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Dodd-Walls Centre member information

Seminar series

The Dodd-Walls Centre is proud to present a series of seminars hosted by our themes on different topics and everyone is welcome to attend.  Anyone can join from anywhere using a laptop, computer, mobile phone or other Zoom capable device.  Look for connection details within each Seminar notice.   

Upcoming Seminars

The Quantum Fluids and Gases (QFG) seminar to be held on Friday 17 August will be presented by Péter Jeszenszki, PhD candidate, Massey University.

Title:     Accelerating the convergence of exact diagonalization with the transcorrelated method: Application to the one-dimensional Fermi gas with contact interactions

When:   Friday 17 August, 12 noon (sharp)

Venues: 320e, Dodd-Walls Centre, Otago, Room 303-610, Auckland and Library 1.22A (Research Collaboration Room) Massey

You can join from anywhere with a laptop, computer or other device using Zoom ID #325 816 342

Abstract:  
Exact diagonalization expansions of quantum gases with contact interactions converge very slowly due to the non-analytic nature of the wave function cusp at particle coalescence. Here we develop a transcorrelated approach where the wave-function cusp is treated exactly with a Jastrow-factor and folded into the many-body Hamiltonian with a similarity transformation, which removes the leading order singularity. The resulting non-hermitian transcorrelated Hamiltonian can be treated numerically with a standard projection approach. We find that the convergence rate for the ground state energy for a one-dimensional Fermi gas is improved dramatically. The energy error scales as 1/M^3 with a single-particle basis set of M plane waves compared to 1/M for the expansion of the original Hamiltonian and 1/M^2 using conventional lattice renormalization.

Past Seminars Held

10 August 2018

PSI Theme:  Ruth Eloisa Sales, PhD Candidate from the Department of Chemistry, University of Otago

Abstract:
The red meat industry has been a principal driver of New Zealand’s economy and identity. To stand ahead of its competitors, it is driven to consistently improve its reputation for quality and innovation in the global market. The continuous development of spectroscopic methods allows the testing of meat quality to move from laboratories to the production line. Spectroscopy is very suitable for the on-line measurement of meat quality parameters as it is fast and non-destructive technique with little or no pre-treatment of sample, and has multivariate output. This study aims to develop non-invasive rapid quantification methods for industry-specified parameters in red meat by selecting the optimal combination of spectroscopic techniques and determining in which is best for which task. This should then guide future development of sensors for in-line measurements of NZ meat. Tests of New Zealand meat samples were done using different techniques: Fourier transform infrared (FT-IR), FT-Raman, 1064 nm Raman via fibre optics, low-frequency Raman (830 nm), low-frequency Raman (785 nm), Raman microscopy and near infrared (NIR). Peaks in the spectra were identified which correspond to specific components of the meat. Multivariate analysis showed that spectral information is correlated to different physio-chemical parameters of the meat.

Lake snow has increasingly become a problem in lakes within the South Island of New Zealand. The increased volume of this biomaterial from blooms result in blocked turbines and can interfere with tourism activities such as water sports and fishing. The same vibrational spectroscopic techniques were also applied to lake snow to investigate the natural variance between and within lakes, and over different depths of the lake. The resulting signatures can help identify the chemical variances expressed in these diatoms between the lakes.

20 July 2018

QFG Theme:  Dr Sophie Shamailov, The University of Auckland

Abstract:

We report on recent progress towards observing Anderson localization in two dimensions (2D). The experimental set-up allows one to load a Bose-Einstein condensate into a large, uniform 2D trap, project an arbitrary repulsive potential around the atoms and image the system after expansion.

Transmission experiments where the atoms pass from one reservoir to another through a customizable channel can be used to demonstrate disorder-induced quantum interference effects, whereby atom flow is significantly suppressed by the presence of disorder.

We measure the atom current flowing into the drain reservoir of the dumbbell, the channel of which contains point-like disordered scatterers. The drain reservoir filling rate is measured for various scatterer fill factors, channel lengths and widths. We find an exponential decrease of this rate with channel length and fill factor, as predicted by Anderson localization.

