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26:14 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

A quantum key distribution system immune to detector attacks

Quantum cryptography promises the distribution of cryptographic keys secured by fundamental laws of quantum physics. However, results in quantum hacking have demonstrated that the information theoretic security of quantum cryptography protocols does not guarantee security for actual implementations. Most notable are attacks against the vulnerabilities of single photon detectors [1-4]. In this talk we will report the first proof-of-principle demonstration of a new protocol that removes the threat of any such attack [5]. We demonstrated the protocol over 80 km of spooled fibre as well as across different locations within the city of Calgary [6], confirming this protocol as a realistic approach to secure communication and demonstrating the possibility for controlled two-photon interference in a real-world environment, which is a remaining obstacle to realizing quantum repeaters and quantum networks. [1] Lamas-Linares, A., Kurtsiefer, C. Breaking a quantum key distribution system through a timing side channel, Opt. Express 15 (15), 9388-9393 (2007). [2] Zhao, Y., Fung, C.-H. F., Qi, B., Chen, C. & Lo, H.-K. Quantum Hacking: Experimental demonstration of time-shift attack against practical quantum key distribution systems. Phys. Rev. A 78, 042333 (2008). [3] Lydersen, L., Wiechers, C., Wittmann, C., Elser, D., Skaar, J. & Makarov, V. Hacking commercial quantum cryptography systems by tailored bright illumination. Nature Photonics 4, 686–689 (2010). [4] Lydersen, L., Wiechers, C., Wittmann, C., Elser, D., Skaar, J. & Makarov, V. Thermal blinding of gated detectors in quantum cryptography. Opt. Express 18 (26), 27938-27954 (2010). [5] Lo, H.-K., Curty, M. & Qi, B. Measurement-device-independent quantum key distribution. Phys. Rev. Lett. 108, 130503 (2012). [6] Rubenok, A., Slater, J. A., Chan, P., Lucio-Martinez, I., & Tittel, W. Proof-of-principle field test of quantum key distribution immune to detector attacks. arXiv:1204.0738v1 (2012).
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
35:34 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

1 Mbps coherent one-way QKD with dense wavelength division multiplexing and hardware key distillation

We present the latest results obtained with a quantum cryptography prototype based on a coherent-one way quantum key distribution (QKD) scheme. To support its continuous high rate secret key generation we developed different low-noise single photon detectors for telecom wavelength based on a sine gating and low-pass-filtering technique, as well as a negative feedback APD in an active hold-off circuit. A newly developed hardware distillation engine allows for continuous operation of secret key distribution up to 1 Mbps. We also present results of our system in a DWDM (dense wavelength-division multiplexing) configuration where only one single fiber is needed to interconnect Alice' and Bob's systems. The final prototype is fully compatible to serve a high-speed encryption device developed in parallel which provides encrypted communication of up to 100 Gbps.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
19:53 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

A min-entropy uncertainty relation for finite size cryptography

Apart from their foundational signicance, entropic uncertainty relations play a central role in proving the security of quantum cryptographic protocols. Of particular interest are thereby relations in terms of the smooth min-entropy for BB84 and six-state encodings. Previously, strong uncertainty relations were obtained which are valid in the limit of large block lengths. Here, we prove a new uncertainty relation in terms of the smooth min-entropy that is only marginally less strong, but has the crucial property that it can be applied to rather small block lengths. This paves the way for a practical implementation of many cryptographic protocols. As part of our proof we show tight uncertainty relations for a family of Renyi entropies that may be of independent interest.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
25:13 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Continuous variable quantum key distribution: finite-key analysis of composable security against coherent attacks

We provide a security analysis for continuous variable quantum key distribution protocols based on the transmission of two-mode squeezed vacuum states measured via homodyne detection. We employ a version of the entropic uncertainty relation for smooth entropies to give a lower bound on the number of secret bits which can be extracted from a finite number of runs of the protocol. This bound is valid under general coherent attacks, and gives rise to keys which are composably secure. For comparison, we also give a lower bound valid under the assumption of collective attacks. For both scenarios, we find positive key rates using experimental parameters reachable today.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
22:40 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Experimental demonstration of continuous-variable quantum key distribution over 80 km of standard telecom fiber

We demonstrate for the first time that long-distance quantum key distribution can be achieved with continuous variables, using only standard telecommunication components. Furthermore, we obtain a positive secret key rate over long distances even when taking into account finite-size effects. These results correspond to a practical implementation guaranteeing the strongest level of security achievable with QKD and show that continuous-variable quantum key distribution is a technology of choice for near-future secure quantum communications.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
40:40 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Quantum Steering: Experiments and Applications

