Two-qubit Entangled State Research Articles

Periodically driven facilitated high-efficiency dissipative entanglement with Rydberg atoms

A time-dependent periodical field can be utilized to efficiently modify the Rabi coupling of system, exhibiting nontrivial dynamics. We propose a scheme to show that this feature can be applied for speeding up the formation of dissipative steady entanglement based on Rydberg anti-blockade mechanism in a simplified configuration, fundamentally stemming from a frequency match between the external-field modulation frequency and the systematic characteristic frequency. In the presence of an optimal modulation frequency that is exactly equal to the central frequency of driving field, it enables a sufficient residence time of the two-excitation Rydberg state for an irreversible spontaneous decay onto the target state, leading to an accelerated high-fidelity steady entanglement ~0.98, with a shorter formation time <400\mu s. We show that, a global maximal fidelity benefits from a consistence of microwave-field coupling and spontaneous decay strengths, by which the scheme promises a robust insensitivity to the initial population distributions. This simple approach to facilitate the generation of dissipative entangled two-qubit states by using periodic drivings may guide a new experimental direction in Rydberg quantum technology and quantum information.

Entanglement restoration using environment-assisted measurement and quantum measurement reversal

Based on the quantum technique of environment-assisted measurement and quantum measurement reversal, we systematically investigate the restoration of quantum entanglement for distributing a two-qubit pure state via three typical kinds of noisy quantum channels-amplitude damping, phase damping and depolarizing quantum channels. Special attention is paid to the unsymmetrical two-qubit arbitrarily entangled states transmission. The analytical or numerical simulations results show that for arbitrarily one-qubit entangled state or two-qubit states transmission, the output distant entanglement could be drastically restored to the initial value in all these three cases.

Experimental demonstration of one-sided device-independent self-testing of any pure two-qubit entangled state

We demonstrate one-sided device-independent self-testing of any pure entangled two-qubit state based on a fine-grained steering inequality. The maximum violation of a fine-grained steering inequality can be used to witness certain steerable correlations, which certify all pure two-qubit entangled states. Our experimental results identify which particular pure two-qubit entangled state has been self-tested and which measurement operators are used on the untrusted side. Furthermore, we analytically derive the robustness bound of our protocol, enabling our subsequent experimental verification of robustness through state tomography. Finally, we ensure that the requisite no-signalling constraints are maintained in the experiment.

Experimental randomness certification with a symmetric informationally complete positive operator-valued measurement

Nonlocal correlations observed from entangled quantum particles imply the existence of intrinsic randomness. Normally, locally projective measurements performed on a two-qubit entangled state can only certify one-bit randomness at most, while non-projective measurement can certify more randomness with the same quantum resources. In this Letter, we carry out an experimental investigation on quantum randomness certification through a symmetric informationally complete positive operator-valued measurement, which in principle can certify the maximum randomness through an entangled qubit. We observe the quantum nonlocal correlations that are close to the theoretical values. In the future, this work can provide a valuable reference for the research on the limit of randomness certification.

Certified Randomness From Steering Using Sequential Measurements

The generation of certifiable randomness is one of the most promising applications of quantum technologies. Furthermore, the intrinsic non-locality of quantum correlations allow us to certify randomness in a device-independent way, i.e., we do not need to make assumptions about the devices used. Due to the work of Curchod et al. a single entangled two-qubit pure state can be used to produce arbitrary amounts of certified randomness. However, the obtaining of this randomness is experimentally challenging as it requires a large number of measurements, both projective and general. Motivated by these difficulties in the device-independent setting, we instead consider the scenario of one-sided device independence where certain devices are trusted, and others are not; a scenario motivated by asymmetric experimental set-ups such as ion-photon networks. We show how certain aspects of previous works can be adapted to this scenario and provide theoretical bounds on the amount of randomness that can be certified. Furthermore, we give a protocol for unbounded randomness certification in this scenario, and provide numerical results demonstrating the protocol in the ideal case. Finally, we numerically test the possibility of implementing this scheme on near-term quantum technologies, by considering the performance of the protocol on several physical platforms.

