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ABSTRACT: A unified theory is presented of dynamically modified decay and decoherence in driven multilevel quantum systems that are weakly coupled to arbitrary zero-temperature reservoirs. Examples of different phase and amplitude modulations are given for two-level systems (qubits). Analysis of modulations on multilevel systems is detailed with a numerical example using quasiperiodic impulsive phase jumps. The merits and disadvantages of the different modulation types are discussed.
Journal of Optics B Quantum and Semiclassical Optics 09/2005; 7(10):S283. · 1.81 Impact Factor
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ABSTRACT: We derive and investigate an expression for the dynamically modified decay of states coupled to an arbitrary continuum. This expression is universally valid for weak temporal perturbations. The resulting insights can serve as useful recipes for optimized control of decay and decoherence.
IEEE Transactions on Nanotechnology 02/2005; · 2.29 Impact Factor
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Quantum Information & Computation. 01/2005; 5:285-317.
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ABSTRACT: We develop a unified theory of dynamically suppressed decay and decoherence by external fields in qubits coupled to arbitrary thermal baths and dephasing sources. This general theory does not invoke the rotating-wave approximation, which fails for ultrafast field-induced modulations of qubit-bath coupling. Considerations for optimizing the dynamical suppression are outlined.
Physical Review Letters 10/2004; 93(13):130406. · 7.37 Impact Factor
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ABSTRACT: We show that the quantum Zeno and anti-Zeno effects are realizable for macroscopic quantum tunneling by current-bias modulation in Josephson junctions (and their analogs in atomic condensates).
Physical Review Letters 06/2004; 92(20):200403. · 7.37 Impact Factor
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ABSTRACT: A unified theory is given of dynamically modified decay and decoherence in driven quantum systems that are coupled to arbitrary finite-temperature reservoirs and undergo random phase fluctuations. Criteria for the optimization of the suppression and the limitations of this approach are obtained. Decay acceleration by frequent measurements (interruptions of the coupling), known as the anti-Zeno effect (AZE) is argued to be much more ubiquitous than its inhibition in one- or two-level systems coupled to reservoirs (continua). In multilevel systems, frequent measurements cause accelerated decay by destroying the multilevel interference, which tends to inhibit decay in the absence of measurements.
Physics and Control, 2003. Proceedings. 2003 International Conference; 09/2003
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ABSTRACT: The impediment towards the successful development of the field of quantum information (QI) is decoherence, i.e., the loss of entanglement by the effect of the environment on the systems of interest. An important challenge is that of QI engineering, by entanglement and decoherence control, in complex systems, such as unimolecular and bimolecular systems, that can simultaneously handle large amounts of QI. Progress towards this goal can be achieved by: (a) decay modification and decoherence suppression in molecules, using laser-induced phase and amplitude modulation of rovibrational levels and inter-mode couplings; (b) transfer of internal-translational entanglement and teleportation of wavepackets via molecular dissociation and collisions.
Journal of Optics B Quantum and Semiclassical Optics 08/2002; 4(4):S294. · 1.81 Impact Factor
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ABSTRACT: Summary form only given. We purport to substantially expand the arsenal of decay control, whether measurement-like (i.e., accompanied by dephasing) or fully coherent. We derive a universal form of the decay rate of unstable states into any reservoir (continuum), modified by weak perturbations with arbitrary time dependence.
Quantum Electronics and Laser Science Conference, 2002. QELS '02. Technical Digest. Summaries of Papers Presented at the; 02/2002
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ABSTRACT: We derive and investigate an expression for the dynamically modified decay of states coupled to an arbitrary continuum. This expression is universally valid for weak temporal perturbations. The resulting insights can serve as useful recipes for optimized control of decay and decoherence.
Physical Review Letters 01/2002; 87(27 Pt 1):270405. · 7.37 Impact Factor
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ABSTRACT: The quantum Zeno effect (QZE) is the striking prediction that the decay of any unstable quantum state can be inhibited by sufficiently frequent observations (measurements). The consensus opinion has upheld the QZE as a general feature of quantum mechanics, which should lead to the inhibition of any decay. The claim of QZE generality hinges on the assumption that successive observations can in principle be made at time intervals too short for the system to change appreciably. However, this assumption and the generality of the QZE have scarcely been investigated thus far. We have addressed these issues by showing that (i) the QZE is principally unattainable in radiative or radioactive decay, because the required measurement rates would cause the system to disintegrate; (ii) decay acceleration by frequent measurements (the anti-Zeno effect -- AZE) is much more ubiquitous than its inhibition. The AZE is shown to be observable as the enhancement of tunneling rates (e.g., for atoms trapped in ramped-up potentials or in current-swept Josephson junctions), fluorescence rates (e.g., for Rydberg atoms perturbed by noisy optical fields) and photon depolarization rates (in randomly modulated Pockels cells).
