Zeng-Bing Chen - Profile on Academia.edu (original) (raw)

Papers by Zeng-Bing Chen

Physical Review A, 2007

We derive the lower and upper bounds on the entanglement of a given multipartite superposition st... more We derive the lower and upper bounds on the entanglement of a given multipartite superposition state in terms of the entanglement of the states being superposed. The first entanglement measure we use is the geometric measure, and the second is the q-squashed entanglement. These bounds allow us to estimate the amount of the multipartite entanglement of superpositions. We also show that two states of high fidelity to one another do not necessarily have nearly the same q-squashed entanglement. PACS numbers: 03.67.-a, 03.67.Mn, 03.65.Ud Recently, Linden et al. [1] raised the following problem: given a certain decomposition of a bipartite state |Γ as a superposition of two other states, what is the relation between the entanglement of |Γ and those of the two states being superposed? They derived upper bounds on the entanglement of |Γ in terms of those of the two states in the superposition, where the entanglement was quantified by the von Neumann entropy. Subsequently, Yu et al. [2] and Ou et al.

Multi-photon interference reveals strictly non-classical phenomena. Its applications range from f... more Multi-photon interference reveals strictly non-classical phenomena. Its applications range from fundamental tests of quantum mechanics to photonic quantum information processing, where a significant fraction of key experiments achieved so far comes from multi-photon state manipulation. We review the progress, both theoretical and experimental, of this rapidly advancing research. The emphasis is given to the creation of photonic entanglement of various forms, tests of the completeness of quantum mechanics (in particular, violations of local realism), quantum information protocols for quantum communication (e.g., quantum teleportation, entanglement purification and quantum repeater), and quantum computation with linear optics. We shall limit the scope of our review to "few photon" phenomena involving measurements of discrete observables.

We present a protocol for deterministic and highly efficient quantum cryptography with entangled ... more We present a protocol for deterministic and highly efficient quantum cryptography with entangled photon pairs in a 4×4-dimentional Hilbert space. Two communicating parties, Alice and Bob first share a both polarization-and path-entangled photon pair, and then each performs a complete Bell-state measurement on their own photon in one of two complementary Bell-state bases. It is demonstrated that each measurement in which both Alice and Bob register a photon can build certain perfect correlation and generate 1.5 key bits on average. The security of our protocol is guaranteed by the non-cloning theorem. PACS numbers: 03.67.Lx, 42.50.Ar The central task for quantum cryptography is to establish secure keys before the transmission of message between two parties [1]. In recent years, both BB84 [2] and Ek-ert91 [3] cryptographic protocols have been successfully demonstrated in numerous experiments . However, one drawback of the above schemes is the nondeterministic feature, that is, only less than 50% of all detected qubits can be further used as key bits. This is a serious practical problem, because in the one-time-pad secretkey cryptosystem, the length of secret key must be the same as the ciphertext . Although some deterministic cryptographic schemes based on orthogonal states have been proposed recently , the long storage rings required, which is an essential ingredient of those schemes, lead to a low efficiency for the transmission of polarized photons. More recently, deterministic quantum cryptography and secret direct communication have also been proposed with single-photon two-qubit states . By exploiting the states in four-or more dimensional Hilbert space, each transmitted photon can be used as a key bit with the help of classical communication; eavesdroppers can be detected within the scheme. However, every photon detected can establish only one key bit.

Physical Review Letters, 2002

We generalize Bell's inequalities to biparty systems with continuous quantum variables. This is a... more We generalize Bell's inequalities to biparty systems with continuous quantum variables. This is achieved by introducing the Bell operator in perfect analogy to the usual spin-1/2 systems. It is then demonstrated that two-mode squeezed vacuum states display quantum nonlocality by using the generalized Bell operator. In particular, the original Einstein-Podolsky-Rosen states, which are the limiting case of the two-mode squeezed vacuum states, can maximally violate Bell's inequality due to Clauser, Horne, Shimony and Holt. The experimental aspect of our scheme is briefly considered.

