What is a Josephson junction?

Bolometers based on superconducting Josephson junctions (JJs) can be used for several types of sensors, including transition edge sensors, Josephson escape sensors, and radiation threshold detectors.

This article first presents the structure of a basic bolometer and then looks at how that structure can be varied to produce different types of sensors.

A bolometer is a device that measures radiation based on the temperature rise of a thermal absorber that is warmed by absorption of the radiation. A bolometric sensor consists of three basic components:

The thermal absorber is usually fabricated from a thin layer of superconductor absorptive elements that are operated at cryogenic temperatures, enabling greater sensitivity.
A resistive thermometer is connected directly to the absorber. Any absorbed energy increases the resistance of the absorber, reducing the voltage drop across the resistive thermometer and enabling measurement of the absorbed energy and identification of the absorbed particle.
Optimized thermal coupling between the absorber and the cooling bath. A low thermal conductance ensures that the temperature of the thermal absorber rises sufficiently to enable measurement of the energy. The thermal coupling, however, needs to be strong enough to rapidly return the absorber to the bath temperature after the measurement has been completed.

Transition edge sensor

A transition edge sensor (TES) is a simple voltage-biased superconducting bolometer. The bias voltage puts the TES in its self-biased region where the power dissipation is constant. When a particle is absorbed, the TES temperature rises, and the energy is removed via negative electrothermal feedback. The absorbed energy causes the resistance of the TES to increase, reducing the current flow and the Joule heating, quickly returning the TES back to an equilibrium state in the self-biased region. Readouts of the varying resistance of the TES are accomplished by inductively coupling the device to a superconducting quantum interference device (SQUID) array that can be connected to room-temperature electronics for signal processing.

Josephson escape sensors

Josephson escape sensors (JES) are a variation on a TES. A JES consists of two superconducting JJs. While the sensitivity and working voltage of a TES are fixed, they are controllable in a JES through changes in the bias current.

In addition to its application in telescopes for Gigahertz (GHz) astronomy, the JES’s turnability can be used in quantum technologies such as sub-terahertz (sub-THz) communications, quantum computing, cryptography, and quantum key distribution.

A typical JES nanosensor is ac-biased. In Figure 1a, RL is a high-impedance load resistor, and the small blue box in the center contains an aluminum/copper (Al/Cu) nanowire (shown in detail as the red wire in the inset) placed between the thick Al leads (yellow). The voltage drop across the nanowire is connected to the metrology electronics through a lock-in amplifier that can extract the signal of a known carrier wave in a noisy environment. Figure 1b shows the I-V characteristics of the JES at temperatures ranging from 20 to 160 mK in 20 mK steps.

Figure 1. Structure and some performance characteristics of a Josephson escape sensor. (Image: *Physical Review Applied*)

Figure 1c shows the controllable temperature range of the critical current (IC) (black line) and the retrapping current (IR) (green line). The inset in the upper right corner of the graph shows how the nanowire switches from its superconducting (non-dissipative) state to its normal (resistive) state at IC and the transition from resistive to non-dissipative state at IR. Hysteresis occurs due to Joule heating. IR is about 26.6 nA and is independent of temperature.

Two bolometers and one JJ

A Josephson radiation threshold detector (JRTD) for near-infrared radiation at 1,550 nm has been designed using two bolometers and one JJ. Figure 2 shows a single superconductor-insulator-superconductor (SIS) JJ between two resistors about 100 μm from the JJ. The green box zooms in on the SIS junction structure, and the red box shows the detail of one of the two hot electron bolometers (HEBs), which consist of a normal metal nano-absorber whose temperature is measured by a normal-metal, insulator, superconductor (NIS) thermometer probe.

Figure 2. A single JJ between two HEBs in a Josephson radiation threshold detector. (Image: *arVix*)

Summary

Superconducting JJs can be used to make a variety of sensors, such as TES and JES. When combined with one or more bolometers, more possibilities open, including using one JJ with two bolometers in a Josephson radiation threshold detector.