The localization length is extracted, and is found to be largely independent of fill factor, taking on a value of about 50 μm. A dimensionless conductivity is similarly extracted from the dependence of the drain reservoir filling rate on fill factor. Both the localization length and the dimensionless conductivity are shown to be independent of the channel width.

Finally, we are able to rule out the possibility of classical trapping as the underlying cause of transport suppression. Thus, evidence suggests that the reduced conductivity in the presence of disorder is attributed to quantum interference effects.

Authors: T. A. Haase, D. H. White , D. J. Brown, S. S. Shamailov and M. D. Hoogerland

15 June 2018

QFG Theme:  Dr Russell N Bisset, University of Trento

Abstract: 

Binary, or two-component, condensates can support a second-order phase transition between miscible (co-spatial) and immiscible states that is driven by a diverging susceptibility to spin fluctuations. We demonstrate, for realistic trapped systems, how measurements of number fluctuations within sample cells provide a direct means to study the spin susceptibility [Bisset et al., PRA 91, 053613 (2015)].

While the spin susceptibility and the associated thermal spin fluctuations diverge as the transition is approached, quantum spin fluctuations, on the other hand, tend to zero. With this in mind, we propose a scheme for small binary condensates, which displaces the transition point from the thermodynamic-limit prediction. In such systems, we show that quantum spin fluctuations now diverge on approach to the transition, and we demonstrate how these can be experimentally distilled from the noisy thermal fluctuations that remain significant well below the condensation temperature [Bisset et al., PRA 97, 023602 (2018)].

08 June 2018

PSI Theme:  Dr Frederique Vanholsbeeck, The University of Auckland

Abstract:  Biophotonics at The University of Auckland

Our research projects focus on different imaging modalities such as fluorescence quantification using the optrode, an all-fibre real time spectroscopic optical probe and optical coherence tomography (OCT) imaging.

OCT is a non-invasive imaging technique based on low coherence interferometry that provides high resolution 3d images of samples with a depth range of a few mm. The depth resolution is inversely
proportional to the light source bandwidth and can be as good as 1 μm in some recent experiments. Recently, we have been working on new swept sources to get a better imaging depth. We also use these sources
to map chromatic dispersion in tissue in order to differentiate tissue type. Lately, we have been working on polarisation sensitive OCT to detect early signs of osteoarthritis.

We are using spectroscopic fluorescence to detect and classify bacteria or monitor their activity. This work has a particular focus on food safety where bacterial enumeration is important. During the talk, I
will present our recent results on these topics obtained by the UoA Biophotonics group with a particular emphasis on our cross-disciplinary work.

18 May 2018

QFG Theme:  Craig Chisholm, MSc student, Department of Physics, University of Otago

Abstract:  A three-dimensional optical tweezer system for ultracold atoms

We present a three-dimensional steerable optical tweezer system based on two pairs of orthogonal acousto-optic deflectors. One pair of acousto-optic deflectors is used to displace horizontally propagating laser beams in a vertical plane while the other pair is used to displace vertically propagating laser beams in the horizontal plane. Radio frequencies used to steer the optical tweezers are generated by direct digital synthesis and multiple cross beam dipole traps can be produced through rapid frequency toggling and time averaging. We demonstrate production of arrays of ultracold atomic clouds in both the horizontal and one vertical plane and use this as an indicator for the three-dimensional nature of this optical tweezer system.

11 May 2018

PSI Theme:  Dr Ben Mallett, Research Fellow, The University of Auckland

Abstract: Spectroscopy on metal-oxide thin films: Ellipsometry, X-ray and Raman studies of superconductor sandwiches
Remarkable, and potentially useful, new behaviour can be realised at the interfaces of metal-oxide thin films. These range from induced magnetism and ferro-electricity, through to a two-dimensional electron gas. Recently we found a particularly interesting such example whereby a superconductor sandwiched between two thin-film layers of specific Manganite, adopts a remarkable new ground state. This `superconductor sandwich' hosts an exotic granular-superconducting state in zero-field. Furthermore, whilst a magnetic field is near universally detrimental to the superconducting state, in our system it rather restores superconducting coherence.

