Nuclear spins for quantum contextuality tests and multiplexed quantum network protocols

Suzanne van Dam,  TU Delft

The nitrogen-vacancy (NV) centre in diamond is a promising node for a quantum network, as it combines a coherent spin state with an optical interface. Moreover, surrounding nuclear spins can be used in quantum computation protocols, for tests of quantum foundations, or as a quantum memory. In this seminar, we employ these spins in two ways. First, I describe the first implementation of repeated three-qubit parity measurements and how we use these to perform foundational quantum contextuality tests [1]. Second, I discuss how the rate of entangling protocols can benefit from multiplexing using nuclear spins in a multi-qubit quantum network node [2].

[1] S.B. van Dam, J. Cramer, T.H. Taminiau, and R. Hanson, arXiv:1902.08842

[2] S.B. van Dam, P.C. Humphreys, F. Rozpedek, S. Wehner, and R. Hanson, Quantum Science and Technology, 2, 034002 (2017)

Coffee/tea and bread rolls from 10:00

Vortex Momentum Distribution in Hydrogen

Guizhong Zhang, Tianjin University, China

This presentation will start with a brief introduction to Tianjin University of China. Following that, the talk will be concerned with attosceond and strong-field physics. As is well known for a few-cycle attosecond light pulse, the carrier envelope phase (CEP) has a direct and significant impact on its electric field and strong field processes induced by isolated attosecond pulse (IAP). By numerically investigating the angular photoelectron momentum spectra generated by overlapping IAP and infrared (IR) pulses, we show a scheme for retrieving the CEP value through the rotation angle of the momentum pattern. Another interesting observation for momentum distributions is their vortex shape created and experimentally corroborated by two time-delayed and oppositely circularly polarized attosecond pulses. We also report our numerical experiments on the formation of the vortex momentum distributions and their symmetry distortion when the dynamic Stark effect is taken into account. All the simulation has been performed by deploying the strong field approximation (SFA) theory and on hydrogen atom which has an analytical expression for the dipole transition matrix element.

Coffee/tea and cake from 15:00

Are there non-trivial quantum effects in Biology? A discussion on light harvesting processes

Susana F. Huelga, Uni-Ulm, Germany

Quantum biology is an emerging field of research that concerns itself with the experimental and theoretical exploration of non-trivial quantum phenomena in biological systems (See references below for recent reviews on the subject). We will present an overview aimed to bring out fundamental assumptions and questions in the field, using light harvesting as a prototypical biological process. We will identify basic design principles and develop a key underlying theme -- the dynamics of quantum dynamical networks in the presence of an environment and the fruitful interplay that the two may enter.

A fundamental element in the discussion is the formulation of a microscopic model able to explain the observed persisting oscillatory features in the spectral response of different pigment-protein complexes at ambient temperatures. Along delocalized electronic excitations, we argue that quantum coherent interactions with near-resonant vibrations are instrumental for explaining long lived coherence and may contribute to light-harvesting performance.

Experimental results on both natural and artificial systems will be shown to be in agreement with this vibronic model which therefore provides an archetypical framework for the field.

S. F. Huelga and M.B. Plenio, Contemp. Phys. 54, 181 (2013) G. D. Scholes et al, Nature  543, 647 (2017)

E. Romero,    V.~I. Novoderezhkin and R. van Grondelle,  Nature 543, 355 (2017)

Quantum sensing with diamond spins

Ulrik Lund Andersen, Department of Physics, Technical University of Denmark

Due to its outstanding electronic properties at room temperature, the Nitrogen-Vacancy (NV) center hosted by a diamond crystal has generated increasing interest in quantum information science and in quantum sensing. The NV center has been deployed in numerous quantum information protocols and in measuring extremely small magnetic fields from e.g. neurons and single protons – all performed at ambient temperature and pressure. In this talk we report on an increased sensing efficiency using optimized measurement protocols and improved designs, and I will outline a path towards measuring neurons from human brains.

Mean-field treatment of continuously probed and pumped atomic ensembles

Klemens Hammerer, University of Hannover, Germany

A number of experiments demonstrated the possibility of observing fascinating quantum effects with continuously probed and pumped atomic ensembles. These experiments deal with collective spin excitations on top of a spin polarized reference state serving as an effective ground state in the space of collective excitations. When collective excitations are generated or probed with CW light a certain degree of CW optical pumping has to be applied compensating probe induced depumping in order to maintain a mean spin polarization. The resulting interplay of continuous probe and pump gives rise to interesting collective dynamics of the atomic spin going well beyond single atom physics in optically dense atomic ensembles. Determining the spin polarized reference state in itself becomes a non-trivial problem of non-equilibrium open-system many-body quantum dynamics. Drawing an analogy to superradiant lasers I will introduce a systematic mean-field treatment taking into account any desired order of n-particle correlations for determining the atomic steady states as well as their two-time correlation functions. I will discuss results for ensembles of Caesium atoms.

