How to implement two-party protocols in the noisy-storage model (2009)
Wehner, Stephanie, Curty, Marcos, Schaffner, Christian, Lo, Hoi-Kwong
The noisy-storage model allows the implementation of secure two-party protocols under the sole assumption that no large-scale reliable quantum storage is available to the cheating party. No quantum...
Bounds on probability of transformations between multi-partite pure states (2009)
Cui, Wei, Helwig, Wolfram, Lo, Hoi-Kwong
For a tripartite pure state of three qubits, it is well known that there are two inequivalent classes of genuine tripartite entanglement, namely the GHZ-class and the W-class. Any two states within...
Qi, Bing, Chi, Yue-Meng, Lo, Hoi-Kwong, Qian, Li
We present a high speed random number generation scheme based on measuring the quantum phase noise of a single mode diode laser operating at a low intensity level near the lasing threshold. A delayed...
Security Analysis of an Untrusted Source for Quantum Key Distribution: Passive Approach (2009)
Zhao, Yi, Qi, Bing, Lo, Hoi-Kwong, Qian, Li
We present a passive approach to the security analysis of quantum key distribution (QKD) with an untrusted source. A complete proof of its unconditional security is also presented. This scheme has...
Upper bounds for the secure key rate of decoy state quantum key distribution (2009)
Curty, Marcos, Moroder, Tobias, Ma, Xiongfeng, Lo, Hoi-Kwong, Lütkenhaus, Norbert
The use of decoy states in quantum key distribution (QKD) has provided a method for substantially increasing the secret key rate and distance that can be covered by QKD protocols with practical...
Exotic phenomena in non-Abelian gauge theories (2008)
This thesis deals with some exotic phenomena in non-Abelian gauge theories. More specifically, we study aspects of non-Abelian vortices, non-Abelian Chern-Simons particles, wormhole physics and...
Possibility, impossibility, and cheat sensitivity of quantum-bit string commitment (2008)
Buhrman, Harry, Christandl, Matthias, Hayden, Patrick, Lo, Hoi-Kwong, Wehner, Stephanie
It had been widely claimed that quantum mechanics can protect private information during public decision in for example the so-called two-party se cure computation. If this were the case, quantum...
Quantum key distribution with triggering parametric down conversion sources (2008)
Parametric down-conversion (PDC) sources can be used for quantum key distribution (QKD). One can use a PDC source as a triggered single photon source. Recently, there are various practical proposals...
Security proof of quantum key distribution with detection efficiency mismatch (2008)
Fung, Chi-Hang Fred, Tamaki, Kiyoshi, Qi, Bing, Lo, Hoi-Kwong, Ma, Xiongfeng
In theory, quantum key distribution (QKD) offers unconditional security based on the laws of physics. However, as demonstrated in recent quantum hacking theory and experimental papers, detection...
Quantum key distribution with an unknown and untrusted source (2008)
Zhao, Yi, Qi, Bing, Lo, Hoi-Kwong
The security of a standard bi-directional "plug & play" quantum key distribution (QKD) system has been an open question for a long time. This is mainly because its source is equivalently controlled...
Efficient Quantum Key Distribution (2007)
Mohammad Ardehali, Gilles Brassard, H. F. Chau, Hoi-Kwong Lo
We devise a simple modification that essentially doubles the e#ciency of a well-known quantum key distribution scheme proposed by Bennett and Brassard (BB84). Our scheme assigns significantly...
It had been widely claimed that quantum mechanics can protect private information during public decision in for example the so-called two-party secure computation. If this were the case, quantum...
Random multiparty entanglement distillation (2007)
We describe various results related to the random distillation of multiparty entangled states - that is, conversion of such states into entangled states shared between fewer parties, where those...
Qi, Bing, Huang, Lei-Lei, Qian, Li, Lo, Hoi-Kwong
In this paper, we present a fully fiber-based one-way Quantum Key Distribution (QKD) system implementing the Gaussian-Modulated Coherent States (GMCS) protocol. The system employs a double...