In this talk, I highlight how we are using spectroscopic measurements to reveal the novel physics in these nm thin films. Spectroscopic ellipsometry, Raman and X-ray scattering all show that charge-ordering plays an important role in causing the exotic granular-superconducting state. The question now is, what exactly is the coupling between the materials in our thin-films couple together that leads to their highly unusual properties, and is there a way to engineer that coupling?

27 April 2018

QMI Theme: Ricardo Gutiérrez Jáuregui, PhD candidate, University of Auckland.

Abstract: Seeing light in a different way: correspondence between the driven Jaynes-Cummings model and a charged Dirac particle 

The interaction between a two-level system and one mode of the radiation field has become a cornerstone of quantum optical models. A prominent example is the experimental evidence of the quantum nature of the electromagnetic field achieved by measuring the nonlinear energy spectra predicted by the Jaynes Cummings model. While we have built our intuition of this fundamental interaction around quantum optical systems, the same set of equations can be used to describe relativistic particles in particular configurations. In this case, instead of atoms emitting and absorbing light, we encounter oscillating electrons whose trajectories are influenced by their spin. The methods used to solve each system must be fundamentally related, yet they would appear more natural in one context over the other. In this talk I will revisit the driven Jaynes Cummings model, where the nonlinear spectra mentioned above is seen to collapse at a critical driving field amplitude giving way to a phase transition of light. I will first solve this system using a quantum optical toolbox by following the approach given by Alsing, Guo, and Carmichael. Afterwards, I will construct a relativistic correspondence and show how the manipulations required to obtain these solutions appear naturally as Lorentz transformations. Through this analogy we can make connections between optical and condensed matter systems, thus allowing us to see the phase transition of light in a new way.

09 April 2018

PSI Theme: Prof Neil Broderick, Physics Department, University of Auckland.

Abstract: Why not use NMR?
In this talk I will present a brief summary of the sensing projects I am currently involved, ranging from temperature and strain sensing for geophysics to new sensors for key-hole surgery. I will try and explain my role and also what benefits optics brings in each case compared to other sensors.

13 March 2018

QFG Theme: Dr Maarten Hoogerland, Physics Department, University of Auckland.

Abstract:  A new measurement of the doubly forbidden transition between the two metastable states of helium

We report on new spectroscopy results on the doubly forbidden triplet to singlet transition frequency in metastable helium, which we find to be 192,510,702,148.72 (0.20) kHz. The relative accuracy of 1 x 10-12 of our results represent a factor of 10 improvement to previous work, and is a sensitive test for state-of-the-art atomic structure models, and provides insight in extensions to the Standard Model of Particle Physics. To achieve this improved accuracy, we use a new spectroscopy laser system, and a ``magic wavelength'' optical trap to reduce Stark shifts. The transition frequency is sensitive to the charge radius of the nucleus of the helium atom, and we re-determine the difference in radius of the 4He and 3He nucleus from our results. The wavelength dependence of the AC Stark shift has also allowed us to sensitively determine the magic wavelength. Moreover, we use the density dependence of the transition frequency to accurately determine the singlet-triplet scattering length for the first time.

Friday 9 March

PSI Theme: Dr Sylwia Kolenderska, Research Fellow, University of Auckland

Abstract: Dispersion mapping as a simple postprocessing step for standard Fourier domain Optical Coherence Tomography (OCT) data
Dispersion, although often considered as detrimental to the performance of an optical system, can be used to identify materials. Recently, optical coherence tomography (OCT) has proved to be a modality enabling characterization of ocular media based on medium’s Group Velocity Dispersion (GVD) value. In the method developed by our group, two broadband sources at different wavelengths are used to produce two separate images. Because of the dispersive character of the imaged material, the thicknesses measured in each image differ. Determination of the thickness difference makes it possible to calculate the average GVD value for the imaged material by means of a simple formula assuming the knowledge of the geometrical thickness of the material and spectral separation of two sources.

