2019_Abstracts

Dr John Bartholomew

Title: Toward rare-earth ion quantum technologies at the single-ion level.

Abstract: Rare-earth ions embedded in crystals are an appealing platform for realising a suite of quantum technologies. In addition to demonstrating high performance devices for optical quantum systems, rare-earth ion technology development has broadened to encompass integration with superconducting quantum systems and spin qubits [1]. This expansion in hardware connectivity has been supported by key material developments and fabrication advances, including the demonstration of long electron spin coherence lifetimes and architectures for on-chip integration [2].

In this talk, I will focus on recent results that use on-chip nanophotonic cavities to create optically addressable spin qubits based on single rare-earth ions [3,4]. By harnessing single ions and miniaturised cavity architectures, these initial steps open up opportunities for scaling up resources in qubit networks through local and remote interactions.

[1] Williamson et al., Physical Review Letters 113, 203601 (2014).

[2] Heshami et al., Journal of Modern Optics 63, 2005 (2016).

[3] Raha et al., arXiv:1907.09992 [quant-ph] (2019).

[4] Kindem et al., arXiv:1907.12161v1 [quant-ph] (2019).

Dr Alexander Pitchford

Title: Live optimisation of quantum states - an example maximising squeezing in an opto-mechanical system.

Abstract: Working with partners in Olomouc (theory) and Vienna (experiment) we are preparing for a trial of Theory Blind Quantum Control optimisation method. We aim at maximisation of the optomechanical two-mode squeezing achievable between the mechanical motion of a levitated nanoparticle inside a cavity and a certain temporal mode of light that leaks from the cavity. The two-mode squeezing is observed when the smallest eigenvalue of the covariance matrix of this bipartite system is below the shot-noise. In preparation we are working with a simulation of the experimental setup to demonstrate how numerically optimising control over the driving laser pulse and homodyne measurement parameters could achieve higher levels of squeezing than those derived through theory alone.

Ass/Prof. Marco Tomamichel

Title: Fault-tolerant quantum advantage with shallow circuits in 3D

Abstract: As larger and larger prototypes of quantum computers are being developed, one of the most exciting challenges in the theory of quantum computing is to find computational problems that can be solved by an intermediate-scale noisy quantum computer, but are beyond the capabilities of existing non-quantum computers. Towards this goal, we construct a class of problems that show a weaker separation: they can be solved by noisy quantum computers of constant circuit depth but require at least logarithmic depth classical circuits.

Prior work has shown that there exists a relation problem which can be solved with certainty by a constant-depth quantum circuit composed of geometrically local gates in two dimensions, but cannot be solved with high probability by any classical constant depth circuit composed of bounded fan-in gates. Here we provide two extensions of this result. Firstly, we show that a separation in computational power persists even when the constant-depth quantum circuit is restricted to geometrically local gates in one dimension. The corresponding quantum algorithm is the simplest we know of which achieves a quantum advantage of this type. It may also be more practical for future implementations. Our second, main result, is that a separation persists even if the shallow quantum circuit is corrupted by noise. We construct a relation problem which can be solved with near certainty using a noisy constant-depth quantum circuit composed of geometrically local gates in three dimensions, provided the noise rate is below a certain constant threshold value. On the other hand, the problem cannot be solved with high probability by a noise-free classical circuit of constant-depth.

Dr Mathieu L. Juan

Title: Collective effects between superconducting qubits: designing an artificial “Dicke molecule”

Mathieu L. Juan¹, Aleksei Sharafiev¹, Juan José García-Ripoll², Gerhard Kirchmair^1,3

¹ Institute for Quantum Optics and Quantum Information, Innsbruck, Austria

² Institute of Fundamental Physics IFF-CSIC, Madrid, Spain

³ Institute for Experimental Physics, Innsbruck, Austria

Collective effects between quantum emitters have been studied extensively theoretically starting from Dicke’s seminal paper. Experimentally however some of the theoretical predictions have never been verified since they require sophisticated experimental techniques. In this context, the novel platform of 3D circuit-QED offers unique opportunities in precisely controlling independent qubits. In particular, we can resolve both in time and in frequency the single photon emitted by a qubit, effectively experimentally measuring its full wave-packet distribution.

Furthermore, using transmon qubits, a large coupling strength between qubits can be obtained by placing them in a sub-wavelength volume. By carefully designing such near-field coupling, we have realized a 4 qubits system with equal all-to-all coupling. Such an artificial Dicke molecule allows us to study Dicke physics in a regime that is otherwise challenging to achieve with more conventional experiments based on atoms. This novel platform opens numerous opportunities, such as the study of the effect of disorder and dephasing on collective effects. In addition, the appearance of spectrally distinguishable bright and dark states allows for the precise control of the Dicke molecule states and in particular the possibility to use the dark subspace for quantum computation.

