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While the advanced coherent control of **qubits** is now routinely carried out in low **frequency** (GHz) systems like single spins, it is far more challenging to achieve for two-level systems in the optical domain. This is because the latter evolve typically in the THz range, calling for tools of ultrafast, coherent, nonlinear optics. Using four-wave mixing micro-spectroscopy, we here measure the optically driven Bloch vector dynamics of a single exciton confined in a semiconductor quantum dot. In a combined experimental and theoretical approach, we reveal the intrinsic Rabi **oscillation** dynamics by monitoring both central exciton quantities, i.e., its occupation and the microscopic coherence, as resolved by the four-wave mixing technique. In the **frequency** domain this **oscillation** generates the Autler-Townes splitting of the light-exciton dressed states, directly seen in the four-wave mixing spectra. We further demonstrate that the coupling to acoustic phonons strongly influences the FWM dynamics on the picosecond timescale, because it leads to transitions between the dressed states.

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While the advanced coherent control of **qubits** is now routinely carried out in low **frequency** (GHz) systems like single spins, it is far more challenging to achieve for two-level systems in the optical domain. This is because the latter evolve typically in the THz range, calling for tools of ultrafast, coherent, nonlinear optics. Using four-wave mixing micro-spectroscopy, we here measure the optically driven Bloch vector dynamics of a single exciton confined in a semiconductor quantum dot. In a combined experimental and theoretical approach, we reveal the intrinsic Rabi **oscillation** dynamics by monitoring both central exciton quantities, i.e., its occupation and the microscopic coherence, as resolved by the four-wave mixing technique. In the **frequency** domain this **oscillation** generates the Autler-Townes splitting of the light-exciton dressed states, directly seen in the four-wave mixing spectra. We further demonstrate that the coupling to acoustic phonons strongly influences the FWM dynamics on the picosecond timescale, because it leads to transitions between the dressed states.

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- Collection

We implement distinct quantum gates in parallel on two entangled **frequency**-bin **qubits** for the first time. Our basic quantum operation controls the spectral overlap between adjacent **frequency** bins, allowing us to observe **frequency**-bin Hong-Ou-Mandel interference with a visibility of 0.971±0.007, a record for two photons of different colors. By integrating this tunability with **frequency** parallelization, we synthesize independent gates on entangled **qubits** in the same optical fiber and flip their spectral correlations. The ultralow noise of our gates preserves entanglement purity, as evidenced by a 9.8σ violation of an entropic separability bound. Our results constitute the first closed, user-defined gates on **frequency**-bin **qubits** in parallel, with application to the development of **frequency**-based quantum information processing.

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Large-scale quantum technologies require exquisite control over many individual quantum systems. Typically, such systems are very sensitive to environmental fluctuations, and diagnosing errors via measurements causes unavoidable perturbations. In this work we present an in situ **frequency** locking technique that monitors and corrects **frequency** variations in single photon sources based on microring resonators. By using the same classical laser fields required for photon generation as a probe to diagnose variations in the resonator **frequency**, our protocol applies feedback control to correct photon **frequency** errors in parallel to the optical quantum computation without disturbing the physical **qubit**. We implement our technique on a silicon photonic device and demonstrate sub 1 pm **frequency** stabilization in the presence of applied environmental noise, corresponding to a fractional **frequency** drift of <1% of a photon linewidth. Using these methods we demonstrate feedback controlled quantum state engineering. By distributing a single local **oscillator** across a single chip or network of chips, our approach enables **frequency** locking of many single photon sources for large-scale photonic quantum technologies.

Data Types:

- Collection

Large-scale quantum technologies require exquisite control over many individual quantum systems. Typically, such systems are very sensitive to environmental fluctuations, and diagnosing errors via measurements causes unavoidable perturbations. In this work we present an in situ **frequency** locking technique that monitors and corrects **frequency** variations in single photon sources based on microring resonators. By using the same classical laser fields required for photon generation as a probe to diagnose variations in the resonator **frequency**, our protocol applies feedback control to correct photon **frequency** errors in parallel to the optical quantum computation without disturbing the physical **qubit**. We implement our technique on a silicon photonic device and demonstrate sub 1 pm **frequency** stabilization in the presence of applied environmental noise, corresponding to a fractional **frequency** drift of <1% of a photon linewidth. Using these methods we demonstrate feedback controlled quantum state engineering. By distributing a single local **oscillator** across a single chip or network of chips, our approach enables **frequency** locking of many single photon sources for large-scale photonic quantum technologies.

