45 research outputs found
Dephasing-assisted transport: quantum networks and biomolecules
Original article can be found at: http://www.iop.org/EJ/journal/1367-2630/1 DOI: 10.1088/1367-2630/10/11/113019Transport phenomena are fundamental in physics. They allow for information and energy to be exchanged between individual constituents of communication systems, networks or even biological entities. Environmental noise will generally hinder the efficiency of the transport process. However, and contrary to intuition, there are situations in classical systems where thermal fluctuations are actually instrumental in assisting transport phenomena. Here we show that, even at zero temperature, transport of excitations across dissipative quantum networks can be enhanced by local dephasing noise. We explain the underlying physical mechanisms behind this phenomenon and propose possible experimental demonstrations in quantum optics. Our results suggest that the presence of entanglement does not play an essential role for energy transport and may even hinder it. We argue that Nature may be routinely exploiting dephasing noise and show that the transport of excitations in simplified models of light harvesting molecules does benefit from such noise assisted processes. These results point toward the possibility for designing optimized structures for transport, for example in artificial nanostructures, assisted by noise.Peer reviewe
Robust creation of entanglement between ions in spatially separate cavities
We present a protocol that allows the generation of a maximally entangled state between individual atoms held in spatially separate cavities. Assuming perfect detectors and neglecting spontaneous emission from the atoms, the resulting idealized scheme is deterministic. Under more realistic conditions, when the atom-cavity interaction departs from the strong coupling regime, and considering imperfect detectors, we show that the scheme is robust against experimental inefficiencies and yields probabilistic entanglement of very high fidelity
Improving the precision of frequency estimation via long-time coherences
In recent years, several estimation strategies have been formulated to determine the value of an unknown parameter in the most precise way, taking into account the presence of noise. These strategies typically rely on the use of quantum entanglement between the sensing probes and they have been shown to be optimal in the asymptotic limit in the number of probes, as long as one performs measurements on shorter and shorter time scales. Here, we present a different approach to frequency estimation, which exploits quantum coherence in the state of each sensing particle in the long time limit and is obtained by properly engineering the environment. By means of a commonly used master equation, we show that our strategy can overcome the precision achievable with entanglement-based strategies for a finite number of probes. We discuss a possible implementation of the scheme in a realistic setup that uses trapped ions as quantum sensors
Coherence and non-classicality of quantum Markov processes
Although quantum coherence is a basic trait of quantum mechanics, the presence of coherences in the quantum description of a certain phenomenon does not rule out the possibility to give an alternative description of the same phenomenon in purely classical terms. Here, we give definite criteria to determine when and to what extent quantum coherence is equivalent to non-classicality. We prove that a Markovian multi-time statistics obtained from repeated measurements of a non-degenerate observable cannot be traced back to a classical statistics if and only if the dynamics generates coherences and subsequently turns them into populations. Furthermore, we show with simple examples that such connection between quantum coherence and non-classicality is generally absent if the statistics is non-Markovian
Accessible coherence in open quantum system dynamics
Quantum coherence generated in a physical process can only be cast as a potentially useful resource if its effects can be detected at a later time. Recently, the notion of non-coherence-generating-and-detecting (NCGD) dynamics has been introduced and related to the classicality of the statistics associated with sequential measurements at different times. However, in order for a dynamics to be NCGD, its propagators need to satisfy a given set of conditions for all triples of consecutive times. We reduce this to a finite set of d(d 121) conditions, where d is the dimension of the quantum system, provided that the generator is time-independent. Further conditions are derived for the more general time-dependent case. The application of this result to the case of a qubit dynamics allows us to elucidate which kind of noise gives rise to non-coherence-generation-and-detection
Probabilistic implementation of universal quantum processors
We present a probabilistic quantum processor for qudits. The processor itself
is represented by a fixed array of gates. The input of the processor consists
of two registers. In the program register the set of instructions (program) is
encoded. This program is applied to the data register. The processor can
perform any operation on a single qudit of the dimension N with a certain
probability. If the operation is unitary, the probability is in general 1/N^2,
but for more restricted sets of operators the probability can be higher. In
fact, this probability can be independent of the dimension of the qudit Hilbert
space of the qudit under some conditions.Comment: 7 revtex pages, 1 eps figur
A quantum gate array can be programmed to evaluate the expectation value of any operator
A programmable gate array is a circuit whose action is controlled by input
data. In this letter we describe a special--purpose quantum circuit that can be
programmed to evaluate the expectation value of any operator acting on a
space of states of dimensions. The circuit has a program register whose
state encodes the operator whose expectation value is to be
evaluated. The method requires knowledge of the expansion of in a basis of
the space of operators. We discuss some applications of this circuit and its
relation to known instances of quantum state tomography.Comment: 4 pages, 3 figures include
Weak force detection using a double Bose-Einstein condensate
A Bose-Einstein condensate may be used to make precise measurements of weak
forces, utilizing the macroscopic occupation of a single quantum state. We
present a scheme which uses a condensate in a double well potential to do this.
The required initial state of the condensate is discussed, and the limitations
on the sensitivity due to atom collisions and external coupling are analyzed.Comment: 12 pages, 2 figures, Eq.(41) has been correcte
Causality in quantum teleportation: information extraction and noise effects in entanglement distribution
Quantum teleportation is possible because entanglement allows a definition of
precise correlations between the non-commuting properties of a local system and
corresponding non-commuting properties of a remote system. In this paper, the
exact causality achieved by maximal entanglement is analyzed and the results
are applied to the transfer of effects acting on the entanglement distribution
channels to the teleported output state. In particular, it is shown how
measurements performed on the entangled system distributed to the sender
provide information on the teleported state while transferring the
corresponding back-action to the teleported quantum state.Comment: 14 pages, including three figures, discussion of fidelity adde
Probabilistic instantaneous quantum computation
The principle of teleportation can be used to perform a quantum computation
even before its quantum input is defined. The basic idea is to perform the
quantum computation at some earlier time with qubits which are part of an
entangled state. At a later time a generalized Bell state measurement is
performed jointly on the then defined actual input qubits and the rest of the
entangled state. This projects the output state onto the correct one with a
certain exponentially small probability. The sufficient conditions are found
under which the scheme is of benefit.Comment: 4 pages, 1 figur