THE QUANTUM INTELLIGENCER

Superconducting Shift Registers Achieve Sub-Landauer Energy Dissipation in Information Propagation In Plain English: This research tackles the problem of how much energy computers need to move information around. Normally, there's a physical limit to how little energy you can use when deleting data, but moving data without deleting it might require much less. The scientists built tiny superconducting circuits that pass bits of information in a loop using magnetic whirlpools. They found one version of this circuit uses less energy per step than the supposed minimum limit—because it’s just moving data, not erasing it. This matters because it shows a path toward computers that use far less power, which could lead to faster, cooler-running, and more efficient electronics in the future. Summary: This study presents experimental results on two types of superconducting circular shift registers that encode and propagate information using Josephson vortices—quantized magnetic flux particles—in discrete Josephson transmission lines (JTLs). The first device is a uniform loop composed of identical JTL sections with a flux pump to inject and control the number of moving vortices. In this system, the authors demonstrate energy dissipation per bit-shift operation below the Landauer limit of $k_BT \ln 2$, achieving as low as sub-Landauer levels up to propagation delays of approximately 0.7 ns, corresponding to operational frequencies up to ~1.4 GHz. Crucially, this does not violate Landauer’s principle because the operation involves information propagation rather than irreversible erasure. The second device is a nonuniform register incorporating sections of standard JTLs and sections using nSQUIDs—dc-SQUIDs engineered with negative mutual inductance between arms—which exhibit a tunable double-well potential suitable for reversible computing. In this configuration, the minimum energy dissipation per bit-shift is measured at about 16 times $k_BT \ln 2$, which the authors attribute to nonuniform vortex motion and energy barriers arising from impedance mismatches between JTL and nSQUID sections. The differences in performance are analyzed through current-voltage characteristics, effective resistance, vortex terminal speed, and their dependence on vortex count. Additionally, the nSQUID-based design introduces a novel type of lossless discrete transmission line with ground-connected inductance, resulting in frequency-dependent impedance and propagation speed—distinct from conventional JTL behavior. These findings highlight the importance of device uniformity and impedance matching in minimizing energy loss and support the feasibility of ultra-low-power classical computing architectures based on superconducting vortex dynamics. Key Points: - The uniform superconducting shift register demonstrates bit-shift energy dissipation below the Landauer limit (k_BT ln2), enabled by reversible information propagation via Josephson vortices. The sub-Landauer operation occurs at propagation delays up to ~0.7 ns and frequencies up to ~1.4 GHz. This does not violate Landauer’s principle because no information is erased during the shift operation. The nonuniform register, incorporating nSQUID sections, exhibits higher energy dissipation (~16 × k_BT ln2) due to impedance mismatches and energy barriers between different sections. Josephson vortex dynamics, including speed and effective resistance, depend on the number of vortices in motion. nSQUIDs introduce a new type of lossless transmission line with frequency-dependent impedance and propagation characteristics. The work demonstrates a physical platform for exploring reversible computing and ultra-low-energy classical information processing. Notable Quotes: - "We demonstrate the energy dissipation per bit-shift operation below the Landauer's thermodynamic limit $E_T=k_BT\ln2$ up to propagation delays of ~0.7 ns, corresponding to the circular information motion with frequencies up to ~1.4 GHz." — From the abstract, highlighting the core experimental achievement. - "This does not contradict Landauer's minimum energy requirement for computations since information is not destroyed." — Clarifying the theoretical consistency with thermodynamics. - "nSQUID inductance connecting JJs to the ground leads to an unusual type of lossless discrete transmission line with frequency-dependent impedance and propagation speed, both different from the regular JTLs." — Emphasizing the novel circuit behavior introduced by nSQUIDs. Data Points: - Energy dissipation per bit-shift in the uniform register: below $k_BT \ln 2$ (sub-Landauer). - Propagation delay threshold: ~0.7 ns. - Maximum operational frequency: ~1.4 GHz. - Minimum energy dissipation in nonuniform register: ~16 × $k_BT \ln 2$. - Device types: uniform JTL loop and nonuniform JTL-nSQUID hybrid register. Controversial Claims: - The claim that energy dissipation per bit-shift falls below Landauer's limit, while technically accurate in this context, may be misinterpreted as violating the principle without proper clarification. Landauer's limit applies strictly to logically irreversible operations (i.e., erasure), not to information propagation or storage. While the paper correctly notes this distinction, broader audiences or media may present this result as 'breaking' a fundamental limit, which could be misleading. Additionally, the characterization of nSQUID-based lines as 'lossless' may be idealized, as real-world fabrication imperfections and material losses could introduce dissipation not fully captured in the measurements. Technical Terms: - Josephson vortex, Josephson transmission line (JTL), nSQUID (negative inductance SQUID), dc-SQUID, Landauer's limit ($k_BT \ln 2$), reversible computing, flux pump, double-well potential, effective resistance, terminal speed of vortices, impedance mismatch, frequency-dependent impedance, lossless transmission line, superconducting shift register, parametric device, vortex dynamics. —Ada H. Pemberley Dispatch from The Prepared E0