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SPIE Quantum Information Science and Technology

Client-friendly continuous variable blind and verifiable quantum computing (Conference Presentation)

Nana Liu, Tommaso F. Demarie, Si-Hui Tan, Leandro Aolita, Joseph F. Fitzsimons

Published Oct 11, 2018• 2 citations
Publisher

Abstract

Limitations due to high costs and technological requirements will require users to access the first quantum processors through the cloud. Delegating a computation, however, raises privacy issues about the clients' data and poses questions about the verifiability of computations in high complexity regimes. These concerns have inspired a plethora of blind and verifiable quantum computation protocols, which allow a client with limited quantum capabilities to delegate a quantum computation to a remote server while hiding her data and preserving the integrity of the computation. The majority of these protocols are constrained to discrete quantum systems, but quantum information can also be processed by continuous-variable architectures. These offer a competitive alternative to their discrete-variable counterparts for numerous practical benefits: They rely on well-established quantum optical techniques, allow for the generation of very large optical resource states, offer higher detection efficiencies, and can be integrated into existing optical-fibre networks, all of which are highly desirable features for cloud quantum computing. In this work we fill this gap by presenting a blind and verifiable quantum computing protocol tailored to the unique features of continuous-variable systems. One such feature is the experimental accessibility of Gaussian operations. Our protocol is then based on the delegation of the experimentally challenging non-Gaussian operations. In this sense, it is experimentally friendly to the client, who only needs to perform Gaussian operations. Furthermore, unlike previous schemes, this protocol does not require repeated interactions between the client and server because it only involves the server sending the client non-Gaussian states. We prove universality and blindness using standard techniques, and we introduce an efficient fidelity test - based on homodyne detection - that allows the client to verify the correctness of the computation. This test is interesting in its own right because it could be employed in the context of state-certification of optical systems. The division of quantum hardware between client and server assumed here is typical of the experimental constraints expected in realistic, commercially useful schemes for continuous-variable cloud quantum computing. As such, we believe our protocol constitutes a significant advance towards their actual realisation.

