Quantum Sensors

The development of sensors based on the quantum mechanical phenomenon of superconductivity can dramatically improve the sensitivity of measurements across the electromagnetic spectrum, from dc magnetic fields to microwaves all the way through gamma rays. By operating at temperatures below 1 kelvin, photon detectors based on superconductivity are achieving unparalleled sensitivity, and they are beginning to bring about a broad revolution in many areas. Superconducting X-ray spectrometers are providing new capabilities in materials analysis for the semiconductor industry. Gamma-ray sensors that can provide more than an order of magnitude better spectral resolution are aiding in analyzing nuclear materials for nuclear non-proliferation, and may be used to stop them from being smuggled across international borders. In the future, improved detectors for measuring the polarization of microwaves will probe the first moments of the universe by measuring the pattern that gravity waves from the Big Bang imprinted on the cosmic microwave background.

Goals

Comparison of resolutions of NIST microcalorimeter EDS to standard semiconductor EDS.

The Quantum Sensors Project develops sensors based on quantum phenomena for spectroscopy, imaging, and other precision measurements for wavelengths from dc through gamma rays. We integrate these sensors with custom superconducting and room temperature electronics, cryogenic structures, and software to create complete measurement systems.  We work with collaborators in industry, academia, and other government agencies to apply this measurement capability to applications including materials analysis, particle physics, astronomy, cosmology, and homeland defense.

We are developing sensors that operate at extremely low temperatures, and that take advantage of quantum phenomena, including quantum interference (SQUIDs), quantum phase transitions (superconducting transition-edge sensors), and quantum tunneling (normal-insulator-superconductor tunnel junctions).

The Quantum Sensors Project has been a world leader in developing these new detector systems.  We have developed transition edge sensors (TES) for use in a variety of applications.  These devices utilize a strip of superconducting material, biased in its transition from normal to superconducting states, as an extremely sensitive thermometer. This thermometer is attached to an absorber that is isolated from a cold (~100 mK) heat sink by a micromachined structure. The heat deposited by incident photons is then measured to accurately determine their energy. 

Many applications require large arrays of quantum sensors. However, it is difficult to route individual wires from many different pixels to room temperature readout electronics. Practical implementation of large superconducting detector arrays requires that the signals be multiplexed at the cold stage. We have developed large-format superconducting integrated circuits based on SQUID switches that make it possible to read out many pixels in one output channed.

Applications for superconducting detectors include high-resolution X-ray spectroscopy for materials analysis for the semiconductor industry, high resolution gamma-ray spectroscopy to analyze nuclear materials to assist in nuclear non-proliferation and border security, and submillimeter and microwave detector arrays for astronomy and cosmological physics. In each of these areas the Quantum Sensors Project is developing detector systems that will redefine the measurement abilities of currently available technology, often by orders of magnitude.  Our goal is to continue developing groundbreaking detector systems for both industry and research groups.

Customer Needs

Improved X-ray detector technology is one of the most important metrology needs for the semiconductor industry. The 2005 International Technology Roadmap for Semiconductors recognize that superconducting X-ray detectors provide important new capability in resolving overlapping X-ray peaks: “Such new X-ray detectors will allow resolution of slight chemical shifts in X-ray peaks providing chemical information such as local bonding environments. These advances over traditional energy-dispersive spectrometers (EDS) and some wavelength dispersive spectrometers can enable particle and defect analysis on SEMs located in the clean room.” The transition-edge sensor (TES) microcalorimeter X-ray detector developed at NIST has been identified as a primary means of realizing these detector advances, which will greatly improve in-line and off-line metrology tools that currently use semiconductor EDS. At present, these metrology tools fail to provide fast and unambiguous analysis for smaller nanoparticles. Improved EDS detectors such as the TES microcalorimeter are necessary to extend the capabilities of existing SEM-based instruments to meet the analytical requirements for future technology generations.  With commercialization and continued development, microcalorimeter EDS should be able to meet both the near-term and the longer-term requirements of the semiconductor industry for improved particle analysis.