Zhao, Yi, Fung, Chi-Hang Fred, Qi, Bing, Chen, Christine, Lo, Hoi-Kwong
Quantum key distribution (QKD) systems can send signals over more than 100 km standard optical fiber and are widely believed to be secure. Here, we show experimentally for the first time a...
Quantum key distribution with entangled photon sources (2007)
Ma, Xiongfeng, Fung, Chi-Hang Fred, Lo, Hoi-Kwong
A parametric down-conversion (PDC) source can be used as either a triggered single photon source or an entangled photon source in quantum key distribution (QKD). The triggering PDC QKD has already...
Quantum Cryptography: from Theory to Practice (2007)
Lo, Hoi-Kwong, Lütkenhaus, Norbert
Quantum cryptography can, in principle, provide unconditional security guaranteed by the law of physics only. Here, we survey the theory and practice of the subject and highlight some recent...
Continuous-Variables Quantum Key Distribution over Standard Telecom Fiber (2006)
Huang, Lei-Lei, Qi, Bing, Qian, Li, Lo, Hoi-Kwong
This paper has been withdrawn
Experimental quantum key distribution with active phase randomization (2006)
Zhao, Yi, Qi, Bing, Lo, Hoi-Kwong
Phase randomization is an important assumption made in many security proofs of practical quantum key distribution (QKD) systems. Here, we present the first experimental demonstration of QKD with...
Quantum key distribution with "dual detectors" (2006)
Qi, Bing, Zhao, Yi, Ma, Xiongfeng, Lo, Hoi-Kwong, Qian, Li
To improve the performance of a quantum key distribution (QKD) system, high speed, low dark count single photon detectors (or low noise homodyne detectors) are required. However, in practice, a fast...
Security of quantum key distribution using weak coherent states with nonrandom phases (2006)
We prove the security of the Bennett-Brassard (BB84) quantum key distribution protocol in the case where the key information is encoded in the relative phase of a coherent-state reference pulse and a...
Security of quantum bit string commitment depends on the information measure (2006)
Buhrman, Harry, Christandl, Matthias, Hayden, Patrick, Lo, Hoi-Kwong, Wehner, Stephanie
Unconditionally secure non-relativistic bit commitment is known to be impossible in both the classical and the quantum world. However, when committing to a string of n bits at once, how far can we...
Curty, Marcos, Zhang, Lucy-Liuxuan, Lo, Hoi-Kwong, Lütkenhaus, Norbert
We investigate limitations imposed by sequential attacks on the performance of differential-phase-shift quantum key distribution protocols that use pulsed coherent light. In particular, we analyze...
Qi, Bing, Tausz, Andrew, Qian, Li, Lo, Hoi-Kwong
We propose and experimentally demonstrate a single-mode fiber length and dispersion measurement system based on a novel frequency-shifted asymmetric Sagnac interferometer incorporating an...
Random bipartite entanglement from W and W-like states (2006)
We describe a protocol for distilling maximally entangled bipartite states between random pairs of parties from those sharing a tripartite W state, and show that, rather surprisingly, the total...
Security proof of a three-state quantum key distribution protocol without rotational symmetry (2006)
Fung, Chi-Hang Fred, Lo, Hoi-Kwong
Standard security proofs of quantum key distribution (QKD) protocols often rely on symmetry arguments. In this paper, we prove the security of a three-state protocol that does not possess rotational...
Discrete Rotational Symmetry and Quantum Key Distribution Protocols (2006)
Shirokoff, David, Fung, Chi-Hang Fred, Lo, Hoi-Kwong
We study the role of discrete rotational symmetry in quantum key distribution by generalizing the well-known Bennett-Brassard 1984 (BB84) and Scarani-Acin-Ribordy-Gisin 2004 (SARG04) protocols. We...
