Quantum Computation and Information Seminar 

In this seminar, we will describe the recent development of a new Quantum Key Distribution platform which is based on the Coherent One Way protocol. We will describe the general features of this protocol and will discuss the practical challenges related to the development of a system. We will also approaches for the security certification of such a system and review current applications.

Over the past 30 years, two different computational paradigms have been developed based on the premise that the laws of quantum mechanics could provide radically new and more powerful methods of information processing.  One of these approaches is to encode the solution of a computational problem into the ground state of a programmable many-body quantum Hamiltonian system. Although, there is empirical evidence for quantum enhancement in certain problem instances, there is not a full theoretical understanding of the conditions for quantum speed up for problems of practical interest, especially hard combinatorial optimization and inference tasks in machine learning. In his talk, I will provide an overview of quantum computing paradigms and discuss the progress at the Google Quantum Artificial Intelligence Lab towards developing the general theory and overcoming practical limitations. Furthermore, I will briefly discuss two recent quantum machine learning primitives that we have developed known as Quantum Principal Component Analysis and Multiqubit Quantum Tunneling.

Light has the remarkable capacity to reveal quantum features under ambient conditions, making exploration of the quantum world feasible in simple laboratory experiments. Further, the availability of high-quality integrated optical components makes it possible to conceive of large-scale quantum states by bringing together many different quantum light sources and manipulating them in a coherent manner and detecting them efficiently. By this route, we can envisage a scalable photonic quantum network, that will facilitate the preparation of distributed quantum correlations among  many light beams. This will enable a new regime of state complexity to be accessed - one in which it is impossible using classical computers to determine the structure and dynamics of the system: Photonic quantum machines will therefore open new frontiers in quantum science and technology.

As microelectronics technology nears the end of exponential growth over time, known as Moore’s law, there is a renewed interest in new computing paradigms.  I will discuss recent research at UCSB on superconducting quantum bits, as well as our recent start at Google to build a useful quantum computer to solve machine learning problems.  Two qubit experiments will be highlighted, one to simulate a chemical reaction that finds a cross section, and a second to extend the lifetime of  a qubit state using quantum error correction. 

Since the transfer of quantum states from one site to another is a key task in quantum information processing, the one-dimensional spin-$1/2$ chain has been extensively studied as a quantum wire for the transfer of qubit states. The ideal scenario is that where we have unit fidelity of transfer, but unfortunately it has been demonstrated that such perfect transfer cannot be achieved within chains with basic nearest-neighbor couplings.

In this talk, I will address the notion of pretty good state transfer, where the requirement is only slightly relaxed: here the requirement is that the fidelity of transfer gets arbitrarily close to one. Then, I will show that any multi-qubit state can be transferred with arbitrarily large fidelity through the uniform XX quantum spin chain if and only if the length of the chain is $n = p − 1$, $n = 2p − 1$, where $p$ is prime, or $n = 2^k − 1$. This result highlights the link between quantum dynamics and primality, and opens the way to using uniformly coupled spin chains as short-distance quantum channels for the transfer of arbitrary states of any dimension. I will discuss how the experimental implementation of this transfer protocol can be carried out with current technology.

We investigate the properties of entanglement between two modes of a free Dirac and Scaler Field as seen by two relatively accelerated parties. A uniformly accelerated observer is unable to access information about the whole of spacetime, therefore there is a loss of information and a corresponding degradation of entanglement. The entanglement degradation in the limit of infinite acceleration for the Dirac field asymptotically reaches a non-vanishing minimum value. While for the case of bososnic field, a bipartite system becomes fully seperable in the limit of infinite acceleration. This means that the state always remains entangled to a degree for fermionic field and can be used in quantum information tasks, such as teleportation, between parties in relative uniform acceleration. We also investigate the effect of relativity on a tripartite maximally entangled GHZ state

The transfer of an unknown quantum state (QST) from a sender to a receiver is one of the main requirements to perform quantum information processing tasks. In this respect, QST of a single qubit by means of spin chains has been widely discussed, and many protocols aiming at performing this task have been proposed (for a review see Refs. [1] and references therein).

In this talk quantum state routing (QSR) and the transfer of the quantum state of $n\gt 1$ qubits ($n$-QST) by means of spin-$1/2$ chains is addressed. In the case of QSR, the aim is to design a transfer scheme such that the sender is in condition to choose one receiver of the QST out of several spins. The transfer of a quantum state of $n\gt 1$ quits consists in achieving a high-fidelity transfer of the quantum state of many qubits via the use of a spin chain [2]. For the $2$-QST, an analytical expression of the average fidelity as a function of the allowed excitations transfer amplitude are derived and, hence, theoretical investigations on the search and optimisation of other QST protocols may be triggered [3].