The preparation and storage of photonic entanglement are central to the achievement of scalable l... more The preparation and storage of photonic entanglement are central to the achievement of scalable linear optical quantum computation 1-3 (LOQC). The most widely used photonic entanglement source (a spontaneous parametric downconversion (SPDC) source) 4,5 is not directly suitable for storage, because its working frequency bandwidth is significantly larger than any available quantum memory. To remedy this problem, cavity-enhanced narrow-band SPDC sources 6-12 have been developed. However, the storage of cavity-enhanced narrow-band entangled photons has not yet been achieved. Also, the spectral correlations between the entangled photons can make them practically useless for scalable LOQC 5,13,14 .

Optics Express, 2010

We report an implementation of decoy-state quantum key distribution (QKD) over 200 km optical fib... more We report an implementation of decoy-state quantum key distribution (QKD) over 200 km optical fiber cable through photon polarization encoding. This is achieved by constructing the whole QKD system operating at 320 MHz repetition rate, and developing high-speed transmitter and receiver modules. A novel and economic way of synchronization method is designed and incorporated into the system, which allows to work at a low frequency of 40kHz and removes the use of highly precise clock. A final key rate of 15 Hz is distributed within the experimental time of 3089 seconds, by using super-conducting single photon detectors. This is longest decoy-state QKD yet demonstrated up to date. It helps to make a significant step towards practical secure communication in long-distance scope.

Physical Review Letters, 2008

We report an experimental realization of a narrow-band polarization-entangled photon source with ... more We report an experimental realization of a narrow-band polarization-entangled photon source with a linewidth of 9.6 MHz through cavity-enhanced spontaneous parametric down-conversion. This linewidth is comparable to the typical linewidth of atomic ensemble based quantum memories. Single-mode output is realized by setting a reasonable cavity length difference between different polarizations, using of temperature controlled etalons and actively stabilizing the cavity. The entangled property is characterized with quantum state tomography, giving a fidelity of 94% between our state and a maximally entangled state. The coherence length is directly measured to be 32 m through two-photon interference.

Based on principle of quantum mechanics, quantum cryptography provides an intriguing way to estab... more Based on principle of quantum mechanics, quantum cryptography provides an intriguing way to establish secret keys between remote parties, generally relying on actual transmission of signal particles.

Physical Review Letters, 2010

We experimentally demonstrate an advanced linear-optical programmable quantum processor that comb... more We experimentally demonstrate an advanced linear-optical programmable quantum processor that combines two elementary single-qubit programmable quantum gates. We show that this scheme enables direct experimental probing of quantum commutation relations for Pauli operators acting on polarization states of single photons. Depending on a state of two-qubit program register, we can probe either commutation or anticommutation relations. Very good agreement between theory and experiment is observed, indicating high-quality performance of the implemented quantum processor.

Physical Review A, 2007

We present a detailed analysis of a robust quantum repeater architecture building on the original... more We present a detailed analysis of a robust quantum repeater architecture building on the original Duan-Lukin-Cirac-Zoller ͑DLCZ͒ protocol ͓L.M. Duan et al. Nature ͑London͒ 414, 413 ͑2001͔͒. The architecture is based on two-photon Hong-Ou-Mandel-type interference which relaxes the long-distance interferometric stability requirements by about seven orders of magnitude, from subwavelength for the single photon interference required by DLCZ to the coherence length of the photons, thereby removing the weakest point in the DLCZ scheme. Our proposal provides an exciting possibility for robust and realistic long-distance quantum communication.

