Coherence Distillation Research Articles

Experimental Virtual Distillation of Entanglement and Coherence.

Noise is, in general, inevitable and detrimental to practical and useful quantum communication and computation. Under the resource theory framework, resource distillation serves as a generic tool to overcome the effect of noise. Yet, conventional resource distillation protocols generally require operations on multiple copies of resource states, and strong limitations exist that restrict their practical utilities. Recently, by relaxing the setting of resource distillation to only approximating the measurement statistics instead of the quantum state, a resource-frugal protocol, "virtual resource distillation," is proposed, which allows more effective distillation of noisy resources. Here, we report its experimental implementation on a photonic quantum system for the distillation of quantum coherence (up to dimension four) and bipartite entanglement. We show the virtual distillation of the maximal superposed state of dimension four from the state of dimension two, an impossible task in conventional coherence distillation. Furthermore, we demonstrate the virtual distillation of entanglement with operations acting only on a single copy of the noisy Einstein-Podolsky-Rosen (EPR) pair and showcase the quantum teleportation task using the virtually distilled EPR pair with a significantly improved fidelity of the teleported state. These results illustrate the feasibility of the virtual resource distillation method and pave the way for accurate manipulation of quantum resources with noisy quantum hardware.

Arbitrary Amplification of Quantum Coherence in Asymptotic and Catalytic Transformation.

Quantum coherence is one of the fundamental aspects distinguishing classical and quantum theories. Coherence between different energy eigenstates is particularly important, as it serves as a valuable resource under the law of energy conservation. A fundamental question in this setting is how well one can prepare good coherent states from low coherent states and whether a given coherent state is convertible to another one. Here, we show that any low coherent state is convertible to any high coherent state arbitrarily well in two operational settings: asymptotic and catalytic transformations. For a variant of asymptotic coherence manipulation where one aims to prepare desired states in local subsystems, the rate of transformation becomes unbounded regardless of how weak the initial coherence is. In a non-asymptotic transformation with a catalyst, a helper state that locally remains in the original form after the transformation, we show that an arbitrary state can be obtained from any low coherent state. Applying this to the standard asymptotic setting, we find that a catalyst can increase the coherence distillation rate significantly-from zero to infinite rate. We also prove that such anomalous transformation requires small but nonzero coherence in relevant modes, establishing the condition under which a sharp transition of the operational capability occurs. Our results provide a general characterization of the coherence transformability in these operational settings and showcase their peculiar properties compared to other common resource theories such as entanglement and quantum thermodynamics.

Virtual Quantum Resource Distillation.

Distillation, or purification, is central to the practical use of quantum resources in noisy settings often encountered in quantum communication and computation. Conventionally, distillation requires using some restricted "free" operations to convert a noisy state into one that approximates a desired pure state. Here, we propose to relax this setting by only requiring the approximation of the measurement statistics of a target pure state, which allows for additional classical postprocessing of the measurement outcomes. We show that this extended scenario, which we call "virtual resource distillation," provides considerable advantages over standard notions of distillation, allowing for the purification of noisy states from which no resources can be distilled conventionally. We show that general states can be virtually distilled with a cost (measurement overhead) that is inversely proportional to the amount of existing resource, and we develop methods to efficiently estimate such cost via convex and semidefinite programming, giving several computable bounds. We consider applications to coherence, entanglement, and magic distillation, and an explicit example in quantum teleportation (distributed quantum computing). This work opens a new avenue for investigating generalized ways to manipulate quantum resources.

Coherence distillation through one-way LICC

Understanding the resource consumption in distributed scenario is one of the main goals in resource theory. Entanglement swapping is an important example in such scenario. It involves at least three parties. Initially two pairs are entangled and other is separable. Performing Bell measurement on common party, resource is swapped in the separable pair. In resource theory of coherence, the initial separable pair may be in free state or in resource state. This feature leads to development of the idea of coherence swapping and coherence concentration. The latter one is the generalization of the first one. Both the protocols have important role in enhancement of coherence as well as coherence fraction and in reveling resource from a noisy state.

