Single-Electron Detection with a SQUID: Toward Quantum-Limited, Relativistic Detection

Benedetta Camarota

A Single-Electron Transistor (SET) is a highly sensitive charge amplifier based on the Coulomb blockade that can operate near the quantum limit: amplification can be accomplished with a back-action close to that required by the uncertainty principle. Its charge sensitivity is ultimately limited to about 1micre/sqrt(Hz) by the shot noise in the source-drain current. Despite the fact that such sensitivity has been approached by some, the shot-noise limit has yet to be reached for either the SET and its RF version, the RF-SET. A scheme is proposed and realized in this thesis for the shot noise-limited detection of a Single-Electron Transistor, based on a DC-SQUID impedance-matched with a DC-biased SET. SQUIDs can operate at the quantum limit, and a microstrip SQUID amplifier has already been demonstrated to operate at almost twice the quantum-limited noise temperature at about 500MHz by M. Muck and collaborators, with a sensitivity sufficient to resolve the SET shot noise. The device has been fabricated and characterized. A 50KOhm resistor has been used as a noise source at low temperature and the measured noise levels have been estimated. Moreover, the detection noise temperature of about 110mK was derived corresponding to a current sensitivity of about 8fA/sqrt(Hz) for the detection of a 100KOhm resistor.

Denis Vasyukov

Jongsoo Yoon, John L. Clarke, Jan M. Gildemeister +5 more

We describe a frequency domain superconducting quantum interference device (SQUID) multiplexer which monitors a row of low-temperature sensors simultaneously with a single SQUID. Each sensor is ac biased with a unique frequency and all the sensor currents are added in a superconducting summing loop. A single SQUID measures the current in the summing loop, and the individual signals are lock-in detected after the room temperature SQUID electronics. The current in the summing loop is nulled by feedback to eliminate direct crosstalk. In order to avoid the accumulation of Johnson noise in the summing loop, a tuned bandpass filter is inserted in series with each sensor. To multiplex 32 Voltage-biased Superconducting Bolometers (VSB) with 1 msec time constants a total bandwidth of 100 kHz is required. The minimum bias frequency is limited to ≳500 kHz by practical limitations in the size of photolithographed inductors and capacitors. The major limitation of our multiplexing scheme is in the slew rate of the readout SQUID. We discuss a “carrier nulling” technique which could be used to increase the number of sensors in a row or to multiplex faster bolometers by reducing the required slew rate for a readout SQUID.

Elena Bartolomé, A. Camón, Jakob Flokstra +4 more

Jani Tuorila

Ling Yun Hao

SQUIDs (superconducting quantum interference devices) have been in use for more than half a century and constitute one of the first macroscopic quantum devices. SQUIDs operating at millikelvin temperatures can act as qubits for quantum computers and the recent reports of more than 50 qubit circuits indicate how far the technology has developed. In this talk, the author will describe two other applications of SQUIDs as quantum detectors, focusing on single spin and single photon energy resolving detection. Most superconducting devices rely on tri-layer Josephson tunnel junctions which are not easily scalable to the nanoscale. We have developed a Josephson junction fabrication method, based on electron beam lithography or focused ion beam milling of a single thin film of superconductor (Nb generally) which can provide sizes down to 50 nm. These devices are particularly relevant for two main applications in quantum technology and metrology. First, by shrinking the size of the SQUID loop and the junctions to around 200 nm, the sensitivity of the SQUID for magnetization measurements is improved to the level where a single electron spin flip may be detected. This is possible at the relatively elevated temperature of 4K.  We are working with Surrey University to implant single magnetic ions within the SQUID loop to provide a platform to test this combination as the basis for a new form of qubit operating at higher temperatures than the conventional Transmon superconducting devices. A second SQUID based detector which we are developing is an inductive transition edge sensor device (ISTED) for energy resolving measurements of single photons. This is based on the development of conventional transition edge sensors where we detect the change of the penetration depth of a small thin film of superconductor when it absorbs a photon. In this way, a major source of noise in conventional TES may be avoided since the absorber remains in the superconducting state at all times. In this way we have demonstrated single photon detection at 633 nm with 0.1eV resolution at operating temperature of 7.5K.