Terahertz-bandwidth switching of heralded single photons.

Optically induced ultrafast switching of single photons is demonstrated by rotating the photon polarization via the Kerr effect in a commercially available single-mode fiber. A switching efficiency of 97% is achieved with a ∼1.7 ps switching time and signal-to-noise ratio of ∼800. Preservation of the single-photon properties is confirmed by measuring no significant increase in the second-order autocorrelation function g(2)(0). These values are attained with only nanojoule-level pump energies that are produced by a laser oscillator with 80 MHz repetition rate. The results highlight a simple device capable of both high-bandwidth operations and preservation of single-photon properties for applications in photonic quantum processing and ultrafast time-gating or switching.

Real-time spectral characterization of a photon pair source using a chirped supercontinuum seed.

Photon pair sources have wide ranging applications in a variety of quantum photonic experiments and protocols. Many of these protocols require well controlled spectral correlations between the two output photons. However, due to low cross-sections, measuring the joint spectral properties of photon pair sources has historically been a challenging and time-consuming task. Here, we present an approach for the real-time measurement of the joint spectral properties of a fiber-based four wave mixing source. We seed the four wave mixing process using a broadband chirped pulse, studying the stimulated process to extract information regarding the spontaneous process. In addition, we compare stimulated emission measurements with the spontaneous process to confirm the technique's validity. Joint spectral measurements have taken many hours historically and several minutes with recent techniques. Here, measurements have been demonstrated in 5-30 s depending on resolution, offering substantial improvement. Additional benefits of this approach include flexible resolution, large measurement bandwidth, and reduced experimental overhead.

Reducing noise in a Raman quantum memory.

Optical quantum memories are an important component of future optical and hybrid quantum technologies. Raman schemes are strong candidates for use with ultrashort optical pulses due to their broad bandwidth; however, the elimination of deleterious four-wave mixing noise from Raman memories is critical for practical applications. Here, we demonstrate a quantum memory using the rotational states of hydrogen molecules at room temperature. Polarization selection rules prohibit four-wave mixing, allowing the storage and retrieval of attenuated coherent states with a mean photon number 0.9 and a pulse duration 175 fs. The 1/e memory lifetime is 85.5 ps, demonstrating a time-bandwidth product of ≈480 in a memory that is well suited for use with broadband heralded down-conversion and fiber-based photon sources.

Quantum Process Tomography of an Optically-Controlled Kerr Non-linearity.

Any optical quantum information processing machine would be comprised of fully-characterized constituent devices for both single state manipulations and tasks involving the interaction between multiple quantum optical states. Ideally for the latter, would be an apparatus capable of deterministic optical phase shifts that operate on input quantum states with the action mediated solely by auxiliary signal fields. Here we present the complete experimental characterization of a system designed for optically controlled phase shifts acting on single-photon level probe coherent states. Our setup is based on a warm vapor of rubidium atoms under the conditions of electromagnetically induced transparency with its dispersion properties modified through the use of an optically triggered N-type Kerr non-linearity. We fully characterize the performance of our device by sending in a set of input probe states and measuring the corresponding output via time-domain homodyne tomography and subsequently performing the technique of coherent state quantum process tomography. This method provides us with the precise knowledge of how our optical phase shift will modify any arbitrary input quantum state engineered in the mode of the reconstruction.

Room-temperature single-photon level memory for polarization states.

An optical quantum memory is a stationary device that is capable of storing and recreating photonic qubits with a higher fidelity than any classical device. Thus far, these two requirements have been fulfilled for polarization qubits in systems based on cold atoms and cryogenically cooled crystals. Here, we report a room-temperature memory capable of storing arbitrary polarization qubits with a signal-to-background ratio higher than 1 and an average fidelity surpassing the classical benchmark for weak laser pulses containing 1.6 photons on average, without taking into account non-unitary operation. Our results demonstrate that a common vapor cell can reach the low background noise levels necessary for polarization qubit storage using single-photon level light, and propels atomic-vapor systems towards a level of functionality akin to other quantum information processing architectures.

Complete characterization of quantum-optical processes.

The technologies of quantum information and quantum control are rapidly improving, but full exploitation of their capabilities requires complete characterization and assessment of processes that occur within quantum devices. We present a method for characterizing, with arbitrarily high accuracy, any quantum optical process. Our protocol recovers complete knowledge of the process by studying, via homodyne tomography, its effect on a set of coherent states, that is, classical fields produced by common laser sources. We demonstrate the capability of our protocol by evaluating and experimentally verifying the effect of a test process on squeezed vacuum.

Experience-driven dose-volume histogram maps of NTCP risk as an aid for radiation treatment plan selection and optimization.

Commonly, the quality of treatment plans is judged by a dose-volume histogram (DVH) in regards to satisfying a series of dose-volume constraints. This paper presents a novel technique for mapping normal tissue complication probabilities (NTCP) onto regions of dose-volume space with statistical considerations of risk. Mapping is done for DVHs specific to one treatment technique for one disease site. In this study, the method is illustrated for simplified intensity modulated arc therapy of the prostate, and the resulting NTCP values apply to complications in the rectum. The method consists of implementing a Monte Carlo algorithm that creates a large set of DVH curves by simulating random walks through dose-volume space. The walks are guided by a base set of clinical DVHs. Grid points in the dose-volume space have an associated NTCP spectrum for curves passing above right of the grid point of interest. After a DVH is simulated and the NTCP estimate calculated using the Lyman model, dose-volume points located to the bottom left of the curve are scored with this NTCP value and contributed to the spectrum of each point. A NTCP tolerance value is then selected and the risk of violating this tolerance is identified by a gray-scale map in regions of dose-volume space. The generated maps distinguish technique-specific, high-risk regions, a feature which is advantageous over fixed single-point dose-volume constraints commonly used. The maps also provide a visualization tool to help select safe and robust treatment plans and open the possibility for improving the efficiency of biologically based plan optimization by focusing on the more critical sections of DVH curves.