Quantum steering allows two parties to verify shared entanglement even if one measurement device is untrusted, as well as convincing an unbelieving party of the existence of entanglement. I will discuss quantum steering in the contexts of recent experiments [1,2,3] steering the polarization degree of freedom for single photons. The historical context as well as modern motivation for steering will be covered, as well as the similarities and differences in the various recent experiments. Our own work [1], demonstrating quantum steering with high efficiency (62%) in two measurement bases, will be discussed in detail, including the technical challenges in certifying the results due to measurement imperfections of various types. We ultimately demonstrate a violation of some 48 standard deviations of the steering inequality most relevant to applications, which also happens to be the one most difficult to violate. The efficiency demonstrated in this experiment (62%) is half again as high as the previous world record for detection efficiency for an experiment in this context. I will conclude with our current research project – to implement semi-device-independent quantum key distribution [4], with security guaranteed by a steering inequality. This lies in the gap between current QKD implementations and the ultimate security given by device-independent QKD, and, in practical situations, requires a detection efficiency some 10% higher again than our previous result, which should be achievable given the advances made since that result was published. 1. DH Smith, G Gillett et al., Nat. Commun. 3:625 (2012) 2. AJ Bennet et al., Phys. Rev. X 2, 031003 (2012) 3. B Wittmann, S Ramelow et al., New J. Phys. 14, 053030 (2012) 4. C Branciard et al., Phys. Rev. A 85, 010301(R) (2012)
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
1:32:56 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Focus Tutorial: Cryptographic primitives

The Annual Conference on Quantum Cryptography (QCRYPT) is a conference for students and researchers working on all aspects of quantum cryptography. This includes theoretical and experimental research on the possibilities and limitations of secure communication and computation with quantum mechanical devices or in the presence of quantum mechanical devices. (The conference includes but is not limited to research on quantum key distribution.) It is the goal of the conference to represent the previous year's best results on quantum cryptography and to support the building of a research community in quantum cryptography. In order to achieve this goal, the conference will feature both invited and contributed talks, selected by the steering committee and programme committee, respectively. In line with this goal, the conference has no published proceedings. QCRYPT welcomes the submission of any interesting and important result, while allowing researchers from a wide range of disciplines to pursue publication in any appropriate venue.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
36:52 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Solid-state quantum memories for quantum repeaters

The maximal transmission distance of optical quantum communication is reaching a hard limit imposed by the intrinsic loss of the transmission medium, e.g. optical fibre. A quantum repeater promises to push that limit towards much longer, potentially intercontinental distances. Its implementation relies on the development of efficient and long-lived quantum memories that can store and retrieve the quantum properties of light. Sources of photonic entanglement, tailored for quantum memories, are also necessary and represent a challenging experimental task. I will review the efforts of our group towards the realization of quantum memories based on rare-earth-ion doped crystals (REIC) as well as a matching source of photon pair. This approach has recently allowed us to demonstrate several features that are of great importance for quantum repeaters, and for quantum networks in general. After a brief introduction, I will show how we have successfully entangled two neodymium-doped crystals in a heralded fashion. I will then show how polarization qubits encoded in true single photons can be stored in such crystals, despite their intrinsic birefringence and polarization-dependant absorption. I will finally present an on-demand quantum memory exploiting the long hyperfine coherence time of europium ions to store light for up to 8 ms. Our results highlight the great potential of REIC for quantum repeaters.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
59:41 Eidgenössische Technische Hochschule (ETH) Zürich English 2011

Composition in Quantum Cryptography

Quantum cryptography aims to achieve security from fundamental physical principles, such as the quantum mechanical phenomena of entanglement and Heisenberg's uncertainty principle. In the last few years significant progress has been made in the theoretical understanding of quantum cryptography and its technological feasibility has been demonstrated experimentally. Quantum cryptography is therefore regarded as one of the most promising candidates for a future quantum technology. -- With QCRYPT we initiate a new conference series on Quantum Cryptography. The first conference in this series will be held at ETH Zurich and the second at CQT in Singapore. The aim of this series is to bring together researchers working on all aspects of the subject (both on the theoretical and experimental sides) and to support the building of a research community in Quantum Cryptography.
  • Published: 2011
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
23:15 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Secure bit commitment from relativistic constraints

