Number resolving superconducting nanowire photon detector on multiple surfaces …

Description (excerpt)

STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon. BACKGROUND OF THE INVENTION The ability of a, very thin and very narrow, section or “nanowire” of superconducting material to detect the impact of a photon has long been known. The basic principle is as follows. A nanowire of a superconducting material is created and electrically wired to a voltage source. As the current flows through the wire it creates heat. If the cooling needed to reach the temperature for superconducting phenomena (T C of approximately 10 kelvin or less) and the heating due to the current density in the nanowire are properly balanced, then the nanowire can be held extremely close to, but under, the superconducting phase transition. Below this critical temperature, T C , the wire effectively has no resistance. Just above the critical temperature the wire is not superconducting and has a finite resistance. When a photon strikes the superconducting nanowire it breaks the cooper pairs in the vicinity and creates a hot spot. If this photon induced hotspot raises the temperature of the segment of nanowire above T C then the hotspot will undergo a phase transition and no longer be superconducting. If the non-superconducting area is large enough, or equivalently the nanowire is small enough, it will block the entire nanowire. This leads to a sudden rise in the resistance of the nanowire. This change in resistance can be detected by various electronic setups and a single photon is counted. Once the hot spot from the incident photon disperses, the wire will return to the superconducting state and the device will be ready to detect another photon. This is called the reset time of the device. Traditional Superconducting Nanowire Single Photon Detectors (SNSPDs) are made from one long nanowire. In order for this single nanowire to cover a useful area a “meander” is created. In effect the wire is folded back and forth across the surface of the desired area, usually about 10 μm by 10 μm (microns). These devices are called “single photon detectors” because the nanowire can only feel the loss of the superconducting condition somewhere along its length. If two photons hit the nanowire at the same time, two hot spots are created but only the increase in resistance is felt so the detector can “see” only one photon. Such detectors are effectively high pass filters; they can detect the presence of 1 or more photons without being able to count them. Similarly the device has no means of measuring spatial resolution. The output signal of the device is not a function of the location of the photon impact. A second drawback of the long nanowire approach is the relaxation time of the detector. It has been shown that the relaxation time, the time for a hot spot to dissipate, is related to the kinetic-inductance and thus the length of the nanowire. This leads to a relaxation time of about 10 ns for a niobium nitrite, NbN, single wire 10 μm by 10 μm meander. The operational repetition rate will need to be significantly slower than the relaxation time of the device to avoid interactions between the relaxation and incoming photons. This leads to experimental repetition rates much slower than current pulsed laser systems which are capable of gigahertz frequencies. To create the number resolution of the overall optical device multiple nanowires detectors are needed. In U.S. Patent Application Publication No. 2009/0050790A1 by Dauler et al., one possible method to gain some amount of number resolution is given, the so called multi element superconducting nanowire single photon detector (MESNSPD). Their method involves interleaving multiple long nanowires in a parallel meander across the surface of a substrate chip. While this solution is somewhat effective in creating number resolution, it has numerous drawbacks common to all current SNSPDs and the limited number of nanowires available, currently 10 or so, limits the number resolution of the device. One of the most important effects in a SNSPD is that of current crowding. Current crowding in superconducting nanowires has been studied by Clem and Berggren. The heating in the superconducting nanowire, as mentioned above, is vital and is determined by the local current density along the wire. Ergo an area with higher current density will be “hotter” than an area of low current density. In order to maintain the necessary superconducting condition along the full length of the wire the maximum bias current applied through any nanowire detector is determined by the point of highest current density and therefore highest temperature. Current crowding as discussed by Clem and Breggen describes the effect of bends and constrictions in the nanowire which increase the local current density, such as the bends in the multi element superconducting nanowire single photon detector (MESNSPD) of Dauler et al. and standard SNSPDs. These bends are then the hottest spots on the detector, i.e. the closest to the critical temperature T C . This is a significant problem as the quantum efficiency of any region of a nanowire is proportional to how close that nanowire region is to T C . The incident single photon carries a very small amount of energy and creates a small amount of heating. The clos