New Quantum Dot Transistor Counts Individual Photons
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A transistor containing quantum dots that can count individual photons (the smallest particles of light) has been designed and demonstrated at the National Institute of Standards and Technology (NIST). The semiconductor device could be integrated easily into electronics and may be able to operate at higher temperatures than other single-photon detectors — practical advantages for applications such as quantum key distribution (QKD) for "unbreakable" encryption using single photons.
The NIST device, described in a new paper,* can accurately count 1, 2 or 3 photons at least 83 percent of the time. It is the first transistor-based detector to count numbers of photons; most other types of single-photon detectors simply "click" in response to any small number of photons. (See table for a comparison of various types of single-photon detectors used at NIST.) Counting requires a linear, stepwise response and low-noise operation. This capability is essential for advanced forms of precision optical metrology — a focus at NIST — and could be used both to detect photons and to evaluate single-photon sources for QKD. The new device also has the potential to be cooled electronically, at much higher temperatures than typical cryogenic photon detectors.
Dubbed QDOGFET, the new detector contains about 1,000 quantum dots, nanoscale clusters of semiconductors with unusual electronic properties. The NIST dots are custom-made to have the lowest energy of any component in the detector, like the bottom of a drain. A voltage applied to the transistor produces an internal current, or channel. Photons enter the device and their energy is transferred to electrons in a semiconductor "absorbing layer," separating the electrons from the "holes" they formerly occupied. As each photon is absorbed, a positively charged hole is trapped by the quantum dot drain, while the corresponding electron is swept into the channel. The amount of current flowing in the channel depends on the number of holes trapped by quantum dots. By measuring the channel response, scientists can count the detected photons. NIST measurements show that, on average, each trapped hole boosts the channel current by about one-fifth of a nanoampere. The detector has an internal quantum efficiency (percentage of absorbed photons that result in trapped holes) of 68 ± 18 percent, a record high for this type of photon detector.
The QDOGFET currently detects single photons at wavelengths of about 800 nanometers. By using different semiconductor materials, NIST researchers hope to make detectors that respond to the longer near-infrared wavelengths used in telecommunications. In addition, researchers hope to boost the external quantum efficiency (percentage of photons hitting the detector that are actually detected), now below 10 percent, and operate the device at faster speeds.
[tags]quantum, quantum computer[/tags]
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Fausto Intilla
October 17th, 2007
at 8:50am
Source: http://www.sciencedaily.com/releases/2007/10/071008103647.htm
Scientists at Florida State University’s National High Magnetic Field Laboratory and the university’s Department of Chemistry and Biochemistry have introduced a new material that could be to computers of the future what silicon is to the computers of today.
The material — a compound made from the elements potassium, niobium and oxygen, along with chromium ions — could provide a technological breakthrough that leads to the development of new quantum computing technologies. Quantum computers would harness the power of atoms and molecules to perform memory and processing tasks on a scale far beyond those of current computers.
“The field of quantum information technology is in its infancy, and our work is another step forward in this fascinating field,” said Saritha Nellutla, a postdoctoral associate at the magnet lab and lead author of the paper published in Physical Review Letters.
Semiconductor technology is close to reaching its performance limit. Over the years, processors have shrunk to their current size, with the components of a computer chip more than 1,000 times smaller than the thickness of a human hair. At those very small scales, quantum effects — behaviors in matter that occur at the atomic and subatomic levels — can start playing a role. By exploiting those behaviors, scientists hope to take computing to the next level.
In current computers, the basic unit of information is the “bit,” which can have a value of 0 or 1. In so-called quantum computers, which currently exist only in theory, the basic unit is the “qubit” (short for quantum bit). A qubit can have not only a value of 0 or 1, but also all kinds of combinations of 0 and 1 — including 0 and 1 at the same time — meaning quantum computers could perform certain kinds of calculations much more effectively than current ones.
How scientists realize the promise of the theoretical qubit is not clear. Various designs and paths have been proposed, and one very promising idea is to use tiny magnetic fields, called “spins.” Spins are associated with electrons and various atomic nuclei.
Magnet lab scientists used high magnetic fields and microwave radiation to “operate” on the spins in the new material they developed to get an indication of how long the spin could be controlled. Based on their experiments, the material could enable 500 operations in 10 microseconds before losing its ability to retain information, making it a good candidate for a qubit.
Putting this spin to work would usher in a technological revolution, because the spin state of an electron, in addition to its charge, could be used to carry, manipulate and store information.
“This material is very promising,” said Naresh Dalal, a professor of chemistry and biochemistry at FSU and one of the paper’s authors. “But additional synthetic and magnetic characterization work is needed before it could be made suitable for use in a device.”
Dalal also serves as an adviser to FSU chemistry graduate student Mekhala Pati, who created the material.
Note: This story has been adapted from material provided by Florida State University.
Fausto Intilla
http://www.oloscience.com