Quantum information technology and correlated photon radiometry

We develop methods and devices to further radiometric techniques and quantum information technology. This includes developing and characterizing new sources with engineered single photon state emission, developing detectors for quantum information and metrology applications, and developing metrological methods to better characterize those sources and detectors.

Detectors that can register individual photons are key to applications in quantum information, metrology, biology, and remote sensing, each having its own distinct detection requirements. We are developing and characterizing detectors with single-photon and photon-number-resolving capabilities. Research efforts include solid-state detectors such as avalanche photodiodes and the operational electronics that accounts for a much of their performance, cryogenic detectors based on microbolometers with true photon-number-resolution, and the multiplexing of these detectors for significant advantages in maximum count rate and deadtime over individual detectors.

As with detectors, sources at the single photon level are critical for many applications. Chip-scale (therefore easy to integrate) photon-pair sources that emit into a single spatial mode (therefore easy to collect) offer significant advantages in the goal for workable photonic computation system. Among various contenders, waveguided spontaneous parametric down-conversion is a promising candidate for the above goal because it can generate correlated photons predominantly in a single spatial mode while having a very small footprint. We systematically study such a waveguide-based photon pair source by measuring important performance parameters such as temperature dependence of photon-pair production and the amount of quantum entanglement produced, which is critical for the success of quantum information applications. We find that a periodically-poled KTP waveguide supports two types of parametric down conversion, known as type-0 and type-II, with the latter being spectrally narrower and brighter than the former and that the entanglement properties can be controlled through the waveguide design. This flexibility in the type of down conversion, the spectral width of the light emitted, and entanglement, suggests that this is a promising photon source for an integrated chip-scale quantum system.

Critical to quantum information applications is the need to store a quantum state while other qbits are created or processed. While photonic systems offer very robust qbits and are excellent for transporting quantum information between locations due to their minimal interactions with their environment, they are inconvenient for the storage of quantum information in one place. As a result there is significant interest in developing matter-based qbits. To implement such a system, one needs a quantum state that is accessible by some controllable means, but interacts only weakly with it surrounding environment. The nuclear hyperfine states are one such system. We are working on storing quantum information in Praseodymium (Pr) ions (doping in a crystalline matrix) which have nuclear hyperfine states with lifetimes of tens of seconds. Current efforts involve using optical beams to prepare these ions in the optimal states for long term storage and are beginning to show promising results.

A motivating factor for our source and detector development is our goal to tie our current radiometry standards, which are designed for relatively high power levels, to detectors and sources that operate at the few photon level. Making this connection offers the possibility of major improvements in the uncertainties achievable with current methods.

For example, we have verified a correlated-photon detector efficiency technique against conventional measurement to the best levels of uncertainty yet achieved. This resulted in the development of a number of transfer standard detectors designed specifically for the low light levels encountered in photon-counting measurements. These detectors will made be available to outside users.

Another effort used correlated photons as a tool to measure source spectral radiance. With this method we demonstrated fundamentally absolute radiometry measurements as far into the infrared as 8 μm, while using only an uncalibrated visible detector.

Establishing entanglement between a photon (flying qbits) and a matter state (stationary qbits) is both an essential component of many quantum information protocols, and an important general theme in quantum information. We are building an interface between a solid-state quantum dot (QD) and a photonic state created in a parametric down conversion (PDC) process. The fundamental issue in interconnecting a photon produced by a matter state to a photon state generated by traditional means is that they differ significantly, primarily in spectral widths. Among all the materials used for handling quantum states in matter, QDs stand out, because the their radiative decay rate is large (100 MHz) insuring a high photon flux and the spectral linewidth is sufficiently large. In addition, recent advances in engineering PDC sources are yielding sources with higher spectral brightness. Therefore, photons produced by a QD and in a PDC process are an excellent choice for an experimental interconnect.

• Tunneling at the single photon level
  We are studying what happens when a single particle (in this case a photon) crosses a tunneling barrier. This is a particularly interesting question because tunneling is a fundamental distinguishing characteristic of quantum mechanics and it implies remarkable properties such as barrier crossing times that are faster than the speed of light. Using a high reflectance stack of dielectric layers as a proxy for the tunneling barrier, super-luminal traversal times were found to be significantly dependent on minimal changes to the dielectric stack (such as the addition or subtraction of a just a single layer), even changing from super- to sub-luminal regimes. We have also developed a theoretical model explaining these variations in terms of surface states. Current work involves measuring traversal times in a true tunnel barrier, using a frustrated total internal reflection gap.
  
• Quantum Enhanced measurements:
  Super-resolution and Sub-Rayleigh beam measurements - Recent work has tested the advantages of a photon-number-resolving detector and classical states in resolving closely spaced optical beams.
  
  Quantum state discrimination measurements - We are beginning to test schemes to optimally measure unknown quantum states for the purpose of surpassing what are typically considered to be the classical limits to such measurements.
  

We made a metrological verification of the correlated photon method for measuring photon counting detection efficiency to an uncertainty of ~0.15 % (k = 1).

We performed a test using a fiber-based entangled photon source that ruled out certain nonlocal hidden variable models as an alternative to quantum mechanics.