On the Controlled Cyclic Quantum Teleportation of an Arbitrary Two-Qubit Entangled State by Using a Ten-Qubit Entangled State

Recently, Li et al. (Int J Theor Phys: 58:1541–1545, 2019) proposed a controlled cyclic quantum teleportation method of the arbitrary two-qubit entangled states. They claimed that their method can help each of three involved participants to teleport an arbitrary two-qubit entangled state to the next participant, and at the same time, none of the involved participants can obtain the teleported state without the controller’s permission. However, this study shows that two involved participants can reconstruct the teleported states without the controller’s permission. To solve this problem, a modification is proposed here.

Entanglement Teleportation of a Two-Qubit System via Correlated Quantum Channels

We address the teleportation of a two-qubit entangled state through quantum channels where successive uses of the channels are correlated, and investigate how memory effect induced by successive uses of the channels influences the entanglement teleportation and fidelity. The analytical expressions of the entanglement teleportation and average fidelity under three different correlated channels are presented. Our results show that, the output entanglement teleportation strongly depends on the source state, the initial entanglement of teleporting state and parameters of noisy channels. However, the average fidelity is only affected by the parameters of the source state and noisy channels.

A Ten-Qubit Solid-State Spin Register with Quantum Memory up to One Minute

Spins associated to single defects in solids provide promising qubits for quantum information processing and quantum networks. Recent experiments have demonstrated long coherence times, high-fidelity operations and long-range entanglement. However, control has so far been limited to a few qubits, with entangled states of three spins demonstrated. Realizing larger multi-qubit registers is challenging due to the need for quantum gates that avoid crosstalk and protect the coherence of the complete register. In this paper, we present novel decoherence-protected gates that combine dynamical decoupling of an electron spin with selective phase-controlled driving of nuclear spins. We use these gates to realize a 10-qubit quantum register consisting of the electron spin of a nitrogen-vacancy center and 9 nuclear spins in diamond. We show that the register is fully connected by generating entanglement between all 45 possible qubit pairs, and realize genuine multipartite entangled states with up to 7 qubits. Finally, we investigate the register as a multi-qubit memory. We show coherence times up to 63(2) seconds - the longest reported for a single solid-state qubit - and demonstrate that two-qubit entangled states can be stored for over 10 seconds. Our results enable the control of large quantum registers with long coherence times and therefore open the door to advanced quantum algorithms and quantum networks with solid-state spin qubits.

Two-Qubit Pure Entanglement as Optimal Social Welfare Resource in Bayesian Game

Entanglement is of paramount importance in quantum information theory. Its supremacy over classical correlations has been demonstrated in a numerous information theoretic protocols. Here we study possible adequacy of quantum entanglement in Bayesian game theory, particularly in social welfare solution (SWS), a strategy which the players follow to maximize sum of their payoffs. Given a multi-partite quantum state as an advice, players can come up with several correlated strategies by performing local measurements on their parts of the quantum state. A quantum strategy is called quantum-SWS if it is advantageous over a classical equilibrium (CE) strategy in the sense that none of the players has to sacrifice their CE-payoff rather some have incentive and at the same time it maximizes sum of all players' payoffs over all possible quantum advantageous strategies. Quantum state yielding such a quantum-SWS is called a quantum social welfare advice (SWA). We show that any two-qubit pure entangled state, even if it is arbitrarily close to a product state, can serve as quantum-SWA in some Bayesian game. Our result, thus, gives cognizance to the fact that every two-qubit pure entanglement is the best resource for some operational task.

Role of maximally entangled states in the context of linear steering inequalities

Linear steering inequalities are useful to check whether a bipartite state is steerable when both the parties are allowed to perform $n$ dichotomic measurements on their parts. In the present study we propose the necessary and sufficient condition under which $2$-settings linear steering inequality will be violated for any given set of spin-$\frac{1}{2}$ observables at trusted and untrusted parties' sides. The important result revealed by the present paper is that maximally entangled two-qubit states give the largest quantum violations of $2$-settings as well as $3$-settings linear steering inequalities attainable for any given set of spin-$\frac{1}{2}$ observables at trusted and untrusted parties' sides (if any violation exists for that given set of spin-$\frac{1}{2}$ observables).

