Yu Ping Huang

Mode-resolved photon counting via cascaded quantum frequency conversion.

Resources for the manipulation and measurement of high-dimensional photonic signals are crucial for implementing qudit-based applications. Here we propose potentially high-performance, chip-compatible devices for such purposes by exploiting quantum frequency conversion in nonlinear optical media. Specifically, by using sum-frequency generation in a χ(2) waveguide, we show how mode-resolved photon counting can be accomplished for telecom-band photonic signals subtending multiple temporal modes. Our method is generally applicable to any nonlinear medium with arbitrary dispersion properties.

Optical sum-frequency generation in a whispering-gallery-mode resonator

We demonstrate sum-frequency generation between a telecom wavelength and the Rb D2 line, achieved through natural phase matching in a nonlinear whispering gallery mode resonator. Due to the strong optical field confinement and ultra high Q of the cavity, the process saturates already at sub-mW pump peak power, at least two orders of magnitude lower than in existing waveguide-based devices. The experimental data are in agreement with the nonlinear dynamics and phase matching theory based on spherical geometry. Our experimental and theoretical results point toward a new platform for manipulating the color and quantum states of light waves for applications such as atomic memory based quantum networking and logic operations with optical signals.

Multidimensional mode-separable frequency conversion for high-speed quantum communication

Quantum frequency conversion (QFC) of photonic signals preserves quantum information while simultaneously changing the signal wavelength. A common application of QFC is to translate the wavelength of a signal compatible with the current fiber-optic infrastructure to a shorter wavelength more compatible with high quality single-photon detectors and optical memories. Recent work has investigated the use of QFC to manipulate and measure specific temporal modes (TMs) through tailoring of the pump pulses. Such a scheme holds promise for multidimensional quantum state manipulation that is both low loss and re-programmable on a fast time scale. We demonstrate the first QFC temporal mode sorting system in a four-dimensional Hilbert space, achieving a conversion efficiency and mode separability as high as 92% and 0.84, respectively. A 20-GHz pulse train is projected onto 6 different TMs, including superposition states, and mode separability with weak coherent signals is verified via photon counting. Such ultrafast high-dimensional photonic signals could enable long-distance quantum communication with high rates.

Quantum optical arbitrary waveform manipulation and measurement in real time

We describe a technique for dynamic quantum optical arbitrary-waveform generation and manipulation, which is capable of mode selectively operating on quantum signals without inducing significant loss or decoherence. It is built upon combining the developed tools of quantum frequency conversion and optical arbitrary waveform generation. Considering realistic parameters, we propose and analyze applications such as programmable reshaping of picosecond-scale temporal modes, selective frequency conversion of any one or superposition of those modes, and mode-resolved photon counting. We also report on experimental progress to distinguish two overlapping, orthogonal temporal modes, demonstrating over 8 dB extinction between picosecond-scale time-frequency modes, which agrees well with our theory. Our theoretical and experimental progress, as a whole, points to an enabling optical technique for various applications such as ultradense quantum coding, unity-efficiency cavity-atom quantum memories, and high-speed quantum computing.

Interaction-Free Quantum Optical Fredkin Gates in

We present novel “interaction-free” realizations of quantum optical Fredkin gates that do not rely on direct physical coupling between the target light (signal) and the control light (pump). The interaction-free feature of such gates allow to overcome the fundamental limits of photon loss and quantum-state decoherence imposed by the signal-pump coupling. This advantage, together with the low inherent quantum-noise level in χ(2) microdisks, gives rise to substantially improved performance over the existing Fredkin-gate designs. Explicitly using lithium-niobate mircrodisks, we present two kinds of interaction-free Fredkin gates, a phase gate and an optical-path gate, both of which are designed with telecom-band applications in mind. For both gates, the threshold pump peak power to achieve a gate contrast >;100 and a signal loss <;10% is hundreds of microwatts for practical parameters of the devices.

\chi^{(2)}

Recent efforts to produce single photons via heralding have relied on creating spectrally factorable two-photon states in order to achieve both high purity and high production rate. Through a careful multimode analysis, we find, however, that spectral factorability is not necessary. Utilizing single-mode detection, a similar or better performance can be achieved with nonfactorable states. This conclusion rides on the fact that even when using a broadband filter, a single-mode measurement can still be realized, as long as the coherence time of the triggering photons exceeds the measurement window of the on-off detector.

Microdisks

We experimentally demonstrate all-optical interaction-free switching using the quantum Zeno effect, achieving a high contrast of 35:1. The experimental data match a zero-parameter theoretical model for several different regimes of operation, indicating a good understanding of the switchs characteristics. We also discuss extensions of this work that will allow for significantly improved performance, and the integration of this technology onto chip-scale devices, which can lead to ultra-low-power all-optical switching, a long-standing goal with applications to both classical and quantum information processing.

Heralding single photons without spectral factorability

We propose an interaction-free scheme for all-optical switching which does not rely on the physical coupling between signal and control waves. The interaction-free nature of the scheme allows it to overcome the fundamental photon-loss limit imposed by the signal-pump coupling. The same phenomenon protects photonic-signal states from decoherence, making devices based on this scheme suitable for quantum applications. Focusing on {chi}{sup (2)} waveguides, we provide device designs for traveling-wave and Fabry-Perot switches. In both designs, the performance is optimal when the signal switching is induced by coherent dynamical evolution. In contrast, when the switching is induced by a rapid dissipation channel, it is less efficient.

Experimental demonstration of interaction-free all-optical switching via the quantum Zeno effect.

We propose a quantum switch for telecom-band applications that is composed of a chi((2)) microdisk coupled to two fibers (or waveguides). The idea is to apply a pump pulse to shift the microdisk out of resonance, thereby switching the device between the cross and bar states in an interaction-free manner. As an example, a 2.5-microm-thick, 10 microm radius GaAs microdisk with an intrinsic Q of approximately 10(8) and a fiber-cavity-coupling Q of approximately 10(4) can achieve low-loss (< or approximately equal to 1%) switching for gigahertz-rate O-band quantum signals with milliwatt-peak-power pumps in the C band.

Interaction-free all-optical switching via the quantum Zeno effect

Realizing optical-nonlinear effects at a single-photon level is a highly desirable but also extremely challenging task, because of both fundamental and practical difficulties. We present an avenue to surmounting these difficulties by exploiting quantum Zeno blockade in nonlinear optical systems. Considering specifically a lithium-niobate microresonator, we find that a deterministic phase gate can be realized between single photons with near-unity fidelity. Supported by established techniques for fabricating and operating such devices, our approach can provide an enabling tool for all-optical applications in both classical and quantum domains.