Bit Of Quantum Information Research Articles

Exponential speedup in measuring out-of-time-ordered correlators and gate fidelity with a single bit of quantum information

Out-of-time-ordered correlators (OTOC) are a quantifier of quantum information scrambling and are useful in characterizing quantum chaos. We propose an efficient quantum algorithm to measure OTOCs that provides an exponential speed-up over the best known classical algorithm provided the OTOC operator to be estimated admits an efficient gate decomposition. We also discuss a scheme to obtain information about the eigenvalue spectrum and the spectral density of OTOCs as well as an efficient algorithm to estimate gate fidelities.

Bidirectional Quantum Teleportation by Using a Four-Qubit GHZ State and Two Bell States

In this paper, a theoretical scheme for bidirectional quantum teleportation by using a four-qubit GHZ state and two Bell states as a quantum channel is proposed. In this scheme, two arbitrary single-qubit quantum states and an unknown three-qubit state can be mutually transmitted between the two communication participants Alice and Bob. The quantum state information can be transmitted if Alice performs four single-qubit measurement operations on her qubits and Bob carries out a four-qubit joint measurement on his qubits, respectively. Then both Alice and Bob can reconstruct the target quantum state by means of appropriate unitary operations. Then we take comparisons with the other bidirectional quantum teleportation protocols in five aspects of quantum information bits transmitted, necessary operations, consumption of quantum resource, consumption of classical resource and intrinsic efficiency. Furthermore, we use channel authentication method to ensure the channel security and prove that our protocol can withstand four attack scenarios. It is concluded that our scheme has significant advantages of more qubits number transmitted, higher intrinsic efficiency and high security.

Bidirectional Quantum Teleportation by Using Six-Qubit Cluster State

Quantum teleportation is considered as the most basic process of quantum communication. It is undoubtedly beneficial for the realization of quantum teleportation while transmitting the maximum number of quantum bits with the minimum number of quantum channel resources. An improved bidirectional quantum teleportation protocol based on a quantum channel of six-qubit cluster state is proposed. In this paper, the user Alice can transmit an unknown three-qubit entangled state to the user Bob, and at the same time, the user Bob can transmit an arbitrary single-qubit state to the user Alice by utilizing GHz-state measurement, bell-state measurement, single-qubit Von Neumann measurement, and unitary operations. Compared with other schemes in quantum resource consumption, necessary operation complexity including operation difficulty and intensity, classical resource consumption, quantum information bits transmitted, and intrinsic efficiency, our scheme has the remarkable advantages in transmitting more quantum information and possesses higher intrinsic efficiency.

Fundamental quantum structures — Conclusions with respect to cosmology and interactions

An explanation can be seen as the process of reducing complicated structures to simple structures. For more than two millennia, the guiding idea was to reduce phenomena to "small particles", to some types of "atoms". In other words: the smaller, the simpler.Up to the atoms of the chemistry this concept was extremely successful. Structures smaller than atomic nuclei, however, require ever higher energies for their production and their analysis. That these ever higher energies lead to ever simpler structures is implausible.The simplest structures that are mathematically possible are abstract bits of quantum information - AQIs. Taken as the ultimate substance, referred to as protyposis, the AQI concept affords a unifying explanation of the structure of space and time and the evolution of the cosmos and its inventory.Some thirty years ago, the model of a closed cosmos expanding at the speed of light was developed deriving from basic quantum-theoretical arguments. In this model, the equation of state reads ρ + 3p = 0 (ρ and p denoting cosmological energy density and pressure). At the core of the model, there are the quantum bits of absolute quantum information (AQI), the simplest of all possible quantum structures.Recent astrophysical investigations declare that the "ρ + 3p = 0"-model is in better agreement with the observation data than the present standard flat-ΛCDM model. Ad hoc assumptions such as Dark Energy and Inflation are dispensable.The protyposis theory has been advanced to account for the formation of relativistic quantum particles from AQI bits. Moreover, a rationalization has been given of the General Theory of Relativity and of the three non-gravitational forces.Ultimately, the protyposis concept will allow us to understand the emergence of both matter and consciousness from the AQIs

Acetylcholine molecular arrays enable quantum information processing

We have found self-assembly of four neurotransmitter acetylcholine (ACh) molecular complexes in a water molecules environment by using geometry optimization with DFT B97d method. These complexes organizes to regular arrays of ACh molecules possessing electronic spins, i.e. quantum information bits.These spin arrays could potentially be controlled by the application of a non-uniform external magnetic field. The proper sequence of resonant electromagnetic pulses would then drive all the spin groups into the 3-spin entangled state and proceed large scale quantum information bits.

