B.J. Davies
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Quantum networks hold the potential to enable new applications, such as secure key distribution, high-precision distributed sensing, and distributed quantum computing. A central functionality of a quantum network is the distribution of entanglement between remote parties. Since experimental implementations remain in an early stage, it is important to understand both the capabilities and limitations of near-term architectures. However, characterising quantum network performance is challenging, due to the complex, stochastic nature of even simple architectures. Analytical studies can therefore play a crucial role: they not only reduce computational cost but also reveal fundamental relationships between performance, the choice of entanglement distribution protocols, and properties of quantum network hardware. In this thesis, we develop analytical methods to study quantumnetwork performance in several important scenarios.
We firstly analyse entanglement buffers, which are systems designed to generate and store high-quality entangled states to be consumed at any time. For this setting, we derive analytical expressions for two key performance metrics. The solutions are computationally efficient, make no restrictive assumptions about the entanglement purification protocol, and allow general insights: for example, that simple purification schemes can outperformmore complex ones previously considered “optimal” in different contexts.
Then,we turn to the problemof entanglement packet generation,where multiple entangled states of sufficient quality must be established simultaneously between network users. The fast generation of entanglement packets is an essential capability for many quantum network protocols. We obtain analytical results for the entanglement packet generation rate under a constant entanglement generation scheme and later extend the analysis to adaptive schemes, where entanglement parameters are tuned dynamically. Using parameter regimes motivated by current experiments, we show that adaptivity can enhance the entanglement packet generation rate by up to a factor of twenty.
Finally, we examine a standard assumption in performance analyses: that the initial states in a quantum repeater chain can be approximated by a symmetrised, or “twirled”, form. We investigate this assumption in the contexts of postselected and non-postselected entanglement swapping, where postselection is performed based on the Bell-state measurement outcomes at the repeaters. A central result is that, in many relevant cases, the twirled approximation is exact for non-postselected swapping. More generally, we provide a systematic framework to determine when the twirled approximation is valid for the initial states of a repeater chain.
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We firstly analyse entanglement buffers, which are systems designed to generate and store high-quality entangled states to be consumed at any time. For this setting, we derive analytical expressions for two key performance metrics. The solutions are computationally efficient, make no restrictive assumptions about the entanglement purification protocol, and allow general insights: for example, that simple purification schemes can outperformmore complex ones previously considered “optimal” in different contexts.
Then,we turn to the problemof entanglement packet generation,where multiple entangled states of sufficient quality must be established simultaneously between network users. The fast generation of entanglement packets is an essential capability for many quantum network protocols. We obtain analytical results for the entanglement packet generation rate under a constant entanglement generation scheme and later extend the analysis to adaptive schemes, where entanglement parameters are tuned dynamically. Using parameter regimes motivated by current experiments, we show that adaptivity can enhance the entanglement packet generation rate by up to a factor of twenty.
Finally, we examine a standard assumption in performance analyses: that the initial states in a quantum repeater chain can be approximated by a symmetrised, or “twirled”, form. We investigate this assumption in the contexts of postselected and non-postselected entanglement swapping, where postselection is performed based on the Bell-state measurement outcomes at the repeaters. A central result is that, in many relevant cases, the twirled approximation is exact for non-postselected swapping. More generally, we provide a systematic framework to determine when the twirled approximation is valid for the initial states of a repeater chain.
...
Quantum networks hold the potential to enable new applications, such as secure key distribution, high-precision distributed sensing, and distributed quantum computing. A central functionality of a quantum network is the distribution of entanglement between remote parties. Since experimental implementations remain in an early stage, it is important to understand both the capabilities and limitations of near-term architectures. However, characterising quantum network performance is challenging, due to the complex, stochastic nature of even simple architectures. Analytical studies can therefore play a crucial role: they not only reduce computational cost but also reveal fundamental relationships between performance, the choice of entanglement distribution protocols, and properties of quantum network hardware. In this thesis, we develop analytical methods to study quantumnetwork performance in several important scenarios.
We firstly analyse entanglement buffers, which are systems designed to generate and store high-quality entangled states to be consumed at any time. For this setting, we derive analytical expressions for two key performance metrics. The solutions are computationally efficient, make no restrictive assumptions about the entanglement purification protocol, and allow general insights: for example, that simple purification schemes can outperformmore complex ones previously considered “optimal” in different contexts.