Nature Photonics, 2011

The preparation and storage of photonic entanglement are central to the achievement of scalable l... more The preparation and storage of photonic entanglement are central to the achievement of scalable linear optical quantum computation (LOQC). The most widely used photonic entanglement source (a spontaneous parametric downconversion (SPDC) source) is not directly suitable for storage, because its working frequency bandwidth is significantly larger than any available quantum memory. To remedy this problem, cavity-enhanced narrow-band SPDC sources have been developed. However, the storage of cavity-enhanced narrow-band entangled photons has not yet been achieved. Also, the spectral correlations between the entangled photons can make them practically useless for scalable LOQC. Here, we report the preparation and storage of frequency-uncorrelated narrowband (5 MHz) entangled photons from a cavity-enhanced SPDC source. The frequency correlation between the entangled photons is eliminated by changing the continuous UV pumping beam to short pulses. The storage of the polarization state of a single photon, and of a photon entangled with another flying in the fibre, is demonstrated. Our work demonstrates a quantum interface between narrow-band entangled photons from cavity SPDC and atomic quantum memory, and thus provides an important tool towards the achievement of all-optical quantum information processing.

Nature Photonics, 2011

The paradigm of measurement-based quantum computation opens new experimental avenues to realizing... more The paradigm of measurement-based quantum computation opens new experimental avenues to realizing a quantum computer, and also deepens our understanding of quantum physics. Measurement-based quantum computation originates with a highly entangled universal resource state. For years, clusters states have been the only known universal resources. Surprisingly, a novel framework, namely quantum computation in correlation space, has opened a new route to implementing measurement-based quantum computation based on quantum states having entanglement properties, which differ from cluster states. Here, we report an experimental demonstration of every building block of such a model. With four-qubit and six-qubit states, which are not in the cluster-state category, we have realized a universal set of single-qubit rotations, two-qubit entangling gates and also Deutsch's algorithm. As well as being of fundamental interest, our experiment proves, in principle, the feasibility of universal measurement-based quantum computation without the use of cluster states, which represents a new approach towards the realization of a quantum computer.

Physical Review Letters, 2009

Anyons are exotic quasiparticles living in two dimensions that do not fit into the usual categori... more Anyons are exotic quasiparticles living in two dimensions that do not fit into the usual categories of fermions and bosons, but obey a new form of fractional statistics. Following a recent proposal [Phys. Rev. Lett. 98, 150404 (2007)], we present an experimental demonstration of the fractional statistics of anyons in the Kitaev spin lattice model using a photonic quantum simulator. We dynamically create the ground state and excited states (which are six-qubit graph states) of the Kitaev model Hamiltonian, and implement the anyonic braiding and fusion operations by single-qubit rotations. A phase shift of π related to the anyon braiding is observed, confirming the prediction of the fractional statistics of Abelian 1/2-anyons.

We demonstrate the decoy-state quantum key distribution over 200 km with photon polarization thro... more We demonstrate the decoy-state quantum key distribution over 200 km with photon polarization through optical fiber, by using superconducting single photon detector with a repetition rate of 320 Mega Hz and a dark count rate of lower than 1 Hz. Since we have used the polarization coding, the synchronization pulses can be run in a low frequency. The final key rate is 14.1 Hz. The experiment lasts for 3089 seconds with 43555 total final bits.

Optics Express, 2009

We present a secure network communication system that operated with decoy-state quantum cryptogra... more We present a secure network communication system that operated with decoy-state quantum cryptography in a real-world application scenario. The full key exchange and application protocols were performed in real time among three nodes, in which two adjacent nodes were connected by approximate 20 km of commercial telecom optical fiber. The generated quantum keys were immediately employed and demonstrated for communication applications, including unbreakable real-time voice telephone between any two of the three communication nodes, or a broadcast from one node to the other two nodes by using one-time pad encryption.

Physical Review A, 2010

We investigate the behavior of the light pulse in Λ-type cold atomic gases with two counterpropag... more We investigate the behavior of the light pulse in Λ-type cold atomic gases with two counterpropagating control lights with equal strength by directly simulating the dynamic equations and exploring the dispersion relation. Our analysis shows that, depending on the length L0 of the stored wave packet and the decay rate γ of ground-spin coherence, the recreated light can behave differently. For long L0 and/or large γ, a stationary light pulse is produced, while two propagating light pulses appear for short L0 and/or small γ. In the γ → 0 limit, the light always splits into two propagating pulses for sufficiently long time. This scenario agrees with a recent experiment [Y.-W. Lin, et al., Phys. Rev. Lett. 102, 213601(2009)] where two propagating light pulses are generated in laser-cooled cold atomic ensembles.