In addition, the astronomy community has an ever-increasing need for instruments capable of supplying extremely high energy-sensitivity coupled with large-format arrays for imaging and photon collection.  TES detector arrays promise to greatly expand the abilities of astronomers to study objects ranging from solar flares to supernova remnants to the formation of galaxies.  The Quantum Sensor project has formed collaborations to transfer our TES technology into astronomical instruments with several institutions, including NASA, Stanford University, the Lockheed-Martin Solar Astrophysics Laboratory and the UK Astronomy Technology Center.

Technical Strategy

The ability to detect photons with high-energy resolution and near-unity quantum efficiency promises to dramatically improve the field of X-ray microanalysis.  Improved energy-dispersive X-ray spectroscopy will be used to solve a wide range of problems in materials analysis. For instance, in semiconductor manufacturing, improved X-ray materials analysis is needed to identify nanoscale contaminant particles on wafers and to analyze very thin layers of materials and minor constituents.

To make this technology available to the materials analysis community, NIST is working on the commercialization of these inventions. Additionally, NIST’s Chemical Science and Technology Laboratory is using a prototype microcalorimeter system constructed by our group to improve its own materials-analysis capability.

The transition-edge-sensor (TES)-based X-ray microcalorimeter developed in our group has been shown to have world-record energy resolution and to have wide application in many areas of X-ray microanalysis.  In trying to deliver maximum benefit of this technology to industrial and scientific users of microanalysis, we are concentrating our efforts in three areas: support of existing systems, development of improved and simplified single detector pixels, and development of arrays of detectors as a means of increasing X-ray collection area and count rate.

The low temperature operation of TES microcalorimeters  (~100 mK) necessitates a fairly complex arrangement of cryogenic and electronic elements in order to construct a complete X-ray spectrometer. In this case, we have developed superconducting electronics to read out the detectors, compact adiabatic demagnetization refrigerators to simplify cooling the detectors to millikelvin operating temperatures, and room-temperature electronics to process the output signals.  

An important part of our effort is to provide support to our existing customers, including the users of the prototype microcalorimeter system in the Chemical Science and Technology Laboratory in Gaithersburg, Maryland. We continue to provide expertise and training in detector and SQUID (readout electronic) operation and optimization, and operation of millikelvin refrigerators to CSTL, including consultations and site visits.  Additionally we continue to make improvements to the components in the spectrometer system.

While the performance of the detectors we have made is much better than existing detectors, additional improvements in energy resolution (particularly at higher energies) are both theoretically possible and desirable by the user community. Improvements in energy resolution require new understanding of the limitations on performance of microcalorimeter detectors.  We have begun to explore novel detector geometries and materials, and to develop the nonequilibrium thermodynamic theory of resistive bolometers to better explain measured noise.

For many X-ray microanalysis applications, improvements in count-rate and collection area are far more important than further improvements in energy resolution. This can be achieved by the creation of multipixel arrays of detectors. In addition to the fabrication difficulties in making such arrays, the cold- and room-temperature electronics to read out the arrays must also be created.

256-pixel TES X-ray calorimeter array and SQUID multiplexer electronics for X-ray material analysis and astronomy.

Another significant focus of the project is the development of gamma-ray detectors for the analysis of nuclear materials. One of the most pressing international priorities is to control the dissemination of nuclear materials that could be used in attacks by terrorists or rogue states. Nuclear materials contain unstable isotopes, which emit X-rays and gamma rays. The characteristic energies of these photons provide a fingerprint of which radioactive isotopes are present. Unfortunately, some isotopes that occur in benign applications emit gamma rays with energies that are very similar to those emitted by materials used in weapons, which lead to ambiguous identifications and false alarms. This problem has been vexing the U.S., which is installing thousands of radiation portal monitors to detect the gamma rays emitted by nuclear materials carried by vehicles crossing the Canadian and Mexican borders. One of our most significant fears is that terrorists might smuggle highly enriched (weapons-grade) uranium into the country to build a crude Hiroshima-style atomic bomb. The primary signature of highly enriched uranium is the 185.7 kiloelectron-volt (keV) gamma ray from uranium-235. This gamma ray, however, has almost the same energy as the 186.1 keV gamma ray emitted from the radium-226 in clay in cat litter and other materials, making it very difficult to distinguish the two. This so-called kitty-litter problem is the largest source of false alarms at the U.S. border.

