Superconducting Single-Photon Detectors (SSPD) invented two decades ago have evolved to a mature technology and have become devices of choice in the advanced applications of quantum optics, such as quantum cryptography and optical quantum computing. In these applications SSPDs are coupled to single-mode fibers and feature almost unity detection efficiency, negligible dark counts, picosecond timing jitter and MHz photon count rate. Meanwhile, there are great many applications requiring coupling to multi-mode fibers or free space. `Classical’ SSPDs with 100-nm-wide superconducting strip and covering area of about 100 µm2 are not suitable for further scaling due to degradation of performance and low fabrication yield. Recently we have demonstrated single-photon counting in micron-wide superconducting bridges and strips. Here we present our approach to the realization of practical photon-counting detectors of large enough area to be efficiently coupled to multi-mode fibers or free space. The detector is either a meander or a spiral of 1-µm-wide strip covering an area of 50x50 µm2. Being operated at 1.7K temperature it demonstrates the saturated detection efficiency (i.e. limited by the absorption in the detector) up to 1550 nm wavelength, about 10 ns dead time and timing jitter in range 50-100 ps.
Achievement of the ultimate sensitivity along with a high spectral resolution is one of the frequently addressed problems, as the complication of the applied and fundamental scientific tasks being explored is growing up gradually. In our work, we have investigated performance of a superconducting nanowire photon-counting detector operating in the coherent mode for detection of weak signals at the telecommunication wavelength. Quantum-noise limited sensitivity of the detector was ensured by the nature of the photon-counting detection and restricted by the quantum efficiency of the detector only. Spectral resolution given by the heterodyne technique and was defined by the linewidth and stability of the Local Oscillator (LO). Response bandwidth was found to coincide with the detector’s pulse width, which, in turn, could be controlled by the nanowire length. In addition, the system noise bandwidth was shown to be governed by the electronics/lab equipment, and the detector noise bandwidth is predicted to depend on its jitter. As have been demonstrated, a very small amount of the LO power (of the order of a few picowatts down to hundreds of femtowatts) was required for sufficient detection of the test signal, and eventual optimization could lead to further reduction of the LO power required, which would perfectly suit for the foreseen development of receiver matrices and the need for detection of ultra-low signals at a level of less-than-one-photon per second.
We report on the design, fabrication and measurement of travelling-wave superconducting nanowire single-photon detectors (SNSPDs) integrated with polycrystalline diamond photonic circuits. We analyze their performance both in the near-infrared wavelength regime around 1600 nm and at 765 nm. Near-IR detection is important for compatibility with the telecommunication infrastructure, while operation in the visible wavelength range is relevant for compatibility with the emission line of silicon vacancy centers in diamond which can be used as efficient single-photon sources. Our detectors feature high critical currents (up to 31 μA) and high performance in terms of efficiency (up to 74% at 765 nm), noise-equivalent power (down to 4.4×10-19 W/Hz1/2 at 765 nm) and timing jitter (down to 23 ps).
KEYWORDS: Superconductors, Quantum efficiency, Single photon detectors, Sensors, Switching, Infrared radiation, Picosecond phenomena, Infrared detectors, Mid-IR, Signal to noise ratio
Superconducting single-photon detector (SSPD) is patterned from 4-nm-thick NbN film deposited on sapphire
substrate as a 100-nm-wide strip. Due to its high detection efficiency, low dark counts, and picosecond timing
jitter SSPD has become a competitor to the InGaAs avalanche photodiodes at 1550 nm and longer wavelengths.
Although the SSPD is operated at liquid helium temperature its efficient single-mode fibre coupling enabled its
usage in many applications ranging from single-photon sources research to quantum cryptography. In our strive
to increase the detection efficiency at 1550 nm and longer wavelengths we developed and fabricated SSPD with
the strip almost twice narrower compared to the standard 100 nm. To increase the voltage response of the device
we utilized cascade switching mechanism: we connected 50-nm-wide and 10-μm-long strips in parallel covering
the area of 10 μmx10 μm. Absorption of a photon breaks the superconductivity in a strip leading to the bias
current redistribution between other strips followed their cascade switching. As the total current of all the strips
about is 1 mA by the order of magnitude the response voltage of such an SSPD is several times higher compared
to the traditional meander-shaped SSPDs. In middle infrared (about 3 μm wavelength) these devices have the
detection efficiency several times higher compared to the traditional SSPDs.
