arXiv subjects

Yuxiang Yang

Publications and source records attributed to Yuxiang Yang.

117 records · Page 7Linked to original sources

arXiv2017

Optimal quantum operations at zero energy cost

Quantum technologies are developing powerful tools to generate and manipulate coherent superpositions of different energy levels. Envisaging a new generation of energy-efficient quantum devices, here we explore how coherence can be manipulated without exchanging energy with the surrounding environment. We start from the task of converting a coherent superposition of energy eigenstates into another. We identify the optimal energy-preserving operations, both in the deterministic and in the probabilistic scenario. We then design a recursive protocol, wherein a branching sequence of energy-preserving filters increases the probability of success while reaching maximum fidelity at each iteration. Building on the recursive protocol, we construct efficient approximations of the optimal fidelity-probability trade-off, by taking coherent superpositions of the different branches generated by probabilistic filtering. The benefits of this construction are illustrated in applications to quantum metrology, quantum cloning, coherent state amplification, and ancilla-driven computation. Finally, we extend our results to transitions where the input state is generally mixed and we apply our findings to the task of purifying quantum coherence.

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arXiv2017

Certifying quantumness: Benchmarks for the optimal processing of generalized coherent and squeezed states

Quantum technology promises revolutionary advantages in information processing and transmission compared to classical technology; however, determining which specific resources are needed to surpass the capabilities of classical machines often remains a nontrivial problem. To address such a problem, one first needs to establish the best classical solutions, which set benchmarks that must be beaten by any implementation claiming to harness quantum features for an enhanced performance. Here we introduce and develop a self-contained formalism to obtain the ultimate, generally probabilistic benchmarks for quantum information protocols including teleportation and approximate cloning, with arbitrary ensembles of input states generated by a group action, so-called Gilmore-Perelomov coherent states. This allows us to construct explicit fidelity thresholds for the transmission of multimode Gaussian and non-Gaussian states of continuous variable systems, as well as qubit and qudit pure states drawn according to nonuniform distributions on the Bloch hypersphere, which accurately model the current laboratory facilities. The performance of deterministic classical procedures such as square-root measurement strategies is further compared with the optimal probabilistic benchmarks, and the state-of-the-art performance of experimental quantum implementations against our newly derived thresholds is discussed. This work provides a comprehensive collection of directly useful criteria for the reliable certification of quantum communication technologies.

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arXiv2016

Optimal compression for identically prepared qubit states

We establish the ultimate limits to the compression of sequences of identically prepared qubits. The limits are determined by Holevo's information quantity and are attained through use of the optimal universal cloning machine, which finds here a novel application to quantum Shannon theory.

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arXiv2016

Quantum superreplication of states and gates

While the no-cloning theorem forbids the perfect replication of quantum information, it is sometimes possible to produce large numbers of replicas with vanishingly small error. This phenomenon, known as quantum superreplication, can take place both for quantum states and quantum gates. The aim of this paper is to review the central features of quantum superreplication, providing a unified view on the existing results. The paper also includes new results. In particular, we show that, when quantum superreplication can be achieved, it can be achieved through estimation, up to an error vanishing with a power law. Quantum strategies still offer an advantage for superreplication, in that they allow for an exponentially faster reduction of the error. Using the relation with estimation, we provide i) an alternative proof of the optimality of the Heisenberg scaling of quantum metrology, ii) a strategy to estimate arbitrary unitary gates with Heisenberg scaling, up to a logarithmic overhead, and iii) a protocol that generates M nearly perfect copies of a generic pure state with a number of queries to the corresponding unitary gate scaling as the square root of M. Finally, we point out that superreplication can be achieved using interactions among k systems, provided that k is large compared to square of the ratio between the numbers of input and output copies.

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arXiv2016

Efficient Quantum Compression for Ensembles of Identically Prepared Mixed States

We present one-shot compression protocols that optimally encode ensembles of $N$ identically prepared mixed states into $O(\log N)$ qubits. In contrast to the case of pure-state ensembles, we find that the number of encoding qubits drops down discontinuously as soon as a nonzero error is tolerated and the spectrum of the states is known with sufficient precision. For qubit ensembles, this feature leads to a 25% saving of memory space. Our compression protocols can be implemented efficiently on a quantum computer.

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arXiv2014

Superactivation of quantum gyroscopes

Quantum particles with spin are the most elementary gyroscopes existing in nature. Can two such gyroscopes help two distant observers find out their relative orientation in space? Here we show that a single pair of gyroscopes in an EPR state gives little clue about the relative orientation, but when two or more EPR pairs are used in parallel, suddenly a common reference frame emerges, with an error that drops quickly with the size of the system, beating than the best classical scaling already for small number of copies. This activation phenomenon indicates the presence of a latent resource hidden into EPR correlations, which can be unlocked and turned into advantage when multiple copies are available.

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arXiv2014

Quantum replication at the Heisenberg limit

No process in nature can perfectly clone an arbitrary quantum state. But is it possible to engineer processes that replicate quantum information with vanishingly small error? Here we demonstrate the possibility of probabilistic super-replication phenomena where N equally prepared quantum clocks are transformed into a much larger number of M nearly perfect replicas, with an error that rapidly vanishes whenever M is small compared to the square of N. The quadratic replication rate is the ultimate limit imposed by Quantum Mechanics to the proliferation of information and is fundamentally linked with the Heisenberg limit of quantum metrology.

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arXiv2014

Is global asymptotic cloning state estimation?

We pose the question whether the asymptotic equivalence between quantum cloning and quantum state estimation, valid at single-copy level, still holds when all the copies are examined jointly. For an N-to-M cloner, we consider the overall fidelity between the state of the M output systems and the state of M ideal copies, and we ask whether the optimal fidelity is attained by a measure-and- prepare protocol in the limit of large M. In order to gain intuition into the general problem, we analyze two concrete examples: i) cloning qubit states on the equator of the Bloch sphere and ii) cloning two-qubit maximally entangled states. In the first case, we show that the optimal measure-and- prepare fidelity converges to the fidelity of the optimal cloner in the limit of large M. In the second case, we restrict our attention to economical covariant cloners, and again, we exhibit a measure- and-prepare protocol that achieves asymptotically the optimal fidelity. Quite counterintuitively, in both cases the optimal states that have to be prepared in order to maximize the overall fidelity are not product states corresponding to M identical copies, but instead suitable M-partite entangled states: the simple protocol where one estimates the input state and re-prepares M identical copies of the estimated state is strictly suboptimal, even in the asymptotic limit.

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arXiv2013

Confusability graphs for symmetric sets of quantum states

For a set of quantum states generated by the action of a group, we consider the graph obtained by considering two group elements adjacent whenever the corresponding states are non-orthogonal. We analyze the structure of the connected components of the graph and show two applications to the optimal estimation of an unknown group action and to the search for decoherence free subspaces of quantum channels with symmetry.

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