Data Types:

- Collection

We implement distinct quantum gates in parallel on two entangled **frequency**-bin **qubits** for the first time. Our basic quantum operation controls the spectral overlap between adjacent **frequency** bins, allowing us to observe **frequency**-bin Hong-Ou-Mandel interference with a visibility of 0.971±0.007, a record for two photons of different colors. By integrating this tunability with **frequency** parallelization, we synthesize independent gates on entangled **qubits** in the same optical fiber and flip their spectral correlations. The ultralow noise of our gates preserves entanglement purity, as evidenced by a 9.8σ violation of an entropic separability bound. Our results constitute the first closed, user-defined gates on **frequency**-bin **qubits** in parallel, with application to the development of **frequency**-based quantum information processing.

Data Types:

- Collection

Among the objectives for large-scale quantum computation is the quantum interconnect: a device that uses photons to interface **qubits** that otherwise could not interact. However, the current approaches require photons indistinguishable in **frequency**—a major challenge for systems experiencing different local environments or of different physical compositions altogether. Here, we develop an entirely new platform that actually exploits such **frequency** mismatch for processing quantum information. Labeled “spectral linear optical quantum computation” (spectral LOQC), our protocol offers favorable linear scaling of optical resources and enjoys an unprecedented degree of parallelism, as an arbitrary N-**qubit** quantum gate may be performed in parallel on multiple N-**qubit** sets in the same linear optical device. Not only does spectral LOQC offer new potential for optical interconnects, but it also brings the ubiquitous technology of high-speed fiber optics to bear on photonic quantum information, making wavelength-configurable and robust optical quantum systems within reach.

Data Types:

- Collection

Among the objectives for large-scale quantum computation is the quantum interconnect: a device that uses photons to interface **qubits** that otherwise could not interact. However, the current approaches require photons indistinguishable in **frequency**—a major challenge for systems experiencing different local environments or of different physical compositions altogether. Here, we develop an entirely new platform that actually exploits such **frequency** mismatch for processing quantum information. Labeled “spectral linear optical quantum computation” (spectral LOQC), our protocol offers favorable linear scaling of optical resources and enjoys an unprecedented degree of parallelism, as an arbitrary N-**qubit** quantum gate may be performed in parallel on multiple N-**qubit** sets in the same linear optical device. Not only does spectral LOQC offer new potential for optical interconnects, but it also brings the ubiquitous technology of high-speed fiber optics to bear on photonic quantum information, making wavelength-configurable and robust optical quantum systems within reach.

Data Types:

- Collection

It is shown that the saw-tooth variation of the cavity length in a photorefractive semilinear coherent **oscillator** can suppress the instability in the **frequency** domain and prevent a bifurcation in the **oscillation** spectrum. To achieve such a suppression the **frequency** of the cavity length modulation should be chosen appropriately. It depends on the photorefractive crystal parameters (electrooptic properties, photoconductivity, dimensions) and on the experimental conditions (pump intensity ratio, orientation of the pump and **oscillation** waves with respect to the crystallographic axes, polarization of the pump waves, etc.). It depends also strongly on a possible misalignment of the two pump waves. On the other hand, within a certain range of the experimental parameters the mirror vibration may lead to a further **frequency** splitting in the already existing two-mode **oscillation** spectrum.

Data Types:

- Collection

It is shown that the saw-tooth variation of the cavity length in a photorefractive semilinear coherent **oscillator** can suppress the instability in the **frequency** domain and prevent a bifurcation in the **oscillation** spectrum. To achieve such a suppression the **frequency** of the cavity length modulation should be chosen appropriately. It depends on the photorefractive crystal parameters (electrooptic properties, photoconductivity, dimensions) and on the experimental conditions (pump intensity ratio, orientation of the pump and **oscillation** waves with respect to the crystallographic axes, polarization of the pump waves, etc.). It depends also strongly on a possible misalignment of the two pump waves. On the other hand, within a certain range of the experimental parameters the mirror vibration may lead to a further **frequency** splitting in the already existing two-mode **oscillation** spectrum.

Data Types:

- Collection