Secure Delegated Quantum Computation (SDQC) protocols are a vital piece of the future quantum information processing global architecture since they allow end-users to perform their valuable computations on remote quantum servers without fear that a malicious quantum service provider or an eavesdropper might acquire some information about their data or algorithm. They also allow end-users to check that their computation has been performed as they have specified it. However, existing protocols all have drawbacks that limit their usage in the real world. Most require the client to either operate a single-qubit source or perform single-qubit measurements, thus requiring them to still have some quantum technological capabilities albeit restricted, or require the server to perform operations which are hard to implement on real hardware (e.g isolate single photons from laser pulses and polarisation-preserving photon-number quantum non-demolition measurements). Others remove the need for quantum communications entirely but this comes at a cost in terms of security guarantees and memory overhead on the server's side. We present an SDQC protocol which drastically reduces the technological requirements of both the client and the server while providing information-theoretic composable security. More precisely, the client only manipulates an attenuated laser pulse, while the server only handles interacting quantum emitters with a structure capable of generating spin-photon entanglement. The quantum emitter acts as both a converter from coherent laser pulses to polarisation-encoded qubits and an entanglement generator. Such devices have recently been used to demonstrate the largest entangled photonic state to date, thus hinting at the readiness of our protocol for experimental implementations.
Secure Delegated Quantum Computation (SDQC) protocols are a vital piece of the future quantum information processing global architecture since they allow end-users to perform their valuable computations on remote quantum servers without fear that a malicious quantum service provider or an eavesdropper might acquire some information about their data or algorithm. They also allow end-users to check that their computation has been performed as they have specified it. However, existing protocols all have drawbacks that limit their usage in the real world. Most require the client to either operate a single-qubit source or perform single-qubit measurements, thus requiring them to still have some quantum technological capabilities albeit restricted, or require the server to perform operations which are hard to implement on real hardware (e.g isolate single photons from laser pulses and polarisation-preserving photon-number quantum non-demolition measurements). Others remove the need for quantum communications entirely but this comes at a cost in terms of security guarantees and memory overhead on the server's side. We present an SDQC protocol which drastically reduces the technological requirements of both the client and the server while providing information-theoretic composable security. More precisely, the client only manipulates an attenuated laser pulse, while the server only handles interacting quantum emitters with a structure capable of generating spin-photon entanglement. The quantum emitter acts as both a converter from coherent laser pulses to polarisation-encoded qubits and an entanglement generator. Such devices have recently been used to demonstrate the largest entangled photonic state to date, thus hinting at the readiness of our protocol for experimental implementations.
Secure Delegated Quantum Computation (SDQC) protocols allow a client to delegate a quantum computation to a powerful remote server while ensuring the privacy and the integrity of its computation. Recent resource-efficient and noise-robust protocols led to experimental proofs of concept. Yet, their physical requirements are still too stringent to be added directly to the roadmap of quantum hardware vendors. To address part of this issue, this paper shows how to alleviate the necessity for the client to have a single-photon source. We describe an SDQC protocol, similar in spirit to the decoy state method used in QKD, which requires only a source of weak coherent pulses. It is noise-robust, composable, and has negligible correctness and security errors. The simplest instantiation of our protocol with only 2 intensities shows improved scaling at low transmittance and adds verifiability to previous proposals.
The scarcity of quantum resources is a significant theme in quantum cryptography and computation. Within quantum cryptography, secure delegated quantum computation (SDQC) allows low-resource users to access quantum computation with privacy. From the study of models of quantum computation, Pauli-based computation (PBC) has emerged as a resource-efficient alternative to conventional models. In this thesis, we merge these two strands of work, presenting the first protocol for SDQC in the PBC model. Our protocol allows a client with minimal quantum resources to execute a PBC on a secret classical input of their choice by interacting with a quantum server. We prove that the protocol gives the client perfect, information-theoretic security. Our protocol inherits the resource efficiencies of PBC, significantly reducing the size of the quantum register required by the server, implying that SDQC could be available sooner than anticipated in a world of scarce quantum resources.
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.
Known protocols for secure delegation of quantum computations from a client to a server in an information theoretic setting require quantum communication. In this work, we investigate methods to reduce communication overhead. First, we establish an impossibility result by proving that server-side local processes cannot decrease quantum communication requirements of secure delegation protocols. We develop no-go results that prohibit such processes within an information theoretic framework. Second, we present a possibility result by introducing Selectively Blind Quantum Computing (SBQC), a novel functionality that allows the client to hide one among a known set of possible computations. We characterize how differences between computations in the protected set influence the number of qubits sent during our SBQC implementation, yielding a communication-optimal protocol. This approach reduces qubit communication drastically and demonstrates the trade-off between information leaked to the server and communication cost.
In this paper, we propose a distributed secure delegated quantum computation protocol, by which an almost classical client can delegate a (dk)-qubit quantum circuit to d quantum servers, where each server is equipped with a 2k-qubit register that is used to process only k qubits of the delegated quantum circuit. None of servers can learn any information about the input and output of the computation. The only requirement for the client is that he or she has ability to prepare four possible qubits in the state of (|0⟩+eiθ|1⟩)/2, where θ∈{0,π/2,π,3π/2}. The only requirement for servers is that each pair of them share some entangled states (|0⟩|+⟩+|1⟩|−⟩)/2 as ancillary qubits. Instead of assuming that all servers are interconnected directly by quantum channels, we introduce a third party in our protocol that is designed to distribute the entangled states between those servers. This would simplify the quantum network because the servers do not need to share a quantum channel. In the end, we show that our protocol can guarantee unconditional security of the computation under the situation where all servers, including the third party, are honest-but-curious and allowed to cooperate with each other.
The rapid evolution of quantum technologies is likely to cause major shifts in the mainstream computing landscape. In order to fully reach their potential in a wide base accessible to any user, remote access of quantum computers and manipulation of data with strong privacy and integrity guarantees are essential. Consider a setting where a client having a fully classical computer wants to determine the result of some quantum computation, but lacks the necessary resources to perform the computation herself. She has access to a more powerful server which has quantum resources and can solve the problem and send the outcome back to the client. However, the client does not trust the powerful server, so she needs to find a way to hide her data. Therefore, the main question that arises is how can we guarantee the client’s privacy of the input and even the computation itself against the server possessing quantum computational capabilities. In the present thesis, we study this problem, denoted here as classical secure delegation of quantum computations (CSDQC) between a fully classical honest client and a quantum untrusted server. We focus on different models of security, analyzing the limitations and potential of each of the settings. Concretely, we first study the CSDQC problem under information-theoretic security. We analyse two categories of quantum computations, decision and sampling problems and in both cases we provide evidence indicating the impossibility of achieving information-theoretic security. Subsequently, we consider relaxing the security framework and specifically, we will analyze this task in the computational security setting (against quantum polynomial-time adversaries). As a result, in the second part of the thesis we put forward the remote state preparation as a key component that would allow us to achieve classical secure delegation of universal quantum computations. We present two protocols realizing the remote state preparation primitive assuming only a classical channel between client and server. The first candidate is shown to be secure in the honest-but-curious model, while the second candidate is proven secure against the server in the malicious setting. The security of both constructions relies on the hardness of the learning with errors problem. Finally, given the important role the remote state preparation plays not only in CSDQC, but also in other quantum communication protocols, we analyze its composable security to determine the privacy loss as a result of using remote state preparation as a sub-module in different protocols.