Qi, Bing, Huang, Lei-Lei, Lo, Hoi-Kwong, Qian, Li
We present a design for a quantum key distribution(QKD) system in a Sagnac loop configuration, employing a novel phase modulation scheme based on frequency shift, and demonstrate stable BB84 QKD...
Decoy state quantum key distribution with two-way classical post-processing (2006)
Ma, Xiongfeng, Fung, Chi-Hang Fred, Dupuis, Frédéric, Chen, Kai, Tamaki, Kiyoshi, Lo, Hoi-Kwong
Decoy states have recently been proposed as a useful method for substantially improving the performance of quantum key distribution protocols when a coherent state source is used. Previously, data...
Quantum Key Distribution Based on Arbitrarily Weak Distillable Entangled States (2006)
Horodecki, Karol, Leung, Debbie, Lo, Hoi-Kwong, Oppenheim, Jonathan
States with private correlations but little or no distillable entanglement were recently reported. Here, we consider the secure distribution of such states, i.e., the situation when an adversary...
Simulation and Implementation of Decoy State Quantum Key Distribution over 60km Telecom Fiber (2006)
Zhao, Yi, Qi, Bing, Ma, Xiongfeng, Lo, Hoi-Kwong, Qian, Li
Decoy state quantum key distribution (QKD) has been proposed as a novel approach to improve dramatically both the security and the performance of practical QKD set-ups. Recently, many theoretical...
Phase-Remapping Attack in Practical Quantum Key Distribution Systems (2006)
Fung, Chi-Hang Fred, Qi, Bing, Tamaki, Kiyoshi, Lo, Hoi-Kwong
Quantum key distribution (QKD) can be used to generate secret keys between two distant parties. Even though QKD has been proven unconditionally secure against eavesdroppers with unlimited computation...
Quantum key distribution based on arbitrarily weak distillable entangled states (2006)
Lo, Hoi-Kwong, Horodecki, Karol, Leung, Debbie, Oppenheim, Jonathan
Time-shift attack in practical quantum cryptosystems (2005)
Qi, Bing, Fung, Chi-Hang Fred, Lo, Hoi-Kwong, Ma, Xiongfeng
Recently, a new type of attack, which exploits the efficiency mismatch of two single photon detectors (SPD) in a quantum key distribution (QKD) system, has been proposed. In this paper, we propose...
Quantum key distribution based on arbitrarily-weak distillable entangled states (2005)
Horodecki, Karol, Leung, Debbie, Lo, Hoi-Kwong, Oppenheim, Jonathan
States with private correlations but little or no distillable entanglement were recently reported. Here, we consider the secure distribution of such states, i.e., the situation when an adversary...
On the performance of two protocols: SARG04 and BB84 (2005)
Fung, Chi-Hang Fred, Tamaki, Kiyoshi, Lo, Hoi-Kwong
We compare the performance of BB84 and SARG04, the later of which was proposed by V. Scarani et al., in Phys. Rev. Lett. 92, 057901 (2004). Specifically, in this paper, we investigate SARG04 with...
Quantum Key Distribution with Vacua or Dim Pulses as Decoy States (2005)
Recently, Hwang has proposed a decoy state method in quantum key distribution (QKD). In Hwang's proposal, the average photon number of the decoy state is about two. Here, we propose a new decoy state...
Conference Key Agreement and Quantum Sharing of Classical Secrets with Noisy GHZ States (2005)
We propose a wide class of distillation schemes for multi-partite entangled states that are CSS-states. Our proposal provides not only superior efficiency, but also new insights on the connection...
Two-way quantum communication channels (2005)
Childs, Andrew M., Leung, Debbie W., Lo, Hoi-Kwong
We consider communication between two parties using a bipartite quantum operation, which constitutes the most general quantum mechanical model of two-party communication. We primarily focus on the...
Phase randomization improves the security of quantum key distribution (2005)
Ideal quantum key distribution (QKD) protocols call for a source that emits single photon signals, but the sources used in typical practical realizations emit weak coherent states instead. A weak...
