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arXiv:2610.04667

Quantum-State Oblivious Transfer over Quantum Erasure Channels

Remi A. Chou

quant-phcs.ITSubmitted Oct 3, 2026• Announced Oct 6, 2026• 0 citations
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Abstract

We study quantum-state oblivious transfer over the quantum erasure channel under information-theoretic security guarantees. Specifically, Alice holds two quantum systems, Bob wishes to recover one of them, and the parties have access to independent channel uses and authenticated public classical communication, with no initial shared entanglement. We consider both honest-but-curious and malicious behavior. We establish finite-blocklength converse and achievability bounds, which match in the honest-but-curious setting. The malicious setting construction combines interactive hashing with tests of whether Bob received selected channel outputs, and distributes each input across several qubits so that erasures conceal an entire input from a dishonest receiver, even when the inputs are entangled with an external reference. We also derive fixed-error asymptotic expansions and show that both malicious and honest-but-curious settings have the same capacity.

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We consider oblivious transfer between Alice and Bob in the presence of an eavesdropper Eve when there is a broadcast channel from Alice to Bob and Eve. In addition to the secrecy constraints of Alice and Bob, Eve should not learn the private data of Alice and Bob. When the broadcast channel consists of two independent binary erasure channels, we derive the oblivious transfer capacity for both 2-privacy (where the eavesdropper may collude with either party) and 1-privacy (where there are no collusions).
Oblivious Transfer (OT) is a broadly studied cryptographic primitive which involves two mutually distrustful users who wish to interact with each other in order to transfer messages in an oblivious manner. Noise is an important resource to realize information-theoretically secure cryptographic protocols including oblivious transfer. However, oftentimes these noisy channels may be poorly characterized and the complete channel characteristics might be unknown to both users. Such channels are studied in the literature through the lens of compound channels. We study 1-out-of-2 oblivious transfer over a compound binary erasure channel (compound-BEC) and characterize the oblivious transfer capacity of compound-BEC. Our OT capacity characterization involves a general alphabet compound-DMC converse, subsequently specialized to compound-BEC along with an optimal and computationally-efficient achievability scheme for honest-but-curious users.
We determine the optimal success probability for transmitting a fixed number of classical messages through a finite number of uses of the quantum erasure channel, assisted by noiseless classical feedback and without initial shared entanglement. For an input dimension ddd, an erasure probability ppp, a blocklength nnn, and MMM equiprobable messages, the optimal success probability is equal to E[min⁡{1,dK/M}]E[\min\{1,d^K/M\}]E[min{1,dK/M}], where KKK is binomial with parameters nnn and 1−p1-p1−p. The converse allows arbitrary adaptive quantum encoders, quantum memories, and receiver instruments. Its main ingredient is an elementary dimension bound for noiseless quantum communication with classical feedback, proved by fixing the classical controls without conditioning the sender's state on the observed transcript. A classical protocol that transmits the base-ddd digits of an integer representing the message, repeating each digit until the receiver acknowledges its reception or the prescribed blocklength is reached, attains the bound for every integer MMM. We also establish a relative-majorization property of erasure-channel outputs and exactly evaluate a hypothesis-testing converse, recovering the same numerical bound without feedback. That converse need not be achievable without feedback: four uses of the qubit erasure channel and four messages give a strict gap. A product-state code employing tetrahedral qubit states nevertheless outperforms every classical binary erasure code with these parameters. We give an explicit message-size formula and show that the bounded-remainder normal approximation and the average-success strong-converse exponent are unchanged without feedback. We also determine the feedback-assisted error exponent below capacity, prove that it agrees with the no-feedback exponent above a critical rate, and give no-feedback bounds at lower rates.
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