2015


   


Ivan Deutsch
Home institution:
University of New Mexico
Visit dates: 
17/12/15 

 

SEMINAR

Time: 2pm, Thursday 17 December
Venue: Room 3024, Sydney Nanoscience Hub, A31 
Topic: Quantum Control and Measurement of Atomic Spins     


Abstract:

We study a comprehensive testbed for quantum information processing based on high-dimensional spins in ensembles of atomic cesium, controlled by magneto-optical fields. With this we explore optimal unitary control, entangling interactions, and fundamental quantum measurements. Particular examples include implementation of high-fidelity SU(16) unitary maps, quantum state tomography via continuous measurement, studies of compressed sensing tomography, and quantum control on the Dicke symmetric subspace of a collection of qubits encoded in the atomic ensemble.  I will give an overview of this QIP testbed, emphasizing the theoretical foundation of this platform, as well as experimental implementations conducted in collaboration with Prof. Poul S. Jessen, University or Arizona (USA), and Dr. Grant Biedermann, Sandia National Labs (USA). 

 


Troy Lee
Home institution:
Nanyang Technological University
Visit dates: 
14/12/15 

 

SEMINAR

Time: 3pm, Monday 14 December
Venue: Room 3024, Sydney Nanoscience Hub, A31
Topic: New separations in query complexity 


Abstract:

For partial boolean functions, whose domain can be a subset of {0,1}^n, exponential separations are known between the number of queries a classical deterministic algorithm needs to compute a function and the number of queries a quantum algorithm needs.  For a total boolean function f, whose domain is all of {0,1}^n, the situation is quite different: the quantum Q(f) and deterministic D(f) query complexities are always polynomially related, in fact D(f) = O(Q(f)^6).  It was widely believed this relation was far from tight, as for 20 years the largest separation known between these two measures has been quadratic, witnessed by Grover's search algorithm.  We exhibit a total boolean function with a 4th power separation between its quantum and deterministic query complexities.  Interestingly, no new quantum algorithms are needed to achieve this separation—our quantum algorithm is based on Grover search and amplitude amplification.

 

 


Matthias Brandl

Home institution:
Innsbruck University
Visit dates:

8-19/12/15



Ben Baragiola
Home institution:
University of New Mexico
Visit dates: 
26/11/15

 

SEMINAR

Time: 4pm, Thursday 26 November
Venue: Room 3024, Sydney Nanoscience Hub, A31
Topic: Quantum networks: Driving a quantum system with propagating Fock states of light


Abstract:

Traveling wave packets of light prepared with a definite number of photons, continuous-mode Fock states, are well-suited for the role of "flying qubits" to relay information for quantum information processing.

Describing their interaction with fundamentally quantum components - e.g. a multilevel atom in a cavity - is complicated by the fact that, in contrast to coherent and vacuum fields, continuous-mode Fock states are endowed with temporal-mode entanglement and drive non-Markovian reduced-state dynamics.

I will describe the Fock-state master equation formalism for the reduced-state dynamics of an arbitrary quantum system, either isolated or a collection of systems comprising a quantum network. In a tutorial fashion, I will attempt to provide a clear physical understanding of their derivation and interpretation using input-output theory.

Time permitting, I will introduce the Fock-state stochastic master equations, where the output fields are continuously monitored rather than traced out. Here, the reduced-state dynamics are conditional, as they depend on the random outcomes and the nature of the measurements (photon counting, homodyne, heterodyne).

 

 


Rainer Blatt

Home institution:
Innsbruck University                                 
Visit dates:
20-25/11/15

 


Howard Wiseman

Home institution:
Griffith University
Visit dates: 
19/11/15

 

SEMINAR

Time: 4pm, Thursday 19 November
Venue: Room 3024, Sydney Nanoscience Hub, A31 
Topic: What is Quantum Markovianity?

