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Register a new userNumeric estimation of resource requirements for a practical polarization-frame alignment scheme for QKD
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Due to physical orientations and birefringence effects, practical quantum information protocols utilizing optical polarization need to handle misalignment between preparation and measurement reference frames. For any such capable system, an important question is how many resources -- e.g., measured single photons -- are needed to reliably achieve alignment precision sufficient for the desired quantum protocol. Here we study the performance of a polarization-frame alignment scheme used in prior laboratory and field quantum key distribution (QKD) experiments by performing Monte Carlo numerical simulations. The scheme utilizes, to the extent possible, the same single-photon-level signals and measurements as for the QKD protocol being supported. Even with detector noise and imperfect sources, our analysis shows that only a small fraction of resources from the overall signal -- a few hundred photon detections, in total -- are required for good performance, restoring the state to better than 99% of its original quality.
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We propose a method for reconfiguring a relay node for polarization encoded quantum key distribution (QKD) networks. The relay can be switched between trusted and untrusted modes to adapt to different network conditions, relay distances, and security requirements. This not only extends the distance over which a QKD network operates but also enables point-to-multipoint (P2MP) network topologies. The proposed architecture centralizes the expensive and delicate single-photon detectors (SPDs) at the relay node with eased maintenance and cooling while simplifying each user node so that it only needs commercially available devices for low-cost qubit preparation.
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We propose and experimentally demonstrate a plug-and-play, practical, and enabling method allowing to synchronize the building blocks of a quantum network in an all-optical way. Our scheme relies on mature and reliable classical telecommunication and non-linear optical technologies and can be implemented in a universal way with off-the-shelf components. Compared to already reported solutions, it allows achieving high-quality synchronization compatible with high network-operation rate and is free from opto-electronic jitters affecting servo-loop based configurations. We test our scheme with a genuine quantum optical method in terms of the interference between two photons coming from two remotely synchronized sources spaced by distances of up to 100 km. Measured visibilities well above 90% confirm the validity of our approach. Due its simplicity and high-quality performance, our scheme paves the way for the synchronization of long-distance quantum networks based on fibre, free-space, as well as hybrid solutions.
In this work we consider practical implementations of Kitaevs algorithm for quantum phase estimation. We analyze the use of phase shifts that simplify the estimation of successive bits in the estimation of unknown phase $varphi$. By using increasingly accurate shifts we reduce the number of measurements to the point where only a single measurements in needed for each additional bit. This results in an algorithm that can estimate $varphi$ to an accuracy of $2^{-(m+2)}$ with probability at least $1-epsilon$ using $N_{epsilon} + m$ measurements, where $N_{epsilon}$ is a constant that depends only on $epsilon$ and the particular sampling algorithm. We present different sampling algorithms and study the exact number of measurements needed through careful numerical evaluation, and provide theoretical bounds and numerical values for $N_{epsilon}$.
Application of the adiabatic model of quantum computation requires efficient encoding of the solution to computational problems into the lowest eigenstate of a Hamiltonian that supports universal adiabatic quantum computation. Experimental systems are typically limited to restricted forms of 2-body interactions. Therefore, universal adiabatic quantum computation requires a method for approximating quantum many-body Hamiltonians up to arbitrary spectral error using at most 2-body interactions. Hamiltonian gadgets, introduced around a decade ago, offer the only current means to address this requirement. Although the applications of Hamiltonian gadgets have steadily grown since their introduction, little progress has been made in overcoming the limitations of the gadgets themselves. In this experimentally motivated theoretical study, we introduce several gadgets which require significantly more realistic control parameters than similar gadgets in the literature. We employ analytical techniques which result in a reduction of the resource scaling as a function of spectral error for the commonly used subdivision, 3- to 2-body and $k$-body gadgets. Accordingly, our improvements reduce the resource requirements of all proofs and experimental proposals making use of these common gadgets. Next, we numerically optimize these new gadgets to illustrate the tightness of our analytical bounds. Finally, we introduce a new gadget that simulates a $YY$ interaction term using Hamiltonians containing only ${X,Z,XX,ZZ}$ terms. Apart from possible implications in a theoretical context, this work could also be useful for a first experimental implementation of these key building blocks by requiring less control precision without introducing extra ancillary qubits.
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