We investigate two-party cryptographic protocols that are secure under assumptions motivated by physics, namely relativistic assumptions (no-signalling) and quantum mechanics. In particular, we discuss split models, i.e. models in which certain parties are not allowed to communicate during certain phases of the protocol, for the purpose of bit commitment. We find the minimal splits that are necessary to evade the Mayers-Lo-Chau no-go argument and present protocols that achieve security in these split models. Furthermore, we introduce the notion of local versus global commands, a subtle issue that arises when the split committer is required to delegate agents to perform the open phase separately, without communication. We argue that classical protocols are insecure in the global command model, even when the committer is split. On the other hand, we provide a rigorous security proof in the global command model for a quantum protocol proposed by Kent. The proof employs two fundamental principles of modern physics, the no-signalling property of relativity and the uncertainty principle of quantum mechanics.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
23:04 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Superposition attacks on cryptographic protocols

Attacks on cryptographic protocols are usually modeled by allowing an adversary to ask queries to an oracle. Security is then defined by requiring that as long as the queries satisfy some constraint, there is some problem the adversary cannot solve, such as compute a certain piece of information. Even if the protocol is quantum, the queries are typically classical, such as a choice of subset of players to corrupt. In this paper, we introduce a fundamentally new model of quantum attacks on protocols, where the adversary is allowed to ask several classical queries in quantum superposition. This is a strictly stronger attack than the standard one, and we consider the security of several primitives in this model. We show that a secret-sharing scheme that is secure with threshold t in the standard model is secure against superposition attacks if and only if the threshold is lowered to t/2. This holds for all classical as well as a large class of quantum secret sharing schemes. We then consider zero-knowledge and first show that known protocols are not, in general, secure in our model by designing a superposition attack on the well-known zero-knowledge protocol for graph isomorphism. We then use our secret-sharing result to design zero-knowledge proofs for all of NP in the common reference string model. While our protocol is classical, it is sound against a cheating unbounded quantum prover and computational zero-knowledge even if the verifier is allowed a superposition attack. Finally, we consider multiparty computation and give a characterization of a class of protocols that can be shown secure, though not necessarily with efficient simulation. We show that this class contains non-trivial protocols that cannot be shown secure by running a classical simulator in superposition.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
28:28 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Security of continuous-variable quantum key distribution against general attacks

We prove that Gaussian continuous-variable quantum key distribution protocols, using a Gaussian distribution of coherent or squeezed states and homodyne or heterodyne measurement, are secure against arbitrary attacks. Our proof exploits the specific symmetries in phase-space of Gaussian QKD protocols to prove that once a simple test over the measurement outcomes succeeds, the global state shared between Alice and Bob is well decribed by assigning a low dimensional Hilbert space to each mode. Then one can use the postselection technique introduced by Christandl, Koenig and Renner for discrete-variable protocols to conclude. Our result greatly improves over previous ones using either a de Finetti theorem or an entropic uncertainty principle which could not be applied to prove the security of protocols in realistic experimental implementations.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
29:18 Eidgenössische Technische Hochschule (ETH) Zürich English 2011

An ultrafast quantum random number generator based on quantum phase fluctuations

Quantum cryptography aims to achieve security from fundamental physical principles, such as the quantum mechanical phenomena of entanglement and Heisenberg's uncertainty principle. In the last few years significant progress has been made in the theoretical understanding of quantum cryptography and its technological feasibility has been demonstrated experimentally. Quantum cryptography is therefore regarded as one of the most promising candidates for a future quantum technology. -- With QCRYPT we initiate a new conference series on Quantum Cryptography. The first conference in this series will be held at ETH Zurich and the second at CQT in Singapore. The aim of this series is to bring together researchers working on all aspects of the subject (both on the theoretical and experimental sides) and to support the building of a research community in Quantum Cryptography.
  • Published: 2011
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
21:29 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Quantum to classical randomness extractors

The goal of randomness extraction is to distill (almost) perfect randomness from a weak source of randomness. When the source yields a classical string X, many extractor constructions are known. Yet, when considering a physical randomness source, X is itself ultimately the result of a measurement on an underlying quantum system. When characterizing the power of a source to supply randomness it is hence a natural question to ask, how much classical randomness we can extract from a quantum system. To tackle this question we here take on the study of quantum to classical randomness extractors (QC-extractors). We provide constructions of QC-extractors based on measurements in a full set of mutually unbiased bases (MUBs), and certain single qubit measurements. As the first application, we show that any QC-extractor gives rise to entropic uncertainty relations with respect to quantum side information. Such relations were previously only known for two measurements. As the second application, we resolve the central open question in the noisy-storage model [Wehner et al., PRL 100, 220502 (2008)] by linking security to the quantum capacity of the adversary’s storage device.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
30:28 Eidgenössische Technische Hochschule (ETH) Zürich English 2011

