2010 International Workshop
on Quantum Information Science
Organized by JST ERATO-SORST QCI Project, in cooperation with the University of Tokyo.

The Workshop had Successfully Ended

Our thanks go to the speakers and the attendees.

The pictures taken during the workshop are uploaded. We apologize that there is no picture for the first session.

Date and Place

Date: March 8th, 2010.
Time: 9:00-19:10

Annex of Ichijo Hall, in the Faculty of Agriculture. The University of Tokyo, Hongo Campus.

Yayoi Auditorium, Annex (Seihoku Gallery) Link to the map of the Yayoi Auditorium.
Please be aware that the annex is located on the other side of the gate (left side on the map).

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Time Table

09:00 - 09:10 Opening Remark
09:10 - 10:50 Session I: Experiments (chair: Dr. Akihisa Tomita)
09:10 - 09:40 Dr. Mile Gu Sharpening Occam's Razor with Quantum Mechanics
09:40 - 10:10 Dr. Hidehiro Yonezawa Adaptive phase measurements of phase-squeezed states
10:10 - 10:30 Dr. Jun-ichi Yoshikawa Experimental demonstrations of Gaussian operations via teleportation-like schemes
10:30 - 10:50 Hugo Benichi Quantum Teleportation of non-Gaussian states of light
10:50 - 11:00Break
11:00 - 13:00 Session II: Experiments & Information (chair: Dr. Francesco Buscemi)
11:00 - 11:30 Dr. Kenji Tsujino Sub-shot-noise-limit discrimination of on-off keyed coherent signals via a quantum receiver with a superconducting transition edge sensor
11:30 - 12:00 Dr. Jun Suzuki Entanglement detection for interference fringes in atom-photon systems
12:00 - 12:30 Dr. Shengmei Zhao A Class of Quantum LDPC Codes Constructed From Cyclic Difference Set
12:30 - 13:00 Dr. Min-Hsiu Hsieh Entanglement-assisted quantum error correction
13:00 - 14:20Lunch Break
14:20 - 15:50 Session III: Information (chair: Dr. Min-Hsiu Hsieh)
14:20 - 15:10 Dr. Francesco Buscemi Private Quantum Decoupling and Secure Disposal of Information
15:10 - 15:50 Dr. Mark Wilde Additivity in quantum Shannon theory
15:50 - 16:00Break
16:00 - 17:50 Session VI: Computation (chair: Dr. Mark Wilde)
16:00 - 16:40 Dr. Kae Nemoto Architecture and system design for quantum computer
16:40 - 17:20 Dr. Takeshi Koshiba On the computational power of BB84 states
17:20 - 17:50 Dr. Kai-Yuen Cheong On quantum based oblivious transfer
17:50 - 18:00Break
18:00 - 19:10 Session V: Cryptography (chair: Dr. Min-Hsiu Hsieh)
18:00 - 18:40 Dr. Harumichi Nishimura Quantum Counterfeit Coin Problems
18:40 - 19:10 Dr. Josef Sprojcar Untraceable quantum ballots?
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Session I: Experiments