Dr Mathieu L. Juan

Title: Quantum Magneto-mechanics

The possibility to operate mechanical systems close to the quantum regime has become central in both fundamental and applied science. Fundamentally, it offers a route to control the quantum state of macroscopic objects paving the way to experiments that may lead to a more profound understanding of quantum mechanics; and from the point of view of applications, quantum opto(magneto)mechanical techniques provide motion and force detection near the fundamental quantum limits. One of the key parameter is the coupling between the mechanical and photonic degrees of freedom which controls how strongly the photons will impact the mechanics (or vice versa). In this context, working in the microwave regime offers the opportunity to achieve very large coupling towards a regime where it becomes the dominant time scale in the system.

In this seminar I will present the recent experimental efforts taking place at IQOQI Innsbruck in the field of magneto-mechanics. The approach is closely related to the typical optomechanical system where a mechanical resonator (membrane, movable mirror) is coupled to an optical cavity. Here, the optical cavity is replaced by a microwave resonator that is inductively coupled to the mechanical resonator. For the mechanical resonator, a simple atomic force microscope cantilever is modified with a hard magnet. Using this system, we demonstrated single photon strong cooperativity, allowing for cavity cooling with less than a photon. Improvement of the current design should enable reaching the single photon strong coupling regime, where complex quantum states of the mechanics could be prepared.

Dr Koji Maruyama

Title: Hilbert space structure induced by quantum probes

Abstract: Considering artificial control operations for many-body quantum system necessarily induce noise to the system, the fewer probes the better. When our direct access is limited to a small subsystem, what is the extent to which the whole system can be controlled? This is a natural question since it has been known that the set of realisable operations on the system depends on the type of the system Hamiltonian and our feasible controls in a complex manner. We have revealed the characteristic structures of the Hilbert space that is induced by limited access. The structure tells us that there is a clear distinction between the cases of d_S=2 and d_S>= 3, where d_S is the dimension of the probe. It also clarifies what draws the line between controllable and uncontrollable subspaces, which leads to the equivalence class of indistinguishable states as well.

Mr Daniel Cohen

Title: Utilising NV based quantum sensing for velocimetry at the nanoscale

Prof Jonathan Dowling

Title: Practical figures of merit and thresholds for entanglement distribution in large-scale quantum repeater networks

Sumeet Khatri, Corey T. Matyas, Aliza U. Siddiqui, Jonathan P. Dowling

(Submitted on 16 May 2019)

Before global-scale quantum networks become operational, it is important to consider how to evaluate their performance so that they can be suitably built to achieve the desired performance. In this work, we consider three figures of merit for the performance of a quantum network: the average global connection time, the average point-to-point connection time, and the average largest entanglement cluster size. These three quantities are based on the generation of elementary links in a quantum network, which is a crucial initial requirement that must be met before any long-range entanglement distribution can be achieved. We evaluate these figures of merit for a particular class of quantum repeater protocols consisting of repeat-until-success elementary link generation along with entanglement swapping at intermediate nodes in order to achieve long-range entanglement. We obtain lower and upper bounds on these three quantities, which lead to requirements on quantum memory coherence times and other aspects of quantum network implementations. Our bounds are based solely on the inherently probabilistic nature of elementary link generation in quantum networks, and they apply to networks with arbitrary topology.

Dr Pedro Contino Da Silva Costa

Title: Quantum Cellular Automata and Quantum Walks to Classical and Quantum Simulation and Computation

Abstract: In this talk, we will briefly show some of our previous works and ongoing projects about quantum cellular automata-(QCA) the quantum counterpart of cellular automata-(CA) and quantum walks-(QWs), the quantum counterpart of random walks.

In one of them [1] we proposed a new type of QCA that we called partitioned unitary quantum cellular automata-(PUQCA) and we showed how this new definition can be used to move from different flavors of QWs to PUQCA. Since this model of computation does not share the same research activity than the QW, we expect that this result increases the use of this model, once there is a straightforward way to describe QCA in terms of qubits, which are the base of most quantum computers' architectures.

In another one, we show how the PUQCA and its classical counterpart can be useful to explore emerging dynamics [2] and to study quantum-classical transitions (ongoing project) via coarse-graining technics.

We will also give some few comments about our ongoing project where the PUQCA is being applied for the density classification problem. It is a well-known problem in CA, where the solution with deterministic CA does not exist [3].  Finally, we present a new type of interaction for quantum walks [4] inspired on a classical model for gas collision.

