February 2018 - ITP-quantumseminare

No meeting today—moved to Thursdays at 3pm
by Joe Renes 26 Feb '18

26 Feb '18

Hi all, We decided to move the meeting to Thursdays at 3. I’ll send out details of this week’s meeting later on. Joe

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talk tomorrow @ 16:45 in E41.1 ; new group meeting time
by Joe Renes 18 Feb '18

18 Feb '18

Hi all, tomorrow we have a guest, Sania Jevtic from Imperial College, who will tell us about recent results on steering. See below for title and abstract. Note the special time; this is also due to the fact that she is only in town tomorrow. Subsequent meetings will be on a different day, and thanks to everyone for filling out the doodle. It looks like Thursday afternoon is perhaps the best option (also Wednesday afternoon, but then there's also the physics colloquium, or Thursday morning, but it's morning. ;) We should make a decision tomorrow. Best, -joe ----------------- Title: Quantum steering with positive operator valued measures Abstract: Quantum steering is one of the three forms of quantum non-locality, the other two being Bell-nonlocality and entanglement. The quantum steering scenario most closely resembles the seminal thought experiment of Einstein, Podolsky, and Rosen (1935). Two distant observers, Alice and Bob, each possess a quantum particle, and these two particles are entangled. Alice can perform measurements on her particle which affect, or “steer”, the state of Bob’s particle. The fascinating aspect of this is that, for certain entangled states of the two particles, Bob’s steered ensembles cannot be described in a classical way, that is, using a model of local hidden states. Such entangled states are called “steerable” (otherwise they are unsteerable). Steerable states have been verified experimentally and have proven advantageous in a variety of quantum information tasks. Nevertheless, the set of steerable states is still very poorly understood. Methods for checking whether a state is steerable have been presented in cases when Alice’s measurements are restricted, for example, she can only perform projective, measurements. There are currently no known efficient methods for tackling steerability when she has the ability to perform generalised measurements, known as “positive-operator valued measures” (POVMs). By viewing the steering as a problem of nested convex objects, we derive an inequality which can help to determine the boundary of steerable vs unsteerable quantum states for all measurements. Given an ansatz u for the "ensemble of local hidden states", we can systematically test whether a given entangled state is unsteerable with respect to u. We test our inequality on a “Werner state” and confirm (numerically to a high precision) a longstanding conjecture that Werner state is unsteerable for all measurements when it is an equal mixture of the maximally mixed state and a singlet. As a novel application, we also test our inequality on states that are mixtures of Bell pairs (“T-states”), and our numerics indicate that here also steerability for all measurements coincides with steerability for projective measurements.

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no meeting today --- new meeting time?
by Joe Renes 12 Feb '18

12 Feb '18

Hi all, in view of the upcoming semester, we need to move the group meeting to avoid conflicting with the Quantum Information Processing course. To find a good common time, please indicate your availability in the doodle at https://doodle.com/poll/5cd7h8r9n9tpszcy. Best, -joe

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tomorrow's group meeting 15:45 E41.1
by Joe Renes 04 Feb '18

04 Feb '18

Hi all, tomorrow we'll hear from Yosi Atia, visiting from Hebrew University. His title and abstract are below. If you want to check out the paper in advance, it's https://arxiv.org/abs/1610.09619. Best, -joe Fast Forwarding of Hamiltonians and Exponentially Precise Measurements Yosi Atia, Dorit Aharonov The time-energy uncertainty relation (TEUR) ∆t∆E ≥ 1/2 holds if the Hamiltonian is completely unknown, but can be violated otherwise; Here we initiate a rigorous study describing when and to what extent such violations can occur. To this end we propose a computational version of the TEUR (cTEUR) in which ∆t is replaced by the computational complexity of simulating the measurement. cTEUR violations are proved to occur if and only if the Hamiltonian can be fast forwarded (FF), namely, simulated for time t with complexity significantly smaller than t. Shor’s algorithm provides an example of exponential cTEUR violations; we show that so do commuting local Hamiltonians and quadratic Fermion Hamiltonians. A general FF method is ruled out, but finding further examples, as well as experimental demonstrations, are left for future work. We discuss possible connections to sensing and quantum gravity. This work initiates a rigorous theory of efficiency versus accuracy in energy measurements using computational complexity language.

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