Alexander Korneev, Alexander Divochiy, Yury Vachtomin, Yulia Korneeva, Irina Florya, Michael Elezov, Nadezhda Manova, Michael Tarkhov, Pavel An, Anna Kardakova, Anastasiya Isupova, Galina Chulkova, Konstantin Smirnov, Natalya Kaurova, Vitaliy Seleznev, Boris Voronov, Gregory Goltsman
Superconducting single-photon detector (SSPD) is a planar nanostructure patterned from 4-nm-thick NbN film
deposited on sapphire substrate. The sensitive element of the SSPD is 100-nm-wide NbN strip. The device is
operated at liquid helium temperature. Absorption of a photon leads to a local suppression of superconductivity
producing subnanosecond-long voltage pulse. In infrared (at 1550 nm and longer wavelengths) SSPD outperforms
avalanche photodiodes in terms of detection efficiency (DE), dark counts rate, maximum counting rate and
timing jitter. Efficient single-mode fibre coupling of the SSPD enabled its usage in many applications ranging
from single-photon sources research to quantum cryptography. Recently we managed to improve the SSPD
performance and measured 25% detection efficiency at 1550 nm wavelength and dark counts rate of 10 s-1. We
also improved photon-number resolving SSPD (PNR-SSPD) which realizes a spatial multiplexing of incident
photons enabling resolving of up to 4 simultaneously absorbed photons. Another improvement is the increase of
the photon absorption using a λ/4 microcavity integrated with the SSPD. And finally in our strive to increase the
DE at longer wavelengths we fabricated SSPD with the strip almost twice narrower compared to the standard
100 nm and demonstrated that in middle infrared (about 3 μm wavelength) these devices have DE several times
higher compared to the traditional SSPDs.
We present a novel concept of photon number resolving detector based on 120-nm-wide superconducting stripes made of
4-nm-thick NbN film and connected in parallel (PNR-SSPD). The detector consisting of 5 strips demonstrate a capability
to resolve up to 4 photons absorbed simultaneously with the single-photon quantum efficiency of 2.5% and negligibly
low dark count rate.
At present superconducting detectors become increasingly attractive for various practical applications. In this paper we
present results on the depelopment of fiber coupled receiver systems for the registration of IR single photons, optimized
for telecommunication and quantum-cryptography. These receiver systems were developed on the basis of
superconducting single photon detectors (SSPD) of VIS and IR wavelength ranges. The core of the SSPD is a narrow
(~100 nm) and long (~0,5 mm) strip in the form of a meander which is patterned from a 4-nm-thick NbN film
(TC=10-11 K, jC=~5-7•106
A/cm2); the sensitive area dimensions are
10×10 μm2. The main problem to be solved while the
receiver system development was optical coupling of a single-mode fiber (9 microns in diameter) with the SSPD
sensitive area. Characteristics of the developed system at the optical input are as follows: quantum efficiency >10 % (at
1.3 μm), >4 % (at 1.55 μm); dark counts rate ≤1 s-1; duration of voltage pulse ≤5 ns; jitter ≤40 ps. The receiver systems
have either one or two identical channels (for the case of carrying out correlation measurements) and are made as an
insert in a helium storage Dewar.
Single-photon detectors (SPDs) are the foundation of all quantum communications (QC) protocols.
Among different classes of SPDs currently studied, NbN superconducting SPDs (SSPDs) are established as the
best devices for ultrafast counting of single photons in the infrared (IR) wavelength range. The SSPDs are
nanostructured, 100 μm2
in total area, superconducting meanders, patterned by electron lithography in ultra-thin
NbN films. Their operation has been explained within a phenomenological hot-electron photoresponse model.
We present the design and performance of a novel, two-channel SPD receiver, based on two fiber-coupled NbN
SSPDs. The receivers have been developed for fiber-based QC systems, operational at 1.3 μm and 1.55 μm
telecommunication wavelengths. They operate in the temperature range from 4.2 K to 2 K, in which the NbN
SSPDs exhibit their best performance. The receiver unit has been designed as a cryostat insert, placed inside a
standard liquid-heliumstorage dewar. The input of the receiver consists of a pair of single-mode optical fibers,
equipped with the standard FC connectors and kept at room temperature. Coupling between the SSPD and the
fiber is achieved using a specially designed, precise micromechanical holder that places the fiber directly on top
of the SSPD nanostructure. Our receivers achieve the quantum efficiency of up to 7% for near-IR photons, with
the coupling efficiency of about 30%. The response time was measured to be < 1.5 ns and it was limited by our
read-out electronics. The jitter of fiber-coupled SSPDs is < 35 ps and their dark-count rate is below 1s-1. The
presented performance parameters show that our single-photon receivers are fully applicable for quantum correlation-type QC systems, including practical quantum cryptography.
