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Register a new userOn-chip interaction-free measurements via the quantum Zeno effect
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Although interference is a classical-wave phenomenon, the superposition principle, which underlies interference of individual particles, is at the heart of quantum physics. An interaction-free measurements (IFM) harnesses the wave-particle duality of single photons to sense the presence of an object via the modification of the interference pattern, which can be accomplished even if the photon and the object havent interacted with each other. By using the quantum Zeno effect, the efficiency of an IFM can be made arbitrarily close to unity. Here we report an on-chip realization of the IFM based on silicon photonics. We exploit the inherent advantages of the lithographically written waveguides: excellent interferometric phase stability and mode matching, and obtain multipath interference with visibility above 98%. We achieved a normalized IFM efficiency up to 68.2%, which exceeds the 50% limit of the original IFM proposal.
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The quantum Zeno effect describes the inhibition of quantum evolution by frequent measurements. Here, we propose a scheme for entangling two given photons based on this effect. We consider a linear-optics set-up with an absorber medium whose two-photon absorption rate $xi_{2gamma}$ exceeds the one-photon loss rate $xi_{1gamma}$. In order to reach an error probability $P_{rm error}$, we need $xi_{1gamma}/xi_{2gamma}<2P_{rm error}^2/pi^2$, which is a factor of 64 better than previous approaches (e.g., by Franson et al). Since typical media have $xi_{2gamma}<xi_{1gamma}$, we discuss three mechanisms for enhancing two-photon absorption as compared to one-photon loss. The first mechanism again employs the quantum Zeno effect via self-interference effects when sending two photons repeatedly through the same absorber. The second mechanism is based on coherent excitations of many atoms and exploits the fact that $xi_{2gamma}$ scales with the number of excitations but $xi_{1gamma}$ does not. The third mechanism envisages three-level systems where the middle level is meta-stable ($Lambda$-system). In this case, $xi_{1gamma}$ is more strongly reduced than $xi_{2gamma}$ and thus it should be possible to achieve $xi_{2gamma}/xi_{1gamma}gg1$. In conclusion, although our scheme poses challenges regarding the density of active atoms/molecules in the absorber medium, their coupling constants and the detuning, etc., we find that a two-photon gate with an error probability $P_{rm error}$ below 25% might be feasible using present-day technology.
The quantum Zeno effect, i.e. the inhibition of coherent quantum dynamics by projective measurements is one of the most intriguing predictions of quantum mechanics. Here we experimentally demonstrate the quantum Zeno effect by inhibiting the microwave driven coherent spin dynamics between two ground state spin levels of the nitrogen vacancy center in diamond nano-crystals. Our experiments are supported by a detailed analysis of the population dynamics via a semi-classical model.
A relation is found between pulsed measurements of the excited state probability of a two-level atom illuminated by a driving laser, and a continuous measurement by a second laser coupling the excited state to a third state which decays rapidly and irreversibly. We find the time between pulses to achieve the same average detection time than a given continuous measurement in strong, weak, or intermediate coupling regimes, generalizing the results in L. S. Schulman, Phys. Rev. A 57, 1509 (1998).
In ``interaction free measurements, one typically wants to detect the presence of an object without touching it with even a single photon. One often imagines a bomb whose trigger is an extremely sensitive measuring device whose presence we would like to detect without triggering it. We point out that all such measuring devices have a maximum sensitivity set by the uncertainty principle, and thus can only determine whether a measurement is ``interaction free to within a finite minimum resolution. We further discuss exactly what can be achieved with the proposed ``interaction free measurement schemes.
We investigate theoretically the suppression of two-body losses when the on-site loss rate is larger than all other energy scales in a lattice. This work quantitatively explains the recently observed suppression of chemical reactions between two rotational states of fermionic KRb molecules confined in one-dimensional tubes with a weak lattice along the tubes [Yan et al., Nature 501, 521-525 (2013)]. New loss rate measurements performed for different lattice parameters but under controlled initial conditions allow us to show that the loss suppression is a consequence of the combined effects of lattice confinement and the continuous quantum Zeno effect. A key finding, relevant for generic strongly reactive systems, is that while a single-band theory can qualitatively describe the data, a quantitative analysis must include multiband effects. Accounting for these effects reduces the inferred molecule filling fraction by a factor of five. A rate equation can describe much of the data, but to properly reproduce the loss dynamics with a fixed filling fraction for all lattice parameters we develop a mean-field model and benchmark it with numerically exact time-dependent density matrix renormalization group calculations.
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