Possibility, Impossibility and Cheat-Sensitivity of Quantum Bit String Commitment (2005)
Buhrman, Harry, Christandl, Matthias, Hayden, Patrick, Lo, Hoi-Kwong, Wehner, Stephanie
Unconditionally secure non-relativistic bit commitment is known to be impossible in both the classical and the quantum worlds. But when committing to a string of n bits at once, how far can we...
Experimental Quantum Key Distribution with Decoy States (2005)
Zhao, Yi, Qi, Bing, Ma, Xiongfeng, Lo, Hoi-Kwong, Qian, Li
To increase dramatically the distance and the secure key generation rate of quantum key distribution (QKD), the idea of quantum decoys--signals of different intensities--has recently been proposed....
Getting Something Out of Nothing (2005)
We study quantum key distribution with standard weak coherent states and show, rather counter-intuitively, that the detection events originated from vacua can contribute to secure key generation...
Unconditionally secure key distillation from multi-photons (2004)
Tamaki, Kiyoshi, Lo, Hoi-Kwong
In this paper, we prove that the unconditionally secure key can be surprisingly extracted from {\it multi}-photon emission part in the photon polarization-based QKD. One example is shown by...
We derive lower limits on the inefficiency and classical communication costs of dilution between two-term bipartite pure states that are partially entangled. We first calculate explicit relations...
Decoy State Quantum Key Distribution (2004)
Lo, Hoi-Kwong, Ma, Xiongfeng, Chen, Kai
There has been much interest in quantum key distribution. Experimentally, quantum key distribution over 150 km of commercial Telecom fibers has been successfully performed. The crucial issue in...
Multi-partite quantum cryptographic protocols with noisy GHZ states (2004)
We propose a wide class of distillation schemes for multi-partite entangled states that are CSS-states. Our proposal provides not only superior efficiency, but also new insights on the connection...
Some Attacks On Quantum-based Cryptographic Protocols (2003)
Quantum-based cryptographic protocols are often said to enjoy security guaranteed by the fundamental laws of physics. However, even carefully designed quantum-based cryptographic schemes may be...
Why Quantum Information Processing (2003)
In this brief note, I will consider the following questions: (1) What is QIP? (2) Why QIP is interesting? (3) What QIP can do? (4) What QIP cannot do? (5) What are the major challenges in QIP?
Security of quantum key distribution with imperfect devices (2002)
Gottesman, Daniel, Lo, Hoi-Kwong, Lütkenhaus, Norbert, Preskill, John
We prove the security of the Bennett-Brassard (BB84) quantum key distribution protocol in the case where the source and detector are under the limited control of an adversary. Our proof applies when...
A tight lower bound on the classical communication cost of entanglement dilution (2002)
Entanglement concentration requires no classical communication, but the best prior art result for diluting to N copies of a partially entangled state requires an amount of communication on the order...
Method for decoupling error correction from privacy amplification (2002)
Entanglement purification provides a unifying framework for proving the security of quantum key distribution schemes. Nonetheless, up till now, a local commutability constraint in the CSS code...
From Quantum Cheating to Quantum Security (2001)
Gottesman, Daniel, Lo, Hoi-Kwong
For thousands of years, code-makers and code-breakers have been competing for supremacy. Their arsenals may soon include a powerful new weapon: quantum mechanics. We give an overview of quantum...
Proof of security of quantum key distribution with two-way classical communications (2001)
Gottesman, Daniel, Lo, Hoi-Kwong
Shor and Preskill have provided a simple proof of security of the standard quantum key distribution scheme by Bennett and Brassard (BB84) by demonstrating a connection between key distribution and...
Proof of unconditional security of six-state quantum key distribution scheme (2001)
We prove the unconditional security of the standard six-state scheme for quantum key distribution (QKD). We demonstrate its unconditional security up to a bit error rate of 12.7 percents, by allowing...