Fast coherent-one way quantum key distribution and high-speed encryption

Quantum cryptography aims to achieve security from fundamental physical principles, such as the quantum mechanical phenomena of entanglement and Heisenberg's uncertainty principle. In the last few years significant progress has been made in the theoretical understanding of quantum cryptography and its technological feasibility has been demonstrated experimentally. Quantum cryptography is therefore regarded as one of the most promising candidates for a future quantum technology. -- With QCRYPT we initiate a new conference series on Quantum Cryptography. The first conference in this series will be held at ETH Zurich and the second at CQT in Singapore. The aim of this series is to bring together researchers working on all aspects of the subject (both on the theoretical and experimental sides) and to support the building of a research community in Quantum Cryptography.
  • Published: 2011
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
33:55 Eidgenössische Technische Hochschule (ETH) Zürich English 2011

Quantum Computing on Encrypted Data

Quantum cryptography aims to achieve security from fundamental physical principles, such as the quantum mechanical phenomena of entanglement and Heisenberg's uncertainty principle. In the last few years significant progress has been made in the theoretical understanding of quantum cryptography and its technological feasibility has been demonstrated experimentally. Quantum cryptography is therefore regarded as one of the most promising candidates for a future quantum technology. -- With QCRYPT we initiate a new conference series on Quantum Cryptography. The first conference in this series will be held at ETH Zurich and the second at CQT in Singapore. The aim of this series is to bring together researchers working on all aspects of the subject (both on the theoretical and experimental sides) and to support the building of a research community in Quantum Cryptography.
  • Published: 2011
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
50:50 Eidgenössische Technische Hochschule (ETH) Zürich English 2011

Public Quantum Communication

Quantum cryptography aims to achieve security from fundamental physical principles, such as the quantum mechanical phenomena of entanglement and Heisenberg's uncertainty principle. In the last few years significant progress has been made in the theoretical understanding of quantum cryptography and its technological feasibility has been demonstrated experimentally. Quantum cryptography is therefore regarded as one of the most promising candidates for a future quantum technology. -- With QCRYPT we initiate a new conference series on Quantum Cryptography. The first conference in this series will be held at ETH Zurich and the second at CQT in Singapore. The aim of this series is to bring together researchers working on all aspects of the subject (both on the theoretical and experimental sides) and to support the building of a research community in Quantum Cryptography.
  • Published: 2011
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
1:01:29 Eidgenössische Technische Hochschule (ETH) Zürich English 2011

Wavelength Division Multiplexing Quantum Key Distribution (WDM-QKD) with high throughput key distillation engine

Quantum cryptography aims to achieve security from fundamental physical principles, such as the quantum mechanical phenomena of entanglement and Heisenberg's uncertainty principle. In the last few years significant progress has been made in the theoretical understanding of quantum cryptography and its technological feasibility has been demonstrated experimentally. Quantum cryptography is therefore regarded as one of the most promising candidates for a future quantum technology. -- With QCRYPT we initiate a new conference series on Quantum Cryptography. The first conference in this series will be held at ETH Zurich and the second at CQT in Singapore. The aim of this series is to bring together researchers working on all aspects of the subject (both on the theoretical and experimental sides) and to support the building of a research community in Quantum Cryptography.
  • Published: 2011
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
29:48 Eidgenössische Technische Hochschule (ETH) Zürich English 2011

Semi-device-independent security of one-way quantum key distribution (QKD)

Quantum cryptography aims to achieve security from fundamental physical principles, such as the quantum mechanical phenomena of entanglement and Heisenberg's uncertainty principle. In the last few years significant progress has been made in the theoretical understanding of quantum cryptography and its technological feasibility has been demonstrated experimentally. Quantum cryptography is therefore regarded as one of the most promising candidates for a future quantum technology. -- With QCRYPT we initiate a new conference series on Quantum Cryptography. The first conference in this series will be held at ETH Zurich and the second at CQT in Singapore. The aim of this series is to bring together researchers working on all aspects of the subject (both on the theoretical and experimental sides) and to support the building of a research community in Quantum Cryptography.
  • Published: 2011
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
31:34 Eidgenössische Technische Hochschule (ETH) Zürich English 2011

Universal composable security of quantum message authentication with key recycling

