170 research outputs found
Quantum Optical Fredkin Gate
A simple optical model to realize a reversible, potentially error-free logic gate a Fredkin gate is discussed. The device dissipates no energy and makes use of the Kerr nonlinearity to produce intensity-dependent phase shifts. The analysis shows that quantum mechanics permits the operation of error-free logic gates which dissipate no energy. However, even though the device is nondissipative, error-free performance only occurs under particular operating conditions
Optimal Quantum Measurements for Phase Estimation
Quantum information theory is applied to practical interferometer-based phase measurements to deduce the optimal phase measurement scheme with two optical modes. Optimal phase measurements, given ideal input states, reveal an asymptotic 1/n decrease in phase uncertainty Delta theta for n the mean photon number of the input state. In contradistinction to previous schemes for realizing the number-phase uncertainty limit, the 1/n limit achieved here is independent of the interferometer phase shift; prior information about the expected phase shift is not necessary to attain this limit. These results apply more generally to su(2) and so(3) phase parameter estimation
Quantum-Theory of Optical Feedback Via Homodyne Detection
We present a quantum theory of feedback in which the homodyne photocurrent alters the dynamics of the source cavity. To the nonlinear stochastic (Ito) evolution of the conditioned system state we add a feedback term linear in the instantaneous stochastic (Stratonovich) photocurrent. Averaging over the photocurrent gives a feedback master equation which has the desired driftlike term, plus a diffusionlike term. We apply the model to phase locking a regularly pumped laser, and show that under ideal conditions the noise spectra of the output light exhibit perfect squeezing on resonance
Interference in hyperbolic space
The interference in a phase space algorithm of Schleich and Wheeler [Nature 326, 574 (1987)] is extended to the hyperbolic space underlying the group SU(1,1). The extension involves introducing the notion of weighted areas. Analytic expressions for the asymptotic forms for overlaps between the eigenstates of the generators of su(1,1) thus obtained are found to be in excellent agreement with the numerical results.[S1050-2947(98)08602-8]
Exploring quantum chaos with a single nuclear spin
Most classical dynamical systems are chaotic. The trajectories of two identical systems prepared in infinitesimally different initial conditions diverge exponentially with time. Quantum systems, instead, exhibit quasiperiodicity due to their discrete spectrum. Nonetheless, the dynamics of quantum systems whose classical counterparts are chaotic are expected to show some features that resemble chaotic motion. Among the many controversial aspects of the quantum-classical boundary, the emergence of chaos remains among the least experimentally verified. Time-resolved observations of quantum chaotic dynamics are particularly rare, and as yet unachieved in a single particle, where the subtle interplay between chaos and quantum measurement could be explored at its deepest levels. We present here a realistic proposal to construct a chaotic driven top from the nuclear spin of a single donor atom in silicon, in the presence of a nuclear quadrupole interaction. This system is exquisitely measurable and controllable, and possesses extremely long intrinsic quantum coherence times, allowing for the observation of subtle dynamical behavior over extended periods. We show that signatures of chaos are expected to arise for experimentally realizable parameters of the system, allowing the study of the relation between quantum decoherence and classical chaos, and the observation of dynamical tunneling
Realization of a single Josephson junction for Bose-Einstein condensates
We report on the realization of a double-well potential for Rubidium-87
Bose-Einstein condensates. The experimental setup allows the investigation of
two different dynamical phenomena known for this system - Josephson
oscillations and self-trapping. We give a detailed discussion of the
experimental setup and the methods used for calibrating the relevant
parameters. We compare our experimental findings with the predictions of an
extended two-mode model and find quantitative agreement
Quantum Interference of Photon Pairs from Two Trapped Atomic Ions
We collect the fluorescence from two trapped atomic ions, and measure quantum
interference between photons emitted from the ions. The interference of two
photons is a crucial component of schemes to entangle atomic qubits based on a
photonic coupling. The ability to preserve the generated entanglement and to
repeat the experiment with the same ions is necessary to implement entangling
quantum gates between atomic qubits, and allows the implementation of protocols
to efficiently scale to larger numbers of atomic qubits.Comment: 4 pages, 4 figure
Quantum Gates and Memory using Microwave Dressed States
Trapped atomic ions have been successfully used for demonstrating basic
elements of universal quantum information processing (QIP). Nevertheless,
scaling up of these methods and techniques to achieve large scale universal
QIP, or more specialized quantum simulations remains challenging. The use of
easily controllable and stable microwave sources instead of complex laser
systems on the other hand promises to remove obstacles to scalability.
Important remaining drawbacks in this approach are the use of magnetic field
sensitive states, which shorten coherence times considerably, and the
requirement to create large stable magnetic field gradients. Here, we present
theoretically a novel approach based on dressing magnetic field sensitive
states with microwave fields which addresses both issues and permits fast
quantum logic. We experimentally demonstrate basic building blocks of this
scheme to show that these dressed states are long-lived and coherence times are
increased by more than two orders of magnitude compared to bare magnetic field
sensitive states. This changes decisively the prospect of microwave-driven ion
trap QIP and offers a new route to extend coherence times for all systems that
suffer from magnetic noise such as neutral atoms, NV-centres, quantum dots, or
circuit-QED systems.Comment: 9 pages, 4 figure
Modern space-time and undecidability
The picture of space-time that Minkowski created in 1907 has been followed by
two important developments in physics not contained in the original picture:
general relativity and quantum mechanics. We will argue that the use of
concepts of those theories to construct space-time implies conceptual
modifications in quantum mechanics. In particular one can construct a viable
picture of quantum mechanics without a reduction process that has outcomes
equivalent to a picture with a reduction process. One therefore has two
theories that are entirely equivalent experimentally but profoundly different
in the description of reality they give. This introduces a fundamental level of
undecidability in physics of a kind that has not been present before. We
discuss some of the implications.Comment: 8 pages, no figures, Revtex, contribution to the volume "Minkowski
spacetime: a hundred years later", edited by Vesselin Petko
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