References

1. S. Bose, Quantum communication through spin chain dynamics: an introductory overview, Contemp. Phys. 48, 13 (2007); T.J.G. Apollaro, S. Lorenzo, F. Plastina, Transport of quantum correlations across a spin chain, Int. J. Mod. Phys. B 27, 1345035 (2013).
2. S. Paganelli, S. Lorenzo, T. J. G. Apollaro, F. Plastina, and G.L. Giorgi, Routing quantum information in spin chains, Phys. Rev. A 87, 062309 (2013).
3. T. J. G. Apollaro, S. Lorenzo, A. Sindona, S. Paganelli, G. L. Giorgi, and F. Plastina, Many-qubit quantum state transfer via spin chains, Phys. Rev. A 91, 042321 (2015).

In this talk, we report a recent work of our group regarding quantum algorithm for phase estimation. Specially, we design a distributed semi-quantum algorithm for phase estimation which has a better time complexity than the conventional quantum algorithm.

Semiconductor diode lasers play a major role in everyday life in our information society. These lasers generate coherent light by stimulated emission of photons. In contrast, laser-like operation can be obtained also by stimulated scattering of bosonic quasiparticles called exciton-polaritons into the ground state of strongly coupled light-matter systems in microcavities. This polariton laser or dynamic and non-equilibrium polariton condensate regime can be reached with pump thresholds lower than conventional lasing. The exciton-polaritons decay by the leakage of photons from a cavity, which produces a monochromatic and coherent light output. In this talk, we discuss the electrical injection of exciton-polaritons into microcavity structures and the formation of exciton-polariton condensates in lattice structures with deep and tailorable confinement potential.

This talk will discuss how the concept of a Majorana Fermion, a particle that is its own anti-particle is at the basis of the presently known models for topological quantum computing. These models include the Fibonacci Model related to the quantum Hall effect and based on properties of the Jones polynomial and other models related to braid group representations that arise from Clifford algebras. This talk will be self-contained and will discuss the concepts of quantum computing, quantum algorithms and how topology and braid group representations are related to fundamental physics.

Quantum data hiding, originally invented as a limitation on local operations and classical communications (LOCC) in distinguishing globally orthogonal states, is actually a phenomenon arising generically in statistics whenever comparing a 'strong' set of measurements (i.e., decision rules) with a 'weak' one. The classical statistical analogue of this would be secret sharing, in which two perfectly distinguishable multi-partite hypotheses appear to be indistinguishable when accessing only a marginal. The quantum versions are richer in that for example LOCC allows for state tomography, so the states cannot be come perfectly indistinguishable but only nearly so, and hence the question is one of efficiency. The issues covered in the talk are going to be the following:

1. Every restriction on the allowed measurements, but with arbitrary processing of the measurement data at the end, gives rise to a norm on density matrices, the "distinguishability norm"; we will review the general theory of these [Matthews/Wehner/AW, CMP 291:813-843,2009].
2. LOCC is perhaps the most natural restriction in multi-partite systems, and we will revisit LOCC data hiding and its efficiency.
3. Gaussian operations and classical computation (GOCC): Not very surprisingly, GOCC cannot distinguish optimally even two coherent states of a single mode [Takeoka & Sasaki, PRA 78:022320, 2008].

But we can find states, each a mixture of multi-mode coherent states, which are almost perfectly distinguishable by suitable measurements, by when restricted to GOCC, i.e. linear optics and postprocessing, the states appear almost identical. The construction is random and relies on coding arguments. Open questions include whether one can give a constructive version of the argument, and whether for instance even thermal states can be used, or how efficient the hiding is.

Using the operational framework of completely positive, trace preserving operations and thermodynamic fluctuation relations, I will derive a lower bound for the heat exchange in a Landauer erasure process on a quantum system. The bound comes from a non-phenomenological derivation of the Landauer principle which holds for generic non-equilibrium dynamics. Furthermore the bound depends on the non-unitality of dynamics, giving it a physical significance that differs from other derivations. I will illustrate the framework to the model of a spin-1/2 system coupled to an interacting spin chain at finite temperature.

I will further investigate the link between information and thermodynamics embodied by Landauer principle in an open-system dynamics embodied by a collision-based mechanism involving a suitable multipartite system and a multi-particle spin reservoir at finite temperature. I will demonstrate that Landauer principle holds, in such an open configuration, in a form that involves the flow of heat dissipated into the environment and the rate of change of the entropy of the system, Quite remarkably, such a principle for heat and entropy power can be explicitly linked to the rate of creation of correlations among the elements of the multipartite system and, in turn, the non-Markovian nature of their reduced evolution. I will illustrate such principle using two paradigmatic cases.