Catalytic transformations in coherence theory

In 2016, Winter and Yang [A. Winter and D. Yang, Phys. Rev. Lett. 116, 120404 (2016)] provided an operational meaning to relative entropy of coherence and coherence of formation by introducing coherence distillation and dilation protocols in an asymptotic setup. Here we have provided relative entropy of coherence [via incoherent operations (IO)] and coherence of formation (via IO) and quantum incoherent relative entropy [via local quantum incoherent operations with classical communications (LQICC)] as a measure using the concept of a catalyst. We have proved an existential correspondence between asymptotic and catalytic state transformation using IO, local incoherent operations with classical communications, and LQICC. We have also discussed two very important protocols, assisted distillation and quantum incoherent state merging using a catalyst. Monotone properties of relative entropy of coherence, coherence of formation, and quantum incoherent relative entropy under the catalytic transformation are also discussed here.

One-shot coherence distillation in superconducting circuit systems

Distilling quantum coherence is important for optimizing the performance of quantum technologies, however, it cannot always be accomplished with certainty. Then, the probabilistic distillation of quantum coherence has been developed and successfully implemented in experiments. We introduce a proposal to realize the one-shot coherence distillation in the superconducting circuit system. The target maximally coherent state can be extracted from a single copy of the prepared state by using appropriate incoherent operations and a finite error tolerance is allowed. It is easy to implement our scheme in the experiment that only a superconducting qubit is required to be the auxiliary system. To demonstrate the feasibility of our scheme, we numerically simulate the distillation process under the influence of dephasing (according to the typical experimental parameters). We find that the distillation rate of the coherence resource can be well achieved with the current experimental technique.

Faithful coherent states

We propose the notion of faithful coherent states based on the fidelity-based coherence witness. The criterion for detecting faithful coherent states can be restricted to a subclass of fidelity-based criterion under unitary transformations for single and bipartite systems. We can realize these unitary transformations by using quantum gates and circuits, and establish the connection of faithful coherence states and coherence distillation, maximum relative entropy of coherence.

Coordinately assisted distillation of quantum coherence in multipartite system

We investigate the issue of assisted coherence distillation in the asymptotic limit, by coordinately performing the identical local operations on the auxiliary systems of each copy. When we further restrict the coordinate operations to projective measurements, the distillation process branches into many sub-processes. Finally, a computable measure of the assisted distillable coherence is derived as the maximal average coherence of the residual states with the maximization taken over all the projective measurements on the auxiliary. The measure can be conveniently used to evaluate the assisted distillable coherence in experiments, especially suitable for the case that the system and its auxiliary are in mixed states. By using the measure, we for the first time study the assisted coherence distillation in multipartite systems. Monogamy-like inequalities are derived to constrain the distribution of the assisted distillable coherence in the subsystems. Taking a three-qubit system for example, we experimentally prepare two types of tripartite correlated states, i.e., the W-type and GHZ-type states in a linear optical setup, and experimentally test the assisted distillable coherence. Theoretical and experimental results agree well to verify the distribution inequalities given by us. Three measures of multipartite quantum correlation are also considered. The close relationship between the assisted coherence distillation and the multipartite correlation is revealed.

Approximate distillation of quantum coherence

Coherence distillation is a basic information-theoretic task in the resource theory of coherence. In this paper, we develop the framework of the approximate coherence distillation under strictly incoherent operations. This protocol considers the situation that we cannot transform an initial state $\rho$ into a target state $\psi$ with certainty; instead, we aim to deterministically transform the initial state $\rho$ into an intermediate state $\phi$ that is most approximate to the target state $\psi$ in terms of fidelity. We also present the explicit conversion strategy of the approximate coherence distillation.