03/2001;
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ABSTRACT: Summary form only given. The quantum Zeno effect (QZE) is the striking possibility of inhibiting the decay of an unstable quantum state by sufficiently frequent measurements. This effect is currently believed to be universal, even though it has only been observed for oscillations between discrete states. Recently we showed that both the QZE and the inverse (anti-) Zeno effect (AZE) can be observed on spontaneous decay in cavities. We develop a general theory of evolution of quantum systems in the presence of frequent measurements. We consider both the ideal case of instantaneous projections of the wave function to the initial state and some possible measurement schemes. The considerations imply the impossibility of the continuous-observation limit necessary to stop quantum evolution (the quantum Zeno paradox). Indeed, this limit leads to an unbounded increase of the energy fluctuations of the system, which would cause the system to disintegrate. The present theory describes different measurement effects, including the QZE and AZE, and allows us to obtain the general criteria of their validity. In particular, we show that the AZE, rather than the QZE, is achievable for spontaneous decay of atoms in free space.
Quantum Electronics and Laser Science Conference, 2001. QELS '01. Technical Digest. Summaries of Papers Presented at the; 02/2001
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ABSTRACT: Novel methods are discussed for the state control of atoms coupled to single-mode and multi-mode cavities and microspheres. (1) Excitation decay control: The quantum Zeno effect, i.e. inhibition of spontaneous decay by frequent measurements, is observable in high-Q cavities and microspheres using a sequence of evolution-interrupting pulses or randomly-modulated CW fields. By contrast, in 'bad' cavities or open space, frequent measurements can only accelerate the decay, causing the anti-Zeno effect. (2) Location-dependent interference of decay channels: Control of two metastable states is feasible via resonant single-photon absorption to an intermediate state, by engineering spontaneous emission in a multimode cavity. (3) Decoherence control by conditionally interfering parallel evolutions: An arbitrary internal state of an atomic wavepacket can be protected from decoherence by interference of its interactions with the cavity over many different time intervals in parallel, followed by the detection of appropriate atomic-momentum observables. The arsenal of control methods described above can advance the state-of-the-art of quantum information storage and manipulation in cavities.
New Journal of Physics 12/2000; 2(1):28. · 4.18 Impact Factor
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ABSTRACT: We discuss a simple, experimentally feasible scheme, which elucidates the principles of controlling ("engineering") the reservoir spectrum and the spectral broadening incurred by repeated measurements. This control can yield either the inhibition (Zeno effect) or the acceleration (anti-Zeno effect) of the quasi-exponential decay of the observed state by means of frequent measurements. In the discussed scheme, a photon is bouncing back and forth between two perfect mirrors, each time passing a polarization rotator. The horizontal and vertical polarizations can be viewed as analogs of an excited and a ground state of a two level system (TLS). A polarization beam splitter and an absorber for the vertically polarized photon are inserted between the mirrors, and effect measurements of the polarization. The polarization angle acquired in the electrooptic polarization rotator can fluctuate randomly, e.g., via noisy modulation. In the absence of an absorber the polarization randomization corresponds to TLS decay into an infinite-temperature reservoir. The non-Markovian nature of the decay stems from the many round-trips required for the randomization. We consider the influence of the polarization measurements by the absorber on this non-Markovian decay, and develop a theory of the Zeno and anti-Zeno effects in this system. Comment: 11 pages, 4 figures
11/2000;
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ABSTRACT: We point out that the quantum Zeno effect, i.e., inhibition of spontaneous decay by frequent measurements, is observable only in spectrally finite reservoirs, i.e., in cavities and waveguides, using a sequence of evolution-interrupting pulses or randomly-modulated CW fields. By contrast, such measurements can only accelerate decay in free space.