The Quantum Sensors Project is developing gamma-ray detectors based on TES technology that have more than ten times better energy resolution than conventional detectors. These detectors can resolve more lines in the complicated gamma-ray spectra of nuclear materials such as uranium and plutonium isotopic mixtures (see figure). The devices are being developed specifically to help in the verification of international nonproliferation treaties, by determining the plutonium content of spent nuclear fuel. But they can also distinguish between the radium-226 in cat litter and the uranium-235 in highly enriched uranium. If a conventional hand-held detector or portal monitor were to detect a gamma-ray signal, one of the superconducting devices could be used as a follow-up tool to distinguish unambiguously between these two isotopes, thus eliminating many false alarms.

Spectrum taken with NIST gamma-ray detector array. When superconducting detectors are used to measure the spectrum of radiation from a plutonium-uranium isotopic mixture, the photons from the different isotopes are clearly resolved, allowing unambiguous identification of the isotopes that are present (dark blue lines). In a spectrum taken with a conventional high-purity germanium (HPGe) detector (red line), the lines are not resolved, and the determination of the isotopic composition is more difficult.

The astronomy community has long been the driving force behind improvements in photon detection systems at all wavelengths.  Because of TES detectors’ extremely good sensitivity, they are obvious candidates to solve many problems faced by this community.  The same X-ray detectors used in our microanalysis efforts, for example, are well suited to analyzing the x-ray spectra of supernova remnants and solar flares.  By redesigning these detectors, they may be used as bolometers to measure far infrared and submillimeter radiation on ground-based telescopes, allowing astronomers to probe the evolution of galaxies and search for planets around other stars. Finally, quantum sensors will be important for the precision measurement of the polarization of the cosmic microwave background, which will make it possible to measure the signal of gravity waves from the Big Bang itself.

The Quantum Sensors Project has collaborations with several institutions to deploy our detectors for use in astronomical applications.  As these collaborations push the technical abilities of our detectors, they often drive us to create improvements that are then applied to our more commercial applications, such as X-ray microanalysis.

Many of the requirements for X-ray astronomy are identical to those for our X-ray microanalysis project: high energy resolution, large arrays, high counting rates and multiplexed readout.   We have two principal collaborators in this area: NASA’s Laboratory for High Energy Astrophysics (LHEA) and the Lockheed-Martin Solar Astrophysics Laboratory (LMSAL).

NASA has an ongoing program to study X-ray astrophysics as part of its Structure and Evolution of the Universe theme.  Following on its successful Chandra mission, Constellation-X is the next generation of X-ray astronomy telescopes.  To accomplish its goals, Constellation-X will need to have an imaging array of X-ray spectrometers to place at the focal plane of an orbiting X-ray telescope.

A similar telescope is planned by LMSAL to study the mechanisms behind solar flares and coronal mass ejections (CMEs).  CMEs cause significant financial impact around the world, as they disrupt satellites in Earth orbit and can knock out power grids on the ground.  Scientists hope to understand and possibly predict these solar phenomenon by study the spectra of solar flares, and LMSAL is working with the Quantum Sensor project to develop TES detectors for this purpose.

In the infrared regime, our TES bolometers have achieved world-record sensitivity. This impressive result confirms the utility of TES technology for this application as well.  For several years, we have been involved in a collaboration with Laboratory for Astronomy nd Solar Physics (LASP), at NASA’s Goddard Space Flight Center, to develop far-infrared bolometers.  A result on early result of this collaboration was the Fabry-Perot Interferometer Bolometer Research Experiment, (FIBRE), the first multiplexed TES bolometer array, which was deployed on the Caltech Submillimeter Observatory at Mauna Kea, Hawaii. 