Long-lived single-frequency oscillations in the local non-equilibrium dynamics of a quantum many-body system is an exceptional phenomenon. In fact, till now, it has never been observed, nor predicted, for the physically relevant case where a system is prepared to be quenched from one quantum phase to another. Here we show how the quench dynamics of the entanglement spectrum may reveal the emergence of such oscillations in a correlated quantum system with Kondo impurities. The oscillations we find are characterized by a single frequency which is proportional to the Kondo temperature of the system. The frequency is universal, being independent of the amount of energy released by the local quench, and scales as $1/N$ for $N$ being the size of the system. Importantly, the universal frequency manifests itself also in local observables, such as the spin-spin correlation function of the impurities.

Strong coupling in quantum mechanical systems occurs when the strength of the coupling is much larger than losses. Such a feature can be found in optical cavities, where the atoms strongly couple to the cavity photonic modes (cavity QED). The same occurs in semi-conductor microcavities, where excitons strongly couple to photons. In these systems, the resulting quasi-particle, the so-called polariton, results from the hybridisation of a typical excitation with photons. In this talk, I will discuss two examples involving ultracold atoms where strong coupling involves a phonon mode, giving rise to an acoustic-like quasi-particle: the phonon-polariton. Some relations with the research in the domain of exciton-polaritons and its applications will be drawn.

Machine learning algorithms look for patterns in data. Frequently, that data comes in the form of large arrays of high-dimensional vectors. Quantum computers are adept at manipulating large arrays of high-dimensional vectors. This talk presents a series of quantum algorithms for big data analysis. The ability of quantum computers to perform Fourier transforms, find eigenvectors and eigenvalues, and invert matrices translates into quantum algorithms for clustering, principal component analysis, and for identifying topological features such as numbers of connected components, holes and voids. These quantum algorithms are exponentially faster than their classical counterparts: complex patterns in datasets of size $N$ can be identified in time $O( \log N)$. The talk will discuss methods for implementing quantum machine learning algorithms on the current generation of quantum information processors.

Recently, atom-like impurities in diamond (colour centers) have emerged as an exceptional system for quantum physics in solid state. In this talk I will discuss recent developments transforming quantum control tools into quantum technologies based on single colour centers. Specially, realization of quantum optical interface between spins and photons and scalable quantum registers in diamond will be presented. New applications of diamond qubits involving nanoscale magnetic resonance and force measurements will be shown. I will discuss single spin NMR paving the way to ultrasensitive MRI and structure determination of single biomolecules. The detection of proteins using nanodiamond sensors will be presented. I will also highlight future directions of research including combination of quantum error correction and sensing protocols and quantum enabled sensing and imaging in living cells.

I will argue for and against possibilities to experimentally test the interface between quantum physics and gravity in a meaningful way. One promising route has been opened by massive mechanical objects that are now becoming available as new systems for quantum science. Devices currently under investigation cover a mass range of more than 17 orders of magnitude - from nanomechanical waveguides of some picogram to macroscopic, kilogram-weight mirrors of gravitational wave detectors. This provides access to a hitherto untested parameter regime of macroscopic quantum physics, eventually at the interface to gravity. My conclusion is therefore going to be an optimistic one.

Integrated photonics circuits have a strong potential to perform quantum information processing. Indeed, the ability to manipulate quantum states of light by integrated devices may open new perspectives both for fundamental tests of quantum mechanics and for novel technological applications. Within this framework we have developed a directional coupler, fabricated by femtosecond laser waveguide writing, acting as an integrated beam splitter able to support polarization-encoded qubits. As following step we addressed the implementation of quantum walk. This represents one of the most promising resources for the simulation of physical quantum systems, and has also emerged as an alternative to the standard circuit model for quantum computing. Up to now the experimental implementations have been restricted to single particle quantum walk, while very recently the quantum walks of two identical phot ons have been reported. For the first time, we investigated how the particle statistics, either bosonic or fermionic, influences a two-particle discrete quantum walk. Such experiment has been realized by adopting two-photon entangled states and integrated photonic circuits. As following step we have exploited this technology to simulate the evolution for disordered quantum systems observing how the particle statistics influences Anderson localization. Finally we will discuss the perspectives of optical quantum simulation: the implementation of the boson sampling to demonstrate the computational capability of quantum systems and the development of integrated architecture with three-dimensional geometries.

Quantum non-demolition information on the number of microwave photons stored in a superconducting cavity can be obtained by sending trough the mode circular Rydberg atoms, interacting dispersively with the field. This information can be used to reveal the quantum jumps of light. We will describe an optimal use of this information, based on the past quantum state formalism. Photon number measurement can also be used to stabilize a non-classical photon number state against decoherence, in a quantum feedback process operating in the steady state. Finally, the principle of quantum feedback can be applied to an improved adaptive QND measurement of the photon number. We shall report on these recent experiments and on the perspectives they open for non-classical state manipulations.