Research On Superconducting NSQUID Circuit For Reversible Computing

Reversible computing is an adiabatic computing with information conservation.Its advantages are low-power and high energy-efficient.It can be used to realize logic operations with bit energy dissipation lower than thermodynamic limit,k_BTln2.It is also compatible to quantum computing,in fact quantum computing is a reversible computing with the ability to perform superposition state.Negative-inductance SQUID?nSQUID?is the key device specifically invented to study reversible computing.Although the existed nSQUID circuits are all voltage-driven because of simple bias strategy,the clock frequency of voltage-driven nSQUID circuit is vulnerable to voltage noise,even thermal voltage noise,which will make the nSQUID circuits unstable.In this thesis,current-driven nSQUID is comprehensively investigated to address this problem.With the practical experience and technology developed in Nb-based Josephson junction and DC-SQUID,we have clarified the principle of current-driven nSQUID.By choosing proper key parameters,the potential energy of current-driven nSQUID can be tuned adiabatically from single-well to double-well,which is the basis for reversible computing.The clock frequency is well controlled and insensitive to thermal voltage noise,therefore the robustness of nSQUID circuit is expected to be highly improved.Superconducting circuit simulator,WRspice,has been used to evaluate and design current-driven nSQUID circuit.4-layer Nb film superconducting circuit fabrication technology has been develop to fabricate current-driven nSQUID circuit.A current-driven nSQUID chip has been successfully fabricated and tested in Oxford dilution refrigerator.The experimental results match the WRspice simulation results well,and the chip passes the functional verification,which is the first experimental realization of current-driven nSQUID.To quantitatively evaluate the energy dissipation of the current-driven nSQUID,we proposed to couple current-driven nSQUID to coplanar waveguide resonator,which is an extremely sensitive device to inner loss change.The coupled circuit has been successfully fabricated and tested in Janis?39?e-3 cryostat.The power dissipation and bit energy dissipation of current-driven nSQUID can be calculated from the transmission spectra,S₂₁,of the coupled system in different microwave input power,which is also confirmed in WRspice simulation.The experimental result shows that bit energy dissipation of current-driven nSQUID operating at 2.2 GHz is about 1.38 zJ,which is 4to 5 orders of magnitude superior to state-of-the-art CMOS logic.This study shows that current-driven nSQUID is a superconducting device with excellent performance and expected to be used to realize logic operations with energy dissipation lower than thermodynamic limit.Besides,we propose a superconductive true random number generator?TRNG?based on current-driven nSQUID,which extracted the entropy generated from symmetry breaking while tuning the potential energy of current-driven nSQUID from single-well to double-well.The proposal has been initially verified at low frequency experiments at low temperature.The autocorrelation function of the generated data has been calculated.High frequency characteristics of the proposed TRNG has been analyzed by WRspice,the simulation result imply that the TRNG can operate at frequency of GHz,if the Josephson junctions with high critical current density and critical damping condition are used.