In addition, the Quantum Sensors Project, in collaboration with the United Kingdom’s Astronomy Technology Center, is developing both sensors and readout technology for the second Submillimeter Common User Bolometer Array (SCUBA-2).  The SCUBA-2 instrument is designed to detect radiation from astronomical sources at wavelengths of 450 µm and 850 µm and will be installed on the James Clerk Maxwell Telescope (JCMT) in Hawaii.  This array will be, by orders of magnitude, the largest bolometer array ever deployed, having over 10,000 individual pixels.  It will allow astronomers to map the sky at speeds 100-1000 times faster than previously achieved.

The detector systems discussed above all share a common technical requirement: large arrays of TES detectors.  This requirement brings with it the complication of reading out such large arrays.  These arrays will all operate at temperatures below 1 K.  If each pixel in the array requires a separate readout all the way to the room temperature electronics, then the heat load on the array’s refrigeration system will rapidly become unmanageably large.  A system to multiplex the readout of these detectors at the cold stage of their refrigerator is thus required to reduce the number of wires from the cold stage to warmer parts of the cryostat.

Fortunately, TES devices are low impedance, which allows them to be read out by superconducting SQUIDs.  Because SQUIDS, in their unbiased “off” state, are superconducting, they may be effectively multiplexed without adding the noise of each individual SQUID to the whole.

The Quantum Sensors Project has been at the forefront of SQUID multiplexing for several years now.  Our first-generation 8-channel SQUID multiplexer (MUX) was successfully deployed with the FIBRE far-infrared bolometer array on the CSO telescope on Mauna Kea, Hawaii.  We have now built upon that success by developing a second generation, 32-channel SQUID MUX that has better bandwidth and 2 orders of magnitude lower power dissipation than the first design. The first fabrication run yielded devices that work at the design specifications.  These MUX chips should have sufficient performance to multiplex X-ray TES detectors, which are much faster than the infrared bolometers in FIBRE. We are now building full 1,280-pixel multiplexed arrays for the SCUBA-2 project.

A hybridized TES subarray for the SCUBA-2 instrument. The array consists of a 1,280-pixel SQUID multiplexer wafer bump bonded to a 1,280-pixel TES bolometer array.

NIST has a long tradition of exploiting physical phenomena that occur at ultralow temperatures to produce electronic devices with properties that cannot be achieved by conventional electronics.  For instance, NIST has developed Transition-Edge Sensor (TES) X-ray sensors which operate at temperatures near 100 mK and provide improved analytical capabilities to the semiconductor industry.  The development of two-stage adiabatic demagnetization refrigerators (ADRs) has made these low operating temperatures significantly more accessible.  Nonetheless, even a two-stage ADR adds considerable complexity, size, and expense to an analytical station.  To overcome this challenge, NIST has begun development of a thin-film refrigerator which is capable of cooling sensors from 300 mK to 100 mK. 

The refrigerator consists of Normal-Insulator-Superconductor (NIS) tunnel junction.  Current flow through the insulating barrier separating the electrodes preferentially removes the hottest electrons from the normal electrode, thereby producing cooling.  When coupled to a Helium-3 cryostat, NIS coolers may provide a significantly smaller, cheaper, and less complex means of reaching 100 mK temperatures. 

Deliverables

• We have provided the Chemical Science and Technology Laboratory (CSTL) of NIST with a complete prototype microcalorimeter system for collaborative use in studying problems of interest to our customers. We will continue to provide support and improvements for this system.
• We will develop models of single-pixel microcalorimeter performance to assist in improving detector sensitivity, and will fabricate and test novel detector designs based on these models.
• We will further develop arrays of X-ray microcalorimeter detectors to demonstrate the increase in collection area and count rate achievable through arrays. We will work with NASA to develop these arrays for use in the Constellation-X project.
• We will develop arrays of detectors optimized for imaging infrared bolometry, along with the necessary wiring, SQUID MUX and room temperature DSP electronics to read the array.

