Multi-stage coherent beam combination of semiconductor optical amplifiers in ready-made fiber couplers.

We report on multi-stage coherent beam combination (CBC) of continuous-wave (CW) outputs from semiconductor optical amplifiers (SOAs) in ready-made fiber couplers. The first CBC stage combines two 120-mW outputs from SOAs seeded by an extended-cavity diode laser (ECDL) at 1458 nm in a 2×2 50%:50% fiber coupler. Two beams generated by two such CBC setups are then combined in the second stage. By concatenating three stages we obtained an output power of 723 mW at 1458 nm from eight SOAs with a total combining efficiency of 75.3%. Stable power generation without interrupts nor degradation over three days was successfully implemented using a simple low-bandwidth servo system. An averaged single-stage combining efficiency of 89.5% deduced from seven CBC setups constituting the three-stage CBC is used to estimate scaling to further stages. As a practical application the output is used to second harmonic generation (SHG) in a nonlinear crystal to achieve an output power of 239 mW at 729 nm.

Frequency ratio of an 115In+ ion clock and a 87Sr optical lattice clock.

We report on the first, to the best of our knowledge, frequency ratio measurement of an 115In+ singleion clock and a 87Sr optical lattice clock. A hydrogen maser serves as a flywheel oscillator to measure the ratio by independent optical combs. From 89,000 s of measurement time, the frequency ratio fIn/fSr is determined to be 2.952 748 749 874 863 3(23) with 7.7×10-16 relative uncertainty. The measurement creates a new connection in the network of frequency ratios of optical clocks.

Long-Distance Single Photon Transmission from a Trapped Ion via Quantum Frequency Conversion.

Trapped atomic ions are ideal single photon emitters with long-lived internal states which can be entangled with emitted photons. Coupling the ion to an optical cavity enables the efficient emission of single photons into a single spatial mode and grants control over their temporal shape. These features are key for quantum information processing and quantum communication. However, the photons emitted by these systems are unsuitable for long-distance transmission due to their wavelengths. Here we report the transmission of single photons from a single ^{40}Ca^{+} ion coupled to an optical cavity over a 10 km optical fiber via frequency conversion from 866 nm to the telecom C band at 1530 nm. We observe nonclassical photon statistics of the direct cavity emission, the converted photons, and the 10 km transmitted photons, as well as the preservation of the photons' temporal shape throughout. This telecommunication-ready system can be a key component for long-distance quantum communication as well as future cloud quantum computation.

Frequency measurement of the clock transition of an indium ion sympathetically-cooled in a linear trap.

We report frequency measurement of the clock transition in an 115In+ ion sympathetically-cooled with Ca+ ions in a linear rf trap. The Ca+ ions are used as a probe of the external electromagnetic field and as the coolant for preparing the cold In+. The frequency is determined to be 1 267 402 452 901 049.9 (6.9) Hz by averaging 36 measurements using an optical frequency comb referenced to the frequency standards located in the same site.

Frequency Measurement System of Optical Clocks Without a Flywheel Oscillator.

We developed a system for the remote frequency comparison of optical clocks. The system does not require a flywheel oscillator at the remote end, making it possible to evaluate optical frequencies even in laboratories, where no stable microwave reference, such as an Rb clock, a Cs clock, or a hydrogen maser exists. The system is established by the integration of several systems: a portable carrier-phase two-way satellite frequency transfer station and a microwave signal generation system by an optical frequency comb from an optical clock. The measurement was as quick as a conventional method that employs a local microwave reference. We confirmed the system uncertainty and instability to be at the low 10-15 level using an Sr lattice clock.

Direct comparison of a Ca+ single-ion clock against a Sr lattice clock to verify the absolute frequency measurement.

Optical frequency comparison of the (40)Ca(+) clock transition ν(Ca)((2)S(1/2-)(2D(5/2), 729 nm) against the (87)Sr optical lattice clock transition ν(Sr) ((1)S(0)-(3)P(0), 698 nm) has resulted in a frequency ratio ν(Ca) / ν(Sr) = 0.957 631 202 358 049 9(2 3). The rapid nature of optical comparison allowed the statistical uncertainty of frequency ratio ν(Ca) / ν(Sr) to reach 1 × 10(-15) in 1000s and yielded a value consistent with that calculated from separate absolute frequency measurements of ν(Ca) using the International Atomic Time (TAI) link. The total uncertainty of the frequency ratio using optical comparison (free from microwave link uncertainties) is smaller than that obtained using absolute frequency measurement, demonstrating the advantage of optical frequency evaluation. We note that the absolute frequency of (40)Ca(+) we measure deviates from other published values by more than three times our measurement uncertainty.

Modulation-free optical locking of an external-cavity diode laser to a filter cavity.

Optical locking to a filter cavity is an effective method to eliminate the limitations of an external-cavity diode laser, such as broad spontaneous emission backgrounds and frequency jitters. Stable operation of the optical locking requires simultaneous control of the feedback phase and the diode-laser frequency. Frequency dither is usually used to extract the two error signals, but this causes extra frequency modulation in the output beam. A modulation-free method for deriving the error signals by modulating the laser-cavity coupling strength is demonstrated with a violet diode laser. A modulation-free linewidth upper limit of about 7 kHz for a 1 s measurement is realized by the method.