Quantum Teleportation of Five-qubit State

We propose a novel quantum teleportation protocol for certain class of five-qubit state with a seven-qubit cluster state as quantum channel. In our scheme, the sender merely needs to perform a seven-qubit von-Neumann projective measurement, the original state with deterministic probability can be reconstructed by the receiver after a series corresponding unitary transformations. Compared with other schemes proposed before, our scheme has the distinct advantages of requiring fewer quantum channels, possessing higher intrinsic efficiency, and transmitting more quantum information bits.

Tripartite Quantum Controlled Teleportation via Seven-Qubit Cluster State

In this paper, a theoretical scheme for tripartite quantum controlled teleportation is presented using the entanglement property of seven-qubit cluster state. This means that Alice wants to transmit a entangled state of particle a to Bob, Charlie wants to transmit a entangled state of particle b to David and Edison wants to transmit a entangled state of particle c to Ford via the control of the supervisor. In the end, we compared the aspects of quantum resource consumption, operation complexity, classical resource consumption, quantum information bits transmitted, success probability and efficiency with other schemes.

Power of one bit of quantum information in quantum metrology

We construct a model of quantum metrology inspired by the computational model known as deterministic quantum computation with one quantum bit (DQC1). Using only one pure qubit together with $l$ fully-mixed qubits we obtain measurement precision at the standard quantum limit, which is typically obtained using the same number of uncorrelated qubits in fully-pure states. The standard quantum limit can be exceeded using an additional qubit, which adds only a small amount of purity. We show that the discord in the final state vanishes only in the limit of attaining infinite precision for the parameter being estimated.

Bidirectional and asymmetric quantum controlled teleportation via maximally eight-qubit entangled state

We propose a new protocol of bidirectional and asymmetric quantum controlled teleportation, using a maximally eight-qubit entangled state as the quantum channel. We compared the aspects of quantum resource consumption, operation complexity, classical resource consumption, quantum information bits transmitted and efficiency with other schemes.

Quantum of solace for black hole mystery

Sean Carroll of the California Institute of Technology in Pasadena and his colleagues approached the problem using techniques gleaned from the art of quantum teleportation. When particles are entangled, measuring the state atone particle instantly influences the state of the other. In an oft-cited example, Alice and Bob each has one of a pair of entangled particles. Bob also sends onebit of information to Alice. She then uses that bit along with her share of the entangled pair to teleport one bit of quantum information, or one bit, from Bob's location to where she is

Saving Space with Quantum Information

A technique that packs the full content of three bits of quantum information into two may be a space saver for quantum computers.

Storing Quantum Information in Schrödinger's Cats

Superposition states created with more than 100 photons enable the storage of multiple bits of quantum information. [Also see Report by Vlastakis et al. ]

Measuring geometric quantum discord using one bit of quantum information

We describe an efficient DQC1-algorithm to quantify the amount of Geometric Quantum Discord present in the output state of a DQC1 computation. DQC1 is a model of computation that utilizes separable states to solve a problem with no known efficient classical algorithm and is known to contain quantum correlations as measured by the discord. For the general case of a (1+n)-qubit DQC1-state we provide an analytical expression for the Geometric Quantum Discord and find that its typical (and maximum) value decreases exponentially with n. This is in contrast to the standard Quantum Discord whose value for typical DQC1-states is known to be independent of n. We experimentally demonstrate the proposed algorithm on a four-qubit liquid-state nuclear magnetic resonance quantum information processor. In the special case of a two-qubit DQC1 model, we also provide an expression for the Quantum Discord that only requires the outcome of the DQC1 algorithm.

On the role of vibrational anharmonicities in a two-qubit system

Anharmonicities of vibrational modes allow controlling state-to-state transitions and encoding quantum information bits into vibrational eigenstates. Resonances between different transitions hinder the control. In this paper all resonances that can occur in a two-qubit system are identified and studied. Computational experiments are carried out using optimal control theory and numerical propagation of vibrational wave packets. Useful insight is obtained into the mechanisms of negative effect of these resonances onto the control tasks. A set of general criteria is derived for evaluation of candidate molecules for practical realization of vibrational two-qubit system. Several simple strategies for avoiding such resonances by choosing suitable vibrational characteristics (mode frequencies and anharmonicities) are proposed.