01/2000;
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ABSTRACT: . -- We show that the injection of spectrally narrow probe wavepackets into quantized parametrically amplifying media can give rise to transient tachyonic wavepackets. Remarkably, their quantum information, albeit causal, is confined to times corresponding to superluminal velocities, which is advantageous for communications. These phenomena are explicitly analyzed for stimulated Raman scattering, parametric downconversion and four-wave mixing. The quest for tachyons, i.e., particles with an imaginary rest mass which propagate superluminally (with group velocities v g ? c) has been motivated by their possible basic role in quantum-field and spacetime theories [1, 2]. A recent Letter coauthored by us [3] has considered an optical process wherein tachyonic behavior should be observable: the propagation of a weak, spectrally-narrow electromagnetic (EM) pulse through an inverted two-level medium, which is governed by the linearized sine-Gordon equation. This work provokes further importan...
12/1999;
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ABSTRACT: Introduction The "watchdog" or quantum Zeno effect (QZE) is a basic manifestation of the influence of measurements on the evolution of a quantum system. The original QZE prediction has been that irreversible decay of an excited state into an open-space reservoir can be inhibited [1], by repeated interruption of the system-reservoir coupling, which is associated with measurements (e.g., the interaction of an unstable particle with its environment on its flight through a bubble chamber) [2, 3]. However, this prediction has not been experimentally verified as yet! Instead, the interruption of Rabi oscillations and analogous forms of nearly-reversible evolution has been at the focus of interest [4--11]. Tacit assumptions have been made that the QZE is in principle attainable in open space, but is technically difficult. We have recently demonstrated [12] that the inhibition of nearly-exponential excited-state decay by the QZE in two-l
07/1999;
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A.G. Kofman
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ABSTRACT: Summary form only given. We have developed a general theory of single-photon bound-free transitions, which takes into account all the states of the quantum system of interest and treats the rotating- and counterrotating-wave terms on equal footing. With the help of the smoothed-dynamics approximation (SDA), which holds until the laser field is much weaker than the intramolecular field, a closed analytical result for the Laplace transform of the initial-state amplitude has been obtained. To perform the inverse Laplace transform, the pole approximation, extended to obtain higher-order corrections, has been employed
Quantum Electronics Conference, 1998. IQEC 98. Technical Digest. Summaries of papers presented at the International; 06/1998
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ABSTRACT: We show that optical tachyonic dispersion corresponding to superluminal
(faster than-light) group velocities characterizes parametrically amplifying
media. The turn-on of parametric amplification in finite media, followed by
illumination by spectrally narrow probe wavepackets, can give rise to transient
tachyonic wavepackets. In the stable (sub-threshold) operating regime of an
optical phase conjugator it is possible to transmit probe pulses with a
superluminally advanced peak, whereas conjugate reflection is always
subluminal. In the unstable (above-threshold) regime, superluminal response
occurs both in reflection and in transmission, at times preceding the onset of
exponential growth due to the instability. Remarkably, the quantum information
transmitted by probe or conjugate pulses, albeit causal, is confined to times
corresponding to superluminal velocities. These phenomena are explicitly
analyzed for four-wave mixing, stimulated Raman scattering and parametric
downconversion.
05/1998;
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ABSTRACT: We theoretically investigate the response of optical phase conjugators to incident probe pulses. In the stable (sub-threshold) operating regime of an optical phase conjugator it is possible to transmit probe pulses with a superluminally advanced peak, whereas conjugate reflection is always subluminal. In the unstable (above-threshold) regime, superluminal response occurs both in reflection and in transmission, at times preceding the onset of exponential growth due to the instability. Comment: 9 pages, 6 figures, RevTex, to appear in Phys. Rev. A
03/1998;
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ABSTRACT: We theoretically analyze wave packet transmission through a phase-conjugating mirror and show that the transmission of a suitably chosen input pulse is superluminal, i.e. the peak of the pulse emerges from the mirror before the time it takes to travel the same distance in vacuum. This pulse reshaping effect can be attributed directly to the dispersion relation in the nonlinear medium constituting the mirror. Thus, for the first time a connection is laid between optical phase conjugation and superluminal behavior. In view of its additional amplifying ability, a phase-conjugating mirror is a most promising candidate for an experimental observation of tachyonic signatures. Comment: 4 pages, 3 figures, RevTex
11/1997;