The prerequisite is to have two OCT images at different wavelength range, which doesn’t necessarily mean using two light sources. Two OCT images can be numerically synthetized from data acquired from any Fourier domain system employing a light source with a sufficiently broad spectral bandwidth. In our new method, every spectrum from the acquired dataset is multiplied by two Gaussian filters which extract two different spectral ranges from the signal and thus create two sub-datasets. The two resulting sub-datasets are processed in a standard way to produce two images which are then used for determination of GVD values. This method has the added advantage to ensure that images are taken at identical position and avoid most artefacts present when using two light sources. We have devised a few simple criteria to optimise the method given the sample thickness and dispersion.

We have successfully trialled the method on several glass objects and on biological media (rat eyes).

Friday 24 November 2017

QMI Theme: Gabriel A. Santamaria-Botello, Universidad Carlos III de Madrid, Leganés, Spain

Abstract: Cosmic microwave background and microwave up-conversion: Limitations and quantum noise
A radiometer concept based on the photonic up-conversion of microwave radiation into the optical domain within nonlinear whispering-gallery mode (WGM) resonators is proposed. Since the parametric up-conversion is intrinsically noiseless for the anti-Stokes generated sideband, this detection technique can potentially achieve a high sensitivity even at room temperature. This technique finds applications in radio astronomy such as the observation of the temperature and spectral characteristics of the cosmic microwave background (CMB) upon which many cosmological predictions rely. The sensitivity of the radiometer is limited by thermal noise coupled to the WGM resonator, and by quantum noise originated from low photon conversion efficiencies. Thermal noise can be reduced by radiatively cooling the microwave mode, whereas high efficiencies can be achieved by engineering the cavities such that the modal overlap is maximized. Other challenges such as the realization broadband up-conversion have to be overcome in order to observe the CMB with high temperature sensitivity. Thus, on the basis of a thermal and quantum noise study, the sensitivity limitations of the radiometer are discussed, while some approaches currently under research to achieve efficient and broadband up-conversion are presented.

Friday 10 November 2017

PSI Theme: Dr Baptiste Auguie, School of Chemical & Physical Sciences, Victoria University of Wellington

Abstract: Optics of interacting nanoparticles and molecules in the coupled-dipole approximation
Recent experiments have demonstrated how optical absorption by dye molecules can be substantially modified upon adsorption on metal colloids, with some molecules experiencing enhanced, lowered, or spectrally-modified absorption. These spectral changes likely originate in multiple physico-chemical effects, some of which could be attributed to electromagnetic interactions. I will present an original electromagnetic model combining a coupled-dipole approximation for dye-dye interactions, together with Mie theory to account for interactions at all multipolar orders with the metal core. This model allows a comprehensive exploration of different spherical core-shell geometries, varying the dye concentration, orientation and uniformity in coverage. The results exhibit a rich variety of spectral modifications that cannot be captured by a simple effective-medium approximation for the shell of dyes.


Friday 26 October 2017

QMI Theme: Professor Michael F Reid, School of Physical and Chemical Sciences, University of Canterbury

Abstract: Smarter Modelling of Lanthanide Electronic Structure. Optical, magnetic, and hyperfine data; Ab-initio calculations
Bulk and nano-scale materials doped with lanthanide (rare-earth) ions are used in a wide variety of applications. Calculations of electronic structure, transition probabilities, and dynamics, are crucial to many of these applications [1].

Single crystals doped with lanthanide ions are candidates for a number of quantum-information applications. Recent work has shown that coherence can be stored for up to six hours [2]. However, the crystals used in that work have very low symmetry, where conventional crystal-field modelling is impossible. However, this difficulty is being overcome by making use of a combination of optical, magnetic, and hyperfine energy-level data [3]. Furthermore, ab-initio calculations are now capable of accurate calculations for lanthanide-doped materials [4].

This seminar will give a general introduction to modelling of lanthanide electronic structure, the determination of crystal-field parameters from ab-initio calculations, our recent work on low-symmetry materials, and plans that Jon Wells and I have for this work over the next three years.

