Future seminars:

Past seminars:

Speaker: Prof John Howell
Title: Weak Values and Precision Measurements
Time: Wednesday, July 6, 11:00.
Location:Holzblat Hall 007
Abstract: Weak values were originally introduced by Aharanov, Albert and Vaidman for understanding the arrow of time in quantum mechanics. Weak values have recently proved useful in amplifying very small effects. I will introduce the ideas of weak values and discuss some of our recent experimental results in which we were able to observe a deflection of a laser beam of less than 1 picoradian (equivalent to measuring a deflection of the width of a hair at the distance of the moon). Perhaps more interestingly, the noise properties lead to a suppression of technical noise and an amplification of the signal to noise ratio to the optimal value for coherent states for standard beam deflection techniques. I will also discuss the use of weak values and precision deflection measurements for precision spectral and phase measurements.

Speaker: Yaron Kedem
Title: Modular values and weak values of quantum observables
Time: Sunaday, June 12, 13:00.
Location:Kaplun 324
Abstract: We will introduce a new concept, the modular value of an observable of a pre- and post-selected quantum system [1]:

Am = <ψ| Exp(-i k A) |φ> / <ψ|φ>,
where <ψ| |φ> is a two-state vector describing the system and k is a constant having dimension such that (k A) is dimensionless. The modular value completely characterizes the effect of a pre- and post-selected quantum system on a qubit that is coupled via an observable A and coupling strength k. For some observables, modular values yield weak values Aw = <ψ| A |φ> / <ψ|φ>, universal properties of pre- and post-selected quantum system weakly coupled to other systems introduced by Aharonov, Albert and Vaidman in 1988. Measurements of modular values, which do not require weak coupling, provide an efficient method of measuring certain weak values. This is particularly important for measuring weak values of nonlocal observables. Modular values have a potential for useful applications in devising and engineering novel quantum protocols.
[1] Y. Kedem, L. Vaidman, Phys. Rev. Lett, 105, 230401 (2010)

Speaker: Avi Marchewka
Title: Bosons and Fermions Escape from a Trapping Beam
Time: Tuesday, May 31, 13:30.
Location:Shenkar 105
Abstract: We investigate the dynamics of pairs of Fermions and Bosons with respect to an arbitrary spatial region and find that their populations have unique generic principles ensuing from the axioms of quantum statistics and symmetries. These principles depend neither on the specific wave equations, such as Schrödinger, Klein-Gordon, Dirac, nor on the specific potential involved. These principles are even independent of the validity of the conservation laws and are time independent. One surprising finding is that outside any arbitrary spatial region, there are always more Boson than Fermion pairs. Moreover, if the initial single particle wave functions have the same parity (odd or even), then there is a higher chance of finding a Boson pair (as opposed to a Fermion one) on two opposite sides of the spatial region as if there is a“repulsion” between them. The case of releasing from infinite trap of pair of Fermions and of Bosons will be taken.

Speaker: Noam Erez
Title: Quantum phases, Maxwell's Demon & Bad apples
Time: Wednesday, May 4, 16:00.
Location:Shenkar 222
Abstract: Statistical mechanics contributed to the discovery of quantum theory (black body problem) and the physical aspects of information (Maxwell's demon). Quantum mechanics, in turn, introduced phases into physical probability amplitudes. I will discuss some surprising aspects of these phases, and their thermodynamic consequences. Whereas the information obtained by measurement (quantum or classical)--in effect 'inside information' on fluctuations--can be used to obtain work from a system in thermal equilibrium, a quantum measurement can also create thermal non-equilibrium allowing work extraction even without such information. In either case, the 2nd law is upheld, but the mechanisms are very different.