Entanglement propagation through spin chains in the presence of a staggered magnetic field

We study the dynamics of entanglement in an $XY$ spin chain subject to a staggered magnetic field and contrast it to the previously discussed uniform field case. We find that in the isotropic $(XX)$ limit a staggered field can provide almost equally favorable conditions for a high-fidelity transfer of entanglement, while even a modest amount of exchange anisotropy appears to have a strong detrimental effect. We also study interactions between different waves of entanglement and assess the possibility of simultaneous transmission of multiple bits of quantum information.

Qubits ride the photon bus

Quantum mechanics using whole electrical circuits might seem a far-fetched idea. But make the circuits superconducting, and they can be used to send and collect single photons, rather like atoms do — only better. Microfabricated superconducting circuit elements can harness the power of quantum behaviour for information processing. Unlike classical information bits, quantum information bits (qubits) can form superpositions or mixture states of ON and OFF, offering a faster, natural form of parallel processing. Previously, direct qubit–qubit coupling has been achieved for up to four qubits, but now two independent groups demonstrate the next crucial step: communication and exchange of quantum information between two superconducting qubits via a quantum bus, in the form of a resonant cavity formed by a superconducting transmission line a few millimetres long. Using this microwave cavity it is possible to store, transfer and exchange quantum information between two quantum bits. It can also perform multiplexed qubit readout. This basic architecture lends itself to expansion, offering the possibility for the coherent interaction of many superconducting qubits. The cover illustrates a zig-zag-shaped resonant cavity or quantum bus linking two superconducting phase qubits.

Entanglement and the power of one qubit

The "Power of One Qubit" refers to a computational model that has access to only one pure bit of quantum information, along with n qubits in the totally mixed state. This model, though not as powerful as a pure-state quantum computer, is capable of performing some computational tasks exponentially faster than any known classical algorithm. One such task is to estimate with fixed accuracy the normalized trace of a unitary operator that can be implemented efficiently in a quantum circuit. We show that circuits of this type generally lead to entangled states, and we investigate the amount of entanglement possible in such circuits, as measured by the multiplicative negativity. We show that the multiplicative negativity is bounded by a constant, independent of n, for all bipartite divisions of the n+1 qubits, and so becomes, when n is large, a vanishingly small fraction of the maximum possible multiplicative negativity for roughly equal divisions. This suggests that the global nature of entanglement is a more important resource for quantum computation than the magnitude of the entanglement.

Accuracy of gates in a quantum computer based on vibrational eigenstates.

A model is developed to study the properties of a quantum computer that uses vibrational eigenstates of molecules to implement the quantum information bits and shaped laser pulses to apply the quantum logic gates. Particular emphasis of this study is on understanding how the different factors, such as properties of the molecule and of the pulse, can be used to affect the accuracy of quantum gates in such a system. Optimal control theory and numerical time-propagation of vibrational wave packets are employed to obtain the shaped pulses for the gates NOT and Hadamard transform. The effects of the anharmonicity parameter of the molecule, the target time of the pulse and of the penalty function are investigated. Influence of all these parameters on the accuracy of qubit transformations is observed and explained. It is shown that when all these parameters are carefully chosen the accuracy of quantum gates reaches 99.9%.

Exponential speedup with a single bit of quantum information: measuring the average fidelity decay.

We present an efficient quantum algorithm to measure the average fidelity decay of a quantum map under perturbation using a single bit of quantum information. Our algorithm scales only as the complexity of the map under investigation. Thus for those maps admitting an efficient gate decomposition, it provides an exponential speedup over known classical procedures. Fidelity decay is important in the study of complex dynamical systems, where it is conjectured to be a signature of eigenvector statistics. Our result also illustrates the role of chaos in the process of decoherence.

Testing integrability with a single bit of quantum information

We show that deterministic quantum computing with a single bit can determine whether the classical limit of a quantum system is chaotic or integrable using $O(N)$ physical resources, where N is the dimension of the Hilbert space of the system under study. This is a square-root improvement over all known classical procedures. Our study relies strictly on the random matrix conjecture. We also present numerical results for the nonlinear kicked top.