[1] M.F. Reid. Theory of Rare-Earth Electronic Structure and Spectroscopy. In Bunzli JCG; Pecharsky VK (Ed.), Handbook on the Physics and Chemistry of Rare Earths: 47. Amsterdam: North Holland (2016).
[2] M. Zhong, M. P. Hedges, R. L. Ahlefeldt, J. G. Bartholomew, S. E. Beavan, S. M. Wittig, J. J. Longdell, M. J. Sellars. Optically addressable nuclear spins in a solid with a six-hour coherence time. Nature 517, 177 (2015).
[3] S. P. Horvath, M. F. Reid, and J.-P. R. Wells, M. Yamaga. High precision wavefunctions for hyperfine states of low symmetry materials suitable for quantum information processing. J. Lumin. 169, 773 (2016).
[4] J. Wen, C.-K. Duan, L. Ning, Y. Huang, S. Zhan, J. Zhang, M. Yin. Spectroscopic distinctions
between two types of Ce3+ ions in X2-Y2SiO5: a theoretical investigation. J. Phys.
Chem. A 118, 4988 (2014).


Friday 13 October 2017

PSI Theme: Caroline Anyi, PhD student, School of Physical and Chemical Sciences, University of Canterbury

Abstract: Experiments with Ring Laser Gyroscope
Large ring laser gyroscopes are highly sensitive rotation sensors whose operating principle is based upon the Sagnac effect. They offer applications in geodesy, geophysics, seismology and fundamental physics. So far, Canterbury’s ring lasers run solely on the 632.8 nm neon transition. We propose to operate large ring lasers on different laser wavelengths because the scale factor of a ring laser depends on the operating wavelength in addition to geometrical factors. In this talk, I will discuss the technical details of a 1.6 m² ring laser gyroscope and overview of various experimental work undertaken to operate this device at shorter lasing wavelengths.


Friday 22 September 2017

QMI Theme: Professor Howard Carmichael, Department of Physics, University of Auckland

Abstract: An Open Systems Framework to Link Optical Resonators and Superconducting Circuits
Although the physics of electromagnetic radiation is explored in the optical and microwave domains using entirely different experimental tools, a surprising commonality has emerged at the theoretical and conceptual level, through recent experiments with superconducting circuits and their links to quantum optics [1-6]. In one sense, the development can be seen as a return to beginnings considering that the optical intensity interferometer from the 1950s (Hanbury Brown and Twiss effect) was a carryover from Robert Hanbury Brown’s involvement with radar and radio astronomy. In this regard, it is interesting to recall the reaction to his proposal with Richard Twiss [7]: “Our work really put the cat amongst the pigeons. The basic problem was that you can think about light in two different ways, as a wave and as particles … to a surprising number of people the idea that the arrival of photons at two different detectors can ever be correlated was not only heretical it was patently absurd, and they told us so in no uncertain terms … If science had a Pope we would have been excommunicated.”. In this tutorial I will review the modern ground that puts any perceived dichotomy between microWAVES and optical PHOTONS (particles) finally in its place. Recent resonator experiments “see” the particles—even the tiny microwave ones—while the resonators themselves are obviously engineered around waves. I will visit experiments from both sides of the border (optical and microwave), building from an introduction that sets out the conceptual links provided by the theory of Markov open quantum systems.


Friday 25 August 2017

QMI Theme: Samuel Ruddell, Doctoral Candidate, Department of Physics, University of Auckland

Abstract: Collective strong coupling of cold atoms to an all-fibre ring resonator
A sub-wavelength diameter optical nanofibre strongly confines light in the transverse direction, with a significant portion of the guided light propagating as an evanescent field extending from the surface of the nanofibre. This allows for efficient coupling of particles near the surface of the nanofibre to the guided modes of the fibre, and is an ideal platform for studying interactions between single atoms and single photons.

We experimentally demonstrate the use of a fibre ring resonator containing a nanofibre section for the enhancement of photon interactions with cold caesium atoms. We cool and trap caesium atoms
near the nanofibre using a magneto-optical trap. By taking full advantage of the thermal properties of the nanofibre in vacuum, we are able to lock the cavity resonance to the atomic transition. We probe the atoms using near resonant light with a variable power and detuning, and observe a non-linear response at the few photon level (~13 intra-cavity photons). Our system displays a collectively
enchanced atom-cavity cooperativity, and we observe splitting of the cavity resonance at low input powers.