Then,we turn to the problemof entanglement packet generation,where multiple entangled states of sufficient quality must be established simultaneously between network users. The fast generation of entanglement packets is an essential capability for many quantum network protocols. We obtain analytical results for the entanglement packet generation rate under a constant entanglement generation scheme and later extend the analysis to adaptive schemes, where entanglement parameters are tuned dynamically. Using parameter regimes motivated by current experiments, we show that adaptivity can enhance the entanglement packet generation rate by up to a factor of twenty.
Finally, we examine a standard assumption in performance analyses: that the initial states in a quantum repeater chain can be approximated by a symmetrised, or “twirled”, form. We investigate this assumption in the contexts of postselected and non-postselected entanglement swapping, where postselection is performed based on the Bell-state measurement outcomes at the repeaters. A central result is that, in many relevant cases, the twirled approximation is exact for non-postselected swapping. More generally, we provide a systematic framework to determine when the twirled approximation is valid for the initial states of a repeater chain.
We firstly analyse entanglement buffers, which are systems designed to generate and store high-quality entangled states to be consumed at any time. For this setting, we derive analytical expressions for two key performance metrics. The solutions are computationally efficient, make no restrictive assumptions about the entanglement purification protocol, and allow general insights: for example, that simple purification schemes can outperformmore complex ones previously considered “optimal” in different contexts.
Then,we turn to the problemof entanglement packet generation,where multiple entangled states of sufficient quality must be established simultaneously between network users. The fast generation of entanglement packets is an essential capability for many quantum network protocols. We obtain analytical results for the entanglement packet generation rate under a constant entanglement generation scheme and later extend the analysis to adaptive schemes, where entanglement parameters are tuned dynamically. Using parameter regimes motivated by current experiments, we show that adaptivity can enhance the entanglement packet generation rate by up to a factor of twenty.
Finally, we examine a standard assumption in performance analyses: that the initial states in a quantum repeater chain can be approximated by a symmetrised, or “twirled”, form. We investigate this assumption in the contexts of postselected and non-postselected entanglement swapping, where postselection is performed based on the Bell-state measurement outcomes at the repeaters. A central result is that, in many relevant cases, the twirled approximation is exact for non-postselected swapping. More generally, we provide a systematic framework to determine when the twirled approximation is valid for the initial states of a repeater chain.
Entanglement buffers are systems that maintain high-quality entanglement, ensuring it is readily available for consumption when needed. We study the performance of a two-node buffer, where each node has one long-lived quantum memory for entanglement storage and multiple short-lived memories for generation. Freshly generated entanglement may be used to purify stored entanglement, which degrades over time. Stored entanglement may be removed due to consumption or failed purification. We derive analytical expressions for the entanglement availability and the average fidelity upon consumption. Our solutions are computationally efficient and provide fundamental bounds to the performance of purification-based entanglement buffers. We also show that purification must be performed as frequently as possible to maximise the average fidelity of entanglement upon consumption, even if this often leads to the loss of high-quality entanglement due to purification failures. Moreover, we obtain heuristics for the design of good purification policies in practical systems.
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Entanglement buffers are systems that maintain high-quality entanglement, ensuring it is readily available for consumption when needed. We study the performance of a two-node buffer, where each node has one long-lived quantum memory for entanglement storage and multiple short-lived memories for generation. Freshly generated entanglement may be used to purify stored entanglement, which degrades over time. Stored entanglement may be removed due to consumption or failed purification. We derive analytical expressions for the entanglement availability and the average fidelity upon consumption. Our solutions are computationally efficient and provide fundamental bounds to the performance of purification-based entanglement buffers. We also show that purification must be performed as frequently as possible to maximise the average fidelity of entanglement upon consumption, even if this often leads to the loss of high-quality entanglement due to purification failures. Moreover, we obtain heuristics for the design of good purification policies in practical systems.