Criterion for a state to be distillable via stochastic incoherent operations

Coherence distillation is a basic information-theoretic task in the resource theory of coherence. In this paper, we present the necessary and sufficient conditions under which a mixed state can be distilled into a pure coherent state via stochastic incoherent operations (sIOs). With the help of this result, we further show the following: (i) Any $2$-dimensional coherent state is distillable via sIOs if and only if it is a pure coherent state; (ii) a state $\rho$ is n-distillable via sIOs if and only if it is 1-distillable; and (iii) the set of distillable states via stochastic maximally incoherent operations is identical to the set of distillable states via sIOs. Finally, we analyze the reason why sIO is stronger than stochastic strictly incoherent operations when we use them to distill a coherent state.

Assisted Coherence Distillation of Certain Mixed States

We study the assisted coherence distillation of two special classes of mixed states including the states with vanished basis-dependent discord in the incoherent basis of the second party and Werner two-qubit states via the sets of operations such as local incoherent operations and classical communication (LICC), local quantum-incoherent operations and classical communication (LQICC), separable incoherent operations (SI), and separable quantum-incoherent operations (SQI). We show that the assisted distillable coherence of the former can reach the upper bound, namely quantum-incoherent (QI) relative entropy via all the sets of operations considered above. In contrast, for the Werner two-qubit states the assisted distillable coherence is strictly smaller than the quantum-incoherent (QI) relative entropy. Our results also mean that the optimal rate of assisted coherence distillation of Werner two-qubit states can be achieved even in one-copy scenarios without the complex joint quantum operations on many copies. Finally, we discuss the coherence location in the multipartite system.

One-Shot Coherence Distillation in Superconducting Circuit Systems

One-Shot Coherence Distillation in Superconducting Circuit Systems

Optimal probabilistic distillation of quantum coherence

We complete the task of optimal probabilistic coherence distillation protocol, whose aim is to transform a general state into a set of n-level maximally coherent states via strictly incoherent operations (SIO). Specifically, we present the necessary and sufficient condition for the transformation from a mixed state into a pure-state ensemble via SIO. With the aid of this condition and the simplex algorithm, we can accomplish the probabilistic distillation protocol by presenting an analytical expression of the maximal average distillable coherence of a general state and constructing the corresponding operation achieving this bound. Our protocol is a universal protocol since it can be applied to any coherence measure.

Experimental Progress on Quantum Coherence: Detection, Quantification, and Manipulation

AbstractQuantum coherence is a fundamental property of quantum systems, separating quantum from classical physics. Recently, there has been significant interest in the characterization of quantum coherence as a resource, investigating how coherence can be extracted and used for quantum technological applications. In this work, the progress of this research is reviewed, focusing in particular on recent experimental efforts. After a brief review of the underlying theory, the main platforms for realizing the experiments are discussed: linear optics, nuclear magnetic resonance, and superconducting systems. Experimental detection and quantification of coherence, experimental state conversion and coherence distillation, and experiments investigating the dynamics of quantum coherence are then considered. Experiments exploring the connections between coherence and uncertainty relations, path information, and coherence of operations and measurements are also reviewed. Experimental efforts on multipartite and multilevel coherence are also discussed.

Experimental demonstration of one-shot coherence distillation: realizing N-dimensional strictly incoherent operations

We experimentally investigate the one-shot distillation of quantum coherence, which focuses on the transformations from a single copy of a given state into maximally coherent states under various incoherent operations. We present a general proposal to realize a type of strictly incoherent operations (SIOs) that can act on N -dimensional ( N ≥ 2 ) states. This proposal is suitable for a variety of quantum devices. Based on a linear optical setup, we experimentally demonstrate that the proposed SIO successfully implements pure-to-pure state transformations, and thus the one-shot coherence distillation process is accomplished. The experimental data agree well with the theoretical results and clearly indicate the distillation fidelities, which bound the regions of different coherence distillation rates versus different superposition parameters of input states and different distillation errors.

