| 1. |
- Anwer, Hammad, et al.
(author)
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Experimental Characterization of Unsharp Qubit Observables and Sequential Measurement Incompatibility via Quantum Random Access Codes
- 2020
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 125:8
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Journal article (peer-reviewed)abstract
- Unsharp measurements are increasingly important for foundational insights in quantum theory and quantum information applications. Here, we report an experimental implementation of unsharp qubit measurements in a sequential communication protocol, based on a quantum random access code. The protocol involves three parties; the first party prepares a qubit system, the second party performs operations that return both a classical and quantum outcome, and the latter is measured by the third party. We demonstrate a nearly optimal sequential quantum random access code that outperforms both the best possible classical protocol and any quantum protocol that utilizes only projective measurements. Furthermore, while only assuming that the involved devices operate on qubits and that detected events constitute a fair sample, we demonstrate the noise-robust characterization of unsharp measurements based on the sequential quantum random access code. We apply this characterization towards quantifying the degree of incompatibility of two sequential pairs of quantum measurements.
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| 2. |
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| 3. |
- Bakhshinezhad, Pharnam, et al.
(author)
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Scalable Entanglement Certification via Quantum Communication
- 2024
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In: PRX Quantum. - 2691-3399. ; 5:2
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Journal article (peer-reviewed)abstract
- Harnessing the advantages of shared entanglement for sending quantum messages often requires the implementation of complex two-particle entangled measurements. We investigate entanglement advantages in protocols that use only the simplest two-particle measurements, namely, product measurements. For experiments in which only the dimension of the message is known, we show that robust entanglement advantages are possible but that they are fundamentally limited by Einstein-Podolsky-Rosen steering. Subsequently, we propose a natural extension of the standard scenario for these experiments and show that it circumvents this limitation. This leads us to prove entanglement advantages from every entangled two-qubit Werner state, evidence its generalization to high-dimensional systems, and establish a connection to quantum teleportation. Our results reveal the power of product measurements for generating quantum correlations in entanglement-assisted communication and they pave the way for practical semi-device-independent entanglement certification well beyond the constraints of Einstein-Podolsky-Rosen steering.
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| 4. |
- Francesco Diotallevi, Giovanni, et al.
(author)
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Steady-state entanglement production in a quantum thermal machine with continuous feedback control
- 2024
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In: New Journal of Physics. - 1367-2630. ; 26:5
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Journal article (peer-reviewed)abstract
- Quantum thermal machines can generate steady-state entanglement by harvesting spontaneous interactions with local environments. However, using minimal resources and control, the entanglement is typically weak. Here, we study entanglement generation in a two-qubit quantum thermal machine in the presence of a continuous feedback protocol. Each qubit is measured continuously and the outcomes are used for real-time feedback to control the local system-environment interactions. We show that there exists an ideal operation regime where the quality of entanglement is significantly improved, to the extent that it can violate standard Bell inequalities and uphold quantum teleportation. In agreement with (Khandelwal et al 2020 New J. Phys. 22 073039), we also find, for ideal operation, that the heat current across the system is proportional to the entanglement concurrence. Finally, we investigate the robustness of entanglement production when the machine operates away from the ideal conditions.
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| 5. |
- Goel, Suraj, et al.
(author)
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Inverse design of high-dimensional quantum optical circuits in a complex medium
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In: Nature Physics. - 1745-2473.
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Journal article (peer-reviewed)abstract
- Programmable optical circuits are an important tool in developing quantum technologies such as transceivers for quantum communication and integrated photonic chips for quantum information processing. Maintaining precise control over every individual component becomes challenging at large scales, leading to a reduction in the quality of operations performed. In parallel, minor imperfections in circuit fabrication are amplified in this regime, dramatically inhibiting their performance. Here we use inverse design techniques to embed optical circuits in the higher-dimensional space of a large, ambient mode mixer such as a commercial multimode fibre. This approach allows us to forgo control over each individual circuit element, and retain a high degree of programmability. We use our circuits as quantum gates to manipulate high-dimensional spatial-mode entanglement in up to seven dimensions. Their programmability allows us to turn a multimode fibre into a generalized multioutcome measurement device, allowing us to both transport and certify entanglement within the transmission channel. With the support of numerical simulations, we show that our method is a scalable approach to obtaining high circuit fidelity with a low circuit depth by harnessing the resource of a high-dimensional mode mixer.
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| 6. |
- Goel, Suraj, et al.