Physical Review Letters, 2010

We experimentally demonstrate an optical controlled-NOT (CNOT) gate with arbitrary single inputs ... more We experimentally demonstrate an optical controlled-NOT (CNOT) gate with arbitrary single inputs based on a 4-photon 6-qubit cluster state entangled both in polarization and spatial modes. We first generate the 6-qubit state, and then, by performing single-qubit measurements the CNOT gate is applied to arbitrary single input qubits. To characterize the performance of the gate, we estimate its quantum process fidelity and prove its entangling capability. In addition, our results show that the gate cannot be reproduced by local operations and classical communication. Our experiment shows that such hyper-entangled cluster states are promising candidates for efficient optical quantum computation.

Physical Review A, 2009

In this letter, we report a realization of synchronization-free quantum teleportation and narrowb... more In this letter, we report a realization of synchronization-free quantum teleportation and narrowband three-photon entanglement through interfering narrowband photon sources. Since both the single-photon and the entangled photon pair utilized are completely autonomous, it removes the requirement of high demanding synchronization technique in long-distance quantum communication with pulsed spontaneous parametric down-conversion sources. The frequency linewidth of the three-photon entanglement realized is on the order of several MHz, which matches the requirement of atomic ensemble based quantum memories. Such a narrowband multi-photon source will have applications in some advanced quantum communication protocols and linear optical quantum computation.

These authors contributed equally to this work.

We propose and demonstrate scheme for direct experimental testing of quantum commutation relation... more We propose and demonstrate scheme for direct experimental testing of quantum commutation relations for Pauli operators. The implemented device is an advanced quantum processor that involves two programmable quantum gates. Depending on a state of two-qubit program register, we can test either commutation or anti-commutation relations. Very good agreement between theory and experiment is observed, indicating high-quality performance of the implemented quantum processor and reliable verification of commutation relations for Pauli operators.

Physical Review A, 2007

We derive the lower and upper bounds on the entanglement of a given multipartite superposition st... more We derive the lower and upper bounds on the entanglement of a given multipartite superposition state in terms of the entanglement of the states being superposed. The first entanglement measure we use is the geometric measure, and the second is the q-squashed entanglement. These bounds allow us to estimate the amount of the multipartite entanglement of superpositions. We also show that two states of high fidelity to one another do not necessarily have nearly the same q-squashed entanglement. PACS numbers: 03.67.-a, 03.67.Mn, 03.65.Ud Recently, Linden et al. [1] raised the following problem: given a certain decomposition of a bipartite state |Γ as a superposition of two other states, what is the relation between the entanglement of |Γ and those of the two states being superposed? They derived upper bounds on the entanglement of |Γ in terms of those of the two states in the superposition, where the entanglement was quantified by the von Neumann entropy. Subsequently, Yu et al. [2] and Ou et al.

Multi-photon interference reveals strictly non-classical phenomena. Its applications range from f... more Multi-photon interference reveals strictly non-classical phenomena. Its applications range from fundamental tests of quantum mechanics to photonic quantum information processing, where a significant fraction of key experiments achieved so far comes from multi-photon state manipulation. We review the progress, both theoretical and experimental, of this rapidly advancing research. The emphasis is given to the creation of photonic entanglement of various forms, tests of the completeness of quantum mechanics (in particular, violations of local realism), quantum information protocols for quantum communication (e.g., quantum teleportation, entanglement purification and quantum repeater), and quantum computation with linear optics. We shall limit the scope of our review to "few photon" phenomena involving measurements of discrete observables.

