[Show abstract][Hide abstract] ABSTRACT: The influences of intracavity nonlinear losses on the intensity noise of output lasers are theoretically and experimentally investigated with an all-solid-state single-frequency laser with high gain. By means of tuning the temperature of a nonlinear crystal deliberately placed inside the laser resonator, the intracavity nonlinear loss of the laser is controlled. The dependence of the frequency and amplitude of the resonant relaxation oscillation peak on the nonlinear loss is studied in detail for both fundamental-wave (FW) and second-harmonic-wave (SHW). We find that, by controlling the temperature of the nonlinear crystal, the intensity noises of the laser can be transferred between FW and SHW. The theoretical predictions are in good agreement with the experimental measurements. The obtained results can be applied to manipulate and suppress the laser noises.
[Show abstract][Hide abstract] ABSTRACT: Quantum error correction protects the quantum state against noise and decoherence in quantum communication and quantum computation, which enables one to perform fault-torrent quantum information processing. We experimentally demonstrate a quantum error correction scheme with a five-wave-packet code against a single stochastic error, the original theoretical model of which was firstly proposed by S. L. Braunstein and T. A. Walker. Five submodes of a continuous variable cluster entangled state of light are used for five encoding channels. Especially, in our encoding scheme the information of the input state is only distributed on three of the five channels and thus any error appearing in the remained two channels never affects the output state, i.e. the output quantum state is immune from the error in the two channels. The stochastic error on a single channel is corrected for both vacuum and squeezed input states and the achieved fidelities of the output states are beyond the corresponding classical limit.
[Show abstract][Hide abstract] ABSTRACT: The generation and storage of entangled photons play important roles in
quantum information technique. Spontaneous Raman scattering (SRS) in atomic
ensembles provides a promising method to generate entangled photons capable of
storage. In the past experiments, a spin-wave-photon entangled state is
produced via SRS in an atomic ensemble, with which a pair of entangled photons
is obtained. Here, we report a scheme of simultaneously generating two
spin-wave-photon entangled states in an atomic ensemble by collecting Stokes
photons at two different directions. Based on the obtained two atom-photon
entangled sources, we generate a three-photon GHZ polarization-entangled state
and conditionally prepare a polarization-entangled photon pair, respectively.
[Show abstract][Hide abstract] ABSTRACT: A balanced homodyne detector, with a maximum common mode rejection ratio and clearance of 75.2 dB and 37 dB, is experimentally obtained with two arbitrary photodiodes of the same model. On the basis of self-subtraction photodetector scheme, we divide the influence of photodiodes on the common mode rejection ratio into two parts, including magnitude and phase of output signal. The discrepancy of quantum efficiency and dark current affects magnitude of output signal of photodiodes, which is compensated by adjusting the splitter ratio. The difference of the equivalent capacitance and resistance affects the phase of output signal of photodiodes, which is compensated by the differential fine tuning circuit and adjustable bias voltage circuit. With these designs, the developed homodyne detector can be used for measuring accurately the squeezed state.
[Show abstract][Hide abstract] ABSTRACT: The non-measurement based coherent feedback control (CFC) is a control method without
introducing any backaction noise into the controlled system, thus is specially
suitable to manipulate various quantum optical systems for preparing nonclassical
states of light. By simply tuning the transmissivity of an optical controller in a
CFC loop attached to a non-degenerate optical parametric amplifier (NOPA), the
quantum entanglement degree of the output optical entangled state of the system is
improved. At the same time, the threshold pump power of the NOPA is reduced also.
The experimental results are in reasonable agreement with the theoretical
[Show abstract][Hide abstract] ABSTRACT: Entangled state of light is one of the essential quantum resources in quantum
information science and technology. Especially, when the fundamental principle
experiments have been achieved in labs and the applications of continuous
variable quantum information in the real world are considered, it is crucial to
design and construct the generation devices of entangled states with high
entanglement and compact configuration. We have designed and built an efficient
and compact light source of entangled state, which is a non-degenerate optical
parametric amplifier (NOPA) with the triple resonance of the pump and two
subharmonic modes. A wedged type-II KTP crystal inside the NOPA is used for
implementing frequency-down-conversion of the pump field to generate the
optical entangled state and achieving the dispersion compensation between the
pump and the subharmonic waves. The EPR entangled state of light with quantum
correlations of 8.4 dB for both amplitude and phase quadratures are
experimentally produced by a single NOPA under the pump power of 75 mW.