Accomplishments

• We have developed an extremely high energy resolution X-ray detector and demonstrated a complete X-ray spectroscopy system. This detector and spectrometer have been made possible by broad expertise within the Division in such fields as superconductivity, device fabrication including silicon micromachining, superconducting electronics, cryogenic engineering, and low-noise, room-temperature electronics. Without expertise in all of these areas, the complete systems that have provided the compelling demonstrations for the power of this technology would not have been possible.
• This system holds the world record for energy resolution for an EDS detector of 2.37 eV ± 0.12 eV at 5900 eV, which is over 30 times better than the best high-resolution semiconductor-based detectors currently available. We have used the system to identify submicrometer particles of materials such as W on Si substrates, an identification problem that is impossible with standard EDS detectors and of great importance to the semiconductor industry. It has also demonstrated energy shifts in the EDS X-ray spectra of materials such as Al, Fe, and Ti, depending on their chemical bonding state, thus allowing differentiation between a particle of Al and Al₂O₃, for example.
• We have fabricated gamma ray detector arrays with the world record energy resolution for an EDS detector of 25 eV at 103 keV (see figure).  This energy resolution was achieved in a 14-pixel multiplexed array. Gamma ray detectors with ultra-high energy resolution will provide superior capabilities for nuclear materials analysis.

World record 25 eV energy resolution achieved with NIST microcalorimeter at 103 keV.

• We have fabricated the first on-chip, solid-state microrefrigerator using Normal metal – Insulator – Superconductor (NIS) tunnel junctions integrated with superconducting transition-edge sensors. Presently, cryogenic instruments that operate near 100 mK use complex, expensive adiabatic demagnetization refrigerators and dilution refrigerators. We have fabricated an NIS refrigerator designed to operate at bath temperatures of 300 mK, which is accessible with relatively simple and inexpensive ³He refrigerators. The normal metal in the junction is an Al thin film doped with ferromagnetic impurities (Mn) in order to suppress superconductivity. These sensors are integrated with TES detectors, making it possible for them to operate from a lower temperature.
• We demonstrated the first multiplexed readout of an X-ray microcalorimeter array.  This breakthrough establishes a clear path to the instrumentation of kilopixel microcalorimeter arrays, which will have wide-ranging implications for fields as diverse as materials analysis and X-ray astronomy.  A challenge in the development of large arrays of X-ray microcalorimeters, which measure the energy, or color, of X-rays with high resolution, is that providing an amplifier channel for each detector in the array requires too many wires to be connected to the cryogenic detectors.  Time-division SQUID multiplexing, a scheme in which the signals from many detectors are read out through a single amplifier, is one way to conquer this challenge. In the experiment, we measured the energy resolution of four microcalorimeters, using both conventional and multiplexed readout techniques. 
• We have demonstrated the first microwave SQUID multiplexer. This breakthrough opens a path to reading out very large arrays of SQUIDs for magnetoencephalography and non-destructive evaluation, and CCD-scale arrays of low-temperature detectors. In this circuit, the outputs of many SQUID amplifiers are simultaneously monitored by a single high electron mobility transistor (HEMT) amplifier channel. Each SQUID amplifier is placed in a high-Q resonant circuit with a different resonant frequency, and excited by a comb of microwave signals at each of the resonant frequencies. The amplitude of the reflected microwave signals at each frequency is a function of the input signal at each SQUID. In an initial experiment, we demonstrated the low-noise readout of SQUIDs using this microwave reflectometer approach, and multiplexed two SQUIDs simultaneously near 500 MHz. Future work will focus on operation at 5 GHz and developing technology for much larger SQUID arrays. Because of the large bandwidth available with microwave measurements, it should be possible in the future to read out many thousands of SQUIDs in a single coaxial cable, and instrument arrays of SQUIDs with thousands or millions of pixels.
• We have delivered science-grade parts for the first full, 1,280-pixel multiplexed submillimeter bolometer subarrays for the SCUBA-2 project. These parts are now being hybridized, and will lead to a significant step forward for the community.