Dr. Mile Gu, The National University of Singapore, Singapore.
Title: Sharpening Occam's Razor with Quantum Mechanics
Abstract: Much of science involves the construction of mathematical models that make predictions about the future, based on relevant information collected from the past. In the spirit of Occam's razor, simpler is often better; should two predictive models equally simulate the future, the one that requires less information from the past is preferred. For a large class of stochastic processes, we show that by encoding possible pasts into non-orthogonal quantum states, we can construct predictive models that require less information about the past than any possible classical approach. This indicates that to construct a device of minimal entropy that perfectly replicates the behavior of such systems, quantum dynamics is a necessity. These results imply that certain phenomena could be significantly simpler than classically possible, should quantum effects be involved.
Dr. Hidehiro Yonezawa, Lecturer, Department of Applied Physics, University of Tokyo, Japan.
(Joint work with Daisuke Nakane, Shuntaro Takeda, Hajime Arao, Trevor Wheatley, Howard Wiseman, Elanor Huntington, and Akira Furusawa)
Title: Adaptive phase measurements of phase-squeezed states
Abstract: Optical phase measurement is of great importance in various fields, e.g., optical communication and gravitational wave detection. Precise phase measurement, however, is not an easy task due to the difficulty of direct phase measurement. Conventional methods of phase measurement, that is, heterodyne or dual homodyne measurements, are simultaneous measurements of conjugate variables (quadrature-phase amplitudes of the field) in which extra noises from the measurement back-action are inevitably added. Adaptive phase measurement is an alternative way to measure phase in which single homodyne measurement and feedback loop are used. Adaptive homodyne measurement enables better phase estimate than the conventional methods. We experimentally demonstrate adaptive phase measurement for a stochastically varying phase on continuous-wave phase-squeezed beam. Here we use phase-squeezed beam to achieve further improvement of the phase estimate. In this presentation we will present these experimental results.
Dr. Jun-ichi Yoshikawa, Department of Applied Physics, University of Tokyo, Japan.
(Joint work with Yoshichika Miwa, Peter van Loock, Radim Filip, and Akira Furusawa)
Title: Experimental demonstrations of Gaussian operations via teleportation-like schemes
Abstract: In optical realization of continuous-variable (CV) quantum teleportation, classical communications of Homodyne measurement outcomes enable transportation of unknown quantum states. Nonclassicality of this procedure comes from the entanglement in ancillas which are shared by the sender and the receiver. Seen from the standpoint of CV quantum information processing, a quantum teleportation is an identity operation. Using the same technique, we demonstrate several nontrivial Gaussian operations, i.e. squeezing operation, quantum nondemolition interaction, etc. Nonclassicality of squeezed vacuum states used as ancillas is converted to the nonclassicality of the processing, via feedforward of Homodyne measurements. The input states that are assumed to be unknown are transformed unitarily up to small excess noise, which is analogous to CV teleportation.
Hugo Benichi, Department of Applied Physics, University of Tokyo, Japan.
(Joint work with Noriyuki Lee, and Akira Furusawa)
Title: Quantum Teleportation of non-Gaussian states of light
Abstract: We report the experimental realization of continuous variable quantum teleportation of a non-Gaussian state of light. The input state similar to a "squeezed photon" is generated by photon-subtraction using the degenerate central modes of an OPO. The OPO cavity bandwidth (7.5 MHz) defines the temporal characteristics of our non-Gaussian state which is a 160 ns long wavepacket of light. Quantum state tomography reveals strong non-classical features for this state and its experimental Wigner function in the origin (negativity) is -0.152, for a density matrix purity of 0.59.
Our experimental broadband teleporter follows the canonical cv teleportation scheme and operates on a 10 MHz bandwidth to teleport all frequency components of the input wavepacket, with -4.5dB equivalent squeezing in the relevant temporal mode. We benchmark our setup using the fidelity F and measure F=0.74 for vacuum input, above the no-cloning limit 2/3. Although the output state for non-Gaussian input does not show negativity yet, we are currently approaching this goal by 1) working on input state negativity 2) increasing the effective squeezing parameter.