[Show abstract][Hide abstract] ABSTRACT: The thermal lens effect of the TGG crystal is investigated theoretically and experimentally. The theoretical analysis is demonstrated by the experimental measurements on a home-made frequency-doubled Nd:YVO4 laser with single-frequency operation. In the presence of the thermal lens effect of the TGG crystal, the output power can be optimized by shortening the distance between the cavity mirrors of M3 and M4 (two plane-concave mirrors placed at two sides of the second-harmonic generator). Consequently, a single-frequency laser with output power of 18.7 W at 532 nm is obtained. The power stability and the beam quality M2 are better than ±0.4% for 5 hours and 1.08, respectively. Meanwhile, we observe and discuss a bistability-like phenomenon of the laser in the cases of increasing and decreasing the incident pump power.
[Show abstract][Hide abstract] ABSTRACT: A single frequency Ti:sapphire (Ti:S) laser with continuous frequency-tuning and low intensity noise is presented, in which an extra nonlinear (NL) loss crystal is placed inside the resonator instead of the traditional etalon locking system. When a NL crystal is inserted into a home-made Ti:S laser resonator, the single frequency laser of 1.27 W at 795 nm with a continuous frequency-tuning range of 48 GHz is realized under the pump level of 11.27 W and the intensity noise at the lower frequencies is successfully suppressed.
[Show abstract][Hide abstract] ABSTRACT: Using a six-mode continuous variable cluster state as ancillary state, we experimentally demonstrate a cascaded Gaussian quantum logic operation consisting of a single-mode squeezing gate and a two-mode CZ gate.
[Show abstract][Hide abstract] ABSTRACT: Single-mode squeezing and Fourier transformation operations are two essential
logical gates in continuous-variable quantum computation, which have been
experimentally implemented by means of an optical four-mode cluster state. In
this paper, we present a simpler and more efficient protocol based on the use
of Einstein-Podolsky-Rosen two-mode entangled states to realize the same
operations. The theoretical calculations and the experimental results
demonstrate that the presented scheme not only decreases the requirement to the
resource quantum states at the largest extent but also enhances significantly
the squeezing degree and the fidelity of the resultant modes under an identical
resource condition. That is because in our system the influence of the excess
noises deriving from the imperfect squeezing of the resource states is
degraded. The gate operations applying two-mode entanglement can be utilized as
a basic element in a future quantum computer involving a large-scale cluster
Physical Review A 03/2014; 89(3). DOI:10.1103/PhysRevA.89.032311 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The optimal physical conditions of single-longitudinal-mode (SLM) operation for continuous-wave all-solid-state lasers with high output powers are investigated theoretically and experimentally. The dependence of the operation conditions on the linear and nonlinear intracavity losses of the laser is numerically calculated. The theoretical analysis is demonstrated by the experimental measurements on a home-made Nd:YVO<sub>4</sub> laser. The stable SLM output up to 33.7 W with optical-optical conversion efficiency of 44.9% at 1064 nm wavelength is recorded for over 7 h. The experimental results are in good agreement with the theoretical expectation.
[Show abstract][Hide abstract] ABSTRACT: Long-lived and high-fidelity memory for photonic polarization qubit (PPQ) is
crucial for constructing quantum networks. Here we present an EIT-based
millisecond storage system in which a moderate magnetic field is applied on a
cold-atom cloud to lift Zeeman degeneracy. PPQ states are stored as two
magnetic-field-insensitive spin waves. Especially, the influence of
magnetic-field-sensitive spin waves on the storage performances is almost
totally avoided. The measured average fidelities of polarization states are
98.6% at 200 us and 78.4% at 4.5 ms, respectively.