A simple proof of the unconditional security of quantum key distribution (2001)
quantum cryptography, quantum information, quantum computation, unconditional security © Copyright Hewlett-Packard Company 1999 Quantum key distribution is the most well-known application of quantum...
Efficient Quantum Key Distribution Scheme And Proof of Its Unconditional Security (2000)
Lo, Hoi-Kwong, Chau, H. F., Ardehali, M.
We devise a simple modification that essentially doubles the efficiency of the BB84 quantum key distribution scheme proposed by Bennett and Brassard. We also prove the security of our modified scheme...
A quantum analog of Huffman coding (2000)
Braunstein, Samuel L., Fuchs, Christopher A., Gottesman, Daniel, Lo, Hoi-Kwong
We analyze a generalization of Huffman coding to the quantum case. In particular, we notice various difficulties in using instantaneous codes for quantum communication. Nevertheless, for the storage...
We study the amount of classical communication needed for distributed quantum information processing. In particular, we introduce the concept of "remote preparation" of a quantum state. Given an...
Will Quantum Cryptography ever become a successful technology in the marketplace? (1999)
We assess the potential of quantum cryptography as a technology. We highlight the fact that academia and real world have rather different perspectives and interests. Then, we describe the various...
A simple proof of the unconditional security of quantum key distribution (1999)
Quantum key distribution is the most well-known application of quantum cryptography. Previous proposed proofs of security of quantum key distribution contain various technical subtleties. Here, a...
Entanglement bits or ``ebits'' have been proposed as a quantitative measure of a fundamental resource in quantum information processing. For such an interpretation to be valid, it is important to...
How to share a quantum secret (1999)
Cleve, Richard, Gottesman, Daniel, Lo, Hoi-Kwong
We investigate the concept of quantum secret sharing. In a ((k,n)) threshold scheme, a secret quantum state is divided into n shares such that any k of those shares can be used to reconstruct the...
A quantum analog of Huffman coding (1998)
Braunstein, Samuel L., Fuchs, Christopher A., Gottesman, Daniel, Lo, Hoi-Kwong
We analyze a generalization of Huffman coding to the quantum case. In particular, we notice various difficulties in using instantaneous codes for quantum communication. Nevertheless, for the storage...
Unconditional Security Of Quantum Key Distribution Over Arbitrarily Long Distances (1998)
Quantum key distribution is widely thought to offer unconditional security in communication between two users. Unfortunately, a widely accepted proof of its security in the presence of source, device...
Efficient Quantum Key Distribution (1998)
Ardehali, M., Chau, H. F., Lo, Hoi-Kwong
We devise a simple modification that essentially doubles the efficiency of a well-known quantum key distribution scheme proposed by Bennett and Brassard (BB84). Our scheme assigns significantly...
Making an empty promise with a quantum computer (1998)
quant-ph/9709053 v2 7
Concentrating entanglement by local actions---beyond mean values (1997)
Suppose two distant observers Alice and Bob share a pure bipartite quantum state. By applying local operations and communicating with each other using a classical channel, Alice and Bob can...
HKUPHYS-HFC-03; quant-ph/9709053 Making An Empty Promise With A Quantum Computer ∗ (1997)
Alice has made a decision in her mind. While she does not want to reveal it to Bob at this moment, she would like to convince Bob that she is committed to this particular decision and that she cannot...
Insecurity of Quantum Secure Computations (1996)
It had been widely claimed that quantum mechanics can protect private information during public decision in for example the so-called two-party secure computation. If this were the case, quantum...
Why quantum bit commitment and ideal quantum coin tossing are impossible (1996)
There had been well known claims of ``provably unbreakable'' quantum protocols for bit commitment and coin tossing. However, we, and independently Mayers, showed that all proposed quantum bit...
Is Quantum Bit Commitment Really Possible? (1996)
We show that all proposed quantum bit commitment schemes are insecure because the sender, Alice, can almost always cheat successfully by using an Einstein-Podolsky-Rosen type of attack and delaying...
