# Institut Transdisciplinaire d'Information Quantique (INTRIQ)

### Meeting de l'INTRIQ (Octobre 2009)

**Meeting de l'INTRIQ ( 2 - 3 octobre 2009 )**

**Program**

**Friday, October 2**

09:00 Welcome breakfast - registration

10:20 Welcome - Michael Hilke

**Session 1 **- Chair: Patrick Hayden

10:30 David Di Vincenzo (IBM) *The quest to build a quantum computer *11:30 Philippe Grangier (Paris)

*Schrödinger's Kittens and Non-Gaussian States of the Light : New Tools*

*for Quantum Communications*

12:30 lunch

14:00 Outdoor activities (pleasse register before 10am if you haven't done so)

16:45 Coffee and Tee

**Session 2 **- Chair: Alain Tapp

17:00 Joe Emerson (IQC) *Is your quantum processor better than the Jones' quantum processor?*18:00 Alexandre Blais (UdeS)

*Demonstration of two-qubit algorithms with a superconducting*qua

*ntum processor*

19:00 Dinner

21:00 Wine and qubits (short talks)

**Saturday, October 3**

07:30 Breakfast

08:30 INTRIQ PIs meet in the conference room

**Session 3 **- Chair: Michael Hilke

09:15 Chris Monroe (Maryland) *Trapped Ion Qubits and Entanglement through the Coulomb Force*10:00 Nicolas Godbout (Polytechnique)

*Flying qubits: photons as carriers and optical fibres as pipes*

10:45 Coffee and Tee

11:00 Chris Monroe (Maryland) *Trapped Ions Photonic Networks*11:45 Louis Salvail (U. Montréal)

*On the Power of Two-Party Quantum Cryptography*

12:30 lunch

**Session 4 **- Chair: Guillaume Gervais

14:00 Daniel Loss (Basel) *A Self-Correcting Quantum Memory in a Thermal Environment*15:00 Mark Eriksson (Madison)

*Electron Spins and Si/SiGe Quantum Dots*

16:00 Coffee and Tee

16:30 Zetian Mi (McGill) *Rolled-up Quantum Dot Tubes for High Efficiency Nanoscale Lasers and **Single Photon Sources*17:15 Michel Pioro-Ladriere (Sherbrooke)

*Electron spin qubits in quantum dots: a micro-magnet*

*approach*

18:00 Closing - Alain Tapp

Departure cocktail

*Speakers*

*Daniel Loss*

**A Self-Correcting Quantum Memory in a Thermal Environment **The ability to store information is of fundamental importance to any computer, be it classical or quantum. Identifying systems for quantum memories which rely, analogously to classical memories, on passive error protection ('self-correction') is of greatest interest in quantum information science. While systems with topological ground states have been considered to be promising candidates, a large class of them was recently proven unstable against thermal fluctuations. Here, we propose new two-dimensional (2D) spin models unaffected by this result [1]. Specifically, we introduce repulsive long-range interactions in the toric code and establish a memory lifetime polynomially increasing with the system size. This remarkable stability is shown to originate directly from the repulsive long-range nature of the interactions. We study the time dynamics of the quantum memory in terms of diffusing anyons and support all our analytical results with extensive numerical simulations. Our findings demonstrate that self-correcting quantum memories can exist in 2D at finite temperatures.

*David Di Vincenzo*

**The quest to build a quantum computer**

I will give an overview of what a quantum computer means for specialists in optics, NMR, atomic physics, quantum dots, and superconductivity. These disparate specialties have struggled mightily with the "simple" requirements for building quantum information processing hardware. They are simple to state, but hard to achieve all at once in a single system. But these achievement are now on the threshold of being realized in a number of areas; I will talk about my own of superconducting qubits in particular. We have reached the stage where the pace of future progress is set as much by non-scientific as scientific factors, and we need clearer discussions about what hardware is most valuable to build from a computer science point of view.

*Mark A. Eriksson*

**Electron Spins and Si/SiGe Quantum Dots**

Si/SiGe quantum dots can now be fabricated with sufficiently high stability and low noise that low temperature transport and charge sensing measurements can be used to study the intrinsic physics of the silicon quantum dot system. I will discuss several recent experiments that have implications for the development of qubits in this system. One key feature of interest is the indirect band-gap of silicon, the resulting presence of multiple conduction band minima (or valleys), and whether or not these valleys play any special role in silicon quantum dots. I will argue, based on experimental data and recent theoretical developments, that such physics is in fact quite interesting, especially in the transition from large to small systems. However, in the small-size limit, such as a quantum dot, I will argue that the valley degeneracy should be regarded as an additional degree of freedom of neither greater nor less complexity or importance than the usual spatial (x, y, z) degrees of freedom. I will also discuss the implications of the relatively large effective mass of electrons in the silicon conduction band, with an emphasis on spin blockade, lifetime-enhanced transport, and energy-dependent tunneling in quantum dots. This work was supported in part by ARO and LPS (W911NF-08-1-0482), by NSF (DMR-0805045), by DOD, and by DOE (DE-FG02-03ER46028). This research utilized NSF-supported shared facilities at the University of Wisconsin-Madison.