Optical continuous-variable qubit.

In a new branch of quantum computing, information is encoded into coherent states, the primary carriers of optical communication. To exploit it, quantum bits of these coherent states are needed, but it is notoriously hard to make superpositions of such continuous-variable states. We have realized the complete engineering and characterization of a qubit of two optical continuous-variable states. Using squeezed vacuum as a resource and a special photon-subtraction technique, we could with high precision prepare an arbitrary superposition of squeezed vacuum and a squeezed single photon. This could lead the way to demonstrations of coherent state quantum computing.

Generation of large-amplitude coherent-state superposition via ancilla-assisted photon subtraction.

We propose and demonstrate a novel method to generate a large-amplitude coherent-state superposition (CSS) via ancilla-assisted photon subtraction. The ancillary mode induces quantum interference of indistinguishable processes in an extended space, widening the controllability of quantum superposition at the conditional output. We demonstrate this by a simple time-separated two-photon subtraction from continuous wave squeezed light. We observe the largest CSS of traveling light ever reported without correcting any imperfections, which will enable various quantum information applications with CSS states.

Compact, filtered diode laser system for precision spectroscopy.

Stable, narrow-linewidth optical sources are necessary in modern atomic physics. An appealing approach to achieving approximately 10 kHz frequency stability is optical feedback. We have designed a compact external cavity diode laser with optical feedback to a filter cavity mounted on a single baseplate and enclosed inside a vacuum sealed box. The design was implemented for three wavelengths addressing the 422 nm cooling, 1091 nm repumping, and 674 nm clock transition lines of Sr(+). We are able to cool a single, trapped strontium ion to approximately 2 mK and observe motional sidebands of the 5S(1/2) <--> 4D(5/2) transition.

Generation of two-mode bright squeezed light using a noise-suppressed amplified diode laser.

We present the generation of nonclassical state using an amplified diode laser as a light source. The intensity noise of an amplified diode laser was significantly suppressed and reached the shot noise limit at 15 MHz using both a filter cavity and resonant optical feedback. Frequency doubling efficiency of 66% and up to 120 mW output power of green has been achieved in cw second-harmonic generation from 1080 nm to 540 nm. Bright two-mode amplitude-squeezed state was generated from a type-II nondegenerate optical parametric amplifier pumped by generated green light. The measured noise reduction is 2.1+/-0.2 dB below the shot-noise level.

Continuous generation of single photons with controlled waveform in an ion-trap cavity system.

The controlled production of single photons is of fundamental and practical interest; they represent the lowest excited quantum states of the radiation field, and have applications in quantum cryptography and quantum information processing. Common approaches use the fluorescence of single ions, single molecules, colour centres and semiconductor quantum dots. However, the lack of control over such irreversible emission processes precludes the use of these sources in applications (such as quantum networks) that require coherent exchange of quantum states between atoms and photons. The necessary control may be achieved in principle in cavity quantum electrodynamics. Although this approach has been used for the production of single photons from atoms, such experiments are compromised by limited trapping times, fluctuating atom-field coupling and multi-atom effects. Here we demonstrate a single-photon source based on a strongly localized single ion in an optical cavity. The ion is optimally coupled to a well-defined field mode, resulting in the generation of single-photon pulses with precisely defined shape and timing. We have confirmed the suppression of two-photon events up to the limit imposed by fluctuations in the rate of detector dark counts. The stream of emitted photons is uninterrupted over the storage time of the ion, as demonstrated by a measurement of photon correlations over 90 min.

Generation of twin beams from an optical parametric oscillator pumped by a frequency-doubled diode laser.

Quantum-correlated twin beams were generated from a triply resonant optical parametric oscillator with an a-cut KTP crystal pumped by a frequency-doubled diode laser. A total output of 5.1 mW was obtained in the classical-nonclassical light-conversion system driven by a 50-mW diode laser at 1080 nm. A quantum-noise reduction of 4.3 dB (63%) in the intensity difference between the twin beams was successfully observed at the detection frequency of 3 MHz.

Generation of 22.8 mW single-frequency green light by frequency doubling of a 50-mW diode laser.

We report frequency doubling of an extended-cavity diode laser in an a-cut KTP crystal. Continuous-wave, single-mode green light at 540 nm with a power of 22.8 mW was generated from an input of 44.2 mW by type II non-critical phase matching. Stable operation with intensity fluctuations smaller than 1 % peak-to-peak for more than two hours was achieved by employing resonant optical feedback. The optical feedback also reduced the fundamental-wave linewidth to the upper limit of 41 kHz.

Quantum channel using photon number correlated twin beams.

We report quantum communications channel using photon number correlated twin beams. The twin beams are generated from a nondegenerate optical parametric oscillator, and the photon number difference is used to encode the signal. The bit error rate of our system will be 0.067 by using the twin beams comparing with 0.217 by using the coherent state as the signal carrier.