DOI
10.1117/12.2326145

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Limitations due to high costs and technological requirements will require users to access the first quantum processors through the cloud. Delegating a computation, however, raises privacy issues about the clients' data and poses questions about the verifiability of computations in high complexity regimes. These concerns have inspired a plethora of blind and verifiable quantum computation protocols, which allow a client with limited quantum capabilities to delegate a quantum computation to a remote server while hiding her data and preserving the integrity of the computation. The majority of these protocols are constrained to discrete quantum systems, but quantum information can also be processed by continuous-variable architectures. These offer a competitive alternative to their discrete-variable counterparts for numerous practical benefits: They rely on well-established quantum optical techniques, allow for the generation of very large optical resource states, offer higher detection efficiencies, and can be integrated into existing optical-fibre networks, all of which are highly desirable features for cloud quantum computing. In this work we fill this gap by presenting a blind and verifiable quantum computing protocol tailored to the unique features of continuous-variable systems. One such feature is the experimental accessibility of Gaussian operations. Our protocol is then based on the delegation of the experimentally challenging non-Gaussian operations. In this sense, it is experimentally friendly to the client, who only needs to perform Gaussian operations. Furthermore, unlike previous schemes, this protocol does not require repeated interactions between the client and server because it only involves the server sending the client non-Gaussian states. We prove universality and blindness using standard techniques, and we introduce an efficient fidelity test - based on homodyne detection - that allows the client to verify the correctness of the computation. This test is interesting in its own right because it could be employed in the context of state-certification of optical systems. The division of quantum hardware between client and server assumed here is typical of the experimental constraints expected in realistic, commercially useful schemes for continuous-variable cloud quantum computing. As such, we believe our protocol constitutes a significant advance towards their actual realisation.
We present a verifiable and blind protocol for assisted universal quantum computing on continuous-variable (CV) platforms. This protocol is experimentally friendly to the client, as it only requires Gaussian-operation capabilities from the latter. Moreover, the server does not require universal quantum-computational power either, its only function being to supply the client with copies of a single-mode non-Gaussian state. Universality is attained based on state injection of the server's non-Gaussian supplies. The protocol is automatically blind because the non-Gaussian resource requested to the server is always the same, regardless of the specific computation. Verification, in turn, is possible thanks to an efficient non-Gaussian state fidelity test where we assume identical state preparation by the server. It is based on Gaussian measurements by the client on the injected states, which is potentially interesting on its own. The division of quantum hardware between client and server assumed here is in agreement with the experimental constraints expected in realistic schemes for CV cloud quantum computing.
Recent experimental achievements motivate an ever-growing interest from companies starting to feel the limitations of classical computing. Yet, in light of ongoing privacy scandals, the future availability of quantum computing through remotely accessible servers pose peculiar challenges: Clients with quantum-limited capabilities want their data and algorithms to remain hidden, while being able to verify that their computations are performed correctly. Research in blind and verifiable delegation of quantum computing attempts to address this question. However, available techniques suffer not only from high overheads but also from over-sensitivity: When running on noisy devices, imperfections trigger the same detection mechanisms as malicious attacks, resulting in perpetually aborted computations. Hence, while malicious quantum computers are rendered harmless by blind and verifiable protocols, inherent noise severely limits their usability. We address this problem with an efficient, robust, blind, verifiable scheme to delegate deterministic quantum computations with classical inputs and outputs. We show that: 1) a malicious Server can cheat at most with an exponentially small success probability; 2) in case of sufficiently small noise, the protocol succeeds with a probability exponentially close to 1; 3) the overhead is barely a polynomial number of repetitions of the initial computation interleaved with test runs requiring the same physical resources in terms of memory and gates; 4) the amount of tolerable noise, measured by the probability of failing a test run, can be as high as 25% for some computations and will be generally bounded by 12.5% when using a planar graph resource state. The key points are that security can be provided without universal computation graphs and that, in our setting, full fault-tolerance is not needed to amplify the confidence level exponentially close to 1.
Blind quantum computing (BQC) allows a client to delegate quantum computations to a remote server without revealing the input, computation, or output. In addition to being blind, the client can sometimes also verify that the server has performed their instructions correctly, a property known as verifiability. A key part of realizing such verifiable BQC (VBQC) is choosing the design of the client device: many architectures have been proposed, each with different hardware requirements, security properties, and performance characteristics, making it difficult to identify which is most suitable for a given implementation. In this work, we present a comparative analysis of client architectures for VBQC with a matter-qubit server. We restrict our analysis to single-server, single-client protocols with information-theoretic security based on measurement-based quantum computation. We identify three main categories of client: emission-based, measurement-based, and rotation-based, each with multiple variants depending on how the client interacts with the server. We evaluate each across different dimensions: we compare guarantees of existing corresponding security proofs, we derive equations for the rate at which each client can execute a protocol, we provide an overview of each architecture's error behaviour, and discuss hardware cost and design considerations. Client architectures implementing measurement-based remote state preparation and reflection-based teleportation emerge as strong default candidates, but as the right choice remains context-dependent, we provide a framework for navigating considerations to guide the selection of the most suitable architecture for a given setting.
Verifiable blind quantum computing allows a client with poor quantum devices to delegate universal quantum computing to a remote quantum server in such a way that the client's privacy is protected and the honesty of the server is verified. In existing protocols, the client has to send single-qubit states to the server. These states might be decohered by the channel noise. Furthermore, the client hides some "trap" qubits in the server's register so that the client can detect the server's deviation. In reality, however, these trap qubits are disturbed by imperfect operations by the server, which reduces the probability that the client accepts the honest server. To solve these problems, we propose a new gadget that allows the client to remotely prepare encoded logical single-qubit states in the server's place. Importantly, in our fault-tolerant verifiable blind quantum computing protocol, the client needs only the ability of physical single-qubit measurements in XXX and ZZZ bases.
Verifiable blind quantum computing is a secure delegated quantum computing where a client with a limited quantum technology delegates her quantum computing to a server who has a universal quantum computer. The client's privacy is protected (blindness), and the correctness of the computation is verifiable by the client despite her limited quantum technology (verifiability). There are mainly two types of protocols for verifiable blind quantum computing: the protocol where the client has only to generate single-qubit states and the protocol where the client needs only the ability of single-qubit measurements. The latter is called the measurement-only verifiable blind quantum computing. If the input of the client's quantum computing is a quantum state, whose classical efficient description is not known to the client, there was no way for the measurement-only client to verify the correctness of the input. Here we introduce a protocol of measurement-only verifiable blind quantum computing where the correctness of the quantum input is also verifiable.
Abstract With the advent of cloud-based quantum computing, it has become vital to provide strong guarantees that computations delegated by clients to quantum service providers have been executed faithfully. Secure—blind and verifiable—delegated quantum computing (SDQC) has emerged as one of the key approaches to address this challenge, yet current protocols lack at least one of the following three ingredients: composability, noise-robustness and modularity. To tackle this question, our paper lays out the fundamental structure of SDQC protocols, namely mixing two components: the computation which the client would like the server to perform and tests that are designed to detect a server’s malicious behaviour. Using this abstraction, our main technical result is a set of sufficient conditions on these components which imply the security and noise-robustness of generic SDQC protocols in the composable Abstract Cryptography framework. This is done by establishing a correspondence between these security properties and the error-detection capabilities of the test computations. Changing the types of tests and how they are mixed with the client’s computation automatically yields new SDQC protocols with different security and noise-robustness capabilities. This approach thereby provides the desired modularity as our sufficient conditions on test computations simplify the steps required to prove the security of the protocols and allows to focus on the design and optimisation of test rounds to specific situations. We showcase this by systematising the search for improved SDQC protocols for bounded-error quantum polynomial-time ( BQP ) computations. The resulting protocols do not require more hardware on the server’s side than what is necessary to blindly delegate the computation without verification, and they outperform all previously known results.