| NSF Org: |
ECCS Division of Electrical, Communications and Cyber Systems |
| Recipient: |
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| Initial Amendment Date: | June 10, 2015 |
| Latest Amendment Date: | June 10, 2015 |
| Award Number: | 1509253 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Dominique Dagenais
ddagenai@nsf.gov (703)292-2980 ECCS Division of Electrical, Communications and Cyber Systems ENG Directorate for Engineering |
| Start Date: | June 15, 2015 |
| End Date: | May 31, 2019 (Estimated) |
| Total Intended Award Amount: | $94,680.00 |
| Total Awarded Amount to Date: | $94,680.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
1 UNF DR JACKSONVILLE FL US 32224-7699 (904)620-2455 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
1 UNF Drive Jacksonville FL US 32224-7699 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | EPMD-ElectrnPhoton&MagnDevices |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.041 |
ABSTRACT
Title: Understanding and Engineering the Timing Precision of Superconducting Nanowire Single Photon Detectors
Superconducting electronics and radiation sensors are exceptional for their speed of operation and precision of timing. As a result, they find application in critical niches such as space communications, metrology, sensing, and computation. The performance of these devices thus sets the limit of what can be achieved in these domains. One type of superconducting detector in particular has demonstrated high speed and timing precision: the superconducting nanowire single photon detector. This type of detector is able to detect the arrival of the smallest amounts of light possible, a single photon. As a result of its excellent speed and precision characteristics, it has found application in a wide variety of areas. For example, quantum key distribution, the secure communications method of the future, crucially relies on timing precision of photon detection in order to guarantee security. In a related field, emerging quantum computing thrusts such as those taking place on photonic integrated circuits rely on the precise detection of single photons. Unfortunately, although the speed limitations of the superconducting nanowire single photodetector are well understood, we do not yet understand what limits timing precision (typically referred to as "jitter"), and thus cannot yet engineer improvement. Many theories have been developed that can explain how these superconducting nanowires function. However, none of these theories can justify the jitter seen in these detectors. In this work, we will investigate the fundamental limits of jitter in superconducting nanowire single-photon detectors, and thus enable improvements in a wide array of application areas. For example, communication data rates depend directly on the jitter because the standard low- power digital communication protocol, pulse-position-modulation, uses timing precision to enhance the data rate. By investigating and characterizing possible sources of timing jitter in these detectors, this work will directly increase the impact of the relevant applications in industry, space, and defense.
Although superconducting nanowires have been studied since the 1970s and have been used as radiation sensors for over 13 years, their picosecond-time-scale dynamics are still not fully understood. Early attempts to explain the timing dynamics in superconducting nanowire single photon detectors focused on possible microscopic origins. In the field of radiation sensors based on superconducting nanowires, some theories related these picosecond-time-scale effects to environmental causes and others to processes intrinsic to the physics of the superconducting nanowires. For example, the hotspot model of the detection mechanism was suggested to explain the time delay between the photon arrival and voltage response as a function of number of incident photons at two different bias currents, but fitting to a theoretical model of gap suppression time was poor and no mention of jitter was made. Later, phase slip centers were purported as the mechanism for the initial hotspot creation but again, no substantive connection to jitter came about from those analyses. In this project, we will probe commonly accepted theories in the field as well as unexplored sources of jitter using both numerical and experimental approaches. We have identified several key components of the nanowire operation that we consider likely sources of jitter: (1) nanowire self-resonance; (2) trapping of vortices; and (3) stochastic elements in the microscopic physics of the hotspot. We intend to characterize the jitter contributions of each of these possible sources, and design modified devices that can reduce these contributions to jitter.
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
The superconducting nanowire single-photon detector (SNSPD) has emerged as the highest performing detector of individual visible and near-IR photons, with near-unity detection efficiency, high count rates, low dark counts, and low timing uncertainty. The timing uncertainty, also known as jitter, is significant because accurate determination of the photon arrival time limits the security of quantum key distribution, the data rate of quantum photonic circuits, and the data rate in classical communication in the photon-starved regime using the standard pulse-position modulation protocol. At the beginning of this project, there was significant variation in the jitter reported by different groups and no clear understanding of what determines the ultimate limit on the achievable jitter in SNSPDs. The scientific and technical goals of this project were to understand the physical mechanisms that determine SNSPD jitter and to use this understanding to engineer new devices with improved timing resolution.
We performed detailed studies of the microwave properties of niobium nitride (NbN) nanowires in a conventional SNSPD close-packed meander geometry in which we showed that the nanowire behaves as a high-impedance transmission line with a propagation velocity approximately 4% the speed of light in free space. This led to a study of the jitter contribution from the variation in the photon detection location along the length of the nanowire. It was shown that this contribution to the jitter can be reduced by using readouts on each end of the nanowire and averaging the pulse detection time from the two readouts. In subsequent work we demonstrated that this dual-ended readout technique can also be used to quantify the jitter contribution from the stochastic nature of the photon-induced resistive hotspot formation process.
We realized that the slow-wave behavior of the nanowire could be exploited to determine the location of photon absorption based on the time difference between the two readout pulses. We call this device a superconducting nanowire single-photon imager (SNSPI). This was achieved using a NbN nanowire in a coplanar waveguide geometry with a total length of 19.7 mm. This device had a spatial resolution of approximately 20 μm, resulting in the ability to resolve photon detection in approximately 600 distinct spatial locations along the nanowire.
One contribution to the total jitter is from the noise of the readout amplifier; the resulting time uncertainty is found by dividing the amplifier voltage noise by the slope of the rising pulse edge (the slew rate). We showed that we could increase the voltage coupling, and hence increase the slew rate, by using an impedance taper to connect the high-impedance nanowire to the 50 Ω readout. Using a meandered taper with a physical length of 78 mm, we found a factor of 3.7 increase in the slew rate and a 25 ps reduction in the total jitter.
The transmission-line properties of the SNSPD were incorporated into a distributed device model that showed good agreement with measured results, e.g. it was able to accurately reproduce the measured pulse shapes from the SNSPI device. One insight gained from this new model was that the dispersion of pulses propagating along the nanowire plays a significant role in both the achievable spatial resolution of the SNSPI as well as the jitter of a conventional SNSPD.
With the realization that nanowire dimensions were the primary contributor to the device jitter, the MIT group, in collaboration with JPL and NIST, fabricated a few-micrometer-long straight nanowire device. This device demonstrated a record low device jitter of 2.7 ps at a detection wavelength of 400 nm and 4.6 ps at a wavelength of 1550 nm.
This collaborative grant was a partnership between the Santavicca group at the University of North Florida (UNF), a primarily undergraduate institution, and the Berggren group at MIT. At UNF, the grant supported the involvement of eight undergraduate research students. Three of these students were co-authors on published papers and four of the students presented their research at conferences with the support of the grant. The grant also supported one UNF undergraduate spending 10 weeks during summer 2017 working at MIT. As of the completion of the grant, three of the UNF research students had gone on to pursue graduate studies in physics. The partnership between UNF and MIT proved to be highly effective at fostering the science and engineering talent pipeline and creating new opportunities for students and researchers at both institutions.
Last Modified: 09/04/2019
Modified by: Daniel F Santavicca
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