Quantum cryptography aims to achieve security from fundamental physical principles, such as the quantum mechanical phenomena of entanglement and Heisenberg's uncertainty principle. In the last few years significant progress has been made in the theoretical understanding of quantum cryptography and its technological feasibility has been demonstrated experimentally. Quantum cryptography is therefore regarded as one of the most promising candidates for a future quantum technology. -- With QCRYPT we initiate a new conference series on Quantum Cryptography. The first conference in this series will be held at ETH Zurich and the second at CQT in Singapore. The aim of this series is to bring together researchers working on all aspects of the subject (both on the theoretical and experimental sides) and to support the building of a research community in Quantum Cryptography.
  • Published: 2011
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
24:11 Eidgenössische Technische Hochschule (ETH) Zürich English 2011

The Garden-Hose Game and Application to Position-Based Quantum Cryptography

Quantum cryptography aims to achieve security from fundamental physical principles, such as the quantum mechanical phenomena of entanglement and Heisenberg's uncertainty principle. In the last few years significant progress has been made in the theoretical understanding of quantum cryptography and its technological feasibility has been demonstrated experimentally. Quantum cryptography is therefore regarded as one of the most promising candidates for a future quantum technology. -- With QCRYPT we initiate a new conference series on Quantum Cryptography. The first conference in this series will be held at ETH Zurich and the second at CQT in Singapore. The aim of this series is to bring together researchers working on all aspects of the subject (both on the theoretical and experimental sides) and to support the building of a research community in Quantum Cryptography.
  • Published: 2011
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
53:17 Eidgenössische Technische Hochschule (ETH) Zürich English 2011

Simplified instantaneous non-local quantum computation with applications to position-based cryptography

Quantum cryptography aims to achieve security from fundamental physical principles, such as the quantum mechanical phenomena of entanglement and Heisenberg's uncertainty principle. In the last few years significant progress has been made in the theoretical understanding of quantum cryptography and its technological feasibility has been demonstrated experimentally. Quantum cryptography is therefore regarded as one of the most promising candidates for a future quantum technology. -- With QCRYPT we initiate a new conference series on Quantum Cryptography. The first conference in this series will be held at ETH Zurich and the second at CQT in Singapore. The aim of this series is to bring together researchers working on all aspects of the subject (both on the theoretical and experimental sides) and to support the building of a research community in Quantum Cryptography.
  • Published: 2011
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
50:16 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Quantum cryptography in Minkowski space

Quantum theory and the relativistic no-signalling principle both give ways of controlling information, in the sense that someone who creates information somewhere in space-time can rely on strict limits both on how much information another party can extract and on where they can obtain it. An increasingly long list of interesting cryptographic applications exploit the power of the no-signalling principle as well as the properties of quantum information. I describe recent work in this area, including secure protocols for bit commitment, quantum tagging (quantum position authentication) and new intrinsically relativistic cryptographic tasks.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
22:48 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Quantum key distribution in the classical authenticated key exchange framework

Key establishment is a crucial primitive for building secure channels: in a multi-party setting, it allows two parties using only public authenticated communication to establish a secret session key which can be used to encrypt messages. But if the session key is compromised, the confidentiality of encrypted messages is typically compromised as well. Without quantum mechanics, key establishment can only be done under the assumption that some computational problem is hard. Since digital communication can be easily eavesdropped and recorded, it is important to consider the secrecy of information anticipating future algorithmic and computational discoveries which could break the secrecy of past keys, violating the secrecy of the confidential channel. Quantum key distribution (QKD) can be used generate secret keys that are secure against any future algorithmic or computational improvements. QKD protocols still require authentication of classical communication, however, which is most easily achieved using computationally secure digital signature schemes. It is generally considered folklore that QKD when used with computationally secure authentication is still secure against an unbounded adversary, provided the adversary did not break the authentication during the run of the protocol. We describe a security model for quantum key distribution based on traditional classical authenticated key exchange (AKE) security models. Using our model, we characterize the long-term security of the BB84 QKD protocol with computationally secure authentication against an eventually unbounded adversary. By basing our model on traditional AKE models, we can more readily compare the relative merits of various forms of QKD and existing classical AKE protocols. This comparison illustrates in which types of adversarial environments different quantum and classical key agreement protocols can be secure.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
1:03:11 Eidgenössische Technische Hochschule (ETH) Zürich English 2011