Friday 18 August 2017

QFG Theme: Lewis Williamson, PhD student, Department of Physics, University of Otago

Abstract: Properties and dynamics of polar-core spin vortices in a ferromagnetic spin-1 condensate
Spinor Bose-Einstein condensates can exist in various ferromagnetic or antiferromagnetic phases depending on the nature of the spin dependent interactions and Zeeman shifts of the spin levels. Associated with this range of symmetry breaking phases is a rich array of topological defects. Topological defects are known to play an essential role in various areas of physics, such as phase ordering dynamics, topological phase transitions and turbulence. In this work we discuss the properties and dynamics of a type of vortex known as a polar-core spin vortex (PCV), which arises in the easy-plane ferromagnetic phase in a spin-1 condensate. Using variational Lagrangian methods, we derive an equation of motion for the point vortex dynamics of PCVs. We find that the PCVs behave as massive charged particles interacting under the two-dimensional Coulomb interaction, with the mass arising from interaction effects within the vortex core. The model provides insights into phase ordering dynamics, as well as possible extensions related to confinement.

Friday 11 August 2017

PSI Theme: Fang Ou (Rachel), PhD candidate, University of Auckland

Abstract: Quantitative fluorescence study of microbiological systems
Determining the ratio of live to dead bacteria in a sample is important for the assessment of antibiotic efficacy, and cleaning procedures in both medical and food processing environments. We are developing an all fibre based fluorimeter, the optrode, for the measurement of fluorescence intensity of each SYTO 9 and propidium iodide; which bind to live and dead cells respectively when added to a bacterial solution together. In order to develop the optrode, the measured fluorescence signals will need to be calibrated to the bacterial concentration measured by a reference method. In this talk, I will present the reference flow cytometry method for bacterial counting and its correlation to the optrode.

Friday 28 July 2017

QMI Theme: Dr Pimonpan (Mim) Sompet, Postdoctoral Fellow, University of Otago

Abstract: Dynamics of Two Atoms Tightly Confined in Optical Tweezers
We experimentally study individual 85Rb-atom pairs and observe their evolution when exposed to external fields. Due to the simplicity of the system (just two atoms in the trap), it can often be simulated directly, thereby revealing detailed insight into it. We find that we can control the energy released from the atomic collisions by tuning the light field parameters. We further apply light-assisted collisions to deterministically prepare individual atoms as a first step towards preparing pure quantum states of individual atoms. As the next step, we cool them down to the vibrational ground state of the optical tweezers using magnetically-insensitive Raman sideband cooling. We achieve 2D cooling in the radial plane with a ground state population of 0.88, which provides a fidelity of 0.73 for the entire procedure. Additionally, we present experimental observations of a strong correlation between the collisionally driven spindynamics of individual atomic pairs.


Friday 21 July 2017

QFG Theme: Ulrich Ebling, Postdoctoral Fellow, Massey University

Abstract: Dynamics of multi-component Fermi gases
I will talk about different types of spinor dynamics in ultracold multi-component trapped Fermi gases. For atoms with total spin larger than 1/2, s-wave collisions can change the spin configuration of two atoms, conserving only the total spin. I will discuss how these scattering processes lead to interesting dynamics like long-lived coherent spin oscillations, as well as the long-time relaxation dynamics of multi-component systems. The second part of my talk is about dipolar Fermi gases. Dipole-dipole interactions violate total spin conservation and therefore lead to additional dynamical processes, such as demagnetization of an initially fully polarized gas. Since total angular momentum must still be conserved, a decrease in magnetization must be lead to an increase in orbital angular momentum, analogous to the Einstein-de Haas (EdH) effect. I show recent results that predict the existence of the EdH effect in dipolar Fermi gases, as well that it is accompanied by an additional twisting motion which is absent in the case of a dipolar BEC, where different spin components rotate in opposite directions.