Quantum networks crucially rely on the availability of high-quality entangled pairs of qubits, known as entangled links, distributed across distant nodes. Maintaining the quality of these links is a challenging task due to the presence of time-dependent noise, also known as decoherence. Entanglement purification protocols offer a solution by converting multiple low-quality entangled states into a smaller number of higher-quality ones. In this work, we introduce a framework to analyse the performance of entanglement buffering setups that combine entanglement consumption, decoherence, and entanglement purification. We propose two key metrics: the availability, which is the steady-state probability that an entangled link is present, and the average consumed fidelity, which quantifies the steady-state quality of consumed links. We then investigate a two-node system, where each node possesses two quantum memories: one for long-term entanglement storage, and another for entanglement generation. We model this setup as a continuous-time stochastic process and derive analytical expressions for the performance metrics. Our findings unveil a trade-off between the availability and the average consumed fidelity. We also bound these performance metrics for a buffering system that employs the well-known bilocal Clifford purification protocols. Importantly, our analysis demonstrates that, in the presence of noise, consistently purifying the buffered entanglement increases the average consumed fidelity, even when some buffered entanglement is discarded due to purification failures.
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Quantum networks crucially rely on the availability of high-quality entangled pairs of qubits, known as entangled links, distributed across distant nodes. Maintaining the quality of these links is a challenging task due to the presence of time-dependent noise, also known as decoherence. Entanglement purification protocols offer a solution by converting multiple low-quality entangled states into a smaller number of higher-quality ones. In this work, we introduce a framework to analyse the performance of entanglement buffering setups that combine entanglement consumption, decoherence, and entanglement purification. We propose two key metrics: the availability, which is the steady-state probability that an entangled link is present, and the average consumed fidelity, which quantifies the steady-state quality of consumed links. We then investigate a two-node system, where each node possesses two quantum memories: one for long-term entanglement storage, and another for entanglement generation. We model this setup as a continuous-time stochastic process and derive analytical expressions for the performance metrics. Our findings unveil a trade-off between the availability and the average consumed fidelity. We also bound these performance metrics for a buffering system that employs the well-known bilocal Clifford purification protocols. Importantly, our analysis demonstrates that, in the presence of noise, consistently purifying the buffered entanglement increases the average consumed fidelity, even when some buffered entanglement is discarded due to purification failures.
Quantum protocols commonly require a certain number of quantum resource states to be available simultaneously. An important class of examples is quantum network protocols that require a certain number of entangled pairs. Here, we consider a setting in which a process generates a quantum resource state with some probability p in each time step and stores it in a quantum memory that is subject to time-dependent noise. To maintain sufficient quality for an application, each resource state is discarded from the memory after w time steps. Let s be the number of desired resource states required by a protocol. We characterize the probability distribution X-{(w,s)} of the ages of the quantum resource states, once s states have been generated in a window w. Combined with a time-dependent noise model, knowledge of this distribution allows for the calculation of fidelity statistics of the s quantum resources. We also give exact solutions for the first and second moments of the waiting time \tau -{(w,s)} until s resources are produced within a window w, which provides information about the rate of the protocol. Since it is difficult to obtain general closed-form expressions for statistical quantities describing the expected waiting time \mathbb {E}(\tau -{(w,s)}) and the distribution X-{(w,s)}, we present two novel results that aid their computation in certain parameter regimes. The methods presented in this work can be used to analyze and optimize the execution of quantum protocols. Specifically, with an example of a blind quantum computing protocol, we illustrate how they may be used to infer w and p to optimize the rate of successful protocol execution.
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Quantum protocols commonly require a certain number of quantum resource states to be available simultaneously. An important class of examples is quantum network protocols that require a certain number of entangled pairs. Here, we consider a setting in which a process generates a quantum resource state with some probability p in each time step and stores it in a quantum memory that is subject to time-dependent noise. To maintain sufficient quality for an application, each resource state is discarded from the memory after w time steps. Let s be the number of desired resource states required by a protocol. We characterize the probability distribution X-{(w,s)} of the ages of the quantum resource states, once s states have been generated in a window w. Combined with a time-dependent noise model, knowledge of this distribution allows for the calculation of fidelity statistics of the s quantum resources. We also give exact solutions for the first and second moments of the waiting time \tau -{(w,s)} until s resources are produced within a window w, which provides information about the rate of the protocol. Since it is difficult to obtain general closed-form expressions for statistical quantities describing the expected waiting time \mathbb {E}(\tau -{(w,s)}) and the distribution X-{(w,s)}, we present two novel results that aid their computation in certain parameter regimes. The methods presented in this work can be used to analyze and optimize the execution of quantum protocols. Specifically, with an example of a blind quantum computing protocol, we illustrate how they may be used to infer w and p to optimize the rate of successful protocol execution.