We present a protocol for deterministic and highly efficient quantum cryptography with entangled ... more We present a protocol for deterministic and highly efficient quantum cryptography with entangled photon pairs in a 4×4-dimentional Hilbert space. Two communicating parties, Alice and Bob first share a both polarization-and path-entangled photon pair, and then each performs a complete Bell-state measurement on their own photon in one of two complementary Bell-state bases. It is demonstrated that each measurement in which both Alice and Bob register a photon can build certain perfect correlation and generate 1.5 key bits on average. The security of our protocol is guaranteed by the non-cloning theorem. PACS numbers: 03.67.Lx, 42.50.Ar The central task for quantum cryptography is to establish secure keys before the transmission of message between two parties [1]. In recent years, both BB84 [2] and Ek-ert91 [3] cryptographic protocols have been successfully demonstrated in numerous experiments . However, one drawback of the above schemes is the nondeterministic feature, that is, only less than 50% of all detected qubits can be further used as key bits. This is a serious practical problem, because in the one-time-pad secretkey cryptosystem, the length of secret key must be the same as the ciphertext . Although some deterministic cryptographic schemes based on orthogonal states have been proposed recently , the long storage rings required, which is an essential ingredient of those schemes, lead to a low efficiency for the transmission of polarized photons. More recently, deterministic quantum cryptography and secret direct communication have also been proposed with single-photon two-qubit states . By exploiting the states in four-or more dimensional Hilbert space, each transmitted photon can be used as a key bit with the help of classical communication; eavesdroppers can be detected within the scheme. However, every photon detected can establish only one key bit.

Physical Review Letters, 2002

We generalize Bell's inequalities to biparty systems with continuous quantum variables. This is a... more We generalize Bell's inequalities to biparty systems with continuous quantum variables. This is achieved by introducing the Bell operator in perfect analogy to the usual spin-1/2 systems. It is then demonstrated that two-mode squeezed vacuum states display quantum nonlocality by using the generalized Bell operator. In particular, the original Einstein-Podolsky-Rosen states, which are the limiting case of the two-mode squeezed vacuum states, can maximally violate Bell's inequality due to Clauser, Horne, Shimony and Holt. The experimental aspect of our scheme is briefly considered.

The preparation and storage of photonic entanglement are central to the achievement of scalable l... more The preparation and storage of photonic entanglement are central to the achievement of scalable linear optical quantum computation 1-3 (LOQC). The most widely used photonic entanglement source (a spontaneous parametric downconversion (SPDC) source) 4,5 is not directly suitable for storage, because its working frequency bandwidth is significantly larger than any available quantum memory. To remedy this problem, cavity-enhanced narrow-band SPDC sources 6-12 have been developed. However, the storage of cavity-enhanced narrow-band entangled photons has not yet been achieved. Also, the spectral correlations between the entangled photons can make them practically useless for scalable LOQC 5,13,14 .

Optics Express, 2010

We report an implementation of decoy-state quantum key distribution (QKD) over 200 km optical fib... more We report an implementation of decoy-state quantum key distribution (QKD) over 200 km optical fiber cable through photon polarization encoding. This is achieved by constructing the whole QKD system operating at 320 MHz repetition rate, and developing high-speed transmitter and receiver modules. A novel and economic way of synchronization method is designed and incorporated into the system, which allows to work at a low frequency of 40kHz and removes the use of highly precise clock. A final key rate of 15 Hz is distributed within the experimental time of 3089 seconds, by using super-conducting single photon detectors. This is longest decoy-state QKD yet demonstrated up to date. It helps to make a significant step towards practical secure communication in long-distance scope.

Physical Review Letters, 2008

We report an experimental realization of a narrow-band polarization-entangled photon source with ... more We report an experimental realization of a narrow-band polarization-entangled photon source with a linewidth of 9.6 MHz through cavity-enhanced spontaneous parametric down-conversion. This linewidth is comparable to the typical linewidth of atomic ensemble based quantum memories. Single-mode output is realized by setting a reasonable cavity length difference between different polarizations, using of temperature controlled etalons and actively stabilizing the cavity. The entangled property is characterized with quantum state tomography, giving a fidelity of 94% between our state and a maximally entangled state. The coherence length is directly measured to be 32 m through two-photon interference.