Is Quantum Bit Commitment Really Possible? (1996)
We show that all proposed quantum bit commitment schemes are insecure because the sender can always cheat successfully by using an EPR-type of attack and delaying her measurement until she opens her...
Quantum Cryptography in Noisy Channels (1995)
We provide a complete proof of the security of quantum cryptography against any eavesdropping attack including coherent measurements even in the presence of noise. Polarization-based cryptographic...
Wavefunctions for Non-Abelian Vortices (1995)
We construct exact wavefunctions of two vortices on a plane, a single vortex on the cylinder and a vortex on the torus. In each case, the physics is shown to be equivalent to a particle moving in a...
Quantum Coding Theorem for Mixed States (1995)
We prove a theorem for coding mixed-state quantum signals. For a class of coding schemes, the von Neumann entropy $S$ of the density operator describing an ensemble of mixed quantum signal states is...
Elusive Order Parameters for Non-Abelian Gauge Theories (1995)
In this Letter, we construct a set of order parameters for non-Abelian gauge theories which probe directly the unbroken group and are free of the deficiencies caused by quantum fluctuations and gauge...
Aharonov-Bohm Order Parameters for Non-Abelian Gauge Theories (1995)
The Aharonov-Bohm effect has been invoked to probe the phase structure of a gauge theory. Yet in the case of non-Abelian gauge theories, it proves difficult to formulate a general procedure that...
Quantum cryptography in noisy channels", manuscript (1995)
We provide a complete proof of the security of quantum cryptography against any eavesdropping attack including coherent measurements even in the pres-ence of noise. Polarization-based cryptographic...
Order Parameters for Non-Abelian Gauge Theories (1994)
Owing to subtle issues concerning quantum fluctuations and gauge fixing, a formulation of a general procedure to specify the realization of non-Abelian gauge symmetry has evaded all earlier attempts....
Is baryon number violated when electroweak strings intercommute? (1994)
We reexamine the self-helicity and the intercommutation of electroweak strings. A plausible argument for baryon number conservation when electroweak strings intercommute is presented. The connection...
Exotic phenomena in non-Abelian gauge theories (1994)
This thesis deals with some exotic phenomena in non-Abelian gauge theories. More specifically, we study aspects of non-Abelian vortices, non-Abelian Chern-Simons particles, wormhole physics and...
Scattering from Electroweak Strings (1994)
The scattering of a charged fermion from an electroweak string is studied. Owing to an amplification of the wave function at the core radius, the cross sections for helicity flip processes can be...
Non-Abelian vortices and non-Abelian statistics (1993)
We study the interactions of non-Abelian vortices in two spatial dimensions. These interactions have novel features, because the Aharonov-Bohm effect enables a pair of vortices to exchange quantum...
Complementarity in Wormhole Chromodynamics (1993)
Lo, Hoi-Kwong, Lee, Kai-Ming, Preskill, John
The electric charge of a wormhole mouth and the magnetic flux ``linked'' by the wormhole are non-commuting observables, and so cannot be simultaneously diagonalized. We use this observation to...
Exact Wavefunctions for non-Abelian Chern-Simons Particles (1993)
Exact wavefunctions for N non-Abelian Chern-Simons (NACS) particles are obtained by the ladder operator approach. The same method has previously been applied to construct exact wavefunctions for...
Non-abelian vortices and non-abelian statistics (1993)
We study the interactions of non-abelian vortices in two spatial dimensions. These interactions have novel features, because the Aharonov-Bohm effect enables a pair of vortices to exchange quantum...
Topological Approach to Alice Electrodynamics (1991)
Bucher, Martin, Lo, Hoi-Kwong, Preskill, John
We analyze the unlocalized ``Cheshire charge'' carried by ``Alice strings.'' The magnetic charge on a string loop is carefully defined, and the transfer of magnetic charge from a monopole to a string...