Friday 23 June 2017

QMI Theme: Nikolett Nemet, PhD student, University of Auckland

Abstract: Enhanced EPR-type entanglement with coherent time-delayed feedback
Coherent time-delayed feedback has a potential to control the squeezing properties of a degenerate parametric amplifier (DPA). However, parametric down-conversion of the pump photons can be considered into two non-degenerate modes as well. The non-degenerate parametric amplifier (NDPA) is a good testbed for observing continuous variable, Einstein-Podolsky-Rosen-type entanglement, which can serve as the basis of continuous variable quantum computation. In this talk we show that similarly to the DPA case, the generalized two-mode squeezing of NDPA can also be controlled with time-delayed feedback. Moreover, the considered setup shows some potential for quantum-enhanced non-linear interferometry as well.


Friday 16 June 2017

QFG Theme: Dr Waltraut Wustmann, Laboratory for Physical Sciences, College Park, USA

Abstract: Reversible logic gates based on fluxon dynamics in Long Josephson Junctions
The exponential performance scaling of digital technology known as Moore's law is limited by the heat generation in logic gate operations. Energy-efficient digital gates are required for further improving performance, and also to tackle the steeply increasing power consumption in high-performance computing. Reversible digital logic, which was developed as a theoretical possibility decades ago, is expected to enable low-power digital computing. Experimental implementations are so far limited to adiabatic reversible logic, where energy efficiency is achieved through a compromise with gate speed.   Here we follow a conceptually different approach which is based on the autonomous dynamics and interactions of flux-solitons (fluxons) in Long Josephson Junctions (LJJ). Using the fluxon polarity to encode bit information enables high bit stability and low-loss information transfer. A challenge is the design of logic gates, where LJJs need to be suitably coupled such that fluxons are not destroyed, captured, or backscattered. We study the scattering of fluxons at special LJJ-interfaces which enable a surprising process of elastic forward-scattering from one LJJ to the other. The fluxon polarity is deterministically either preserved or inverted, and therefore such processes map to the elementary 1-bit gates Identity and NOT. These processes are near-reversible, with an energy loss of only a few percent of the fluxon energy, and robust. We then demonstrate a 2-bit NSWAP gate in a circuit which is related by symmetry to both 1-bit gates.


Thursday 01 June 2017

QMI Theme: Professor Murray Holland, from JILA, University of Colorado, Boulder, USA

Abstract: Supercooling of atoms based on steady-state superradiance
The development of laser cooling methods in the 1980's revolutionized atomic physics and has been an enabling technology for many fields, including quantum gas physics, precision measurements and quantum information. There are various types of laser cooling based on schemes that typically involve the interaction of single atoms with multiple laser fields. In this talk, I will present a new theoretical proposal for "supercooling" of atoms in an optical cavity, which is a novel example of many-body or collective laser cooling. I will show that the motion of the atoms could be subject to a giant frictional force that derives from the growth of many-atom dipole correlations in steady-state superradiance. This effect leads to a final temperature that could be orders of magnitude lower than the conventional limit for laser cooling of atoms in an optical cavity. The superradiant effects lead to a rapid cooling rate that is greatly enhanced by collective emission into the cavity mode. If successfully observed in experiment, this prediction could have important application for the next generation of atomic clocks and in ultrastable lasers.

Friday 26 May 2017

QMI Theme: Jelena Rakonjac, MSc, University of Otago

Abstract: Measuring hyperfine coherence times of 167Er:YSO around zero magnetic field
Rare-earth ion-doped crystals have been shown to have long coherence times for nuclear spin states when using hyperfine transitions with zero first order Zeeman shift (ZEFOZ). This minimizes dephasing from nuclear spin flips. However, this technique has only been utilised with transition energies in an external magnetic field. One particular rare-earth crystal, erbium-167-doped yttrium orthosilicate (167Er:YSO), has a hyperfine structure that allows ZEFOZ transitions to occur even with no external magnetic field. Using a spin Hamiltonian to model the hyperfine transitions of the ground state of 167Er:YSO, we determined which transition would have the longest coherence time. We then investigated the hyperfine structure of 167Er:YSO using a tunable cavity and Raman heterodyne spectroscopy. Although we were unable to measure coherence times for any of the ZEFOZ transitions found, we have measured coherence times of 13 and 50 µs for two transitions with zero external magnetic field.