We present our latest generation of superconducting single-photon detectors (SSPDs) patterned from 4-nm-thick NbN films, as meander-shaped ~0.5-mm-long and ~100-nm-wide stripes. The SSPDs exhibit excellent performance parameters in the visible-to-near-infrared radiation wavelengths: quantum efficiency (QE) of our best devices approaches a saturation level of ~30% even at 4.2 K (limited by the NbN film optical absorption) and dark counts as low as 2x10-4 Hz. The presented SSPDs were designed to maintain the QE of large-active-area devices, but, unless our earlier SSPDs, hampered by a significant kinetic inductance and a nanosecond response time, they are characterized by a low inductance and GHz counting rates. We have designed, simulated, and tested the structures consisting of several, connected in parallel, meander sections, each having a resistor connected in series. Such new, multi-element geometry led to a significant decrease of the device kinetic inductance without the decrease of its active area and QE. The presented improvement in the SSPD performance makes our detectors most attractive for high-speed quantum communications and quantum cryptography applications.
We have fabricated fiber-coupled superconducting single-photon detectors (SSPDs), designed for quantum-correlationtype
experiments. The SSPDs are nanostructured (~100-nm wide and 4-nm thick) NbN superconducting meandering
stripes, operated in the 2 to 4.2 K temperature range, and known for ultrafast and efficient detection of visible to nearinfrared
photons with almost negligible dark counts. Our latest devices are pigtailed structures with coupling between
the SSPD structure and a single-mode optical fiber achieved using a micromechanical photoresist ring placed directly
over the meander. The above arrangement withstands repetitive thermal cycling between liquid helium and room
temperature, and we can reach the coupling efficiency of up to ~33%. The system quantum efficiency, measured as the
ratio of the photons counted by SSPD to the total number of photons coupled into the fiber, in our early devices was
found to be around 0.3 % and 1% for 1.55 &mgr;m and 0.9 &mgr;m photon wavelengths, respectively. The photon counting rate
exceeded 250 MHz. The receiver with two SSPDs, each individually biased, was placed inside a transport, 60-liter
liquid helium Dewar, assuring uninterrupted operation for over 2 months. Since the receiver's optical and electrical
connections are at room temperature, the set-up is suitable for any applications, where single-photon counting capability
and fast count rates are desired. In our case, it was implemented for photon correlation experiments. The receiver
response time, measured as a second-order photon cross-correlation function, was found to be below 400 ps, with
timing jitter of less than 40 ps.
We report on our progress in research and development of ultrafast superconducting single-photon detectors (SSPDs) based on ultrathin NbN nanostructures. Our SSPDs were made of the 4-nm-thick NbN films with Tc ~11 K, patterned as meander-shaped, 100-nm-wide strips, and covering an area of 10×10 μm2. The detectors exploit a combined detection mechanism, where upon a single-photon absorption, a hotspot of excited electrons and redistribution of the biasing supercurrent, jointly produce a picosecond voltage transient signal across the superconducting nanostripe. The SSPDs are typically operated at 4.2 K, but their sensitivity in the infrared radiation range can be significantly improved by lowering the operating temperature from 4.2 K to 2 K. When operated at 2 K, the SSPD quantum efficiency (QE) for visible light photons reaches 30-40%, which is the saturation value limited by the optical absorption of our 4-nm-thick NbN film. With the wavelength increase of the incident photons,the QE of SSPDs decreases significantly, but even at the wavelength of 6 μm, the detector is able to count single photons and exhibits QE of about 10-2 %. The dark (false) count rate at 2 K is as low as 2x10-4 s,-1 which makes our detector essentially a background-limited sensor. The very low dark-count rate results in a noise equivalent power (NEP) below 10-18 WHz-1/2 for the mid-infrared range (6 μm). Further improvement of the SSPD performance in the mid-infrared range can be obtained by substituting NbN for another, lower-Tc materials with a narrow superconducting gap and low quasiparticles diffusivity. The use of such superconductors should shift the cutoff wavelength below 10 μm.