Thermodynamics and Information at the nanoscale

Janet Anders, University of Exeter, UK  

Thermodynamic laws have been key for the design of useful everyday devices from car engines and fridges to power plants and solar cells. Technology’s continuing miniaturisation to the nanoscale is expected to soon enter regimes where standard thermodynamic laws do not apply. I will briefly introduce quantum thermodynamics, an emerging research field that aims to uncover the thermodynamic laws that govern small ensembles of systems that follow non-equilibrium dynamics and can host quantum properties [1]. I will then discuss a nanoscale thermodynamic experiment with heated optically trapped nanospheres in a dilute gas [2]. By developing a new theoretical model that captures the non-equilibrium situation of the particles, we were able to measure the surface temperature of the trapped spheres and observe temperature gradients on the nanoscale. In the second part of the talk I will discuss recent theoretical advances in defining thermodynamic work in the quantum regime. By introducing a process that removes quantum coherences we were able to show that work cannot only be extracted from classical non-equilibrium systems, additional work can be extracted from quantum coherences [3].

[1] Quantum thermodynamics, S. Vinjanampathy, J. Anders, Contemporary Physics 57, 545 (2016).
[2] Nanoscale temperature measurements using non-equilibrium Brownian dynamics of a levitated nanosphere, J. Millen, T. Deesuwan, P. Barker, J. Anders, Nature Nanotechnology 9, 425 (2014). 
[3] Coherence and measurement in quantum thermodynamics, P. Kammerlander, J. Anders, Scientific Reports 6, 22174 (2016).

‘New’ tools in open quantum systems: Collision Models

Steve Campbell, INFN Sezione di Milano & Università degli Studi di Milano

In this talk we will examine the details of collision models, which are versatile tools for modelling open quantum systems. First, we use them to identify the relevant system-environment correlations that lead to a non-Markovian evolution. Using this information equivalent expressions of the dynamics can be formulated with regards to these "relevant" correlations and we will introduce the notion of memory depth where these correlations are established between the system and a suitably sized memory rendering the overall system+memory evolution Markovian. Furthermore, we will extend the framework to assess the thermodynamic aspects of non-Markovian open quantum systems and show some other exciting applications of these tools.

Large deviations and non-equilibrium in open quantum systems

Juan Garrahan, University of Nottingham, UK

There will be coffee, tea and bread rolls from 10:00

Compressed quantum computation and multipartite entanglement

Barbara Kraus, University of Innsbruck, Austria

QUSCOPE Seminar

Programme

Coherent Population Oscillation-based Light Storage

Etienne Brion, Laboratoire Aimé Cotton, France

Mini-symposium with VKR Centre QMATH

See programme

Refocusing schemes of two qubit gates for trapped ions

Itsik Cohen,  Hebrew University of Jerusalem, Israel

Entanglement between two trapped ions is carried via a vibrational phonon mediator. This relatively slow, second order interaction is usually most fragile to noise effects, e.g., single body noise terms and two body noise terms. To compensate for single body noise effects during the two-qubit interaction, dynamical decoupling pulses are often applied at times when the qubits are disentangled from the phonon bus. Therefore, applying dynamical decoupling pulses with  Ω_r repetition rate, to refocus a lower frequency noise, would prolong the gate duration linearly with  Ω_r. In the first part of my talk I will show how to utilize dynamical decoupling pulses, such that the phonon-mediated entangling gate is performed in the strong coupling regime; namely, the gate duration is increased by a reduced factor of less than π/2.

On the other hand, compensating for the amplitude noise, which inflicts a random two qubit term, has proven more challenging. The main obstacle is that the two body noise time scale is shorter than the two qubit gate itself which prevents the use of standard refocusing methods. In the second part of my talk I will present two approaches to tackle this problem. The first one makes the use of composite pulses, applied as dynamical decoupling on the building blocks of ultrafast entangling gates; whereas the second approach uses a measurement and feedback based method to refocus two qubit gates.