[1] https://doi.org/10.1007/s11128-018-1983-x

[2] arXiv:1905.10391

[3] https://doi.org/10.1103/PhysRevLett.74.5148

[4] arXiv:1906.07293

Dr Benjamin Brown

Title: A fault-tolerant non-Clifford gate for the surface code in two dimensions

Abstract: Performing non-Clifford gates with magic state distillation will consume an overwhelming majority of the resources of a two-dimensional fault-tolerant quantum computing architecture. Here we show how to perform a fault-tolerant non-Clifford gate with the surface code; a quantum error-correcting code now under intensive experimental development. This alleviates the need for distillation or higher-dimensional components to complete a universal set of quantum logical gates. The operation uses local transversal gates and code deformations on a two-dimensional architecture over a time that scales with the size of the qubit array. An important component of the gate is a just-in-time decoder. Such decoding algorithms allow us to draw upon the advantages of a three-dimensional model using only a two-dimensional array of live qubits. Remarkably, our gate is completed using parity checks of weight no greater than four. As such, we expect it to be experimentally amenable with technology that is now under development. As this gate circumvents the need for magic-state distillation, it may reduce the resource overhead of surface-code quantum computation dramatically.

Mr Nathan McMahon

Title: Cluster States for Holographic Quantum Codes

Abstract: Holographic quantum codes are a class of codes first proposed by Pastawski et al, and motivated by ideas from the holographic principle. The construction of these codes betrays this origin since each code arises from concatenating a sequence of ‘seed codes’ from the centre of a 2D hyperbolic disc to some boundary. This construction results in a visualisation of these codes as a mapping of logical qubits embedded in a hyperbolic 2D disc onto physical qubits located along the boundary of the disc.

These codes have recently drawn interest due to the fact that they have finite encoding rates and a high resistance to loss errors (particularly the logical qubits located near the centre of the disc). However, these codes have practical issues since the construction inadvertently leads to the presence of stabilisers with support of order of the number of physical qubits of the code. One way we may get around this is to use cluster states, an architecture independent method of constructing quantum states using only controlled phase gates.

In this talk, I will introduce holographic quantum codes and how they arise from concatenating seed codes. I will also discuss how to design finite weight cluster states for arbitrary sized holographic codes, and given time will discuss some aspects of the fault tolerance of such codes.

Dr Wei Wei Zhang

Title: Realizing quantum exotic properties with dynamics

Abstract: Due to the rich controllability in both theoretical and experimental study of quantum walks (QW), researchers find it is a fertile platform for the analysis, simulation and engineering of exotic quantum properties. In this talk, I will give an introduction to the recent QW based schemes for generating cat states, characterising topological materials, generating topological biphoton states and machine-learned properties, and the corresponding experimental implementations in linear optical, photonic chips, silicon waveguide, cold atoms and ion traps.

Ms Qi Yu

Title: On Hybrid Quantum Filtering and Capability of Quantum Sensors

Abstract: This talk spans two problems: quantum hybrid filtering for systems subject to classical disturbances; the determination of the capability of a class of quantum sensors.

The first topic is the hybrid filtering problem where the goal is to derive a filter for a class of quantum systems which are subject to classical disturbances. The task is to obtain a filter to estimate both the quantum system state and the classical signal simultaneously. A composite quantum analog system is employed to represent the hybrid quantum-classical model.

Another topic is to analyse the information extraction capability of a specially designed qubit sensor for a spin chain system. The object system is a qubit detector which is employed to extract information of a spin chain system with unknown structure parameters. The capability of a sensor is defined as the ability of estimating all the unknown parameters.

Mr Yuan Su

Title: Toward the first quantum simulation with quantum speedup

Abstract: Simulating the Hamiltonian dynamics of a quantum system is one  of the most natural applications of a quantum computer. Efficient quantum algorithms for quantum simulation have been known for over two decades. We investigate the possibility of using these algorithms for practical simulation that is beyond the reach of current classical computers. To this end, we consider quantum simulation of spin systems, which could be applied to understand condensed matter phenomena. We synthesize explicit circuits for three leading simulation algorithms, employing diverse techniques to tighten error bounds and optimize circuit implementations. We discuss further improvements in quantum simulation that lead to algorithms with nearly optimal dependence on the system size.

Dr Chris Jackson

Title: How to implement a generalized-coherent-state POVM

Abstract: In quantum physics, observables are quantized by Hilbert spaces which can carry a unitary representation of their group of transformations.  The most common examples are the quantization of a particle, a field amplitude, or a spin.  If the group is continuous and the representation is irreducible, then every Hamiltonian is a polynomial in the infinitesimal generators representing the group’s Lie algebra.  Generalized-coherent states are the non degenerate ground states of linear Hamiltonians.  Equivalently, they are states in the group orbit of a lowest weight state.  These states define an over-complete basis as well as a tomographically complete POVM.  For a bosonic field amplitude, this POVM is implemented through heterodyne detection.  However, a practical implementation of the generalized-coherent-state POVM has in general been unknown, including the spin-coherent-state POVM.  In this talk, I will explain that the spin-coherent-state measurement can be implemented by measuring spin-components isotropically, continuously, and non adaptively.