YQIS-17: 3RD INTERNATIONAL CONFERENCE FOR YOUNG QUANTUM INFORMATION SCIENTISTS
PROGRAM FOR WEDNESDAY, OCTOBER 4TH
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10:15-10:30Coffee Break
10:30-12:00 Session 8: Quantum Simulation and Computation
10:30
Realization of Shor’s algorithm at Room Temperature

ABSTRACT. Shor’s algorithm can find prime factors of a large number more efficiently than any known classical algorithm. Understanding the properties that gives the speedup is essential for a general and scalable construction. Here we present a realization of Shor’s algorithm, that does not need any of the simplifications presently needed in current experiments and also gives smaller systematic errors than any former experimental implementation. Our realization is based on classical pass-transistor logic, runs at room temperature, and uses the same amount of resources as a scalable quantum computer. In this paper, the focus is not on the result of the factorization, but to compare our realization with current state-of-the-art experiment, factoring 15. Our result gives further insight to the resources needed for quantum computation, aiming for a true understanding of the subject.

10:45
Towards quantum simulation with circular Rydberg atoms
SPEAKER: Brice Ravon

ABSTRACT. Quantum simulation offers the possibility to understand phenomena involved in many-body physics. Because of the size of the relevant Hilbert space quantum phase transitions and transport are out of reach from both numerical and analytical treatment, even for supercomputers, when the system involves a few tens of spins. Numerous platforms have been developed to study quantum spin-1/2 systems. We propose a new platform to simulate a Heisenberg XXZ spin chain based on the laser-trapping of circular Rydberg atoms. Benefiting from their long intrinsic lifetime and inhibiting their microwave spontaneous emission, we expect to prepare a defect-free chain of 40 atoms having a lifetime of a few seconds. This versatile simulator provides tunability of all its parameters over the entire range of interest allowing us to follow the dynamics over 10^5 interaction cycles. This promising platform would open the way to simulations beyond the grasp of classical computation.

11:00
Quantum mechanics and the efficiency of simulating classical complex systems

ABSTRACT. The development of tools allowing us to infer models from observed data, and thus to simulate possible future outputs, has a central role in several fields. Many fundamental questions in nature and society can be addressed only by isolating indicators of future behavior in highly complex systems. However, even the most efficient constructions often require information about the past that is uncorrelated with future predictions. In terms of energetic costs, this brings a waste of resources in the computer simulations based on such models. Even if the systems to simulate are completely classical, it has been proved that quantum information can reduce this waste beyond classical limits. During the talk I will sketch this scenario, and present some of the aforementioned results. In particular, I will describe a theoretical proposal for the possible implementation of a quantum model that breaks this classical bound. Such proposal exploits tools and schemes already used for the implementation of quantum walks in linear-optics setups, and has been now experimentally demonstrated in a lab.

11:15
Flow Ambiguity: A Path Towards Classically Driven Blind Quantum Computation
SPEAKER: Atul Mantri

ABSTRACT. Blind quantum computation protocols allow a user to delegate a computation to a remote quantum computer in such a way that the privacy of their computation is preserved, even from the device implementing the computation. To date, such protocols are only known for settings involving at least two quantum devices: either a user with some quantum capabilities and a remote quantum server or two or more entangled but noncommunicating servers. In this work, we take the first step towards the construction of a blind quantum computing protocol with a completely classical client and single quantum server. Specifically, we show how a classical client can exploit the ambiguity in the flow of information in measurement-based quantum computing to construct a protocol for hiding critical aspects of a computation delegated to a remote quantum computer. This ambiguity arises due to the fact that, for a fixed graph, there exist multiple choices of the input and output vertex sets that result in deterministic measurement patterns consistent with the same fixed total ordering of vertices. This allows a classical user, computing only measurement angles, to drive a measurement-based computation performed on a remote device while hiding critical aspects of the computation.

11:30
Fractal Properties of Magic State Distillation
SPEAKER: Patrick Rall

ABSTRACT. All quantum computers must protect quantum data from decoherence. However, the goals of data protection and data manipulation are fundamentally at odds. No error correction code can support a universal set of transversal operations, operations that we know how to implement fault-tolerantly. Luckily, an incomplete operation set can be elevated to universality by supplying certain 'magic' states.

To manufacture magic states we use 'magic state distillation' to assemble many copies of low fidelity states into more useful states. Magic state distillation protocols have a complicated non-linear nature. Analysis of protocols is therefore usually restricted to one-parameter families of states which aids tractability.

We show that if we lift this one-parameter restriction and embrace the complexity, distillation exhibits fractal properties. By studying these fractals we demonstrate that some known protocols are significantly more effective when not restricted. Low fidelity states that are usually worthless for distillation are now usable, and fewer iterations of the protocols are needed to reach high fidelity. Additionally we calculate the fractal dimension of some common protocols as a measure of complexity. The fractal nature of distillation also explains why it is so challenging to design protocols with desired properties.

11:45
On the implausibility of classical client blind quantum computing

ABSTRACT. Suppose a large scale quantum computer becomes available over the Internet. Could we delegate universal quantum computations to this server, having only classical communication between client and server, in a way that is information-theoretically blind (the server learns nothing about the input apart from its size, with no cryptographic assumptions)? We give indications that the answer is no. In more detail, we observe that if there exist information-theoretically secure classical schemes for performing universal quantum computations on encrypted data, then we get unlikely containments between complexity classes, such as BQP ⊂ NP/poly. Moreover, we prove that having such schemes for delegating quantum sampling problems, such as Boson Sampling, would lead to unlikely upper bounds for circuits computing the permanent. We then provide a complexity theoretic upper bound for schemes allowing one round of quantum communication and polynomially many rounds of classical communication (generalizing blind quantum computation). This upper bound then lets us show that, under plausible complexity assumptions, such a protocol is no more useful than classical schemes for delegating NP-hard problems. Lastly, we comment on the implications of these results for the prospect of verifying a quantum computation through classical interaction with the server.