Specialized iNANO Lecture: Intramolecular dynamics of single molecules in free diffusion

Hideo Mabuchi, Stanford University

Biomolecular systems such as multiprotein complexes or biopolymers can span several tens to several hundreds of nanometers, but the dynamics of such “mesocale” molecules remain challenging to probe. We have developed a single-molecule technique that uses Tracking Fluorescence Correlation Spectroscopy (tFCS) to measure the conformation and dynamics of molecular assemblies specifically at the mesoscale level (∼100-1000 nm). tFCS is non-perturbative, as molecules, which are tracked in real-time, are untethered and freely diffusing. To achieve sub-diffraction spatial resolution, we use a feedback scheme which allows us to maintain the molecule at an optimal position within the laser intensity gradient. We find that tFCS is sufficiently sensitive to measure the distance fluctuations between two sites within a DNA molecule separated by distances as short as 1000 bp. We demonstrate that tFCS detects changes in the compaction of reconstituted chromatin, and can assay transient protein mediated interactions between distant sites in an individual DNA molecule. Our measurements highlight the impact that tFCS can have in the study of a wide variety of biochemical processes involving mesoscale conformational dynamics.

See: http://biorxiv.org/content/early/2017/03/24/120311

Mabuchi Lab: https://minty2.stanford.edu/wp/

Ceramics

Hideo Mabuchi, Stanford

  Macro- and micro-crystalline glazes are well known in the ceramic arts.  We are much less aware of the key role that *nanoscale* crystals, which are too small to see individually with the unaided eye, play in the development of surface color in atmospheric firing – wood, salt and soda – of unglazed clay.  In this talk I will show and discuss microscope images that illustrate our contemporary understanding of “flashing” and related phenomena based on nano-crystal growth during kiln cooling.  The story will be told in simple non-technical terms, but at the end of the talk I will indicate some intriguing potential connections (via zeolites and nanoparticle chains) to high-tech materials that engineers are investigating for power storage and for removal of chemical contaminants from gas emissions and water.  Some loose analogies will likewise emerge between atmospheric firing and experimental petrology.

Strong coupling of an electron ensemble on the surface of liquid helium to a microwave cavity

Denis Konstaninov, Okinawa Institute of Science and Technology, Okinawa, Japan

Recently there has been a significant interest in the strong coupling of an ensemble of quantum particles to the electro-magnetic modes of a resonator. Besides traditional systems used in cavity QED experiments such as Rydberg atoms, strong coupling regime has been recently studied in various electron and nuclear spin ensembles, as well as two-dimensional electron systems (2DES) in semiconductors [1,2]. The hallmark of the strong coupling regime is the splitting in the resonator spectrum revealed in the signal reflected from or transmitted through the resonator. In case of a collection of N quantum particles this splitting scales as the square root of N. Besides general interest in the fundamental problem of light-matter interaction, the particular interest in the strong coupling regime comes from the quantum information processing as strong coupling to a high-Q resonator enables coherent information transfer between, for example, a qubit and quantum system excitations. Therefore, most of the recent observations of strong coupling have been interpreted as pure quantum phenomena. However, it is rarely mentioned that the strong coupling between large N-particle ensemble and the coherent state of electromagnetic mode in a resonator can be described completely classically in many cases. We present experimental observation of the strong coupling between cyclotron mode of 2DES on the surface of liquid helium and a microwave cavity resonator. The splitting in the eigenspectrum of coupled motion is observed in the cavity reflection signal, as well as in the ac current of electrons detected by measuring their bolometric photoresponse [3]. A simple model based on classical mechanics and electrodynamics accounts for all experimental features including the observed splitting. The square root scaling of the splitting follows naturally from our model.  Thus, our work reproduces all main features of the strong coupling regime for a large N-particle 2DES, including those reported in Refs. [1,2], but puts them on a completely classical ground.

[1] G. Scalari, Science 335, 1323 (2012).

[2] Q. Zhang et al., Nature Physics 12, 1005 (2016).

[3] L. V. Abdurakhimov et al., Phys. Rev. Lett. 117, 056803 (2016).

Optimization of real-world qubit measurements

Benjamin D'Anjou, Department of Physics, McGill University, Montreal QC, Canada

The last decade has witnessed the rapid development of a variety of quantum bit (qubit) implementations for use in emerging quantum information technologies ranging from fault-tolerant quantum computation to quantum metrology. In these applications, it is often necessary or desirable to read out the state of the qubits with the highest accuracy and in the shortest amount of time. Achieving these goals generally requires 1) an understanding of the physics of the measurement noise and 2) an optimal inference procedure tailored to that noise. In this talk, I will discuss various aspects of the optimization of qubit readout, including single-shot readout [1], adaptive decisions [2], and soft-decision decoding [3]. I will illustrate these aspects with the help of several experimentally relevant examples.

[1] B. D'Anjou & W.A. Coish, "Optimal post-processing for a generic single-shot qubit readout", Phys. Rev. A 89 012313 (2014)
[2] B. D'Anjou, L. Kuret, L. Childress & W.A. Coish, "Maximal adaptive-decision speedups in quantum-state readout", Phys. Rev. X 6 011017 (2016)
[3] B. D'Anjou & W.A. Coish, "Soft decoding of a qubit readout apparatus", Phys. Rev. Lett 113 230402 (2014).