Session II: Experiments & Information

Dr. Kenji Tsujino, Researcher, ERATO-SORST project, JST, Japan.
Title: Sub-shot-noise-limit discrimination of on-off keyed coherent signals via a quantum receiver with a superconducting transition edge sensor
Dr. Tsujino Abstract: We demonstrate a sub-shot-noise-limit discrimination of on-off keyed coherent signals by an optimal displacement quantum receiver in which a superconducting transition edge sensor is installed. Use of a transition edge sensor and a fiber beam splitter realizes high total detection efficiency and high interference visibility of the receiver and the observed average error surpasses the shot-noise-limit in a wider range of the signal power. Our technique opens up a new technology for the sub-shot-noise limit detection of coherent signals in optical communication channels.
Dr. Jun Suzuki, Postdoctoral Research Fellow, NII, Japan. (Joint work with Christian Miniatura, and Kae Nemoto)
Title: Entanglement detection for interference fringes in atom-photon systems
Dr. Suzuki Abstract: A measurement scheme of atomic qubits pinned at given positions is studied by analyzing the interference pattern obtained when they emit photons spontaneously. In the case of two qubits, a well-known relation is revisited, in which the interference visibility is equal to the concurrence of the state in the infinite spatial separation limit of the qubits. By taking into account the super-radiant and sub-radiant effects, it is shown that a state tomography is possible when the qubit spatial separation is comparable to the wavelength of the atomic transition. In the case of three qubits, the relations between various entanglement measures and the interference visibility are studied, where the visibility is defined from the two-qubit case. A qualitative correspondence among these entanglement relations is discussed. In particular, it is shown that the interference visibility is directly related to the maximal bipartite negativity.
Dr. Shengmei Zhao, Professor, Institute of Signal Processing & Transmission, Nanjing University of Posts & Telecommunications, China.
Title: A Class of Quantum LDPC Codes Constructed From Cyclic Difference Set
Dr. Zhao Abstract: Low-density parity check (LDPC) codes are a significant class of classical codes with many applications. Several good construction methods have been proposed for their quantum counterparts, normally they are based on CSS codes which need to construct self-containing classical block codes. Considering the cyclic difference set codes can be decoded with message passing algorithm, we propose a novel method, based on Stabilizer code, to construct quantum low density parity check code by extending the concept of cyclic difference set in this paper. We give the detail construction steps and prove the codes constructed by this method have commutativity property. This method has high code rate and less constraint. It is also surprising we can construct the famous [5,1,3] stabilizer code by this method. At last, we present some examples of quantum stabilizer codes by our construction method.
Dr. Min-Hsiu Hsieh, Researcher, ERATO-SORST, JST, JAPAN.
Title: Entanglement-assisted quantum error correction
Dr. Hsieh Abstract: Entanglement-assisted quantum error-correcting codes (EAQECCs) make use of pre-existing en-tanglement between the sender and receiver to boost the rate of transmission. It is possible to construct an EAQECC from any classical linear code, unlike standard QECCs which can only be constructed from dual-containing codes. Operator quantum error-correcting codes (OQECCs) allow certain errors to be corrected (or prevented) passively, reducing the complexity of the correction procedure. We combine these two extensions of standard quantum error correction into unified entanglement-assisted quantum error correction formalism. This new scheme, which we call entanglement-assisted operator quantum error correction (EAOQEC), is the most general and powerful quantum error-correcting technique known, retaining the advantages of both entanglement-assistance and passive correction. We present the formalism, show the considerable freedom in constructing EAOQECCs from classical codes, and demonstrate the construction with examples.

Session III: Information

Dr. Francesco Buscemi, Associate Professor, Institute for Advanced Research, Nagoya University, Japan.
Title: Private Quantum Decoupling and Secure Disposal of Information
Dr. Buscemi Abstract: Given a bipartite system, correlations between its subsystems can be understood as the information that each one carries about the other. In order to give a model-independent description of secure information disposal, I propose here the paradigm of private quantum decoupling, corresponding to locally reducing correlations in a given bipartite quantum state without transferring them to the environment. In this framework, the concept of private local randomness naturally arises as a resource, and total correlations are divided into eliminable and ineliminable ones. I will prove upper and lower bounds on the quantity of ineliminable correlations present in an arbitrary bipartite state, and show that, in tripartite pure states, ineliminable correlations satisfy a monogamy constraint, making apparent their quantum nature. A relation with entanglement theory is provided by showing that ineliminable correlations constitute an entanglement parameter. In the limit of infinitely many copies of the initial state provided, I compute the regularized ineliminable correlations to be measured by the coherent information, which is thus equipped with a new operational interpretation. Results, in particular, imply that two subsystems can be privately decoupled if their joint state is separable.
Dr. Mark Wilde, Postdoctoral Fellow, Quantum Computing Group, School of Computer Science, McGill University, Canada.
Title: Additivity in quantum Shannon theory
Dr. Wilde Abstract: A noisy quantum channel has many different capacities that characterize its ability to transmit information. These capacities correspond to different operational tasks, such as the transmission of classical information or quantum information with or without the assistance of shared entanglement. Additivity of a channel's capacity is equivalent to a complete understanding of the channel because it implies that its calculation is a tractable optimization problem. The only capacity that we can really claim to understand at this point is the mutual information of a quantum channel because the formula for the capacity is additive for all quantum channels. All suggested formulas for other capacities at this point are additive only for a handful of channels or there exist known counterexamples that violate additivity of the given formulas. In this tutorial overview, I survey the landscape of additivity, covering the different capacities of a quantum channel, reviewing classes of channels for which additivity of a particular capacity formula holds, and mentioning the counterexamples for which additivity does not hold.