12:00-13:00Lunch Break
13:00-14:30 Session 9: Quantum Correlations and Networks
13:00
Remote polarization-entanglement generation by local dephasing and frequency up-conversion

ABSTRACT. We introduce a scheme for remote entanglement generation for the photon polarization. The technique is based on transferring the initial frequency correlations to specific polarization-frequency correlations by local dephasing and their subsequent removal by frequency up-conversion. On fundamental level, our theoretical results show how to create and transfer entanglement, to particles which never interact directly. This possibility stems from the multipath interference and its control in frequency space. For applications, the developed techniques and results allow for the remote generation of entanglement with distant parties without Bell state measurements and open the perspective to probe frequency-frequency entanglement by measuring the polarization state of the photons.

13:15
Entanglement scaling at a first order phase transition
SPEAKER: Abel Yuste

ABSTRACT. First order quantum phase transitions (1QPTs) are signaled, in the thermodynamic limit, by discontinuous changes in the ground state properties. These discontinuities affect expectation values of observables, including spatial correlations. When a 1QPT is crossed in the vicinity of a second order one, the increase of the correlation length associated to the later strongly modifies the properties of the corresponding ground state and it becomes increasingly difficult to determine the order of the transition when the size of the system is finite. Here we show that, in such situations, it is possible to apply finite size scaling to entanglement measures in 1QPTs, as it has recently been done for the order parameter and the energy gap to obtain the correct thermodynamic limit. We show how this finite size scaling can unambigously discriminate between first and second order phase transitions in the vicinity of multricritical point even when the singularities displayed by the measures of entanglement lead to controversial results.

13:30
Convex optimization over classes of multiparticle entanglement

ABSTRACT. A well-known strategy to characterize multiparticle entanglement utilizes the notion of stochastic local operations and classical communication (SLOCC), but characterizing the resulting entanglement classes is very difficult. Given a multiparticle quantum state, we first show that Gilbert's algorithm can be adapted to prove separability or membership in a certain entanglement class. We then present two reliable algorithms for convex optimization over SLOCC classes. The first algorithm uses a simple gradient approach, while the other one employs the accelerated projected-gradient method. For demonstration, the algorithms are applied to the likelihood-ratio test using experimental data on bound entanglement of a noisy four-photon Smolin state [Phys. Rev. Lett. 105, 130501 (2010)]. Our work not only sheds new light on the separability problem, but also provides a reliable tool for experimentalists to characterize the entanglement property of their quantum systems with confidence.

Ref: J. Shang and O. Gühne, Convex optimization over classes of multiparticle entanglement, Under review at Phys. Rev. Lett.; eprint arXiv:1707.02958 [quant-ph] (2017).

13:45
Entanglement and quantum combinatorial designs
SPEAKER: Zahra Raissi

ABSTRACT. Orthogonal Arrays(OA) are combinatorial arrangements which have close connection to error correcting codes, and Latin squares. Applications of orthogonal arrays are given in statistics and design of experiments. An r*n array A with entries taken from a set with q elements is said to be an OA with q levels, strength k and index l if every r*k subarray of A contains each k-tuple of symbols exactly l times as a row. I will have an overview on OAs and then introduce the notion of Quantum Orthogonal Arrays(QOA) as a natural generalization of OAs. Entries of these arrangements are composed by pure quantum states where different columns are allowed to be entangled. Additionally we will see that quantum Latin squares are naturally derived from QOA in the same way as Latin squares arise from OAs. It is important to discuss about the connection between the classical combinatorial arrangement to quantum mechanics, it shows how to find further applications for quantum protocols. I will demonstrate QOA of strength k composed by n columns are one-to-one connected with k-uniform states. A pure quantum state of n subsystems is called k-uniform if all its reductions to k<=floor{n/2} qudits are maximally mixed.

14:00
Efficient Device-independent Entanglement Detection for Multipartite Systems

ABSTRACT. Entanglement is one of the most peculiar aspects of quantum theory and is a key feature for several quantum information protocols. However, detecting its presence in multipartite states remains challenging both experimentally and theoretically. The first barrier towards entanglement detection is the exponential amount of information required to reconstruct the system's state. The second, is that, even if the quantum state is known, the available methods are computationally too demanding even for systems composed of few particles. 



In this talk (based on [1]) I will introduce a device-independent technique for entanglement detection that is both computationally and experimentally efficient. It involves a number of experimental configurations that grows only polynomially with the size of the system, which makes it applicable to states of up to a few tens of particles. Moreover, it is based on the knowledge of few-body correlators, hence being amenable to practical implementations. 
I will also report several examples of implementation of the method for well-known multipartite states, showing that the introduced technique has a promising range of applications.

[1] F. Baccari et al. , Phys. Rev. X 7, 021042 (2017)

14:15
Certification of quantum network functionality based on multi-round teleportation

ABSTRACT. Quantum communication is a core element of quantum information science. The most general communication scenario involves separated nodes exchanging quantum information at a large distance, which defines a quantum network. Recently, various operational stages of network functionality were defined in terms of classes of protocols that can be performed with certain resources. Importantly, there exists a class of protocols, involving e.g. blind quantum computing or generating anonymous entanglement, which crucially requires storing a quantum state while being able to perform universal local operations. However, state of the art implementations can only achieve either good local control or good quantum memory. We propose a simple testing protocol, which we call memory and control test, that provides an explicit certification of attainment of both tasks simultaneously. Specifically, we present a protocol based on multi-round teleportation in the presence of universal local gates. We adapt it to experimentally feasible scenarios, where imperfections naturally arise, and provide explicit parameters for estimating the quality of universal gates in the presence of noisy memory. Moreover, based on the test we estimate the performance of other protocols relevant for the stage.

14:30-14:45Coffee Break
16:15-16:30Coffee Break
16:30-18:15 Session 12: Poster Session - II
16:30
Generalized Probabilistic Description of Noninteracting Identical Particles

ABSTRACT. Generalized probabilistic theories (GPTs) allow to study nonclassical phenomena on a purely operational level and independently of quantum theory. Its basic idea consists in restricting the set of all imaginable measurement outcome probabilities by physically motivated principles in order to retrieve quantum predictions. For instance, the celebrated Popescu-Rohrlich box stems from a GPT investigation of correlations restricted by the nonsignaling principle. This talk aims to introduce a GPT description of bosonic bunching in optical multiports. In particular, a Popescu-Rohrlich box analog within the realm of identical particles will be presented. Furthermore, the simplest nontrivial case of three photons bunching on a tritter (3-port) will be fully explained with straightforward principles.

