We introduce a simple photonic probing scheme of remote nondestructive parity measurement (RNPM) on a pair of matter qubits. The protocol works as a single module for key operations such as entanglement generation, Bell measurement, and parity check measurement, which are sufficient not only for building up a quantum repeater but also for equipping it with entanglement distillation. Moreover, the RNPM protocol can also be used for generating cluster states toward measurement-based quantum computation.
We introduce a simple photonic probing scheme of remote nondestructive parity measurement (RNPM) on a pair of matter qubits. The protocol works as a single module for key operations such as entanglement generation, Bell measurement, and parity check measurement, which are sufficient not only for building up a quantum repeater but also for equipping it with entanglement distillation. Moreover, the RNPM protocol can also be used for generating cluster states toward measurement-based quantum computation.
Parity measurement is a key step in many entanglement generation and quantum error correction schemes. We propose a protocol for nondestructive parity measurement of two remote qubits, i.e., macroscopically separated qubits with no direct interaction. The qubits are instead dispersively coupled to separate resonators that radiate to shared photodetectors. The scheme is deterministic in the sense that there is no fundamental upper bound on the success probability. In contrast to previous proposals, our protocol addresses the scenario where number-resolving photodetectors are available but the qubit-resonator coupling is time independent and only dispersive.
Measurement-based quantum repeaters employ entanglement distillation and swapping across links using locally prepared resource states of minimal size and local Bell measurements. In this Letter, we introduce a systematic protocol for measurement-based entanglement distillation and its application to repeaters that can leverage any stabilizer code. Given a code, we explicitly define the corresponding resource state and derive an error-recovery operation based on all Bell measurement outcomes. Our approach offers deeper insights into the impact of resource state noise on repeater performance while also providing strategies for efficient preparation and fault-tolerant preservation of resource states. As an application, we propose a measurement-based repeater protocol based on quantum low-density parity-check (QLDPC) codes, enabling constant-yield Bell state distribution over arbitrary distances. Numerical simulations identify a fault-tolerant threshold on the total physical error per repeater segment-including errors on resource states, remotely generated Bell states, and Bell measurements-and confirm that increasing the QLDPC code size further suppresses the logical error while maintaining a fixed encoding rate. This Letter establishes a scalable backbone for future global-scale fault-tolerant quantum networks.
Future quantum networks will enable the realisation of a large family of quantum protocols, hence allowing for implementation of various tasks ranging from quantum cryptography through quantum computing to quantum metrology.In this thesis we analyse and develop the building blocks of quantum repeater networks.These networks consist of quantum repeater nodes, which are equipped with quantum memories enabling storage of quantum information.The repeater nodes are connected using optical fibres that enable for the transmission of photonic quantum signals between the repeater nodes.We specifically focus on the two building blocks: remote entanglement generation and entanglement distillation.We then investigate various proof of principle quantum repeater schemes used for generating shared secret key between distant parties.The repeater schemes that we consider utilise one to three memory nodes capable of generating memory-photon entanglement.Finally, we introduce a novel framework for investigating a foundational aspect in the use of quantum communication related to the uncertainty principle.Efficient remote entanglement generation is an indispensable building block of most near-term quantum repeater networks.In this thesis we consider various remote entanglement generation schemes that permit multiplexing, which aims at increasing the throughput of this procedure.We find that for the specific schemes that are suitable for platforms with a single communication qubit and multiple memory qubits, the performance of the multiplexing approach is strongly dependent on the time it takes to perform local operations within the memory nodes.Entanglement distillation enables for filtering out the noise from imperfect entangled states using only local operations and classical communication.Using a general framework for optimising such schemes, which we have developed, we have proven optimality of specific generic and experimentally relevant distillation schemes.We have developed a general framework for optimising such schemes, which we have used to prove optimality of various generic and experimentally relevant distillation schemes.In designing proof of principle quantum repeater schemes we assume an informationtheoretic approach, which enables us to assess their performance in a hardware-agnostic way.We find that all the schemes that we consider can prove useful in specific parameter regimes for the nitrogen-vacancy centre platform.Moreover, we find that one scheme which has already been realised experimentally in the context of remote entanglement generation, is expected to significantly outperform every possible system based on direct communication without repeaters.However, our analysis shows that encapsulating the nitrogen-vacancy centres in optical cavities for the enhancement of the emission rates is a necessary requirement for such a successful demonstration of the first proofof-principle quantum repeater.xi xii SUMMARY Finally, we investigate the quantum uncertainty principle which is a fundamental feature of quantum mechanics, allowing for security in many quantum cryptographic protocols.We find that in a particular scenario, relevant from the perspective of quantum key distribution, a significant part of the observed uncertainty is in fact due to lack of information rather than intrinsic.If the eavesdropper could access that information, she would be able to much better, or in some cases even with certainty, guess the value of the raw bit of the generated key.
Entangled coherent states can be prepared remotely by subtracting nonlocally a single photon from two quantum superpositions of coherent states, the so-called “Schrödinger’s cat” state. Such entanglement can further be distributed over longer distances by successive entanglement swapping operations using linear optics and photon-number resolving detectors. The aim of this paper is to evaluate the performance of this approach to quantum repeaters for long-distance quantum communications. Despite many attractive features at first sight, we show that, when using state-of-the-art photon counters and quantum memories, they do not achieve higher entanglement generation rates than repeaters based on single-photon entanglement. We discuss potential developments, which may take better advantage of the richness of entanglement based on continuous variables, including in particular efficient parity measurements.
We propose a scheme to measure the parity of two distant qubits, while ensuring that losses on the quantum channel between them does not destroy coherences within the parity subspaces. This capability enables deterministic preparation of highly entangled qubit states whose fidelity is not limited by the transmission loss. The key observation is that, for a probe electromagnetic field in a particular quantum state; namely, a superposition of two coherent states of opposite phases, the transmission loss stochastically applies a near-unitary backaction on the probe state. This leads to a parity measurement protocol where the main effect of the transmission losses is a decrease in the measurement strength. By repeating the nondestructive (weak) parity measurement, one achieves a high-fidelity entanglement in spite of a significant transmission loss.
Entanglement is a fundamental resource not only for quantum communication but also for distributed quantum computation. Especially, entanglement including only one type of error is favorable, compared with one including multiple types of noise. In this talk, we consider protocol that presents single-error-type entanglement for distant qubits via coherent-state transmission over lossy channels. This protocol is considered to be a subroutine to serve entanglement for larger protocol to yield a final output, such as ebits or pbits. A protocol based on remote non-destructive parity measurement (RNPM) [K. Azuma, H. Takeda, M. Koashi, and N. Imoto, Phys. Rev. A 85, 062309 (2012)] is identified as a subroutine which achieves the global optimal for typical yield functions monotonically non-decreasing with respect to the singlet fraction, such as an arbitrary convex function of a singlet fraction and two-way distillable entanglement/key.