Quantum algorithms and computational models primarily focus on processing quantum states via qubit manipulations
and measurements. While this allows for hardware independent algorithm development, it does not necessarily reflect
the full capabilities of even imperfect physical implementations - which typically have access to additional degrees of
freedom not routinely considered in quantum algorithm development. In analogy with electrical mixed-signal (analog
and digital) processing, here we investigate the prospects of incorporating the strengths of the native physical platform
into the quantum information processor. Although the treatment here will be limited to optical systems the general
approach should apply to other physical systems as well.
The externally applied bias operation of linear
mode avalanche photodetectors (APDs) is extended
and compared to single photon avalanche photodetectors
(SPADs). The minimum bias required for linear operation
is calculated using the effective voltage field in the
photodetector through Mach-Zehnder modulator (MZM)
interference which allows for quantifiable incident peak
and dc optical powers. This mode of operation allows for a
quantifiable minimum bias on the diode, may enable a dualuse
for single photon detectors at higher optical powers,
and ensures RF signal conversion.
Although two-photon absorption (TPA) is a nonlinear optical process, it is not typically considered a fundamental
resource for optical quantum information processing (QIP). We have recently shown that TPA and the quantum Zeno
effect can be used to make deterministic quantum logic devices (Zeno gates) from an otherwise linear optical system. In
a Zeno gate, TPA is used to suppress the failure events that would normally occur in a linear optics device when multiple
photons exit the device in the same optical mode. We have also recently shown that additional two-photon absorbing
media can be used in a more conventional manner, along with a Zeno gate, to convert weak laser pulses into heralded
single photon pulses. Sources of this kind could have many potential benefits; however, the use of detectors and
classical switches could limit their scalability, and therefore their usefulness in larger QIP systems. Here we describe
how TPA and Zeno gates alone could be used to make more efficient, and possibly scalable, single-photon sources.
Because the Zeno gates also rely on TPA, we show that the only critical enabling resource for this approach is an
efficient TPA medium.
Quantum cryptography systems can operate over relatively long distances in standard telecommunications fiber by
taking advantage of the low transmission losses in these fibers at 1.3 or 1.5 microns. Although there has been much
progress toward the development of highly efficient and low-noise detectors for these wavelengths, silicon avalanche
photodiodes currently offer superior single photon counting performance, but only at visible and near IR wavelengths
where the fiber transmission is poor. For ranges typical of local area networks, it is possible that a quantum key
distribution (QKD) system operating below 850nm could be optimal, even though standard telecommunications fiber
supports multiple optical modes at these wavelengths. We have recently developed an optical mode filter that allows
efficient higher order mode rejection from standard telecommunications fiber near 830nm. We have used this type of
filter to launch and recover QKD signals from a polarization-based system implementing the BB84 quantum
cryptography protocol. Here we present results from testing and operation in installed fiber links ranging up to 3km.
These results demonstrate that the filters can attenuate the higher order modes over 35dB while having a minimal
(<1dB) impact on the fundamental mode carrying the QKD signal.
We review an experimental demonstration of a simple irreversible circuit of two probabilistic exclusive-OR (XOR) gates for single-photon qubits. We describe the operation of the individual linear-optics gates and the overall circuit in terms of two-photon and three-photon quantum interference effects. We also discuss future plans for quantum circuits using single-photon qubits from stored parametric down-conversion sources.
Although there has been tremendous progress in the development of true “on-demand” single-photon sources, periodic or “pseudodemand” single-photon sources can be a sufficient resource for many optical quantum information processing applications. Here we review a recent experimental demonstration of a periodic single-photon source based on parametric down-conversion photon pairs, optical storage loops, and high-speed switching. We also review an experiment in which high speed switching and storage loops were used to implement a periodic quantum memory device for polarization-encoded single-photon qubits. Finally, we describe a method in which two of these periodic quantum memory devices are used to facilitate the production of a periodic source of entangled photon pairs. These experiments and proposals are all motivated within the context of linear optics quantum computing.
Ever since Knill, Laflamme and Milburn [Nature (London) 409, 46 (2001)] showed that nondeterministic quantum logic operations could be performed with linear optical elements, additional
photons (ancilla) and projective measurements, the idea of linear-optics quantum computation has attracted considerable interest. Our group has recently demonstrated several devices of this kind. We give an overview of recent experimental results, including the quantum parity check, the destructive controlled-NOT, and a cyclical quantum memory. The need for high-efficiency detection of single photons, and for detectors capable of distinguishing photon number will be discussed. Some experimental improvements towards meeting that need will be presented.
A magnetic sensor system has been developed to measure the 3-D location and orientation of a rigid body relative to an array of magnetic dipole transmitters. A generalized solution to the measurement problem has been formulated, allowing the transmitter and receiver parameters (position, orientation, number, etc.) to be optimized for various applications. Additionally, the method of images has been used to mitigate the impact of metallic materials in close proximity to the sensor. The resulting system allows precise tracking of high-speed motion in confined metal environments. The sensor system was recently configured and tested as an abdomen displacement sensor for an automobile crash-test dummy. The test results indicate a positional accuracy of approximately 1 mm rms during 20 m/s motions. The dynamic test results also confirmed earlier covariance model predictions, which were used to optimize the sensor geometry. A covariance analysis was performed to evaluate the applicability of this magnetic position system for tracking a pilot's head motion inside an aircraft cockpit. Realistic design parameters indicate that a robust tracking system, consisting of lightweight pickup coils mounted on a pilot's helmet, and an array of transmitter coils distributed throughout a cockpit, is feasible. Recent test and covariance results are presented.