Bidirectional Quantum Controlled Teleportation of Three-Qubit State by Using GHZ States

In this paper, we present a scheme of bidirectional quantum controlled teleportation of three-qubit state by using GHZ states. Alice transmits an unknown three-qubit entangled state to Bob, and Bob transmit an unknown three-qubit entangled state to Alice via the control of the supervisor Charlie. In order to facilitate the implementation in the experimental environment, the preparation method of quantum channel is given. This scheme is based on that three-qubit entangled state are transformed into two-qubit entangled state and single qubit superposition state by using Toffoli Gate and Controlled-NOT operation, receivers can by introducing the appropriate unitary transformation and auxiliary particles to reconstruct the initial state. Finally, this paper is implemented a scheme of bidirectional quantum controlled teleportation of more than two qubits via the control of the supervisor Charlie.

Experimental realization of more quantum randomness generation based on non-projective measurement

Quantum random numbers are fundamental resources (Rev. Mod. Phys. 89 015004) for the fields of communications and information security. Usually, projective measurements can be applied on a two-qubit entangled state to generate quantum random numbers. However, it cannot certify more than one bit of randomness for one set of measurements. On contrast, non-projective measurements in-principle may be able to generate more random bits, and therefore show some merits among present methods. Here, we report the implementation of a quantum random number generator based on a symmetric informationally complete positive operator-valued measure. In our experiment, we prepare a maximally entangled state of two-qubits through spontaneous parametric down-conversion processes, and observe the violation of Gisin’s elegant Bell inequality with the value of 6.8138 ± 0.0822, exceeding the classical bound of 6. Through randomness extractors, including a Toeplitz extractor and a parallel linear feedback shift register based extractor, we finally extract a random string with an autocorrelation coefficient below 10−3 and the string can pass all randomness tests of the National Institute of Science and Technology (NIST).

Heralded Bell State of Dissipative Qubits Using Classical Light in a Waveguide.

Maximally entangled two-qubit states (Bell states) are of central importance in quantum technologies. We show that heralded generation of a maximally entangled state of two intrinsically open qubits can be realized in a one-dimensional (1D) system through strong coherent driving and continuous monitoring. In contrast to the natural idea that dissipation leads to decoherence and so destroys quantum effects, continuous measurement and strong interference in our 1D system generate a pure state with perfect quantum correlation between the two open qubits. Though the steady state is a trivial product state that has zero coherence or concurrence, we show that, with carefully tuned parameters, a Bell state can be generated in the system's quantum jump trajectories, heralded by a reflected photon. Surprisingly, this maximally entangled state survives the strong coherent state input-a classical state that overwhelms the system. This simple method to generate maximally entangled states using classical coherent light and photon detection may, since our qubits are in a 1D continuum, find application as a building block of quantum networks.

Probabilistic Resumable Quantum Teleportation of a Two-Qubit Entangled State.

We explicitly present a generalized quantum teleportation of a two-qubit entangled state protocol, which uses two pairs of partially entangled particles as quantum channel. We verify that the optimal probability of successful teleportation is determined by the smallest superposition coefficient of these partially entangled particles. However, the two-qubit entangled state to be teleported will be destroyed if teleportation fails. To solve this problem, we show a more sophisticated probabilistic resumable quantum teleportation scheme of a two-qubit entangled state, where the state to be teleported can be recovered by the sender when teleportation fails. Thus the information of the unknown state is retained during the process. Accordingly, we can repeat the teleportion process as many times as one has available quantum channels. Therefore, the quantum channels with weak entanglement can also be used to teleport unknown two-qubit entangled states successfully with a high number of repetitions, and for channels with strong entanglement only a small number of repetitions are required to guarantee successful teleportation.