Based on principle of quantum mechanics, quantum cryptography provides an intriguing way to estab... more Based on principle of quantum mechanics, quantum cryptography provides an intriguing way to establish secret keys between remote parties, generally relying on actual transmission of signal particles.

Physical Review Letters, 2010

We experimentally demonstrate an advanced linear-optical programmable quantum processor that comb... more We experimentally demonstrate an advanced linear-optical programmable quantum processor that combines two elementary single-qubit programmable quantum gates. We show that this scheme enables direct experimental probing of quantum commutation relations for Pauli operators acting on polarization states of single photons. Depending on a state of two-qubit program register, we can probe either commutation or anticommutation relations. Very good agreement between theory and experiment is observed, indicating high-quality performance of the implemented quantum processor.

Physical Review A, 2007

We present a detailed analysis of a robust quantum repeater architecture building on the original... more We present a detailed analysis of a robust quantum repeater architecture building on the original Duan-Lukin-Cirac-Zoller ͑DLCZ͒ protocol ͓L.M. Duan et al. Nature ͑London͒ 414, 413 ͑2001͔͒. The architecture is based on two-photon Hong-Ou-Mandel-type interference which relaxes the long-distance interferometric stability requirements by about seven orders of magnitude, from subwavelength for the single photon interference required by DLCZ to the coherence length of the photons, thereby removing the weakest point in the DLCZ scheme. Our proposal provides an exciting possibility for robust and realistic long-distance quantum communication.

Nature Photonics, 2011

The preparation and storage of photonic entanglement are central to the achievement of scalable l... more The preparation and storage of photonic entanglement are central to the achievement of scalable linear optical quantum computation (LOQC). The most widely used photonic entanglement source (a spontaneous parametric downconversion (SPDC) source) is not directly suitable for storage, because its working frequency bandwidth is significantly larger than any available quantum memory. To remedy this problem, cavity-enhanced narrow-band SPDC sources have been developed. However, the storage of cavity-enhanced narrow-band entangled photons has not yet been achieved. Also, the spectral correlations between the entangled photons can make them practically useless for scalable LOQC. Here, we report the preparation and storage of frequency-uncorrelated narrowband (5 MHz) entangled photons from a cavity-enhanced SPDC source. The frequency correlation between the entangled photons is eliminated by changing the continuous UV pumping beam to short pulses. The storage of the polarization state of a single photon, and of a photon entangled with another flying in the fibre, is demonstrated. Our work demonstrates a quantum interface between narrow-band entangled photons from cavity SPDC and atomic quantum memory, and thus provides an important tool towards the achievement of all-optical quantum information processing.

Nature Photonics, 2011

The paradigm of measurement-based quantum computation opens new experimental avenues to realizing... more The paradigm of measurement-based quantum computation opens new experimental avenues to realizing a quantum computer, and also deepens our understanding of quantum physics. Measurement-based quantum computation originates with a highly entangled universal resource state. For years, clusters states have been the only known universal resources. Surprisingly, a novel framework, namely quantum computation in correlation space, has opened a new route to implementing measurement-based quantum computation based on quantum states having entanglement properties, which differ from cluster states. Here, we report an experimental demonstration of every building block of such a model. With four-qubit and six-qubit states, which are not in the cluster-state category, we have realized a universal set of single-qubit rotations, two-qubit entangling gates and also Deutsch's algorithm. As well as being of fundamental interest, our experiment proves, in principle, the feasibility of universal measurement-based quantum computation without the use of cluster states, which represents a new approach towards the realization of a quantum computer.

Physical Review Letters, 2009

Anyons are exotic quasiparticles living in two dimensions that do not fit into the usual categori... more Anyons are exotic quasiparticles living in two dimensions that do not fit into the usual categories of fermions and bosons, but obey a new form of fractional statistics. Following a recent proposal [Phys. Rev. Lett. 98, 150404 (2007)], we present an experimental demonstration of the fractional statistics of anyons in the Kitaev spin lattice model using a photonic quantum simulator. We dynamically create the ground state and excited states (which are six-qubit graph states) of the Kitaev model Hamiltonian, and implement the anyonic braiding and fusion operations by single-qubit rotations. A phase shift of π related to the anyon braiding is observed, confirming the prediction of the fractional statistics of Abelian 1/2-anyons.