16:30
Multiqubit State Tomography with Finite Data
SPEAKER: Lukas Knips

ABSTRACT. We show that for finite set of data the statistical nature of measurements is an almost unavoidable reason for unphysical estimates in multiqubit quantum state tomography. The usual multinomial or Poissonian noise results in eigenvalue distributions converging to the Wigner semicircle distribution for already a modest number of qubits. This fact has to be taken into account for the evaluation of tomographic data. It forms the basis to determine which eigenvalues of the raw density matrix obtained via tomography are relevant and which ones are irrelevant as they are the result of statistical effects or cannot be distinguished from it. We introduce a method to obtain a physical estimate, without using constrained optimization. This approach allows to directly obtain also error bars for both the state estimate as well as for interesting figures of merit such as the fidelity with minimal numerical effort.

16:30
Transient Response of Bistable Systems
SPEAKER: Paul Brookes

ABSTRACT. Bistability is a common phenomenon in driven nonlinear oscillators. For sufficiently strong drive powers there is a range of drive frequencies close to resonance over which the oscillator has two possible steady state modes of oscillation. These modes have different amplitudes and phases. Here we study bistability in a cavity coupled to a transmon qubit. The transient response of this system shows a preference for different steady states depending on the initial state of the qubit. Over time random switching between between bistable states causes the system to lose its memory of its initial state and this approach to thermal equilibrium is much longer than any time constant in the parameters of the system. This is known as critical slowing down.

16:30
Multipartite entanglement transformations with local operations and finite rounds of classical communication

ABSTRACT. We consider generic pure n-qubit states and a general class of pure states of arbitrary dimensions and arbitrarily many subsystems. We characterize those states which can be reached from some other state via local operations assisted by finitely many rounds of classical communication (LOCC_N). For n>3 qubits we show that this set of states is of measure zero. That is, almost no state can be reached if restricted to the practical scenario of LOCC_N. We also identify classes where any separable transformation can be realized by a protocol in which each step is deterministic. Such transformations are natural generalizations of bipartite transformations. We show, however, that in general there exist state transformations which require a probabilistic step within the protocol. This highlights the difference between bipartite and multipartite LOCC and shows that multipartite LOCC transformations are more complex than the transformations considered in the literature so far. Finally, we obtain easily computable lower bounds on some entanglement measures by restricting to LOCC_N.

References: C. Spee, J. I. de Vicente, D. Sauerwein, and B. Kraus, Phys. Rev. Lett. 118, 040503 (2017) ; J. I. de Vicente, C. Spee, D. Sauerwein, and B. Kraus Phys. Rev. A 95, 012323 (2017)

16:30
Commercializing continuous-variable quantum key distribution
SPEAKER: Imran Khan

ABSTRACT. Conventional cryptography serves as a means to protect important communication from unauthorized access. It has been shown throughout history that advances in mathematics can weaken or even break ciphers deemed secure previously. This can be remedied by finding new ciphers. If a powerful adversary however keeps the cipher‘s weakness secret, he may use it to break existing encryption. It is also known that a powerful quantum computer renders some of the existing cryptographic algorithms insecure.

A solution to this problem is quantum key distribution (QKD), which unlike conventional cryptography does not rely on assumptions of computational complexity, nor is it insecure against a quantum computer. Instead, it is based on security proofs using quantum mechanics to provide information-theoretic security.

Recent advances in continuous-variable QKD demonstrated the feasibility of using existing high-speed telecom technology for its implementation. In parallel, photonic integrated circuits – mainly intended for telecommunication purposes – are being developed on a European scale using InP technology. Furthermore, first experiments have demonstrated QKD with satellites to bridge global distances. At the Max-Planck-Institute for the Science of Light, we are launching a startup that takes advantage of these developments in order to commercialize and apply QKD on a wide scale.

16:30
SLOCC hierarchy for generic states in 2 x m x n level systems

ABSTRACT. We consider three partite pure states in the Hilbert space of dimensions 2,m,n and investigate to which states a given state can be locally transformed with a non-vanishing probability. Whenever the initial and final state are elements of the same Hilbert space, the problem is solved via the characterization of the SLOCC classes. However, when considering transformations from higher to lower dimensional Hilbert spaces, a hierarchy among the states can be found. We build on results presented in [1], where a connection to linear matrix pencils has been drawn in order to study SLOCC classes in 2,m,n systems. We first show that a generic set of states of dimensions 2,m,n, where n=m is the union of infinitely many SLOCC classes. However, for n≠m, there exists a single SLOCC class which is generic. Using this result, we derive a hierarchy of SLOCC classes for generic states. We also investigate common resource states, which are those states which can be transformed to any state (not excluding any zero-measure set) in the smaller dimensional Hilbert space.

[1] E. Chitambar, C.A. Miller, and Y. Shi, J. Math. Phys. 51, 072205 (2010)

16:30
Quantum Sensor Networks with NV centers
SPEAKER: Helmut Frasch

ABSTRACT. We develop theoretical methods for using a NV-based sensor spin network for faithful reconstruction of spatio-temporal signals originating from macroscopic samples (e.g., brain) with micro/nano scale resolution. A high degree controllability of the sensors with applied microwaves and good optical readout allows us to establish a network structure for phase acquisition on individual sensors or groups of sensors with optimized pulse sequences. Further the correlation of all encoded phases makes it possible to extract the desired spatio-temporal information with a higher sensitivity and good SNR compared to sequential measurements. We will show how the developed methods could lead to neuronal imaging with high spatial resolution on a wide frequency range. We will discuss the physical constraints for the experimental realization of this method and obtain bounds on the maximum attainable sensitivities in sensing and localizing the signal.

