quantum states into fewer qubits while retaining high fidelity. In this work, we compare two pivotal strategies for compressing an i.i.d. quantum source under small-scale (few-qubit) regimes. The first strategy relies on a projector-based compression method that discards low-probability components in the state’s spectral decomposition, achieving near-optimal asymptotic compression in principle. The second strategy employs a quantum autoencoder (QAE), a variational circuit that learns to embed essential information into fewer qubits. We implement both approaches in simulation, gauge their performance, and explore how many qubits can be ”saved” in practice. Our results highlight how projector-based compression provides a near-optimal yet analytically prescriptive solution, whereas autoencoders offer a machine-learning–style route that can adapt to varied input states.

DOI
10.1117/12.3062823

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Quantum compression is a cornerstone of quantum information science, promising efficient encoding of large quantum states into fewer qubits while retaining high fidelity. In this work, we compare two pivotal strategies for compressing an i.i.d. quantum source under small-scale (few-qubit) regimes. The first strategy relies on a projector-based compression method that discards low-probability components in the state’s spectral decomposition, achieving near-optimal asymptotic compression in principle. The second strategy employs a quantum autoencoder (QAE), a variational circuit that learns to embed essential information into fewer qubits. We implement both approaches in simulation, gauge their performance, and explore how many qubits can be ”saved” in practice. Our results highlight how projector-based compression provides a near-optimal yet analytically prescriptive solution, whereas autoencoders offer a machine-learning–style route that can adapt to varied input states.
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Quantum autoencoders (QAEs) are learning architectures that compress quantum data into a low-dimensional latent state while preserving the information needed for reconstruction. We study blind single-copy compression of quantum states through a kkk-qubit bottleneck and investigate the minimal circuit width required to attain the information-theoretic optimum under average infidelity. Between the conventional architecture, which is narrow but nonuniversal, and fully general \emph{completely positive and trace preserving} (CPTP) realizations, which are universal but overparameterized, we identify a balanced regime. We prove that for every distribution of pure nnn-qubit states, there exists a QAE with kkk encoder ancillas and nnn decoder ancillas that achieves the optimal fidelity over all CPTP encoder--decoder pairs. The encoder-side statement is sharp in that we construct source families for which every optimal scheme necessarily uses at least kkk encoder ancillas, thereby determining the universal encoder threshold exactly. On the decoder side, we show that isometric decoders are optimal for several analytically tractable source families, but we also exhibit an explicit counterexample demonstrating that decoder isometry is not universally sufficient. Nevertheless, numerical experiments indicate that the performance gap is practically negligible.
Abstract We examine information loss, resource costs, and run time from practical application of quantum data compression. Compressing quantum data to fewer qubits enables efficient use of resources, as well as applications for quantum communication and denoising. In this context, we provide a description of the quantum and classical components of the hybrid quantum autoencoder algorithm, implemented using IBMs Qiskit language. Utilizing our own data sets, we encode bitmap images as quantum superposition states, which correspond to linearly independent vectors with density matrices of discrete values. We successfully compress this data with near-lossless compression using simulation, and then run our algorithm on an IBMQ quantum chip. We describe conditions and run times for training and compressing our data on quantum devices, and relate trainability to specific characteristics and performance metrics of our parametric quantum circuits.
With quantum resources a precious commodity, their efficient use is highly desirable. Quantum autoencoders have been proposed as a way to reduce quantum memory requirements. Generally, an autoencoder is a device that uses machine learning to compress inputs, that is, to represent the input data in a lower-dimensional space. Here, we experimentally realize a quantum autoencoder, which learns how to compress quantum data using a classical optimization routine. We demonstrate that when the inherent structure of the dataset allows lossless compression, our autoencoder reduces qutrits to qubits with low error levels. We also show that the device is able to perform with minimal prior information about the quantum data or physical system and is robust to perturbations during its optimization routine.
Abstract We present the enhanced feature quantum autoencoder, or EF-QAE, a variational quantum algorithm capable of compressing quantum states of different models with higher fidelity. The key idea of the algorithm is to define a parameterized quantum circuit that depends upon adjustable parameters and a feature vector that characterizes such a model. We assess the validity of the method in simulations by compressing ground states of the Ising model and classical handwritten digits. The results show that EF-QAE improves the performance compared to the standard quantum autoencoder using the same amount of quantum resources, but at the expense of additional classical optimization. Therefore, EF-QAE makes the task of compressing quantum information better suited to be implemented in near-term quantum devices.
Abstract This work introduces the Schmidt quantum compressor, an innovative approach to quantum data compression that leverages the principles of Schmidt decomposition to encode quantum information efficiently. In contrast to traditional variational quantum autoencoders, which depend on stochastic optimization and face challenges such as shot noise, barren plateaus, and non-convex optimization landscapes, our deterministic method substantially reduces the complexity and computational overhead of quantum data compression. We evaluate the performance of the compressor through numerical experiments, demonstrating its ability to achieve high fidelity in quantum state reconstruction compared to variational quantum algorithms. Furthermore, we demonstrate the practical utility of the Schmidt quantum compressor in one-class classification tasks.