40 research outputs found
Near-term quantum-repeater experiments with nitrogen-vacancy centers: Overcoming the limitations of direct transmission
Quantum channels enable the implementation of communication tasks
inaccessible to their classical counterparts. The most famous example is the
distribution of secret key. However, in the absence of quantum repeaters, the
rate at which these tasks can be performed is dictated by the losses in the
quantum channel. In practice, channel losses have limited the reach of quantum
protocols to short distances. Quantum repeaters have the potential to
significantly increase the rates and reach beyond the limits of direct
transmission. However, no experimental implementation has overcome the direct
transmission threshold. Here, we propose three quantum repeater schemes and
assess their ability to generate secret key when implemented on a setup using
nitrogen-vacancy (NV) centers in diamond with near-term experimental
parameters. We find that one of these schemes - the so-called single-photon
scheme, requiring no quantum storage - has the ability to surpass the capacity
- the highest secret-key rate achievable with direct transmission - by a factor
of 7 for a distance of approximately 9.2 km with near-term parameters,
establishing it as a prime candidate for the first experimental realization of
a quantum repeater.Comment: 19+17 pages, 17 figures. v2: added "Discussion and future outlook"
section and expanded introduction, published versio
Enumerating all bilocal Clifford distillation protocols through symmetry reduction
Entanglement distillation is an essential building block in quantum
communication protocols. Here, we study the class of near-term implementable
distillation protocols that use bilocal Clifford operations followed by a
single round of communication. We introduce tools to enumerate and optimise
over all protocols for up to (not necessarily equal) Bell-diagonal states
using a commodity desktop computer. Furthermore, by exploiting the symmetries
of the input states, we find all protocols for up to copies of a Werner
state. For the latter case, we present circuits that achieve the highest
fidelity. These circuits have modest depth and number of two-qubit gates. Our
results are based on a correspondence between distillation protocols and double
cosets of the symplectic group, and improve on previously known protocols.Comment: 13 pages main text, 5 pages appendices, 8 figure
FOR REAL: Forming Resilience and Employability through Authentic Learning, 2015 action research report
How 'real world' learning aids student learning, resilience and employabilit
Protocols for creating and distilling multipartite GHZ states with Bell pairs
The distribution of high-quality Greenberger–Horne–Zeilinger (GHZ) states is at the heart of many quantum communication tasks, ranging from extending the baseline of telescopes to secret sharing. They also play an important role in error-correction architectures for distributed quantum computation, where Bell pairs can be leveraged to create an entangled network of quantum computers. We investigate the creation and distillation of GHZ states out of nonperfect Bell pairs over quantum networks. In particular, we introduce a heuristic dynamic programming algorithm to optimize over a large class of protocols that create and purify GHZ states. All protocols considered use a common framework based on measurements of nonlocal stabilizer operators of the target state (i.e., the GHZ state), where each nonlocal measurement consumes another (nonperfect) entangled state as a resource. The new protocols outperform previous proposals for scenarios without decoherence and local gate noise. Furthermore, the algorithms can be applied for finding protocols for any number of parties and any number of entangled pairs involved
Dimethyl fumarate in patients admitted to hospital with COVID-19 (RECOVERY): a randomised, controlled, open-label, platform trial
Dimethyl fumarate (DMF) inhibits inflammasome-mediated inflammation and has been proposed as a treatment for patients hospitalised with COVID-19. This randomised, controlled, open-label platform trial (Randomised Evaluation of COVID-19 Therapy [RECOVERY]), is assessing multiple treatments in patients hospitalised for COVID-19 (NCT04381936, ISRCTN50189673). In this assessment of DMF performed at 27 UK hospitals, adults were randomly allocated (1:1) to either usual standard of care alone or usual standard of care plus DMF. The primary outcome was clinical status on day 5 measured on a seven-point ordinal scale. Secondary outcomes were time to sustained improvement in clinical status, time to discharge, day 5 peripheral blood oxygenation, day 5 C-reactive protein, and improvement in day 10 clinical status. Between 2 March 2021 and 18 November 2021, 713 patients were enroled in the DMF evaluation, of whom 356 were randomly allocated to receive usual care plus DMF, and 357 to usual care alone. 95% of patients received corticosteroids as part of routine care. There was no evidence of a beneficial effect of DMF on clinical status at day 5 (common odds ratio of unfavourable outcome 1.12; 95% CI 0.86-1.47; p = 0.40). There was no significant effect of DMF on any secondary outcome
Multipartite Entanglement in Quantum Networks using Subgraph Complementations
Quantum networks are important for quantum communication and consist of
entangled states that are essential for many tasks such as quantum
teleportation, quantum key distribution, quantum sensing and quantum error
correction. Graph states are a specific class of multipartite entangled states
that can be represented by graphs. We propose a novel approach for distributing
graph states across a quantum network. We show that the distribution of graph
states can be characterised by a system of subgraph complementations, which we
also relate to the minimum rank of the underlying graph and the degree of
entanglement quantified by the Schmidt-rank of the quantum state. We analyse
resource usage for our algorithm and show it to match or be improved in the
number of qubits, bits for classical communication and EPR pairs utilised, as
compared to prior work. The number of local operations is efficient, and the
resource consumption for our approach scales linearly in the number of
vertices. This presents a quadratic improvement in completion time for several
classes of graph states represented by dense graphs, and implies a potential
for improved fidelity in the presence of noise. Common classes of graph states
are classified along with the optimal time for their distribution using
subgraph complementations. We also provide a framework to similarly find the
optimal sequence of operations to distribute an arbitrary graph state, and
prove upper bounds along with providing approximate greedy algorithms.Comment: Background section is condensed and requires further clarificatio