16:30
Simulation of non-Pauli channels
SPEAKER: Thomas Cope

ABSTRACT. We consider the simulation of a quantum channel by two parties who share a resource state and may apply local operations assisted by classical communication (LOCC). One specific type of such LOCC is standard teleportation, which is however limited to the simulation of Pauli channels. Here we show how we can easily enlarge this class by means of a minimal perturbation of the teleportation protocol, where we introduce noise in the classical communication channel between the remote parties. By adopting this noisy protocol, we provide a necessary condition for simulating a non-Pauli channel. In particular, we characterize the set of channels that are generated assuming the Choi matrix of an amplitude damping channel as a resource state. Within this set, we identify a class of Pauli-damping channels for which we bound the two-way quantum and private capacities.

16:30
Optical trapping of nano-particles with a deep parabolic mirror

ABSTRACT. We demonstrate the optical levitation of nano-particles in the focus of a dipole mode [1]. The linear-dipole mode is generated by focusing a radially polarized, so called ‘doughnut’-mode with a deep parabolic mirror that covers 94% of radiation pattern of a linear dipole. In the experiment, CdSe/CdS dot-in-rod nano-particles were trapped. We analyze the emission of the trapped particles in terms of second-order intensity correlation functions and test the dependence of the strength of the observed anti-bunching on the number of trapped particles. We also discuss the characteristics of the trapping potential and provide a comparison between theoretical estimations and measured trap parameters.

[1] V.Salakhutdinov et al., Optica 3(11), 1181(2016)

16:30
Entanglement in quantum spin networks with defects
SPEAKER: Himadri Dhar

ABSTRACT. A key aspect in designing an effective and scalable quantum network is generating entanglement between its nodes, which is robust against defects in the network. We consider a bipartite quantum network of spin-1/2 particles with a finite fraction of defects, where the corresponding wave function of the network is rotationally invariant under the action of local unitaries. By using quantum information-theoretic concepts like strong sub-additivity of von Neumann entropy and approximate quantum telecloning, we prove analytically that in the presence of defects, caused by loss of a finite fraction of spins, the network, comprised of a fixed numbers of lattice sites, sustains genuine multisite entanglement, and at the same time may exhibit finite moderate-range bipartit entanglement, in contrast to the network with no defects.

16:30
Negatively charged state on phosphorus atom in the silicon quantum computer architecture

ABSTRACT. Negatively charged donor states (D-) play the important role in quantum computing devices based on nuclear and electron spins in silicon. In particular, read-out via the D- state is two orders of magnitude faster than via the neutral donor state due to the weaker bounding of the second electron with donor. Ability to perform manipulations with single donor orbital requires the knowledge of the bound electron wave function and its modification under applied fields. Negatively charged donors have been extensively studied in bulk semiconductors, however, transport spectroscopy measurements of the field effect transistor indicate that D- states on impurities close to an interface differ qualitatively from D- states in bulk semiconductors. In the present study, we investigated theoretically the D- state in silicon near the oxide interface in uniform electric and magnetic fields, both applied perpendicular to the surface. Wave functions and the energy spectrum of donor and interface states have been obtained analytically within effective mass theory. The hybridization of the negatively charged state between the potential of its donor atom and the triangular quantum well near Si/SiO2 interface was studied. The parameters which allow to control the electron density distribution in direction perpendicular to the interface were calculated.

16:30
Tripled frequency photon generation in the focus of a deep parabolic mirror in argon gas

ABSTRACT. We investigate the generation of tripled frequency photons under the condition of very tight focusing. Our experiments show that the number of tripled frequency photons in such a scenario has fifth power dependence on the intensity of fundamental beam. Considering six-wave-mixing as the responsible process, the number of tripled frequency photons per pulse is calculated theoretically. The behavior of tripled frequency photon generation for the transition from the paraxial regime to focusing from full solid angle is studied experimentally and theoretically. We find a good qualitative agreement between the experiment and a numerical analysis.

16:30
Quantum Algorithm for Perfect Matching Problem

ABSTRACT. In this poster, we consider the problem of detecting an existence of perfect matching in a bipartite graph. We limit our attention to graphs embeddable into a 2D-grid.

For such graphs, it is known that time complexity of the best quantum algorithm for a maximal matching problem, a more general version of the problem, is $|V|\log^2|V|$ for $|V|$ being the number of nodes. The best known deterministic algorithm has time complexity $|V|^{3/2}$.

We suggest a quantum walk algorithm (QWA) with time complexity $|V|\log|V|$. Our algorithm uses recently discovered phenomena of exceptional configurations of marked vertices of QWA with the Grover’s coin. The main idea of our algorithm is as follows. We embed the original graph into a 2D-grid and mark all its vertices. The embedding is such that the marked vertices form the exceptional configuration iff the original graph has the perfect matching. We run a QWA for the 2D-grid. If the algorithm finds a marked vertex then the configuration is not exceptional and, thus, the original graph has no perfect matching.

We also present preliminary results on extending the idea to bipartite graphs embeddable into other regular graphs.

16:30
Bell Inequalities in Continuous Variable Systems for General 4-mode Gaussian States
SPEAKER: Gaurav Saxena

ABSTRACT. Continuous Variable Bell Inequalities for 4-mode systems are studied. The reason for choosing 4-modes over 2-modes is due to the fact that for every propagation direction, two polarizations can be associated and with each of these four modes, annihilation operators a1, a2, a3, a4, can be associated.

The inequalities used are known as multiphoton inequalities and were given by Arvind et.al in 1998. In our study, we have analyzed the non-locality of a general 4-mode Gaussian state by including noise due to a beam splitter and for different temperatures. We have also analyzed our results for different squeezing for different modes and by changing the entanglement between the modes.

We also observe that with the use of beam splitter, a non-local state does not lose its character, i.e., it remains non-local. Even when we increase the leakage due to the beam splitter, i.e., we decrease the transmitivity, there is still some violation of the inequality. Hence, it can be concluded that the action of the beam splitter preserves the non-locality of the transmitted state which is our main result.