[Show abstract][Hide abstract] ABSTRACT: Measurement-based one-way quantum computation using cluster states as resources provides an efficient model to perform computation and information processing of quantum codes. Arbitrary Gaussian quantum computation can be implemented sufficiently by long single-mode and two-mode gate sequences. However, continuous variable gate sequences have not been realized so far due to an absence of cluster states larger than four submodes. Here we present the first continuous variable gate sequence consisting of a single-mode squeezing gate and a two-mode controlled-phase gate based on a six-mode cluster state. The quantum property of this gate sequence is confirmed by the fidelities and the quantum entanglement of two output modes, which depend on both the squeezing and controlled-phase gates. The experiment demonstrates the feasibility of implementing Gaussian quantum computation by means of accessible gate sequences.
[Show abstract][Hide abstract] ABSTRACT: We present a diode-pumped broadband tunable single-frequency and frequency-doubling Nd:YVO<sub>4</sub>/LBO laser with high output power of 10.5 W in all tuning ranges around 532 nm. An etalon placed inside the resonator and the laser gain medium in a wedge shape are used for the coarse- and fine-tuning elements, respectively. By independently scanning the temperatures of the two tuning elements, broadband tunable ranges of 12 and 24 GHz have been achieved, respectively, for the fundamental and the second-harmonic waves.
[Show abstract][Hide abstract] ABSTRACT: Multi-color entangled states of light including low-loss optical fiber
transmission and atomic resonance frequencies are essential resources for
future quantum information network. We present the experimental achievement on
the three-color entanglement generation at 852 nm, 1550 nm and 1440 nm
wavelengths for optical continuous variables. The entanglement generation
system consists of two cascaded non-degenerated optical parametric oscillators
(NOPOs). The flexible selectivity of nonlinear crystals in the two NOPOs and
the tunable property of NOPO provide large freedom for the frequency selection
of three entangled optical beams, so the present system is possible to be
developed as practical devices used for quantum information networks with
atomic storage units and long fiber transmission lines.
[Show abstract][Hide abstract] ABSTRACT: The preparation of multipartite entangled states is the prerequisite for
exploring quantum information networks and quantum computation. In this letter,
we present the first experimental demonstration of eight-partite spatially
separated CV entangled states. The initial resource quantum states are eight
squeezed states of light, through the linearly optical transformation of which
two types of the eight-partite cluster entangled states are prepared,
respectively. The generated eight entangled photonic qumodes are spatially
separated, which provide valuable quantum resources to implement more
complicated quantum information task.
[Show abstract][Hide abstract] ABSTRACT: Electromagnetically induced transparency (EIT) techniques are important tools for the storage of the quantum states of light fields in atomic ensembles and for enhancement of the interaction between photons. In this paper, we briefly summarize the recent experimental studies conducted by our group on enhanced cross-phase modulation based on double EIT effects, the quantum interference of stored dual-channel spin-wave excitations and the coherent manipulation of the spin wave vector for the polarization of photons in a single tripod atomic system. The work presented here has potential application in the developing field of quantum information processing.
Chinese Science Bulletin 06/2012; 57(16). DOI:10.1007/s11434-012-5090-6 · 1.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The superactivation of multipartite bound entanglement (BE) is a special
protocol proposed by Shor et al. in 2003, which can distill
Einstein-Podolsky-Rosen (EPR) entanglement states between two subsystems
of two multipartite BE states. Here we present the first experimental
realization of the superactivation of the BE state, in which two copies
of the four-partite unlockable BE state in a continuous-variable regime
are used. Coupling two thermal states with Gaussian noises into two
submodes of an EPR entangled state on two 50-50 beam splitters
respectively, the four output optical modes form a four-partite
unlocklable BE state. Using two EPR entangled states, we experimentally
produce two BE states first. Then through a superactivation operation
involving measurements and feedback on the two BE states, an EPR
entangled state is distilled out between two designated parties of the
two four-partite BE states. The experiment demonstrates the
superadditivity of quantum entanglement as the individual BE state
cannot be distilled, only two BE states together can be distilled.
[Show abstract][Hide abstract] ABSTRACT: Cluster state is the essential resource for one-way quantum computing. Here, we present the latest experimental achievement on the preparation of eight-partite linear and two-diamond shape cluster states with continuous variable entanglement.