Session VI: Computation

Dr. Kae Nemoto, Associate Professor, NII (National Institute of Informatics), Japan.
(Joint work with W. J. Munro, Simon Devitt, Ashley Stephens, Chun Hsu Su, Andrew Greentree)
Title: Architecture and system design for quantum computer
Dr. Nemoto Abstract: Qubus computation is a type of quantum information processing (QIP) where computational qubits couple through a quantum bus (qubus). Such computation is flexible in terms of physical systems, properties and it scales exceptionally well. In this talk we introduce a number of new designs for qubus-type quantum devices and discuss system designs and architecture for scalable large-scale quantum computer.
Dr. Takeshi Koshiba, Associate Professor, Department of Information and Computer Sciences School of Engineering, Saitama University, Japan.
Title: On the computational power of BB84 states
Dr. Koshiba Abstract: BB84 states play an essential role in BB84 quantum key distribution protocol. We show that BB84 states are a powerful tool even to construct quantum cryptography other than QKD. In classical cryptography, interactive hashing (and interactive hashing theorem) is one of powerful tools to construct classical cryptographic protocols. We will see that BB84 states act as the interactive hashing. BB84 states are usually used in the non-interactive fashion. This is beneficial to construct round-efficient quantum cryptographic protocols.
Dr. Kai-Yuen Cheong, Researcher, ERATO-SORST project, JST, Japan.
Title: On quantum based oblivious transfer
Dr. Cheong Abstract: Due to the famous impossibility results, unconditional security is seen as impossible for oblivious transfer in the quantum world. In this paper, we try to overcome the known impossibility results by proposing a protocol which is not perfectly secure, but is unconditionally secure in practical sense. The relation to the impossibility results is discussed. Other advantages of our protocol include the fact that the honest players do not need quantum memory and entanglement.

Session V: Cryptography

Dr. Harumichi Nishimura, Department of Mathematics and Information Sciences, Graduate School of Science, Osaka Prefecture University, Japan.
(Joint work with Kazuo Iwama, Rudy Raymond, and Junichi Teruyama)
Title: Quantum Counterfeit Coin Problems
Dr. Nishimua Abstract: The counterfeit coin problem requires us to find all false coins from a given bunch of coins using a balance scale. We assume that the balance scale gives us only ``balanced'' or ``tilted'' information and that we know the number $k$ of false coins in advance. The balance scale can be modeled by a certain type of oracle and its query complexity is a measure for the cost of weighing algorithms (the number of weightings). In this paper, we study the quantum query complexity for this problem. Let $Q(k,N)$ be the quantum query complexity of finding all $k$ false coins from the $N$ given coins.
We show that for any $k$ and $N$ such that $k < N/2$, $Q(k,N)=O(k^{1/4})$, contrasting with the classical query complexity, $\Omega(k\log(N/k))$, that depends on $N$. So our quantum algorithm achieves a {\it quartic} speed-up for a natural problem.
Dr. Josef Sprojcar, Researcher, ERATO-SORST project, JST, Japan.
Title: Untraceable quantum ballots?
Dr. Sprojcar Abstract: We start by presenting the cryptographic task of secret ballot elections in its most general way. We concentrate on four of its security properties and after their brief analysis, we show how they impose restrictions on honest party's behavior. These restrictions turned out to be so strong that a no-go theorem can be derived from them. This no-go theorem practically forbids the usage of quantum states for implementing ballots (and taking advantage of quantum no-cloning theorem) as a way of preventing ballot copying.

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