16:30
Improved quantum advantage with shallow circuits via non-local games
SPEAKER: Zachary Webb

ABSTRACT. A recent result by Bravyi, Gosset, and Koenig gave an explicit example of a relational problem solvable by a constant depth quantum circuit, but that any classical circuit that succeeded with probability above 3/4 required logarithmic depth. In this paper, we modify their problem slightly, and show that this new problem is still solvable with a constant depth quantum circuit, but that any classical circuit that solves the problem with larger than superpolynomially small probability requires logarithmic depth. The proof proceeds by constructing a strategy for the repeated Mermin magic square game from the classical circuit, and using known results that such a strategy cannot exist.

16:30
Artificial subsystems in multilevel single-part systems and new informaion-entropic inequalities for Clebsch-Gordan coefficients
SPEAKER: Zhanat Seilov

ABSTRACT. Correlations in quantum system play essential role in quantum communication and quantum computation. Recently the notion of hidden quantum correlations was introduced. These correlations are defined for indivisible N-level quantum system (qudit) can be expressed as entropic inequalities analogous to the properties of composite quantum systems, such as subadditivity condition and strong subadditivity condition.

In our work using constructed bijective map between integers y = 1,..., N and set of variables x1,x2, …,xn we obtain the partition of indivisible N-level system into n subsystems. Such approach allows employing the properties of composite system (bipartite, tripartite) of n subsystems to any indivisible N-level quantum system. This approach can be employed to any partition of multilevel system into "artificial" subsystems.

Using obtained functions detecting the hidden correlations in the system, we found new inequalities for special functions that determine the matrix elements of the SU(2)-group irreducible representation and the Clebsch-Gordan (C-G) coefficients for quantum angular momentum. In this approach we used representation of squares of C-G coefficients as probabilities.

16:30
Games and Monogamy in the relativistically causal correlations

ABSTRACT. Recently a new paradigm for the physically realizable correlations among observables was introduced. The corresponding set of correlations includes the convex set of no-signaling correlations and extends it to include some signaling correlations when the number of parties is equal or more than three. This signaling correlations are called Relativistic Causal (RC) because they cannot be used to send information from an event to other causally preceding event in a given spacetime.

Here we characterize this new resource by computing its advantage on a set of probabilistic games. First we show that for Guest your neighbor’s input (GYNI) game RC boxes provide a maximum success probability which is higher than for the best no-signaling box under several different promises. Furthermore, we show that for all unique games the monogamy relation induced by that game is violated by RC correlations. On the other hand, we show that some inequalities still preserve their monogamy for RC correlations, thus showing that exist a strict hierarchy among monogamy relations. We also compute the extremal boxes for the RC correlation polytope in the three partite, two input, two output scenario.

16:30
Simulating Markov Transition Probabilities in a Quantum Environment
SPEAKER: Carla Silva

ABSTRACT. Quantum mechanics provides the mathematical explanation of the motion and interaction with energy at atoms level, and quantum physics allows particles to be in two states at the same time. Those are concepts that together built up quantum computing assumption. Through quantum simulators we believe faster results can be attained if conducted in a quantum-based approach in a specific environment. We performed experiments with Markov chains by modeling them into a quantum algorithm capable of being simulated on a quantum computer. We used the Google Quantum Computing Playground, a GPU-accelerated quantum computer with a 3D quantum state visualization, a browser-based WebGL Chrome using the language QScript. We implemented a Markov chain assuming that the process is homogeneous in time and built the transition matrix by estimating Markov probabilities for the states. We modeled a transition in which the process does not stay in the same state, eliminating the self-loop transitions and normalizing the remaining probabilities. We notice that sigmaX gate (quantum not) that flips states 0 and 1 for given qbit 0, and hadamard gate that creates superposition of states 0 and 1 for given qbit 2, gave us faster results than for other setting environments in a quantum level.

16:30
Wide Field Imaging of Atomic Spins using Nitrogen-Vacancy Centers in Diamond
SPEAKER: Marwa Garsi

ABSTRACT. Electron and nuclear magnetic resonance are essential techniques in medicine, life sciences and material sciences to obtain deep insights into composition, structure, and function of (bio-)structures. Traditional induction-based techniques achieve volume resolutions on the order of several cubic micrometers to millimeters which excludes them from being applied at the nanoscale level for label-free imaging and single-molecule analysis. To understand fundamental processes such as membrane channel transport and to resolve the structure and folding mechanisms of proteins, novel sensors are crucially required. Here, we show a nanoscale imaging technique based on nitrogen-vacancy centers in diamond towards the detection of small ensembles of electronic [1,2] and nuclear [3,4] spins at ambient conditions. Using optically detected magnetic resonance tomography in a high resolution wide-field microscopy setup we achieve a B-field sensitivity of 100 nT/√(Hz µm^2 ), combined with a spatial resolution of 400 nm. Our results pave the way towards understanding fundamental process at the single-molecule levels.

[1] Steinert, S. et al., Nat. Commun. 4, 1607 (2013).

[2] Ziem, F. C., et al., Nano Lett. 13, 4093–8 (2013).

[3] Mamin, H. J. et al., Science 339, 557 (2013).

[4] Staudacher, T. et al., Science 339, 561 (2013).

16:30
quantum discord between two distant Bose-Einstein condensates with Bell-like detection

ABSTRACT. We propose a technique that enables the creation of quantum discord between two distant nodes,each containing a cavity consist of the Bose-Einstein condensate, by applying a non-ideal Bell-like detection on the output modes of optical cavities. We find the covariance matrix of the system after the non-ideal Bell-like detection, showing explicitly that one enables manipulation of the quantum correlations, and particularly quantum discord, between remote Bose-Einstein condensates. We also find that the non-ideal Bell-like detection can create entanglement between distant Bose-Einstein condensates at the two remote sites.

