Direct current superconducting quantum interferometers with asymmetric shunt resistors

We have investigated asymmetrically shunted Nb/Al-AlO\(_x\)/Nb direct current (dc) superconducting quantum interference devices (SQUIDs). While keeping the total resistance \(R\) identical to a comparable symmetric SQUID with \(R^{-1} = R_1^{-1} + R_2^{-1}\), we shunted only one of the two Josephson junctions with \(R = R_{1,2}/2\). Simulations predict that the optimum energy resolution \(\epsilon\) and thus also the noise performance of such an asymmetric SQUID can be 3--4 times better than that of its symmetric counterpart. Experiments at a temperature of 4.2\,K yielded \(\epsilon \approx 32\,\hbar\) for an asymmetric SQUID with an inductance of \(22\,\rm{pH}\). For a comparable symmetric device \(\epsilon = 110\,\hbar\) was achieved, confirming our simulation results.