Fundamental phenomena like Quantum Zeno effect (QZE) and Anti-Zeno Effect (AZE) have been recognized as relevant tools for quantum control. Along this line, here we present two experiments in which we demonstrate the capability to extract information on noise events by exploiting QZE and AZE. In the first experiment, we realize noise diagnostics by frequent measurement, showing how a single photon undergoing a noise process (e.g., random polarization fluctuations) can diagnose non-Markovian temporal correlations within such a noise. In the second one, instead, we show how, by protecting via QZE a photonic qubit in a noisy quantum channel, it is possible to estimate the statistical distribution of the microscopic noise (decoherence) events by using the qubit itself as a probe. These techniques can become indispensable under extremely faint illumination, when traditional interferometric methods are usually ineffective.
We consider single photons propagating along two paths, with the polarization correlated to the path. Two
information related aspects of this translational-internal entanglement (TIE) are analyzed: a) Using the polarization
to record the path (a "flying detector" scheme), we characterize the tradeoff between path- and phaseinformation.
b) We investigate the effects of non-Markovian noise on the two-qubit quantum channel consisting
of the photon path and polarization (that are both used to encode information), and suggest noise protection
schemes.
Using quantum channels to transmit classical information has been proven to be advantageous in several scenarios.
These channels have been assumed to be memoryless, meaning that consecutive transmissions of information
are uncorrelated. However, as shown experimentally, such correlations do exist, and thereby retain memory
of previous information. This memory complicates the protection of entangled-information transmission from
decoherence.
We have recently addressed these fundamental questions by developing a generalized master equation for multipartite
entangled systems coupled to finite-temperature baths and subject to arbitrary external perturbations
whose role is to provide dynamical protection from decay and decoherence.
Here we explore and extend the foregoing strategy to quantum optical communication schemes wherein
polarization-entangled photons traverse a bit-flip channel with temporal and spatial memory, such that the
two channels experience cross-decoherence. We introduce a novel approach to the protection of the entangled
information from decoherence in such schemes. It is based on selectively modulating the photon polarizations in
each channel.
We show that by applying selective modulation, one can independently control the symmetry and spatial
memory attributes of the channel. We then explore the effects of these attributes on the channel capacity.
Remarkably, we show that there is a nontrivial interplay between the effects of asymmetry and memory on the
channel capacity.
KEYWORDS: Modulation, Quantum communications, Particles, Entangled states, Phase shift keying, Quantum information, Phase modulation, Control systems, Chemical species, Chemical elements
A unified theory is given of dynamically modified decay and decoherence of field-driven multilevel multipartite entangled states that are weakly coupled to zero-temperature baths. The theory allows for arbitrary local differences in their coupling to the environment. Due to such differences, the optimal driving-field modulation to ensure maximal fidelity is found to substantially differ from conventional π-phase flips of the single-qubit evolution.
A scalable multatom entangled system, capable of high-performance quantum computations, can be realized by resonant dipole-dipole interacting dopants in a solid state host. In one realization, the qubits are represented by ground and subradiant states of effective dimers formed by pairs of closely spaced two-level systems (TLS). Such qubits are highly robust against radiative decay. The two-qubit entanglement in this scheme relies on coherent excitation exchange between the dimers by external laser fields. This scheme is challenging because of the nanosize control and addressability it requires. Another realization involves dipole-dipole interacting TLS whose resonance frequency lies in a photonic band gap of a dielectric photonic crystal. A sequence of abrupt changes of the resonance frequency can produce controlled entanglement (logic gates) with improved protection from radiation decay and decoherence.
In this paper, we show how the non-holomic control technique can be employed to build completely controlled quantum devices. Examples of such controlled structures are provided.
In this paper, we present a coherence protection method based upon a multidimensional generalization of the Quantum Zeno Effect, as well as ideas from the coding theory. The non-holonomic control technique is employed as a physical tool which allows its effective implementation. The two limiting cases of small and large quantum systems are considered.
In this paper, we present a universal control technique, the non-holonomic control, which allows us to impose any arbitrarily prescribed unitary evolution to any quantum system through the alternate application of two well-chosen perturbations.
In this paper, we present a realistic application of the coherence protection method proposed in the previous article. A qubit of information encoded on the two spin states of a Rubidium isotope is protected from the action of electric and magnetic fields.
We introduce and discuss two schemes for generation and transfer of photon-photon and atom-atom entanglement. First we propose a method to achieve a large conditional phase shift of a probe field in the presence of a single-photon control field whose carrier frequency is within the photonic band gap created by spatially-periodic modulation of the electromagnetically induced transparency resonance. Then we present the concept of a reversible transfer of the quantum state of two internally-translationally entangled fragments, formed by molecular dissociation, to a photon pair. Our scheme allows, in principle, high-fidelity state transfer from the entangled dissociated fragments to light, thereby producing a highly correlated photon pair. This process can be followed by its reversal at a distant node of a quantum network resulting in the recreation of the original two-fragment entangled state. The proposed schemes may have advantageous applications in quantum teleportation, cryptography, and quantum computation.
The inversionless free electron laser having a drift region consisting of two magnets is analyzed. Performing numerical simulations of electron motion inside wigglers and drift region, we have shown that this system has a positive mean gain over the entire energy distribution of the electron beam. We study the influence of emittance and the spread of electron energies on the gain in small- and high-gain regimes.
We survey basic quantum optical processes that undergo modifications in photonic crystals doped with resonant atoms: (a) Solitons and multi-dimensional localized 'bullets' propagating at photonic band gap frequencies. These novel entities differ substantially from solitons in Kerr-nonlinear photonic crystals. (b) Giant photon-photon cross-coupling that can give rise to fully entangled two-photon states. We conclude that doped photonic crystals have the capacity to form efficient networks for high-fidelity classical and quantum optical communications.
Nonlinear optical processes in spherical microdroplets whose studies have been pioneered by Chang and co-workers are intriguing from the point of view of applications because of the extremely low thresholds they exhibit for the generation of stimulated output, but also from the theoretical point of view, because they require a synthesis between the methods of nonlinear optics and Mie scattering theory. The basis for such a synthesis has been given in the semiclassical theory of Kurizki and Nitzan, which implies that the nonlinearly amplified component of the scattered field is spatially orthogonal ('out of phase') with the linearly scattered field component (the ordinary Mie solution). This approach allows the calculation of amplification coefficients in various nonlinear processes, as a function of the Mie resonant denominators and spatial overlap of the input and output spherical waves. Recently, this approach has been extended to quantized solutions for parametric amplification and oscillation via four-wave mixing in microdroplets. The very low thresholds for oscillation at Mie resonances are predicted to correspond to strong squeezing, i.e., suppression of the photocurrent noise associated with the detection of the output waves below the noise level associated with an ideal laser. Even well below threshold, the spatially orthogonal output waves allow for strong squeezing at periodically recurring distances from the droplet.
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