16:30
Selection of multipartite spin states in a nuclear bath 

ABSTRACT. Multipartite entangled states are key for many quantum information protocols. Generation of these states and their protection against various noise sources is quintessential for successful quantum computing and sensing applications. Here we discuss the generation and characterization of multipartite entangled nuclear spin states coupled to a single Nitrogen-Vacancy center (NV) in diamond. At low temperatures, resonant optical excitation of the NV becomes possible allowing for high fidelity projective spin readout and spin initialization. In combination with spin-lifetimes many orders of magnitude higher compared to ambient conditions, generation of multipartite entangled states becomes plausible by measurement based methods. We present a scheme and evaluate the fidelities for the initialization of the nuclear spin bath of a NV center.

16:30
Nanoscale thermometry and magnetometry for a new generation of hard disk recording heads

ABSTRACT. State of the art hard disk recording heads use magnetic fields to encode data as magnetization on small sectors on a recording medium. To further reduce the bits’ physical size a new generation of recording heads, called Heat Assisted Magnetic Recording (HAMR) [1], applies an additional induced heat spot to reduce the coercivity of a single bit on the recording medium. Although HAMR heads are not yet available in commercial drives this technology may increase storage capacities by orders of magnitude.

The development of the device hinges on suitable sensors for the nanoscale characterization of the produced heat and magnetic field. Individual nitrogen-vacancy (NV) defect centers in diamond are the ideal candidate for this task. They are both efficient single-spin magnetometers [2] and sensitive temperature sensors [3, 4] on a nanoscale dimension. Here we present our work to bring a NV nanoscale thermometer and magnetometer to the hard disk industry. Furthermore, we show possible uses of hard disk recording heads for quantum information and sensing applications.

[1] M. Krayder et al., Proceedings of the IEEE, 2008 [2] I. Jakobi et al., Nature Nanotechnology, 2016 [3] P. Neumann et al., Nano Letters, 2013 [4] A. Laraoui et al., Nature Communications, 2015

16:30
Private key against side channel attacks
SPEAKER: Omer Sakarya

ABSTRACT. A private state consist of a key part AB and a shield part A'B'. The key is secure because of the fact that each private state shared by Alice and Bob has a shield. We consider the scenario where Eve has partial access to the shield of a private bit via some quantum channel (called therefore a side channel). Our goal is twofold: we first check by how much the distillable key decreases under certain side channels. Next, we propose axioms for a well protecting shield.

16:30
The problem of quantum information's transmission through a field

ABSTRACT. In this paper, we investigate the transmission of information through the environment between two widely separated quantum systems, modeled as harmonic oscillators .We focus on quantum Brownian motion models in which N separated oscillators interact with a quantum scalar field. We compute the solutions for the homogeneous equation of motion and the dissipation and noise kernel, of which is constructed the density matrix propagator of the system. We demonstrate that the Markovian approximation fails for this system. We explore the interplay between characteristic non-Markovian phenomena, such as memory effects and quantum correlations, and the quantum phenomena of superposition and entanglement.

16:30
Two-photon interference of vapor-delayed single quantum-dot photons

ABSTRACT. Beside their enormous flux, quantum dots (QDs) allow for high photon indistinguishability and photonic entanglement generation, and their use as *flying qubits* for the quantum communication of the future. One limitation of QDs is the missing long lasting quantum memory. Here, we focus on the approach of storing light in an cesium (Cs)-vapor by slowing down single photons. High dispersion between ground-state hyperfine resonances of Cs-vapors enables lower group velocities, while maintaining transmission. Using a Cs-vapor as slow light-medium, we present a variable delay up to 27 ns for 500 ps photons from resonantly excited QD’s. Increasing the temperature in the vapor changes the dispersion, which allows us to control the amount of delay experienced by the photons. The crucial measure for implementing a quantum repeater, various quantum information protocols, and entanglement distribution is given by the single photon purity and indistinguishability. Therefore we eventually investigate the single-photon emission and compare the two-photon interference of delayed and undelayed photons.

16:30
Estimation of CHSH Inequality for Unknown Quantum State via C-SPSA

ABSTRACT. There are various methods to optimize the violation of Bell inequalities. These are used to find the optimal measurement bases. However, most of them require to know the system state beforehand. Here we study a complementary problem, that is, to verify if an unknown quantum state can violate the CHSH inequality. In this case, conventional optimization methods can not be employed. We address this problem by using an iterative optimization algorithm named “Complex Simultaneous Perturbation Stochastic Approach” (CSPSA). This is based on an estimate of the complex Wirtinger gradient of a target function that is used to generate a sequence of complex estimates approaching the minimizer. Magnitude and direction of the gradient’s estimation are calculated as the difference between the target function evaluated at two different points and as a complex vector whose components are randomly, independently generated, respectively. We considered the CHSH inequality as the objective function. Given a starting point (i.e., a random set of observables), and if the state is able to violate the CHSH inequality, the algorithm finds the necessary measurement bases for which the unknown input quantum state maximizes its violation. We anticipate these findings can be used for assisting several experimental realizations.

16:30
Optimizing Quantum Walk Search on a Reduced Uniform Complete Multi-Partite Graph

ABSTRACT. A recent work by Novo et al. (Sci. Rep. 5, 13304, 2015) shows an invariant subspace method applied to the study of continuous-time quantum walk (CTQW). This method reduces a graph into a simpler version that allows more transparent analyses of the quantum walk model. We adopt the aforementioned method to investigate the optimality of a quantum walk search of a marked element on a complete multi-partite graph. We formulate the eigenbasis that would facilitate the transport between the two lowest energy eigenstates and demonstrate how to set the appropriate coupling factor to achieve optimality.

The notion of invariant subspaces of continuous-time quantum walk (CTQW) problems is a powerful technique that simplifies the analyses of various quantum walk related studies such as the spatial search algorithm, quantum transport, and quantum state transfer. In this work, we apply this technique and generalize the result from complete graphs (CG), complete bipartite-graphs (CBG) and star graphs (SG) to uniform complete multiple-partite graphs (UCMG). More specifically, we (1) derive the formula for the coupling factor and (2) show that CTQW constructed based on our choice of coupling factor will remain optimal.