Speaker: Erez Zohar
Title: The Fermi Problem in Discrete Systems
Time: Tuesday, April 12, 1:30.
Location:Shenkar 105
Abstract: The Fermi two-atom problem illustrates an apparent causality violation in Quantum Field Theory which has to do with the nature of the built in correlations in the vacuum. It has been a constant subject of theoretical debate and discussions during the last few decades. Nevertheless, although the issues at hand could in principle be tested experimentally, the smallness of such apparent violations of causality in Quantum Electrodynamics prevented the observation of the predicted effect. We show that the problem can be simulated within the framework of discrete systems that can be manifested, for instance, by trapped atoms in optical lattices or trapped ions. Unlike the original continuum case, the causal structure is no longer sharp. Nevertheless, as we show, it is possible to distinguish between "trivial" effects due to "direct" causality violations, and the effects associated with Fermi's problem, even in such discrete settings. The ability to control externally the strength of the atom-field interactions, enables us also to study both the original Fermi problem with "bare atoms", as well as correction in the scenario that involves "dressed" atoms. Finally, we show that in principle, the Fermi effect can be detected using trapped ions.

Speaker: Haggai Landa
Title: Strongly Correlate Systems with Cold Atoms
Time: Wednesday, November 17, 10:00 – 12:00.
Location: Dan David 210
Abstract: I will give a general overview of the subject of BEC with cold atoms and optical lattices, focusing on the famous superfluid to Mott-insulator experiment by the Greiner/Bloch group (2002).

Speaker: Noam Erez
Title: Ruling out non-linear extensions of quantum mechanics?
Time: Wednesday, November 10, 10:00 – 12:00.
Location: Dan David 210
Abstract: Historically, every physical theory that has been around long enough has eventually been replaced by refinements. Will quantum mechanics (core features such as the superposition principle, as opposed to particular quantum theories) forever remain an exception? Various non-linear modifications have been proposed and some have been put to rigorous experimental tests. It has been shown that generically, such 'corrections' to QM run afoul of the no-signaling principle ('EPR phone') or lead to more subtle, potentially unphysical effects ('Everett phone'). However, it is also known that extensions that satisfy no-signaling do exist. I will give an informal survey of some of the theories that have been proposed, and experiments that have been carried out to look for specific non-linearities and general violations of Born's rule.

Speaker: Judy Kupferman
Title: Black Hole Entropy Divergence and the Uncertainty Principle
Time: Wednesday, November 3, 10:00 – 12:00.
Location: Dan David 210
Abstract: Black hole entropy has been shown by t'Hooft to diverge at the horizon. The region near the horizon is in a thermal state, so entropy is linear to energy. We first show that a similar divergence due to an infinitely sharp boundary occurs for the energy and entanglement entropy of relativistic and non-relativistic field theories, extending previous results in quantum mechanics. We argue that the origin of the divergence is the position/momentum uncertainty relations and show that when the boundary is smoothed the divergence is tamed. Then we apply our ideas to black hole entropy. We show that the "brick wall" which renders the entropy finite can be interpreted as smoothing of the sharp boundary at the horizon and that any other smoothing of the boundary will result in the same finite answer for the entropy.
The talk is based on a collaboration with Ram Brustein: arXiv:1010.4157

Speaker: Yaron Kedem
Title: Amplification using Weak measurements
Time: Wednesday, October 20, 10:00 – 12:00.
Location: Dan David 210
Abstract: In the last few years the technique of weak measurements was used to measure tiny quantities which seemed undetectable otherwise. I will present the result of Hosten and Kviat, observing the Spin Hall Effect of Light, and the results of Dixon et. al. measuring small beam deflection and also optimizing its SNR. If time allow we will also discuss the meaning of these findings in a more general theoretical framework.

Speaker: Aharon Brodutch
Title: What can we learn from quantum discord?
Time: Sunday, September 26, 11:00 - 13:00.
Location: Kaplun 324.
Abstract: Quantum discord was proposed as a measure of the "quantumness" of correlations, and can be used to quantify the difference between various quantum (non local) and classical (local) protocols for various tasks such as Szilard's engine. The precise definition of discord can be changed to suite the various protocols we want to analyze. Most measures of discord are asymmetric and require some optimization over all possible local measurements. We show that these asymmetric versions of discord must vanish simultaneously and that a simple redefinition of discord which does not require optimization gives us a simple method to test for zero discord. This type of discord is related to a one way communication protocol for extracting work out of a system. Since all versions of discord vanish simultaneously, the definition of zero discord states as quasi-classical is consistent for all types of discord. We look at some of the properties of zero discord and non zero discord states, especially those related to completely positive maps, computational speed up and entanglement. We show that in some tasks it is impossible to increase the discord without at least some entanglement resources.