(author)
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Inverse-Design of High-Dimensional Quantum Optical Circuits in a Complex Medium
- 2023
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In: Frontiers in Optics : Proceedings Frontiers in Optics + Laser Science 2023, FiO, LS 2023 - Proceedings Frontiers in Optics + Laser Science 2023, FiO, LS 2023. - 9781957171296
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Conference paper (peer-reviewed)abstract
- We program high-dimensional linear optical circuits within a commercial multi-mode fibre, turning it into a generalised multi-outcome measurement device, which allows us to both transport and certify high-dimensional entanglement within the transmission channel itself.
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| 7. |
- Hameedi, Alley, et al.
(author)
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Communication Games Reveal Preparation Contextuality
- 2017
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 119:22
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Journal article (peer-reviewed)abstract
- A communication game consists of distributed parties attempting to jointly complete a task with restricted communication. Such games are useful tools for studying limitations of physical theories. A theory exhibits preparation contextuality whenever its predictions cannot be explained by a preparation noncontextual model. Here, we show that communication games performed in operational theories reveal the preparation contextuality of that theory. For statistics obtained in a particular family of communication games, we show a direct correspondence with correlations in spacelike separated events obeying the no-signaling principle. Using this, we prove that all mixed quantum states of any finite dimension are preparation contextual. We report on an experimental realization of a communication game involving three-level quantum systems from which we observe a strong violation of the constraints of preparation noncontextuality.
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| 8. |
- Mironowicz, Piotr, et al.
(author)
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Increased certification of semi-device independent random numbers using many inputs and more post-processing
- 2016
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In: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 18
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Journal article (peer-reviewed)abstract
- Quantum communication with systems of dimension larger than two provides advantages in information processing tasks. Examples include higher rates of key distribution and random number generation. The main disadvantage of using such multi-dimensional quantum systems is the increased complexity of the experimental setup. Here, we analyze a not-so-obvious problem: the relation between randomness certification and computational requirements of the post-processing of experimental data. In particular, we consider semi-device independent randomness certification from an experiment using a four dimensional quantum system to violate the classical bound of a random access code. Using state-of-the-art techniques, a smaller quantum violation requires more computational power to demonstrate randomness, which at some point becomes impossible with today's computers although the randomness is (probably) still there. We show that by dedicating more input settings of the experiment to randomness certification, then by more computational postprocessing of the experimental data which corresponds to a quantum violation, one may increase the amount of certified randomness. Furthermore, we introduce a method that significantly lowers the computational complexity of randomness certification. Our results show how more randomness can be generated without altering the hardware and indicate a path for future semi-device independent protocols to follow.
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| 9. |
- Morelli, Simon, et al.
(author)
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Resource-Efficient High-Dimensional Entanglement Detection via Symmetric Projections
- 2023
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In: Physical Review Letters. - 0031-9007. ; 131:17
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Journal article (peer-reviewed)abstract
- We introduce two families of criteria for detecting and quantifying the entanglement of a bipartite quantum state of arbitrary local dimension. The first is based on measurements in mutually unbiased bases and the second is based on equiangular measurements. Both criteria give a qualitative result in terms of the state's entanglement dimension and a quantitative result in terms of its fidelity with the maximally entangled state. The criteria are universally applicable since no assumptions on the state are required. Moreover, the experimenter can control the trade-off between resource-efficiency and noise-tolerance by selecting the number of measurements performed. For paradigmatic noise models, we show that only a small number of measurements are necessary to achieve nearly-optimal detection in any dimension. The number of global product projections scales only linearly in the local dimension, thus paving the way for detection and quantification of very high-dimensional entanglement.
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| 10. |
- Muhammad, Sadiq, et al.
(author)
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Quantum Bidding in Bridge
- 2014
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In: Physical Review X. - 2160-3308. ; 4:2
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Journal article (peer-reviewed)abstract
- Quantum methods allow us to reduce communication complexity of some computational tasks, with several separated partners, beyond classical constraints. Nevertheless, experimental demonstrations of this have thus far been limited to some abstract problems, far away from real-life tasks. We show here, and demonstrate experimentally, that the power of reduction of communication complexity can be harnessed to gain an advantage in a famous, immensely popular, card game-bridge. The essence of a winning strategy in bridge is efficient communication between the partners. The rules of the game allow only a specific form of communication, of very low complexity (effectively, one has strong limitations on the number of exchanged bits). Surprisingly, our quantum technique does not violate the existing rules of the game (as there is no increase in information flow). We show that our quantum bridge auction corresponds to a biased nonlocal Clauser-Horne-Shimony-Holt game, which is equivalent to a 2 -> 1 quantum random access code. Thus, our experiment is also a realization of such protocols. However, this correspondence is not complete, which enables the bridge players to have efficient strategies regardless of the quality of their detectors.
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