16:30
New entropic functions for qubit and Gaussian states
SPEAKER: Julio Lopez

ABSTRACT. The Tsallis relative entropy $S_q (\hat{\rho},\hat{\sigma})$ measures the distance between two arbitrary density matrices $\hat{\rho}$ and $\hat{\sigma}$. In this work the approximation to this quantity when $q=1+\delta$ ($\delta\ll 1$) is obtained. It is shown that the resulting series is equal to the von Neumann relative entropy when $\delta=0$. Analyzing the von Neumann relative entropy for arbitrary $\hat{\rho}$ and a thermal equilibrium state $\hat{\sigma}=e^{- \beta \hat{H}}/{\rm Tr}(e^{- \beta \hat{H}})$ is possible to define a new inequality relating the energy, the entropy, and the partition function of the system. From this inequality, a parameter that measures the distance between the two states is defined. This distance is calculated for a general qubit system and for an arbitrary unimodal Gaussian state.

16:30
Interference of single photons on a system of coupled waveguides
SPEAKER: Ali Angulo

ABSTRACT. Engineering quantum states by tailoring the properties of quantum correlations can be used for several applications as quantum computation, quantum cryptography and quantum telecommunication. Using the quantum state generated by an SPDC experiment on a system of coupled waveguides we can tailor the state of couple of photons to spread the HIlbert space and make multidimensional state useful for several applications.

16:30
Taming finite statistics for device-independent quantum information
SPEAKER: Pei-Sheng Lin

ABSTRACT. The device-independent approach to physics is one where conclusions are drawn directly and solely from the observed correlations between measurement outcomes. In quantum information, this approach allows one to make strong statements about the properties of the underlying devices via the observation of Bell-inequality-violating correlations. However, since one can only perform a finite number of experimental trials, statistical fluctuations necessarily accompany any estimation of these correlations. Consequently, an important gap remains between the many theoretical tools developed for the asymptotic scenario and the experimentally obtained raw data. In particular, a sensible way to estimate the underlying quantum distribution has so far remained elusive. Here, we propose a few methods to bridge this gap. Under the assumption that the experimental trials are independent and identically distributed, our methods allow one to achieve the analog of quantum state estimation in the device-independent setting by generating unique point estimates of the true quantum distribution. We further demonstrate how these estimates of the true quantum distribution can be used to provide sensible estimates of certain desired properties of the system, such as the amount of entanglement present in the measured system.

16:30
Simulating boson sampling in free space

ABSTRACT. Boson sampling constitutes a promising model of quantum computation, which probably for the first time will show post-classical calculations, even though it is restricted and non-universal. Here, we investigate a particularly simple boson sampling device consisting of freely propagating photons of different statistics. We relate the boson sampling probabilities to correlation functions in free-space setups. We find that the structure of the multi-photon interferences and spatial correlation functions reflect exactly those occurring in boson sampling and thus are characterized by the permanent of a propagation matrix. As a result, we are able to simulate boson sampling-like computations in free space. We also investigate the computational complexity of such a setup.

16:30
Generalised cluster states of mechanical oscillators for universal quantum computation

ABSTRACT. Mechanical oscillators are good candidates for many quantum technologies, namely continuous variables quantum computation. It had been shown in [O. Houhou, H. Aissaoui, and A. Ferraro, Phys. Rev. A 92, 063843 (2015)] that the quantum state of an array of mechanical oscillators can be engineered in order to obtain Gaussian cluster states, a resource useful in measurement based quantum computation over continuous variables. Here, we propose a protocol for the deterministic preparation of a class of non Gaussian cluster states containing both squeezed and cubic phase state elements. This cluster state is hosted in an array of mechanical oscillators of an optomechanical system consisting of a cavity mode driven by classical fields. By a suitable choice of the intensities and phases of the driving fields, the mechanics is driven towards a steady state that is the target cluster. Our protocol can generate generalised continuous variables cluster states with arbitrary size and shape by just tuning the properties of the driving fields. These cluster states allow the implementation of universal quantum computation by only performing Gaussian operations on them.

16:30
Quantum correlations in an optomechanical cavity with a periodic modulation
SPEAKER: Vahid Ameri

ABSTRACT. Proposing an optomechanical cavity with two mirrors where one of them is periodically modulated, we discuss the effect of periodic modulation on the quantum correlations between mirrors and between cavity field and mirrors. We find a synchronization of the periodic modulation between mirrors and also some interesting connections between the synchronization and quantum correlations of mirrors.

16:30
Robust and efficient control of spins in a complex (biological) environment

ABSTRACT. Nanodiamonds (ND) had been shown to be of excellent biocompatibility and additionally can host color centers. One of the most prominent representatives is the so-called nitrogen vacancy center (NV) that features, due to its unique spin system, the intrinsic capacity to sense for example magnetic or electric fields and temperature [1]. Furthermore, intense progress in functionalization of ND surface in past years promises a manifold of different applications in life science [2]. However, keeping control of the NV spin sublevel remains challenging. Especially in complex environments a ND can change for example its orientation, such that variations in excitation strength are expectable. To overcome this obstacle several techniques had been developed exhibiting a certain robustness against such fluctuations [3,4,5]. To this end, we utilize optimum control theory in combination with so-called cooperative pulse schemes [5]. In our work, we present a systematical study exploring the efficiency of such pulses for NVs in NDs. [1] R. Schirhagl et al., ARPC 65: 83-105 2014 [2] D.G. Lim et al., Int. J. Pharm. 514: 41-51 2016 [3] M. Garwood et al., JMR. 153: 155-177 2001 [4] A. M. Souza et al., PRL 106: 240501 2011 [5] M. Braun et al.., NJP 16 2014

16:30
Security analysis of thermal and continuous-variable measurement-device-independent quantum key distribution incorporating finite size effects

ABSTRACT. As continuous-variable (CV) quantum key distribution (QKD) moves towards realistic implementations it is essential to assess the impact of finite-size (FS) effect on the security performance. In fact, in realistic situation the parties can exchange only a finite number of signals, which must be used for both estimating the channel parameters and obtaining the secret key. In this work we extend the security analysis of thermal one-way and CV measurement-device independent (MDI) QKD, including the impact of finite-size effects. In order to perform this analysis we adapt a method described in a previous work for one-way protocols with coherent states. Thermal protocols are interesting because they allow the parties to use trusted thermal noise to both increase the security performance against noisy attacks, and extend quantum private communication to frequencies of practical utility (e.g. microwave). By contrast CV-MDI-QKD is very important because it allows two parties to establish private communication even when they cannot access a direct link. This is possible exploiting an intermediate relay assisting the parties in sharing the signals. As such MDI setup represents a first step toward implementation of more complex network configuration. We found that using blocks with 10^9 signal-points can provide high-rate usable key.