Hierarchy of quantum operations in manipulating coherence and entanglement

Quantum resource theory under different classes of quantum operations advances multiperspective understandings of inherent quantum-mechanical properties, such as quantum coherence and quantum entanglement. We establish hierarchies of different operations for manipulating coherence and entanglement in distributed settings, where at least one of the two spatially separated parties are restricted from generating coherence. In these settings, we introduce new classes of operations and also characterize those maximal, i.e., the resource-non-generating operations, progressing beyond existing studies on incoherent versions of local operations and classical communication and those of separable operations. The maximal operations admit a semidefinite-programming formulation useful for numerical algorithms, whereas the existing operations not. To establish the hierarchies, we prove a sequence of inclusion relations among the operations by clarifying tasks where separation of the operations appears. We also demonstrate an asymptotically non-surviving separation of the operations in the hierarchy in terms of performance of the task of assisted coherence distillation, where a separation in a one-shot scenario vanishes in the asymptotic limit. Our results serve as fundamental analytical and numerical tools to investigate interplay between coherence and entanglement under different operations in the resource theory.

Coherence of assistance and assisted maximally coherent states

Coherence and entanglement are fundamental concepts in resource theory. The coherence (entanglement) of assistance is the coherence (entanglement) that can be extracted assisted by another party with local measurement and classical communication. We introduce and study the general coherence of assistance. First, in terms of real symmetric concave functions on the probability simplex, the coherence of assistance and the entanglement of assistance are shown to be in one-to-one correspondence. We then introduce two classes of quantum states: the assisted maximally coherent states and the assisted maximally entangled states. They can be transformed into maximally coherent or entangled pure states with the help of another party using local measurement and classical communication. We give necessary conditions for states to be assisted maximally coherent or assisted maximally entangled. Based on these, a unified framework between coherence and entanglement including coherence (entanglement) measures, coherence (entanglement) of assistance, coherence (entanglement) resources is proposed. Then we show that the coherence of assistance as well as entanglement of assistance are strictly larger than the coherence of convex roof and entanglement of convex roof for all full rank density matrices. So all full rank quantum states are distillable in the assisted coherence distillation.

One-shot assisted distillation of coherence via one-way local quantum-incoherent operations and classical communication

We obtain the general formula for the optimal rate on one of the subsystems that can be distilled from any bipartite quantum state by one-way local quantum-incoherent operations and classical communication. We provide lower and upper bounds on the assisted distillation of coherence using so-called one-shot entropies, and we establish a hierarchy of information quantities that can be used to calculate an assisted distillable rate in the asymptotical limit. We show that they asymptotically coincide with the quantum-incoherent relative entropy, and our findings imply a novel operational interpretation of quantum-incoherent relative entropy.

Coherent preorder of quantum states

As an important quantum resource, quantum coherence play key role in quantum information processing. It is often concerned with manipulation of families of quantum states rather than individual states in isolation. Given two pairs of coherent states $(\rho_1,\rho_2)$ and $(\sigma_1,\sigma_2)$, we are aimed to study how can we determine if there exists a strictly incoherent operation $\Phi$ such that $\Phi(\rho_i) =\sigma_i,i = 1,2$. This is also a classic question in quantum hypothesis testing. In this note, structural characterization of coherent preorder under strongly incoherent operations is provided. Basing on the characterization, we propose an approach to realize coherence distillation from rank-two mixed coherent states to $q$-level maximally coherent states. In addition, one scheme of coherence manipulation between rank-two mixed states is also presented.

Probabilistic coherence distillation with assisted setting

We consider the distillation of quantum coherence in the one-shot setting. The protocol is based on the probabilistic coherence distillation assisted by another party. In this context, coherent states including pure states and mixed states can be transformed into maximally coherent pure state with some probability and the maximal probability is defined as the maximal success probability of assistance. Then we show an upper bound for the maximal success probability of assistance for transforming d-dimensional mixed state to d-dimensional maximally coherent pure state. This upper bound is reached by all mixed states in two- and three-dimensional system and all incoherent states. A lower bound of the maximal success probability of assistance for the full-rank quantum states is also proposed. We also show an upper bound for the maximal success probability of assistance for transforming d-dimensional mixed state to $$m\ (m\le d)$$ -dimensional maximally coherent pure state in general case.