*Joseph Emerson*

**Is your quantum processor better than the Jones' quantum processor?**

I will describe a family of randomized protocols for characterizing and benchmarking the performance of quantum information processors. I will explain how the results of these protocols can be relevant to identifying the applicability of quantum error correcting codes and fault-tolerant thresholds. I will also describe some interesting open problems (to challenge the theorists) for extending the scope of these protocols.

*Michel Pioro-Ladrière*

**Electron spin qubits in quantumd dots: a micro-magnet approach**Electron spins isolated in quantum dots are promising candidates for the physical implementation of quantum information processing in the solid-state. After introducing how quantum information can be stored and manipulated with electron spins in modern quantum dots, I will review the recent experimental successes and the difficulties that have been encountered. Building on these results, I will demonstrate how the integration of micron-size ferromagnets can help in solving the problem of addressability and at the same time in enabling scalable, all-electrical, one-qubit gates.

*Zetian Mi*

**Rolled-up Quantum Dot Tubes for High Efficiency Nanoscale Lasers and Single Photon Sources*** *High efficiency nanoscale lasers and single photon sources have a broad range of applications in quantum computing and quantum information processing. One of the most promising approaches to realize such devices is to use high quality optical micro- or nano-cavities, with the incorporation of nanoscale emitters such as quantum dots. In this regard, significant progress has been made in the development of photonic crystal, microdisk, and micropillar based lasers and single photon sources. In an effort to achieve such nanoscale devices with high efficiency and well controlled emission properties, we have recently developed rolled-up quantum dot micro- and nanotube based nanophotonic devices, which are formed when a coherently strained self-organized quantum dot layer is selectively released from the host substrate by selective etching. Combining the advantages of both top-down and bottom-up fabrication processes, this powerful approach offers an exceptional flexibility for precisely tailoring the 3- dimensionally confined optical modes and hence the coupling between the dots and the optical cavity. In this presentation, I will also report on our recent demonstration of the first semiconductor tube lasers in the world, which exhibit an ultralow threshold of ~ 4 μW at room temperature. The achievement of quantum dot tube based single photon sources and their integration with Si nanophotonic integrated circuits will also be discussed.

*Christopher Monroe*

**Trapped Ion Qubits and Entanglement through the Coulomb Force*** *Collections of electromagnetically trapped atomic ions are among the most promising candidates for a quantum information processor. Internal electronic energy levels can store qubits for extremely long times, underlying their use in the world's most stable atomic clocks. Moreover, the strong Coulomb interaction between the ions allows entangling quantum gates to be implemented through laser forces that couple the internal qubit level to the external motion of the ions. Using this architecture, all of the rudiments of quantum computing have been demonstrated with trapped ions, and this lecture will concentrate on those fundamental interactions, including recent demosntrations of simple quantum algorithms and quantum simulations. The central challenge now is how to scale the system to larger numbers of qubits, and there are several promising directions that may allow the complete control of hundreds or more trapped ions.

*Christopher Monroe*

**Trapped Ions Photonic Networks*** *The local manipulation and entanglement of nearby atomic ion qubits through their Coulomb interaction is now established as one of the most reliable ways to build entangled states. Trapped ions can also be coupled through a photonic channel, allowing for various remote probabilistic ion-ion entanglement protocols. Recent experiments have shown entanglement, a Bell inequality violation, teleportation, and operation of a two-qubit quantum gate between two ions separated by 1 meter. Despite the probabilistic nature of this ion/photon network, it can be efficiently scaled to much larger numbers of ions for distributed large-scale quantum computing and long-distance quantum communication, especially when accompanied by local Coulomb-mediated deterministic quantum gates. Future work will couple photons emitted from trapped ions into optical cavities, and perhaps interface trapped ion qubits with other optically-active qubits such as quantum dots.

*Louis Salvail*

**On the Power of Two-Party Quantum Cryptography**

We study quantum protocols among two distrustful parties. Under the sole assumption of correctness---guaranteeing that honest players obtain their correct outcomes---we show that every protocol implementing a non-trivial primitive necessarily leaks information to a dishonest player. This extends known impossibility results to all non-trivial primitives. We provide a framework for quantifying this leakage and argue that leakage is a good measure for the privacy provided to the players by a given protocol. Our framework also covers the case where the two players are helped by a trusted third party. We show that despite the help of a trusted third party, the players cannot amplify the cryptographic power of any primitive. All our results hold even against quantum honest-but-curious adversaries who honestly follow the protocol but purify their actions and apply a different measurement at the end of the protocol. As concrete examples, we establish lower bounds on the leakage of standard universal two-party primitives such as oblivious transfer.

*Nicolas Godbout*

**Flying qubits: photons as carriers and optical fibres as pipes **The capabilities of photons for quantum information communications and processing are reviewed. The emphasis is placed on the use of optical fibres as the transmission medium. Optical fibres are shown to provide more than passive light pipes. Different encoding techniques, fibre systems that can convert between encodings and universal one-qubit gates are presented. Recent results on the generation of entangled photons and heralded single photons are presented. The prospect of photons as a qubit for quantum processing is finally discussed.