Quantum Memories for Quantum Networks and Device-Independent QKD

Quantum cryptography aims to achieve security from fundamental physical principles, such as the quantum mechanical phenomena of entanglement and Heisenberg's uncertainty principle. In the last few years significant progress has been made in the theoretical understanding of quantum cryptography and its technological feasibility has been demonstrated experimentally. Quantum cryptography is therefore regarded as one of the most promising candidates for a future quantum technology. -- With QCRYPT we initiate a new conference series on Quantum Cryptography. The first conference in this series will be held at ETH Zurich and the second at CQT in Singapore. The aim of this series is to bring together researchers working on all aspects of the subject (both on the theoretical and experimental sides) and to support the building of a research community in Quantum Cryptography.
  • Published: 2011
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
1:07:57 Eidgenössische Technische Hochschule (ETH) Zürich English 2011

Quo vadis, quantum cryptography

Quantum cryptography aims to achieve security from fundamental physical principles, such as the quantum mechanical phenomena of entanglement and Heisenberg's uncertainty principle. In the last few years significant progress has been made in the theoretical understanding of quantum cryptography and its technological feasibility has been demonstrated experimentally. Quantum cryptography is therefore regarded as one of the most promising candidates for a future quantum technology. -- With QCRYPT we initiate a new conference series on Quantum Cryptography. The first conference in this series will be held at ETH Zurich and the second at CQT in Singapore. The aim of this series is to bring together researchers working on all aspects of the subject (both on the theoretical and experimental sides) and to support the building of a research community in Quantum Cryptography.
  • Published: 2011
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
40:23 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Randomness and quantum non-locality

The Annual Conference on Quantum Cryptography (QCRYPT) is a conference for students and researchers working on all aspects of quantum cryptography. This includes theoretical and experimental research on the possibilities and limitations of secure communication and computation with quantum mechanical devices or in the presence of quantum mechanical devices. (The conference includes but is not limited to research on quantum key distribution.) It is the goal of the conference to represent the previous year's best results on quantum cryptography and to support the building of a research community in quantum cryptography. In order to achieve this goal, the conference will feature both invited and contributed talks, selected by the steering committee and programme committee, respectively. In line with this goal, the conference has no published proceedings. QCRYPT welcomes the submission of any interesting and important result, while allowing researchers from a wide range of disciplines to pursue publication in any appropriate venue.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
41:05 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Quantum Money from Hidden Subspaces

Forty years ago, Wiesner pointed out that quantum mechanics raises the striking possibility of money that cannot be counterfeited according to the laws of physics. We propose the first quantum money scheme that is (1) public-key, meaning that anyone can verify a banknote as genuine, not only the bank that printed it, and (2) cryptographically secure, under a “classical” hardness assumption that has nothing to do with quantum money. Our scheme is based on hidden subspaces, encoded as the zero-sets of random multivariate polynomials. A main technical advance is to show that the “black-box” version of our scheme, where the polynomials are replaced by classical oracles, is unconditionally secure. Even in Wiesner's original setting – quantum money that can only be verified by the bank – we are able to use our techniques to patch a major security hole in Wiesner's scheme. We give the first private-key quantum money scheme that allows unlimited verifications and that remains unconditionally secure, even if the counterfeiter can interact adaptively with the bank. Our money scheme is simpler than previous public-key quantum money schemes, including a knot-based scheme of Farhi et al. The verifier needs to perform only two tests, one in the standard basis and one in the Hadamard basis – matching the original intuition for quantum money, based on the existence of complementary observables.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
1:25:35 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Rise of the Quantum Age

Quantum mechanics is perhaps the most profound revolution ever to occur in our understanding of Nature. While born with last century, it appeared for many decades to be just a game, played by physicists strictly for the sake of advancing pure knowledge, without any impact on everyday life. And then, quantum mechanics gave birth to the transistor, and society was transformed forever. The physicists’ “game” ushered in the Information Age, which is the signature of the 20th century, just as the 19th century was the Machine Age. But we are poised to experience a second quantum revolution in which the full power of the quantum world will be unleashed in ways never before thought possible, transmitting and processing information with unconditionally secure communication and computers powerful beyond imagination. Indeed, the 21st century will go down in history as the Quantum Age.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
23:53 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Complete insecurity of quantum protocols for classical two-party computation

A fundamental task in modern cryptography is the joint computation of a function which has two inputs, one from Alice and one from Bob, such that neither of the two can learn more about the other's input than what is implied by the value of the function. In this work we show that any quantum protocol for the computation of a classical deterministic function that outputs the result to both parties (two-sided computation) and that is secure against a cheating Bob can be completely broken by a cheating Alice. Whereas it is known that quantum protocols for this task cannot be completely secure, our result implies that security for one party implies complete insecurity for the other. Our findings stand in stark contrast to recent protocols for weak coin tossing, and highlight the limits of cryptography within quantum mechanics. We remark that our conclusions remain valid, even if security is only required to be approximate.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
25:01 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Memory attacks on device-independent quantum