Collaborations

UK Astronomy Technology Center,  SCUBA2 camera
University of Edinburgh /Scottish Microelectronics Centre, SCUBA2 camera
Raytheon Vision Systems Inc., SCUBA2 camera
University of Cardiff, SCUBA2 camera
NIST Division 837, X-ray microcalorimeter for microanalysis
Intel, X-ray microanalysis using our microcalorimeter system
Lockheed Martin, X-ray detector development for solar physics
Stanford University, X-ray detector development for solar physics
Jet Propulsion Laboratory,  SQUID multiplexer for IR detectors
NASA Goddard Space Flight Center,  SQUID multiplexer for IR detectors
NASA Goddard Space Flight Center,  X-ray microcalorimeter arrays
NASA Goddard Space Flight Center, Magnetic Microcalorimters
JILA, Microwave SQUID multiplexer
Star Cryoelectronics,  X-ray detector development

Selected Publications

Download our publications

"Seeing with Superconductors", Kent D. Irwin, Scientific American, November, 2006, pp. 86-94.

"Transition-Edge Sensors", K.D. Irwin and G. C. Hilton, in Cryogenic Particle Detection, C. Enss (Ed.), Topics Appl. Phys. 99, 63-149 (2005), Springer-Verlag Berlin Heidelberg 2005, ISBN: 3-540-20113-0

"Electron Probe Microanalysis with Cryogenic Detectors", D.E. Newbury, K.D. Irwin, G.C. Hilton, D.A. Wollman, J.A. Small and J.M. Martinis, in Cryogenic Particle Detection, C. Enss (Ed.), Topics Appl. Phys. 99, 267-312 (2005), Springer-Verlag Berlin Heidelberg 2005, ISBN: 3-540-20113-0

"A Prototype System for SQUID Multiplexing of Large-Format Transition-Edge Sensor Arrays", C.D. Reintsema, J. Beyer, S.W. Nam, S. Deiker, G.C. Hilton, K.D. Irwin, J.M. Martinis, J. Ullom, L.R. Vale, M. MacIntosh, Rev. Sci. Instrum. 74, 4500-4580 (Oct. 2003)

"Practical electron-tunneling refrigerator", A. M. Clark, A. Williams, S. T. Ruggiero, M. L. ven den Berg, and J. N. Ullom, Appl. Phys. Lett. 84, 625-627; 26 January 2004.

"Characterization and reduction of unexplained noise in superconducting transition-edge sensors", J.N. Ullom, W.B. Doriese, G.C. Hilton, J.A. Beall, S. Deiker, W.D. Duncan, L.Ferreira, K.D. Irwin, C.D. Reintsema, and L.R. Vale, Appl. Phys. Lett. 84, 4206-4208 (May 24, 2004).

"Cryogenics on a Chip", J.P. Pekola, R.J. Schoelkopf, and J.N. Ullom, Physics Today 57, 41, May 2004.

"Transition edge sensor using dilute AlMn alloys", S.W. Deiker, W. Doriese, G.C. Hilton, K.D. Irwin, W. H. Rippard, J.N. Ullom, L.R. Vale, S.T. Ruggiero, J.N. Ullom, L.R. Vale, S.T. Ruggiero, A. Williams, and B.A. Young, Appl. Phys. Lett. 85, 2137-2139 (2004).

"Microwave SQUID multiplexe"r, K.D. Irwin and K.W. Lehnert, Appl. Phys. Lett. 85, 2107-2109 (2004).

Time-division multiplexing of high-resolution TES X-ray microcalorimeters: Four pixels and beyond, W.B. Doriese, J.A. Beall, S. Deiker, W.D. Duncan, L. Ferreira, G.C. Hilton, K.D. Irwin, C.D. Reintsema, J.N. Ullom, L.R. Vale, and Y. Xu, Appl. Phys. Lett., in press.

Microwave reflectometer readout for large-format superconductor-insulator-normal metal bolometer arrays, D. R. Schmidt, A. M. Clark, W. Duncan, K. D. Irwin, N. Miller, J. N. Ullom, and K. W. Lehnert, Submitted to Applied Physics Letters.