16:30
Role of cross-talk in multimode continuous-variable quantum communication

ABSTRACT. One of the ways to increase effectiveness of quantum key distribution and quantum communication in general is to use multiplexing of channels. We consider multiplexed continuous-variable quantum communication using multimode entangled states of light. We study the effect of cross-talk between the signal modes on secret key rate and entanglement which can be shared over lossy and noisy Gaussian channels. We show how presence of cross-talk between the modes destroys key and entanglement and determine the optimal state variance for given characteristics of the channel that maximizes secure key rate and entanglement. To restore security of the protocol we find a sequence of local operations that can cancel or limit the negative effect of cross-talk. We show that for channels with the same transmittance rate for all signal modes negative effects of cross-talk in principle can be fully eliminated by proper mode coupling prior to detection on the remote side. For channels with transmittance that differs for different modes, the cross-talk can be at least partially compensated by applying optimized squeezing operation before coupling on the remote side.

16:30
Cryogenic Fabry-Perot cavities for ultrastable lasers

ABSTRACT. Ultrastable lasers are in demand in different fields of physics and technology: radio astronomy, gravitational wave detection, navigation and telecommunication. Besides, such laser systems are the key elements of today’s best optical clocks, whose stability is now limited by laser frequency noise due to the Dick effect. High level of laser’s frequency stability can be achieved by stabilizing it to a well-isolated reference Fabry-Perot cavity. Cutting edge systems demonstrate fractional frequency instabilities on the level of 1*10-17 at τ=1-1000 s [3]. This value is fundamentally limited by thermal noise of the cavity’s spacer, mirror coating and substrate, that causes fluctuations of the cavity length. The level of thermal noise for cavities, made up of different materials and of different size and shape can be predicted with the help of Levin’s formulation of fluctuation-dissipation theorem [2]. We apply this method to many types of cavities. The configuration of resonators corresponds to those described in [3]. We are working on a 1,5 μm laser frequency stabilization systems, based on silicon cavities (Table 1, lines 2 & 3). Resonators with both types of mirrors – dielectric and crystalline - are used in our experiment. Crystalline AlGaAs mirrors are, surely, the most promising. High stability of the resonator’s length to thermal fluctuations in this case is ensured by very high mechanical Q-factors of monocrystalline silicon (Q=10 8 ) and GaAs/AlGaAs mirrors and low operating temperature – 124 K which is zero CTE point of silicon. To cool the cavity, we use a vacuum cryostat. Its temperature stability, which is better than 10 mK is enough for reaching the desirable level of laser’s frequency stability: 10 -16 and even lower at averaging time τ = 1-1000 s.

[1]  Matei  D.G. et  al. ”1.5  μm  Lasers  with  Sub-10 mHz  Linewidth”,  Phys.  Rev.  Lett. 118,  263202 (2017) 
[2]  Yu. Levin “Internal thermal noise in the LIGO test masses: A direct approach”, Phys. Rev. D 57, 659 (1998) 
[3]  Zhadnov  N.O.  et  al.  “A  new  generation  of cryogenic  high-Q  Fabry–Perot  resonators  for ultrastable  lasers”,  Quantum  Electronics,  47 (5):421 (2017) 

16:30
Reduction of many-body terms for quantum computations with fermions

ABSTRACT. Quantum computing is a very promising new computer paradigm with direct applications in computational sciences and simulations. In the future quantum computers may be able to simulate electronic systems with many-body fermionic systems. The direct analog implementation of such systems suffers, however, from the existence of experimentally unfeasible k-local terms, introduced by the encoding of the fermions into a spin system via the Jordan-Wigner transformation [1]. In order to physically realize these terms, principles of perturbation theory may be exploited, to map the physical k-local interactions to a higher dimension system consisted of only 2-local interactions. These techniques are known as Perturbative Hamiltonian Gadgets (PHGs)[2,3,4]. In order to increase the Hilbert space one or more auxiliary qubits (ancillas) are introduced, creating two distinct subspaces (physical and ancilla) in the final enlarged system. The energy difference between these two subspaces needs to be large enough so that the mixing between the states is negligible and the properties of the physical system are fully preserved. This approach gives rise to Hamiltonians consisting of coupling terms of different orders of magnitude (ranging from 1 to 108), making the implementation in an experimental set up very difficult. In this work we present a new approach that exploits the numerical optimization schemes to reduce k-local terms. We have developed a scheme to compare the eigenstates of the enlarged system to the ones of the physical system and to optimize the coupling parameters. Applying this scheme, we find a reduction of the range of coupling parameters by 6 orders of magnitude (final set of parameters varies from 1 to 102). This technique is scalable to multi-qubit systems and it makes the implementation of many-body terms feasible because of the reduced coupling range.

[1] P. Jordan and E. Wigner. Über das paulische Äquivalenzverbot. Zeitschrift für Phys., 298(47):631–651, 1928. [2] J. Kempe, A. Kitaev, and O. Regev. The complexity of the local Hamiltonian problem. SIAM Journal on Computing, 35(5):30, 2004. [3] S. Bravyi, D.P. DiVincenzo amd D. Loss, and B. M. Terhal. Quantum simulation of many-body hamiltonians using perturbation theory with bounded-strength interactions. Physical Review Letters, 1101(070503), 2008.[4] Y. Cao, R. Babbush J. Biamonte, and S.Kais. Hamiltonian gadgets with reduced resource requirements. Physical Review A, 91(012315), 2015. [5] R. Babbush, P.J. Love and A. Aspuru-Guzik. Adiabatic quantum simulation of quantum chemistry. Scientific Reports, 4(6603), 2014.