Abstract: Markovianity versus non-Markovianity is a well-established distinction for classical systems. The same cannot be said for quantum systems. Different people use “quantum non-Markovianity” to mean very different things. We argue that, to avoid confusion, it is best to avoid attributing that term any definite meaning at this stage. However, that does not mean that there is nothing to say about non-Markovianity for open quantum systens. We discuss a large number of concepts that have been, or could logically be, used to define quantum non-Markovianity, and prove hierarchical relations between them. These include (in order) “quantum white noise”, “composability”, “divisibility”, and “non-Markovianity witnesses”. We also prove relations between these and other properties of interest for open quantum systems, such as the applicability of dynamical decoupling to preserve quantum information, the existence of (quantum) information backflow from the environment, and the physical reality of stochastic pure-state trajectories.

 


Andrew Darmawan

Home institution:
University of Sherbrooke
Visit dates:
31/10 – 30/11

 

SEMINAR

Time: 3pm, Thursday 12 November
Venue: Room 3024, Sydney Nanoscience Hub, A31
Topic: Numerical study of the surface code under realistic noise

Abstract: The surface code is a promising candidate for quantum error correction in many architectures, requiring only nearest-neighbour interactions on a two-dimensional square lattice. Numerical simulations of the surface code have found a high threshold relative to other error correction schemes. However, most previous studies have assumed Pauli noise, a restricted noise model that allows efficient simulation within the stabilizer formalism. In this talk I will discuss simulation schemes for the surface code under local non-Pauli noise. Our schemes are based on the tensor network description of the surface code as a projected entangled pair state (PEPS). Logical error rates can be computed by contracting an appropriate tensor network. Exact contraction cannot always be performed efficiently, however approximate schemes may be used provided the error is not too large.

 


Robin Blume-Kohout

Home institution:
Sandia National Laboratories
Visit dates:
4-10/11/15


SEMINAR

Date: 3pm, Thursday 5 November,
Venue:
Room 3024, Sydney Nanoscience Hub, A31
Topic:
What is the error rate of a quantum gate?

Abstract: Abstract: I will try to convince you that the two titles of this talk are, in fact, synonymous - that the “error rate” and “distinguishability of quantum processes” are the same thing. Whether or not I succeed, I will go on to discuss (1) the various ways that this has been quantified, (2) the state of the art in doing so, and (3) why I’m not (and you shouldn’t be) satisfied. Having spent 45 minutes just establishing what the “right” problem is, I will then propose to solve it by sandwiching “distinguishability” between “distillable distinguishability” and “distinguishability of formation”. To demonstrate the utility of this approach, I’ll prove that the diamond norm is not always the right measure of distinguishability (or even close to it!). I will then do a 180-degree turn and argue that for most of the case that we care about, the diamond norm is a good measure of distinguishability, and finally conclude with another 180-degree turn in which I argue that maybe it’s not.

 

 

 


Christopher Wood

Home institution:
IBM
Visit dates:
5/11/15

 

SEMINAR

Date: 3pm Thursday 5 November,
Venue: Room 3024, Sydney Nanoscience Hub, A31
Topic: Initialization and Characterization Open Quantum Systems

Abstract: Abstract: In any real world experiment quantum systems are inevitably coupled to their environment. This environment encapsulates all degrees of freedom that are not directly accessible to the experimenter and typically acts as a source of noise that needs to be characterized and constrained. Under certain conditions it may also be harnessed as a resource, for example in initializing the system to a state that may be otherwise unobtainable. In the first half of this talk I will present recent work for using a collective coupling to an environment for the initialization of a spin-based quantum information processor to a low entropy state. This approach applies cavity cooling techniques to engineer a collective T1 relaxation process of the spin ensemble. By including a local dephasing noise process on the spins I will show how one may transform the collective T1 relaxation process to a local T1 relaxation process enabling cooling to the ground state of the spin ensemble. In the second half I will introduce the super-channel formalism - a generalized channel which takes another quantum operation, rather than a quantum state, as its input. This enables an operational description of a strictly larger set of system-environment interactions than the standard formalism. I will briefly describe techniques for constructing superchannel descriptions of a sequence of quantum channels and show how they may be applied to characterizing simple cases of non-Markovian noise process.