It is shown that a fixed measurement setting, e.g., a measurement in the computational basis, can detect all entangled states by preparing multipartite quantum states, called network states. We present network states for both cases to construct decomposable entanglement witnesses (EWs) equivalent to the partial transpose criteria and also non-decomposable EWs that detect undistillable entangled states beyond the partial transpose criteria. Entanglement detection by state preparation can be extended to multipartite states such as graph states, a resource for measurement-based quantum computing. Our results readily apply to a realistic scenario, for instance, an array of superconducting qubits. neutral atoms, or photons, in which the preparation of a multipartite state and a fixed measurement are experimentally feasible.
Quantum search algorithms show quadratic speedups over their classical counterparts, and the speedup also turns out to be optimal. They share the common structure that an iteration contains two inversions, one with respect to a target state and the other to an initial state, and such iterations are applied O( √ N) times for an unsorted database of N items. In this work, we present the characterization to an iteration that leads to exact quantum search with a quadratic speedup. We show that the fixed-point quantum search algorithm with a quadratic speedup contains iterations that are not QAAOs whereas exact quantum algorithms are sequences of QAAOs. We also demonstrate 3-qubit QAAOs in cloud-based quantum computing services, IBMQ and IonQ.
Network information theory establishes communication protocols among multiple senders and multiple receivers in the presence of correlations introduce noise over a network. We here present a framework of non-local network coding of multiple senders whereby network communication capacities improve over their classical counterparts. The framework exploits a Bell scenario and shows the usefulness of non-local and quantum resources in network information theory. Two-sender and two-receiver interference channels are considered in particular, for which network coding is characterized by two-input and four-outcome Bell scenarios. Resources for the network coding are classified It is shown that non-signaling (quantum) correlations lead to strictly higher channel capacities in general than quantum (local) correlations. It is also shown that, however, more non-locality does not necessarily imply a higher channel capacity. The framework can be generally applied to network communication protocols.
In the distribution of quantum states over a long distance, not only are quantum states corrupted by interactions with an environment but also a measurement setting should be re-aligned such that detection events can be ensured for the resulting states. We here present a method of protecting a measurement in quantum key distribution against the interactions quantum states experience during the transmission, without the verification of a channel. As a result, a receiver does not have to revise the measurement that has been prepared in a noiseless scenario since it would remain ever optimal. The measurement protection is achieved by applications of local unitary transformations before and after the transmission, that leads to a supermap transforming an arbitrary channel to a depolarization one. An experimental demonstration is presented with the polarization encoding on photonic qubits. It is shown that the security bounds for prepare-and-measure protocols can be improved.
Quantum state discrimination is a fundamental task in quantum information theory and also an important building block in practical quantum information applications. We here consider optimal state discrimination in a noisy environment in which quantum states may be corrupted due to the intervention of an environment. We show that even if there exists an unwanted interaction with an environment and the channel describing the interaction is not yet identified, a measurement prepared in the beginning for optimal state discrimination can be preserved as an optimal measurement ever after a channel use. This means that in a practical realization of quantum communication, verification of a channel can be circumvented and the cost of quantum tomography is saved. We also show a protocol of preserving an optimal measurement for state discrimination.
Entanglement detection is one of the most fundamental and practical tasks for quantum information processing. The framework of entanglement witnesses provides an experimentally feasible method detecting entangled states. Although It is clear that no entanglement witness per se can detect all entangled states, little is known about how useful a single entanglement witness is. In this work, we show that an entanglement witness can construct another entanglement witness. This means that the same measurement outcomes can be repeatedly applied to constructing different entanglement witnesses.
In this work, we show that measurement in quantum 2-design, such as mutually unbiased bases (MUBs) or
symmetric, informationally complete states (SICs), improves the capability of detecting entangled states both
theoretically and experimentally. On the theoretical side, we show that measurement in quantum 2-design can
detect entangled states twice compared to entanglement witnesses. On the implementation side, we present the
scheme of entanglement detection with two detectors only of a Hong-Ou-Mandel interferometer. The experimental
scheme applies single-copy level measurement followed by post-processing of measurement outcomes, which
is feasible with current technologies.
The partial transpose by which a subsystem's quantum state is solely transposed is of unique importance in quantum information processing from both fundamental and practical point of view. In this work, we present a practical scheme to realize a physical approximation to the partial transpose using local measurements on individual quantum systems and classical communication. We then report its linear optical realization and show that the scheme works with no dependence on local basis of given quantum states. A proof-of-principle demonstration of entanglement detection using the physical approximation of the partial transpose is also reported.
The partial transpose by which a subsystem's quantum state is solely transposed is of unique importance in
quantum information processing from both fundamental and practical point of view. In this work, we present a
practical scheme to realize a physical approximation to the partial transpose using local measurements on individual
quantum systems and classical communication. We then report its linear optical realization and show that
the scheme works with no dependence on local basis of given quantum states. A proof-of-principle demonstration
of entanglement detection using the physical approximation of the partial transpose is also reported.
The universal transpose of quantum states is an anti-unitary transformation that is not allowed in quantum
theory. In this work, we investigate approximating the universal transpose of quantum states of two-level
systems (qubits) using the method known as structural physical approximation. We also report its experimental
implementation in linear optics. The scheme is optimal in that the maximal fidelity is attained, and also practical
as measurement and preparation of quantum states that are experimentally feasible within current technologies
are solely applied.
The universal transpose of quantum states is an anti-unitary transformation that is not allowed in quantum
theory. In this work, we investigate approximating the universal transpose of quantum states of two-level
systems (qubits) using the method known as structural physical approximation. We also report its experimental
implementation in linear optics. The scheme is optimal in that the maximal fidelity is attained, and also practical
as measurement and preparation of quantum states that are experimentally feasible within current technologies
are solely applied.
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