Quantum space-depth tradeoffs for coherent block encodings

2026-07-02

AI Summary

This study addresses the challenge of reducing the number of ancilla qubits required for quantum block encoding while preserving optimal circuit depth. By leveraging the linear combination of unitaries (LCU) and Suzuki decomposition techniques, combined with quantum circuit complexity analysis and the DQC1 model, this work systematically investigates space-depth trade-off mechanisms in block encoding. It proposes low-ancilla block encoding constructions and establishes corresponding theoretical lower bounds. Furthermore, it develops a quantitative trade-off theory characterizing space-depth and space-query complexities within Suzuki simulation architectures and repeated-query LCU models. Notably, this research achieves depth optimization with O(1) approximation error and yields an optimal algorithm exhibiting linear complexity with respect to approximation precision for trace estimation tasks in the DQC1 model.

Abstract

Block encodings are a basic interface between quantum algorithms and linear algebra. Standard LCU constructions achieve optimal circuit depth but typically require logarithmically many ancilla qubits. We ask how much quantum workspace can be reduced without sacrificing circuit depth, and study this tradeoff from both algorithmic and lower-bound perspectives. For a Hermitian decomposition $A=\sum_{j=1}^L \alpha_j H_j$, with $\|H_j\|=1$ and $\alpha=\sum_j|\alpha_j|$, we give two coherent $\varepsilon$-approximate block-encoding constructions. The first uses one ancilla qubit and has depth $\widetilde O(L(\alpha/\varepsilon)^{o(1)})$, while the second uses $O(\log\log(\alpha/\varepsilon))$ ancillas and achieves depth $\widetilde O(L)$. For a broad Suzuki-based coherent simulation architecture, we prove an ancilla-depth tradeoff. In the polynomial-resource regime and for a constant number of coherent rounds, $\log(1/\varepsilon)\le O((\log Q)^2+2^a\log Q)$, where $Q$ is depth normalized by the number of Hamiltonian terms and $a$ is the ancilla count. Thus polylogarithmic dependence on $1/\varepsilon$ requires more than constantly many ancillas within this architecture. In a separate repeated-query LCU model, for balanced coefficients $1/L$ and error $\varepsilon=\eta/L$ with fixed $0<\eta<1$, we prove $2^a=\Omega_\eta(L^2/(T+L))$, where $T$ is the number of oracle queries. Hence $a=\Omega(\log L)$ when $T=O(L^\alpha)$ for some $\alpha<2$. Moreover, in the exact case, $a\ge \log L$ regardless of $T$. We also extend this tradeoff to arbitrary nonnegative coefficients. Finally, we apply our low-ancilla constructions to normalized trace estimation in DQC1, obtaining an optimal algorithm linear in the approximate degree together with a matching query lower bound. Together, these results establish quantitative space-depth and space-query tradeoffs in two natural circuit models.

Problem

• block encoding
• space-depth tradeoff
• ancilla qubits
• quantum circuit complexity
• linear combination of unitaries

Innovation

• block encoding
• space-depth tradeoff
• ancilla qubits
• LCU construction
• trace estimation