Device-independent quantum cryptographic schemes aim to guarantee security to users based only on the output statistics of any components used, and without the need to verify their internal functionality. Since this would protect users against untrustworthy or incompetent manufacturers, sabotage or device degradation, this idea has excited much interest, and many device-independent schemes have been proposed. Here we identify a critical weakness of device-independent quantum cryptographic protocols that rely on public communication between secure laboratories. Untrusted devices may record their inputs and outputs and reveal information about them via publicly discussed outputs during later runs. Reusing devices thus compromises the security of a protocol and risks leaking secret data. Possible defences include securely destroying or isolating used devices. However, these are costly and often impractical. We briefly consider other possible defences available in scenarios where device reuse is restricted.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
26:45 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Frequency-multiplexed photon storage and read-out on demand using an atomic frequency comb-based quantum memory

The ability to send quantum information encoded in photons over large distances is hampered by the unavoidable loss in the communication channel. In classical communication, channel-loss is alleviated by amplifying the information carrier, however, due to the no-cloning theorem for quantum states, this approach is not viable for quantum communication channels. Instead long-distance quantum communication can be enabled by quantum-repeaters, which serve to distribute entanglement over the entire channel by means of entanglement swapping between subdivision of the channel [1]. In order to synchronize the process of entanglement swapping between adjacent subdivisions, quantum repeaters must incorporate quantum memories [2]. A quantum memory is a device that has the ability to (reversibly) map quantum states between photons and atoms [3]. In most of the quantum repeater architectures proposed to date, it is required that quantum memories feature recall on demand. Other desirable attributes of a quantum memory are high fidelity and efficiency, long storage times, and the possibility to simultaneously store multiple carriers of quantum information, i.e. record multiple photonic modes. The combination of a quantum state storage protocol based on an atomic frequency comb (AFC) [4] with rare-earth-ion doped crystals cooled to cryogenic temperatures as storage materials [5] has been shown to meet many of these requirements. In particular, it is well suited for storage of temporally multiplexed photons [6,7]. Yet, despite first proof-of-principle demonstrations [8], recalling the quantum information at a desired time (i.e. read-out on demand) with broadband, single-photon-level pulses remains an outstanding challenge. Fortunately, the AFC protocol allows not only for multimode storage in the time domain, but also in the frequency domain. Here, we will present the first experimental demonstration of frequency-multiplexed storage of attenuated laser pulses followed by read-out on demand in the frequency domain, pointing to a quantum repeater architecture based on frequency multiplexing. Our work is based on the AFC protocol and employs a Tm-doped LiNbO3 waveguide cooled to 4 K [9,10]. Using a serrodyne sideband chirping technique we prepare several frequency-combs in the atomic absorption spectrum. Each section of AFC is a few 100 MHz wide and since we vary the comb-tooth spacing in each section we prepare them with different storage times on the order of 20-150 ns. After the AFC preparation, we send a probe pulse, which is modulated to contain several frequency components that correspond to the centre frequencies of the AFC sections. The mean photon number in each mode is set to be around one. As the probe pulse is mapped to our quantum memory the different frequency modes are mapped to different sections of the AFC and thus recalled at different times. The recalled pulses pass through a frequency filter with a bandwidth matching a single frequency mode. Before frequency filtering we are able to impart again a frequency shift on the recalled pulses, which can hence be set to pass the spectral filter. This constitutes recall on demand of a particular frequency mode. Our multimode quantum memory is highly flexible and can be set to recall all modes at the same time, and adapted to broader or narrower frequency modes. In addition it has been shown to faithfully store time bin qubits in pure and entangled states and preserve all degrees of freedom of the photonic wavefunction [9,11]. Finally, we will argue that, in view of a quantum repeater, our approach based on a multimode memory with read-out on demand in the frequency domain is equivalent to temporal multiplexing and read-out on demand in the temporal domain. This overcomes one further obstacle to building quantum repeaters using rare-earth-ion doped crystals as memory devices. [1] H.-J. Briegel, et al., Phys. Rev. Lett., 81, 5932 (1998) [2] N. Sangouard, et al., Rev. Mod. Phys., 83(1), 33 (2011) [3] A. I. Lvovsky, B. C. Sanders, and W. Tittel, Nat. Photon, 3(12), 706 (2009) [4] M. Afzelius, et al., Phys. Rev. A, 79(5), 052329 (2009) [5] W. Tittel, et al., Laser & Photonics Rev., 4(2), 244 (2010) [6] I. Usmani, et al., Nat Commun, 1 (2010) [7] M. Bonarota, J.-L. Le Gouët, and T. Chanelière, New J. of Phys., 13(1), 013013 (2011) [8] M. Afzelius, et al., Phys. Rev. Lett., 104, 040503 (2010) [9] E. Saglamyurek, et al., Nature, 469(7331), 512 (2011) [10] N. Sinclair, et al., J. of Luminescence, 130(9), 1586 (2010) [11] E. Saglamyurek, et al., Phys. Rev. Lett., 108, 083602 (2012)
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
53:01 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Probing the reality of quantum state