We demonstrate the decoy-state quantum key distribution over 200 km with photon polarization thro... more We demonstrate the decoy-state quantum key distribution over 200 km with photon polarization through optical fiber, by using superconducting single photon detector with a repetition rate of 320 Mega Hz and a dark count rate of lower than 1 Hz. Since we have used the polarization coding, the synchronization pulses can be run in a low frequency. The final key rate is 14.1 Hz. The experiment lasts for 3089 seconds with 43555 total final bits.

Optics Express, 2009

We present a secure network communication system that operated with decoy-state quantum cryptogra... more We present a secure network communication system that operated with decoy-state quantum cryptography in a real-world application scenario. The full key exchange and application protocols were performed in real time among three nodes, in which two adjacent nodes were connected by approximate 20 km of commercial telecom optical fiber. The generated quantum keys were immediately employed and demonstrated for communication applications, including unbreakable real-time voice telephone between any two of the three communication nodes, or a broadcast from one node to the other two nodes by using one-time pad encryption.

Physical Review A, 2010

We investigate the behavior of the light pulse in Λ-type cold atomic gases with two counterpropag... more We investigate the behavior of the light pulse in Λ-type cold atomic gases with two counterpropagating control lights with equal strength by directly simulating the dynamic equations and exploring the dispersion relation. Our analysis shows that, depending on the length L0 of the stored wave packet and the decay rate γ of ground-spin coherence, the recreated light can behave differently. For long L0 and/or large γ, a stationary light pulse is produced, while two propagating light pulses appear for short L0 and/or small γ. In the γ → 0 limit, the light always splits into two propagating pulses for sufficiently long time. This scenario agrees with a recent experiment [Y.-W. Lin, et al., Phys. Rev. Lett. 102, 213601(2009)] where two propagating light pulses are generated in laser-cooled cold atomic ensembles.

Physical Review Letters, 2010

We experimentally demonstrate an optical controlled-NOT (CNOT) gate with arbitrary single inputs ... more We experimentally demonstrate an optical controlled-NOT (CNOT) gate with arbitrary single inputs based on a 4-photon 6-qubit cluster state entangled both in polarization and spatial modes. We first generate the 6-qubit state, and then, by performing single-qubit measurements the CNOT gate is applied to arbitrary single input qubits. To characterize the performance of the gate, we estimate its quantum process fidelity and prove its entangling capability. In addition, our results show that the gate cannot be reproduced by local operations and classical communication. Our experiment shows that such hyper-entangled cluster states are promising candidates for efficient optical quantum computation.

Physical Review A, 2009

In this letter, we report a realization of synchronization-free quantum teleportation and narrowb... more In this letter, we report a realization of synchronization-free quantum teleportation and narrowband three-photon entanglement through interfering narrowband photon sources. Since both the single-photon and the entangled photon pair utilized are completely autonomous, it removes the requirement of high demanding synchronization technique in long-distance quantum communication with pulsed spontaneous parametric down-conversion sources. The frequency linewidth of the three-photon entanglement realized is on the order of several MHz, which matches the requirement of atomic ensemble based quantum memories. Such a narrowband multi-photon source will have applications in some advanced quantum communication protocols and linear optical quantum computation.

These authors contributed equally to this work.

We propose and demonstrate scheme for direct experimental testing of quantum commutation relation... more We propose and demonstrate scheme for direct experimental testing of quantum commutation relations for Pauli operators. The implemented device is an advanced quantum processor that involves two programmable quantum gates. Depending on a state of two-qubit program register, we can test either commutation or anti-commutation relations. Very good agreement between theory and experiment is observed, indicating high-quality performance of the implemented quantum processor and reliable verification of commutation relations for Pauli operators.