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This Semester

 

:: Wed 4/2, 4:30 PM in 6C-442

Renato Renner (ETH)
Reliable Quantum State Tomography

Quantum state tomography is the task of estimating the state of a quantum system using measurements. Typically, one is interested in the (unknown) state generated during an experiment which can be repeated arbitrarily often in principle. However, the number of actual runs of the experiment, from which data is collected, is always finite (and often small). As pointed out recently, this may lead to unjustified (or even wrong) claims when employing standard statistical tools without care. In this talk, I will present a method for obtaining reliable estimates from finite tomographic data. Specifically, the method allows the derivation of confidence regions, i.e., subsets of the state space in which the unknown state is contained with probability almost one.


:: Fri 4/11, 1:30 PM in 6C-442

Keith Lee (Perimeter Institute)
Quantum Algorithms, Quantum Field Theory, and Computational Complexity

Quantum field theory provides the framework for the Standard Model of particle physics and plays a key role in physics. However, calculations are generally computationally complex and limited to weak interaction strengths. After an introduction to quantum field theory, I'll describe polynomial-time quantum algorithms for computing relativistic scattering amplitudes in both scalar and fermionic quantum field theories. The algorithms achieve exponential speedup over known classical methods. One of the motivations for this work comes from computational complexity theory. Ultimately, we wish to know what is the computational power of our universe. Studying such quantum algorithms probes whether a universal quantum computer is powerful enough to represent quantum field theory; in other words, is quantum field theory in BQP? Conversely, one can ask whether quantum field theory can represent a universal quantum computer; is quantum field theory BQP-hard? I'll describe our approach to addressing the question of BQP-hardness.


:: Fri 4/18, 1:30 PM in 6C-442

Stephanie Wehner (National University of Singapore)
The Second Laws of Quantum Thermodynamics

The second law of thermodynamics tells us which state transformations are so statistically unlikely that they are effectively forbidden. Its original formulation, due to Clausius, states that "Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time." The second law applies to systems composed of many particles, however, we are seeing that one can make sense of thermodynamics in the regime where we only have a small number of particles interacting with a heat bath, or when we have highly correlated systems and wish to make non-statistical statements about them. Is there a second law of thermodynamics in this regime? Here, we find that for processes which are cyclic or very close to cyclic, thesecond law for microscopic or highly correlated systems takes on a very different form than it does at the macroscopic scale, imposing not just one constraint on what state transformations are possible, but an entire family of constraints. In particular, we find that the quantum Renyi relative entropy distances to the equilibrium state can never increase. We further find that there are three regimes which govern which family of second laws govern state transitions, depending on how cyclic the process is. In one regime one can cause an apparent violation of the usual second law, through a process of embezzling work from a large system which remains arbitrarily close to its original state.


:: Fri 4/25, 1:30 in 6C-442

Isaac Kim (Perimeter Institute)
On the Informational Completeness of Local Observables

For a general multipartite quantum state, we formulate a locally checkable condition, under which the expectation values of certain nonlocal observables are completely determined by the expectation values of some local observables. The condition is satisfied by ground states of gapped quantum many-body systems in one and two spatial dimensions, assuming a widely conjectured form of area law is correct. Its implications on quantum state tomography, quantum state verification, and quantum error correcting code is discussed.


:: Fri 5/2, 1:30 in 6C-442

Sergei Bravyi (IBM)
TBA

TBA


:: Fri 5/9, 1:30 PM in 6C-442

Andris Ambainis (University of Latvia)
TBA

TBA


:: Fri 5/16, 1:30 PM in 6C-442

Robin Kothari (University of Waterloo)
Exponential improvement in precision for simulating sparse Hamiltonians

We provide a quantum algorithm for simulating the dynamics of sparse Hamiltonians with complexity sublogarithmic in the inverse error, an exponential improvement over previous methods. Unlike previous approaches based on product formulas, the query complexity is independent of the number of qubits acted on, and for time-varying Hamiltonians, the gate complexity is logarithmic in the norm of the derivative of the Hamiltonian. Our algorithm is based on a significantly improved simulation of the continuous- and fractional-query models using discrete quantum queries, showing that the former models are not much more powerful than the discrete model even for very small error. We also significantly simplify the analysis of this conversion, avoiding the need for a complex fault correction procedure. Our simplification relies on a new form of "oblivious amplitude amplification" that can be applied even though the reflection about the input state is unavailable. Finally, we prove new lower bounds showing that our algorithms are optimal as a function of the error. This is joint work with Dominic W. Berry, Andrew M. Childs, Richard Cleve, and Rolando D. Somma. Available at http://arxiv.org/abs/1312.1414


:: Fri 5/23, 1:30 PM in 6C-442

Robin Blume-Kohout (tentative) (Sandia National Labs)
TBA

TBA


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