Is the quantum state real – a property of the system it is assigned to? Or does it represent only our (incomplete) knowledge of the system? It is possible that the second alternative – the epistemic character of the quantum state – comes about because quantum mechanics is obtained by some statistical averaging over a “complete” theory of nature. Such models are often called “hidden variable” models, because the true variables describing the system, the ontic state, are not accessible. Recently Pusey, Barrett and Rudolph [1] showed that, assuming the natural assumption of “preparation independence”, epistemic models of the quantum state are in contradiction with the predictions of quantum theory. “Preparation independence” means that independent preparations of systems correspond to a joint distribution (over the ontic states) is the product of individual distributions. Here we adopt a different approach. We show that, assuming both a form of continuity and separability (a weak form of preparation independence), epistemic interpretations of the quantum state are in contradiction with quantum theory. We also discuss some implications of “hidden-variable” models for cryptography. We then describe a simple high-precision experiment optics experiment that tests some of the predictions of continuous and separable epistemic models. The experiment is particularly simple. It involves attenuated coherent states in time bins of dimension up to 80 propagating in optical fibres. Our experimental results are in agreement with the predictions of quantum theory and provide strong constraints on possible epistemic extensions of quantum mechanics. These results are reported in [2]. [1] M. F. Pusey, J. Barrett, and T. Rudolph, On the reality of the quantum state, Nature Physics, 2309, (2012). [2] M. K. Patra, L. Olislager, F. Duport, J. Safioui, S. Pironio and S. Massar, Experimentally probing the reality of the quantum state, submitted (2012)
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
1:27:15 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Smooth entropies - A Tutorial

The Annual Conference on Quantum Cryptography (QCRYPT) is a conference for students and researchers working on all aspects of quantum cryptography. This includes theoretical and experimental research on the possibilities and limitations of secure communication and computation with quantum mechanical devices or in the presence of quantum mechanical devices. (The conference includes but is not limited to research on quantum key distribution.) It is the goal of the conference to represent the previous year's best results on quantum cryptography and to support the building of a research community in quantum cryptography. In order to achieve this goal, the conference will feature both invited and contributed talks, selected by the steering committee and programme committee, respectively. In line with this goal, the conference has no published proceedings. QCRYPT welcomes the submission of any interesting and important result, while allowing researchers from a wide range of disciplines to pursue publication in any appropriate venue.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
31:04 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Improving the maximum transmission distance of continuous-variable quantum key distribution using a noiseless amplifier

We show that the maximum transmission distance of continuous-variable quantum key distribution in presence of a Gaussian noisy lossy channel can be arbitrarily increased using a heralded noiseless linear amplifier. We explicitly consider a protocol using amplitude and phase modulated coherent states with reverse reconciliation. Assuming that the secret key rate drops to zero for a line transmittance Tlim, we find that a noiseless amplifier with amplitude gain g can improve this value to Tlim/g2, corresponding to an increase in distance proportional to logg.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
23:45 Eidgenössische Technische Hochschule (ETH) Zürich English 2012

Infrared NbN superconducting single-photon detector for quantum cryptography and quantum information processing

We present the overview of our recent results in research and development of superconducting single-photon detector (SSPD) practical applications such as quantum cryptography. By optimization of fabrication process and usage of high-quality silicon wafers with SiO2 layer acting as a microcavity we managed to reach up to 35.6% detection efficiency at 1500 nm wavelength. Also we extended its wavelength range beyond 1800 nm by the usage of the fluoride ZBLAN fibres.
  • Published: 2012
  • Publisher: Eidgenössische Technische Hochschule (ETH) Zürich
  • Language: English
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