Abstract
An indistinguishability obfuscator is a polynomial-time probabilistic algorithm that takes a circuit as input and outputs a new circuit that has the same functionality as the input circuit, such that for any two circuits of the same size that compute the same function, the outputs of the indistinguishability obfuscator are indistinguishable. Here, we study schemes for indistinguishability obfuscation for quantum circuits. We present two definitions for indistinguishability obfuscation: in our first definition (\(qi\mathcal {O}\)) the outputs of the obfuscator are required to be indistinguishable if the input circuits are perfectly equivalent, while in our second definition (\(qi\mathcal {O}_\mathbf{D}\)), the outputs are required to be indistinguishable as long as the input circuits are approximately equivalent with respect to a pseudo-distance D. Our main results provide (1) a computationally-secure scheme for \(qi\mathcal {O}\) where the size of the output of the obfuscator is exponential in the number of non-Clifford (\(\mathsf{T}\) gates), which means that the construction is efficient as long as the number of \(\mathsf{T}\) gates is logarithmic in the circuit size and (2) a statistically-secure \(qi\mathcal {O}_\mathbf{D},\) for circuits that are close to the \(k\)th level of the Gottesman-Chuang hierarchy (with respect to D); this construction is efficient as long as \(k\) is small and fixed.
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Notes
- 1.
The correction is a tensor products of Pauli operators, which is computed as a function of \(C_q\) and of the teleportation outcome.
- 2.
If two different circuits are close in functionality but not identical, then we have no guarantee that their canonical forms are close.
- 3.
PSPACE is the class of decision problems solvable by a Turing machine in polynomial space and QSZK is the class of decision problems that admit a quantum statistical zero-knowledge proof system.
- 4.
coQMA is the complement of QMA, which is the class of decision problems that can be verified by a one-message quantum interactive proof.
- 5.
Recall that polynomial-time uniformity means that there exists a polynomial-time Turing machine which, on input \(n\) in unary, prints a description of the \(n\)th circuit in the family.
- 6.
We make a few design choices that are more appropriate for our situation, where we show the possibility of \(i\mathcal {O}\) against quantum adversaries: our adversary is a probabilistic polynomial-time quantum algorithm, we dispense with the mention of the random oracle, and note that our indistinguishability notions are defined to hold for all inputs.
- 7.
An operator is admissible if its action on density matrices is linear, trace-preserving, and completely positive. A operator’s type is \((n,m)\) if it maps \(n\)-qubit states to \(m\)-qubit states.
- 8.
A circuit is of type \((i,j)\) if it maps \(i\) qubits to \(j\) qubits.
- 9.
This is without loss of generality, since a \(qi\mathcal {O}\) for a generalized quantum circuit can be obtained from a \(qi\mathcal {O}\) for a reversible version of the circuit, followed by a trace-out operation (see [16]).
- 10.
It would be unreasonable to allow an obfuscator that outputs a circuit on \(n\) qubits, but of depth super-polynomial in \(n\).
- 11.
Their algorithm outputs a canonical form (unique form) provided it runs on the standard initial tableau see pages 8–10 of [2].
- 12.
Circuits that compute update functions for Clifford circuits, see [16].
- 13.
The set \(\{\mathsf{H}, \mathsf{T}\}\) is universal for 1-qubit unitaries [33].
- 14.
We note that, on top of being equal, the circuits that compute the update functions \(F_{C_{q_1}},\) \(F_{C_{q_2}}\) can be assumed to be of the same size. This follows by an argument very similar to the one in [16].
- 15.
A circuit is of type \((i,j)\) if it maps \(i\) qubits to \(j\) qubits.
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Acknowledgements
We thank an anonymous reviewer for pointing out the work of [35]; we would also like to thank Yfke Dulek for related discussions. This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-20-1-0375, Canada’s NFRF, Canada’s NSERC, an Ontario ERA, and the University of Ottawa’s Research Chairs program.
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Broadbent, A., Kazmi, R.A. (2021). Constructions for Quantum Indistinguishability Obfuscation. In: Longa, P., Ràfols, C. (eds) Progress in Cryptology – LATINCRYPT 2021. LATINCRYPT 2021. Lecture Notes in Computer Science(), vol 12912. Springer, Cham. https://doi.org/10.1007/978-3-030-88238-9_2
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