Robust and efficient transport of two-qubit entanglement via disordered spin chains

We investigate how robust is the modified XX spin-1/2 chain of [R. Vieira and G. Rigolin, Phys. Lett. A 382, 2586 (2018)] in transmitting entanglement when several types of disorder and noise are present. First, we consider how deviations about the optimal settings that lead to almost perfect transmission of a maximally entangled two-qubit state affect the entanglement reaching the other side of the chain. Those deviations are modeled by static, dynamic, and fluctuating disorder. We then study how spurious or undesired interactions and external magnetic fields diminish the entanglement transmitted through the chain. For chains of the order of hundreds of qubits, we show for all types of disorder and noise here studied that the system is not appreciably affected when we have weak disorder (deviations of less than 1% about the optimal settings) and that for moderate disorder it still beats the standard and ordered XX model when deployed to accomplish the same task.

Experimental investigation of partially entangled states for device-independent randomness generation and self-testing protocols

Previous theoretical works showed that all pure two-qubit entangled states can generate one bit of local randomness and can be self-tested through the violation of proper Bell inequalities. We report an experiment in which nearly pure partially entangled states of photonic qubits are produced to investigate these tasks in a practical scenario. We show that small deviations from the ideal situation make low entangled states impractical to self-testing and randomness generation using the available techniques. Our results show that in practice lower entanglement implies lower randomness generation, recovering the intuition that maximally entangled states are better candidates for deviceindependent quantum information processing.

Controlled Cyclic Quantum Teleportation of an Arbitrary Two-Qubit Entangled State by Using a Ten-Qubit Entangled State

We propose a scheme of controlled cyclic quantum teleportation of an arbitrary two-qubit entangled state by using a ten-qubit entangled state. In our scheme, Alice can teleport an arbitrary two-qubit entangled state to Bob, Bob can teleport an arbitrary two-qubit entangled state to Charlie and at the same time Charlie can teleport an arbitrary two-qubit entangled state to Alice with the help of the controller David.

Demonstration of Fidelity Improvement Using Dynamical Decoupling with Superconducting Qubits.

Quantum computers must be able to function in the presence of decoherence. The simplest strategy for decoherence reduction is dynamical decoupling (DD), which requires no encoding overhead and works by converting quantum gates into decoupling pulses. Here, using the IBM and Rigetti platforms, we demonstrate that the DD method is suitable for implementation in today's relatively noisy and small-scale cloud-based quantum computers. Using DD, we achieve substantial fidelity gains relative to unprotected, free evolution of individual superconducting transmon qubits. To a lesser degree, DD is also capable of protecting entangled two-qubit states. We show that dephasing and spontaneous emission errors are dominant in these systems, and that different DD sequences are capable of mitigating both effects. Unlike previous work demonstrating the use of quantum error correcting codes on the same platforms, we make no use of postselection and hence report unconditional fidelity improvements against natural decoherence.

Scheme for Asymmetric and Deterministic Controlled Bidirectional Joint Remote State Preparation**Supported by the National Natural Science Foundation of China under Grant No. 11604115

The scheme for asymmetric and deterministic controlled bidirectional joint remote state preparation by using one ten-qubit entangled state as the quantum channel is proposed. In this scheme, Alice and David want to remotely prepare an arbitrary single-qubit state at Bob’s site, at the same time, Bob and Eve wish to help Alice remotely prepare an arbitrary two-qubit entangled state. Alice and Bob can simultaneously prepare the desired states with the cooperation of David and Eve under the control of Charlie.

Many-particle interferometry and entanglement by path identity

We introduce a general scheme of many-particle interferometry in which two identical sources are used and ``which-way information'' is eliminated by making the paths of one or more particles identical (path identity). The scheme allows us to generate many-particle entangled states. We provide general forms of these states and show that they can be expressed as superpositions of various Dicke states. We illustrate cases in which the scheme produces maximally entangled two-qubit states (Bell states) and maximally three-tangled states (three-particle Greenberger-Horne-Zeilinger-class states). A striking feature of the scheme is that the entangled states can be manipulated without interacting with the entangled particles; for example, it is possible to switch between two distinct Bell states. Furthermore, each entangled state corresponds to a set of many-particle interference patterns. The visibility of these patterns and the amount of entanglement in a quantum state are connected to each other. The scheme also allows us to change the visibility and the amount of entanglement without interacting with the entangled particles and, therefore, has the potential to play an important role in quantum information science.