QUSCOPE Seminar

Program

Encryption and quantum computing with continuous variables

Ulrik Lund Andersen, Technical University of Denmark

The role of quantum measurement in quantum thermodynamics

Alexia Auffèves, Institut Néel - CNRS, Grenoble, France

In this talk I will propose a new formalism to investigate stochastic thermodynamics in the quantum regime, where stochasticity and irreversibility primarily come from quantum measurement. In the absence of any bath, a purely quantum component to heat exchange is defined, that corresponds to energy fluctuations caused by measurement back-action. Energetic and entropic signatures of measurement induced irreversibility are then investigated for canonical experiments of quantum optics, and the energetic cost of counter-acting decoherence is characterized on a simple state-stabilizing protocol. By placing quantum measurement in a central position, our formalism contributes to bridge a gap between experimental quantum optics and quantum thermodynamics.

September 14-15  in 1520-626

Thesis Defense and Workshop on Strong-field and Ultrafast Physics

June 22 15:15-16:00 in 1525-323

Clean quantum and classical communication protocols

Matthias Christandl, Department of Mathematical Sciences, University of Copenhagen

To compute a two-party function cleanly , the players must not only correctly calculate the function's value but also return all registers to their initial states at the end of the communication protocol. Such protocols provide methods for implementing distributed computations, can be safely run in coherent superposition and provide lower bounds on the communication complexity of their non-clean counterparts.

Here we present clean protocols for calculating the Inner Product of two n-bit strings, showing that (in the absence of pre-shared entanglement) at most n+O(√n) bits or n + 4 qubits of communication are required. These provide new methods for implementing distribute CNOT or controlled-Z gates in parallel whilst minimizing the amount of communication. While evaluating a function cleanly is harder to perform than just computing the function, it can always be done using at most 2 n bits of communication for functions of n-bit strings. In contrast to the Inner Product, we show that nearly all Boolean functions require close to the maximal 2 n bits of classical communication to compute cleanly.

There will be coffee, tea and cake from 15:00.

April 5 15:15-16:00 in 1525-323

Transfer of spectral purity from the optics to the microwave, THz and Optics domains using an optical frequency comb

Yann Le Coq,  LNE-SYRTE, Observatoire de Paris, France

Optical frequency combs are used routinely in national metrology institutes and elsewhere for measuring and comparing optical frequency standards (optical clocks). Beyond this "traditional" use, they can also be utilized to transfer the spectral purity of a state-of-the-art optical source to other frequency domains, thereby generating an ultra-low phase noise source in this other frequency domain. I will describe our experiments that do so, targeting the microwave (~10 GHz), teraHertz (~30 THz) and optics (~200 THz) domains. I will describe several techniques that are used to ensure the comb adds a negligible extra noise in the frequency conversion and show how these techniques allow to create, both in the microwave and Teraherz domains, spectrally ultra-pure sources with record low phase noise levels, several orders of magnitude below more traditional technologies.

March 10 9:30-18:30 in 1525-626

QUSCOPE WORKSHOP ON ”ULTRAFAST AND ULTRASTRONG”

Final programme

January 8 10:15 in 1520-732

One-dimensional Bose gases in momentum space : equilibrium and out-of-equilibrium behavior

Isabelle Bouchoule, Institut d’Optique, France

On our atom-chip experiment, using the focussing technique, we are able to observe single one-dimensional (1D) Bose gases in momentum-space. We investigated the momentum distribution at equilibrium across the quasi-condensation crossover. Moreover, analyzing statistical noise on the pictures, we measured the two-body correlations in momentum-space. While the momentum distribution does not qualitatively change across the quasi-condensate transition, due to the lack of long-range order in finite temperature 1D Bose gas, the two-body correlations in momentum-space present a clear signature of the quasi-condensation phenomena. Finally, measurements in momentum space are very valuable to study out-of-equilibrium dynamics. In our experiment, we revisited the breathing mode of a harmonically confined 1D Bose gas and show for the first time the self-reflection mechanism that occurs in the quasi-condensate regime.

January 7 11:15 in 1520-732

Dipolar QED (dQED): an alternative paradigm for quantum optics

Charles Adams, Durham University, UK

In the strong coupling regime of cavity QED, a single photon modifies the optical response of the atom-cavity system to subsequent photons. An analogous effect can occur in atomic ensembles without the need of a cavity, if the atoms interact strongly. For example a single photon stored as a Rydberg excitation can switch the optical response of the ensemble to subsequent photons. In this talk we review some applications of this dipolar QED effect in both the quantum and classical regimes.