Friday 19 May 2017

QFG Theme: Stuart Masson, PhD student, University of Auckland

Abstract:  Engineering spinor dynamics via cavity-assisted Raman transitions
Spinor Bose-Einstein condensates (BECs) offer possibilities for the investigation of topics from magnetism and superfluidity to quantum entanglement and metrology. Here, we propose a method to emulate the collisional dynamics found in spinor BECs using schemes of cavity-assisted Raman transitions. We will show how we derive these effective models, and then discuss two possible applications of the method: the production of spin-nematic squeezing and the probabilistic preparation of a highly entangled ground state.


Friday 21 April 2017 

QFG Theme: Associate Professor Niels Kjærgaard, Department of Physics, University of Otago

Abstract:  Scattering experiments on atoms with a Feshbach resonance above threshold
Studies of magnetically tunable Feshbach resonances in ultracold atomic gases have predominantly been carried out in the zero collision-energy limit. In this brief talk I will describe our recent experiments on threshold collisions at well-defined energies in the vicinity of a narrow magnetic Feshbach resonance by means of a laser-based collider. Using our collider we can track the magnetic field shift in resonance position as the energy is tuned. For a narrow resonance this becomes a challenge due to inherent broadening mechanisms of the collider. We find, however, that the narrow Feshbach scattering feature becomes imprinted on the spatial distribution of atoms in a fashion that allows for an accurate determination of resonance position as a function of collision energy through a shift in center-of-mass position of the outgoing clouds.

Wednesday 12 April 2017

QFG Theme: Dr Amita Deb, Postdoctoral Fellow, Department of Physics, University of Otago

Abstract:  Electrometers based on Rydberg atoms
Rydberg atoms have electron(s) in highly excited states, and therefore are extremely responsive to electric fields. The field can be that of a neighbouring Rydberg atom or a deliberately applied external source. In this talk, I will present a proposal to implement a miniaturized electric field sensor based on thermal Rydberg atoms in a vapour cell. I will start by briefly outlining the basic physics of Rydberg atoms and how its extraordinarily high polarizability has proved to be a promising tool for quantum manipulations, such as single-photon phase modulations, quantum chemistry and precise electrometry. For electrometric purposes, Rydberg atoms have already proved their worth in RF and microwave domains, which I will describe. I will then discuss near-dc electrometry using Rydberg atoms and how we propose to implement it. It should be mentioned that the idea is expected to form the core of a protected intellectual right in future.

Friday 24 February 2017

Presenter:  Ricardo Gutierrez Jauregui, University of Auckland

Abstract: Dissipative Phases of Cavity-Mediated Photon Interactions
The exquisite control acquired over quantum s ystems in recent years has provided a playground for studies of transitions between different phases of light and matter[1;2]. The realization of the
Bose-Einstein condensate opened the door for quantum optics experiments using matter waves, while the advent of circuit quantum electrodynamics has allowed for strongly interacting systems to be simulated by light fields[3]. However, due to dissipation, the duality between light and matter systems is not complete. Dissipation affects both the evolution and the physical properties of a quantum system in a fundamental way.   In this seminar we address the question of how phase transitions in equilibrium relate to their driven dissipative analogues. This is done by contrasting the phases acquired by a quasi-conservative system, interacting BEC in an optical trap, with a driven-dissipative system, two driven cavities presenting a Kerr nonlinearity. We present the phases the system can acquire in both scenarios. First, for the single cavity limit where tunnelling is suppressed, then for the full Hamiltonian where competition of J and g leads to different phases of the system. The effect of quantum fluctuations on the phases of the system is highlighted.

[1] M. Greiner, O. Mandel, T. Esslinger, T. W. Hansch, I. Bloch, Nature 415, 39 (2002).
[2] A. D. Greentree, C. Tahan, J. H. Cole, and L. C. L. Hollenberg, Nature Physics 2, 856 (2006).
[3] G. Kirchmair, B. Vlastakis, Z. Leghtas, S. E. Nigg, H. Paik, E. Ginossar, M. Mirrahimi, Lu. Frunzio, S. M. Girvin
& R. J. Schoelkopf, Nature 495, 205209 (2013).