We present the design and performance of a novel, two-channel single-photon receiver, based on two fiber-coupled NbN superconducting single-photon detectors (SSPDs). The SSPDs are nanostructured superconducting meanders covering an area of 100 μm2 and are known for ultrafast and efficient counting of single, visible-to-infrared photons. Their operation has been explained within a phenomenological hot-electron photoresponse model. Our receiver is intended for fiber-based quantum cryptography and communication systems, operational at near-infrared (NIR) telecommunication wavelengths, λ = 1.3 μm and λ = 1.55 μm. Coupling between the NbN detector and a single-mode optical fiber was achieved using a specially designed, micromechanical photoresist ring, positioned directly over the SSPD active area. The positioning accuracy of the ring was below 1 μm. The receiver with SSPDs was placed (immersed) in a standard liquid-helium transport Dewar and kept without interruption for over two months at 4.2 K. At the same time, the optical fiber inputs and electrical outputs were kept at room temperature. Our best system reached a system quantum efficiency of up to 0.3 % in the NIR radiation range, with the detector coupling efficiency of about 30 %. The response time was measured to be about 250 ps and was limited by our read-out electronics. The measured jitter was close to 35 ps. The presented performance parameters show that our NIR single photon detectors are suitable for practical quantum cryptography and for applications in quantum-correlation experiments.
We present our studies on quantum efficiency (QE), dark counts, and noise equivalent power (NEP) of the latest generation of nanostructured NbN superconducting single-photon detectors (SSPDs) operated at 2.0 K. Our SSPDs are based on 4 nm-thick NbN films, patterned by electron beam lithography as highly-uniform 100÷120-nm-wide meander-shaped stripes, covering the total area of 10x10 μm2 with the meander filling factor of 0.7. Advances in the fabrication process and low-temperature operation lead to QE as high as ~30-40% for visible-light photons (0.56 μm wavelength)-the saturation value, limited by optical absorption of the NbN film. For 1.55 μm photons, QE was ~20% and decreased exponentially with the wavelength reaching ~0.02% at the 5-μm wavelength. Being operated at 2.0-K temperature the SSPDs revealed an exponential decrease of the dark count rate, what along with the high QE, resulted in the NEP as low as 5x10-21 W/Hz-1/2, the lowest value ever reported for near-infrared optical detectors. The SSPD counting rate was measured to be above 1 GHz with the pulse-to-pulse jitter below 20 ps. Our nanostructured NbN SSPDs operated at 2.0 K significantly outperform their semiconducting counterparts and find practical applications ranging from noninvasive testing of CMOS VLSI integrated circuits to ultrafast quantum communications and quantum cryptography.
We present a new class of single-photon devices for counting of both visible and infrared photons. Our superconducting single-photon detectors (SSPDs) are characterized by the intrinsic quantum efficiency (QE) reaching up to 100%, above 10 GHz counting rate, and negligible dark counts. The detection mechanism is based on the photon-induced hotspot formation and subsequent appearance of a transient resistive barrier across an ultrathin and submicron-wide superconducting stripe. The devices are fabricated from 3.5-nm-thick NbN films and operate at 4.2 K, well below the NbN superconducting transition temperature. Various continuous and pulsed laser sources in the wavelength range from 0.4 μm up to >3 μm were implemented in our experiments, enabling us to determine the detector QE in the photon-counting mode, response time, and jitter. For our best 3.5-nm-thick, 10×10 μm2-area devices, QE was found to reach almost 100% for any wavelength shorter than about 800 nm. For longer-wavelength (infrared) radiation, QE decreased exponentially with the photon wavelength increase. Time-resolved measurements of our SSPDs showed that the system-limited detector response pulse width was below 150 ps. The system jitter was measured to be 35 ps. In terms of the counting rate, jitter, and dark counts, the NbN SSPDs significantly outperform their semiconductor counterparts. Already identifeid and implemented applications of our devices range from noninvasive testing of semiconductor VLSI circuits to free-space quantum communications and quantum cryptography.
We report our studies on the performance of new NbN ultrathin-film superconducting single-photon detectors (SSPDs). Our SSPDs exhibit experimentally measured quantum efficiencies from ~ 5% at wavelength λ = 1550 nm up to ~10% at λ = 405 nm, with exponential, activation-energy-type spectral sensitivity dependence in the 0.4-μm - 3-μm wavelength range. Using a variable optical delay setup, we have shown that our NbN SSPDs can resolve optical photons with a counting rate up to 10 GHz, presently limited by the read-out electronics. The measured device jitter was below 35 ps under optimum biasing conditions. The extremely high photon counting rate, together with relatively high (especially for λ > 1 μm) quantum efficiency, low jitter, and very low dark counts, make NbN SSPDs very promising for free-space communications and quantum cryptography.
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