The expected emergence of cryptographically relevant quantum computers (CRQCs) will represent a singular discontinuity in the history of digital security, with wide ranging impacts. This Perspective seeks to elucidate specific implications that the capabilities of developing quantum architectures have on blockchain vulnerabilities and potential mitigation strategies. First, we provide new resource estimates for breaking the 256-bit Elliptic Curve Discrete Logarithm Problem over the secp256k1 curve, the core of modern blockchain cryptography. We demonstrate that Shor’s algorithm for this problem can execute with either ≤ 1200 logical qubits and ≤ 90 million Toffoli gates or ≤ 1450 logical qubits and ≤ 70 million Toffoli gates. In the interest of responsible disclosure, we use a zero-knowledge proof to validate these results without disclosing attack vectors. On superconducting architectures with 10 − 3 physical error rates and planar connectivity, those circuits can execute in minutes using fewer than half a million physical qubits. We introduce a critical distinction between “fast-clock” (such as superconducting and photonic) and “slow-clock” (such as neutral atom and ion trap) architectures. Our analysis reveals that the first fast-clock CRQCs would enable “on-spend” attacks on public mempool transactions of some cryptocurrencies. We survey major cryptocurrency vulnerabilities through this lens, identifying systemic risks associated with advanced features in some blockchains such as smart contracts, proof-of-stake consensus, and data availability sampling mechanism, as well as the enduring concern of “abandoned” assets. We argue that technical solutions would benefit from accompanying public policy and discuss various frameworks of “digital salvage” to regulate the recovery or destruction of dormant assets while preventing adversarial seizure. We also discuss implications for other digital assets and tokenization as well as challenges and successful examples of the ongoing transition to post-quantum cryptography (PQC). Finally, we urge all vulnerable cryptocurrency communities to join the migration to PQC without delay.
The knowledge of the energy spectrum completely defines the dynamics of a quantum system for a given initial state. This makes spectroscopy a key characterization technique when studying or designing qubits and complex quantum systems. In semiconductor quantum dots, the electronic quantum states can be probed through charge transport or charge sensing, but the technique is practically limited to gate-defined quantum dots. Epitaxial quantum dots benefit from excellent optical properties, but are usually incompatible with charge transport, while alternative spectroscopy techniques provide only limited information. Here we demonstrate a spectroscopy technique which utilizes nuclear spins as a non-invasive probe. By using the cotunneling-assisted transfer of spin angular momentum instead of the charge currents we achieve near-equilibrium probing. Experiments are conducted on low-strain GaAs/AlGaAs epitaxial dots, revealing energy spectra for charge configurations with up to seven electrons and with resolution limited only by the thermal broadening. Beyond the few-electron energy spectroscopy, we apply non-invasive probing via nuclear spins to reveal the subtle many-body physics of the quantized electronic states. We map out the magnetic field dependence of the electron spin lifetimes, revealing an unusual inverse quadratic dependence. The rich variety of further observations includes the long-lived spin-qubit states in the p shell, electron ground-state phase transitions, strong spin-orbit coupling regimes, and an anomalously fast nuclear spin diffusion process. Experiments are backed up by good agreement with the first-principles configuration interaction numerical modeling. Our work uncovers few-electron states as a new operating regime for optically active quantum dots. Accurate control and probing of many-body states offers a test-bed system for fundamental physics studies, while prospective technological applications include electron spin qubits with extended coherence and scalable electrical control.
Drive-induced unwanted state transitions (DUST) are limiting both for microwave readout and parametric operations of superconducting qubits. Among them, measurement-induced state transitions (MIST) are due to intrinsic resonances described by the readout Hamiltonian. They were previously studied with a qubit linearly coupled to its readout mode, which constitutes the usual readout Hamiltonian. Since MIST can appear even at moderate powers, they limit the readout signal-to-noise ratio and the quantum nondemolition readout fidelity. In this work, we study the high-power readout regime in a different transmon readout scheme, implementing a nonlinear coupling called the cos gyp-coupling. This coupling stems from a transmon molecule circuit and has symmetry properties that suppress nonparity-conserving MIST. We succeed in performing multistate single-shot readout up to the fifth excited state of the transmon, which enables us to identify leakage pathways from the computational subspace. The measurements indicate that the system is free of MIST up to high powers, with more than 300 photons in the readout mode. The MIST can be controllably turned on by breaking the parity symmetry of the coupling using flux-tuning. These experimental results are corroborated by branch analysis and simulations of the classical chaotic dynamics, showing that the cos gyp-coupling is very robust to readout photons compared to the usual transverse coupling.
We propose a decoder for quantum low-density parity-check (LDPC) codes based on a beam search heuristic guided by belief propagation (BP). Our beam search decoder applies to all quantum LDPC codes and achieves different speed-accuracy tradeoffs by tuning its parameters such as the beam width. We perform numerical simulations under circuit level noise for the [[144,12,12]] bivariate bicycle (BB) code at noise rate p=10^{−3} to estimate the logical error rate and the 99.9 percentile runtime and we compare with the BP-OSD decoder which has been the default quantum LDPC decoder for the past six years. A variant of our beam search decoder with a beam width of 64 achieves a 17× reduction in logical error rate. With a beam width of 8, we reach the same logical error rate as BP-OSD with a 26.2× reduction in the 99.9 percentile runtime. We identify the beam search decoder with beam width of 32 as a promising candidate for trapped ion architectures because it achieves a 5.6× reduction in logical error rate with a 99.9 percentile runtime per syndrome extraction round below 1 ms at p=5×10^{−4}. Remarkably, this is achieved in software on a single core, without any parallelization or specialized hardware (FPGA, ASIC), suggesting one might only need three 32-core CPUs to decode a trapped ion quantum computer with 1000 logical qubits.
Precise state-dependent control of optical potentials is of great importance for various applications utilizing cold neutral atoms. In particular, tune-out wavelengths for the clock state pair in alkaline-earth(-like) atoms provide maximally state-selective trap conditions that hold promise for the realization of novel approaches in quantum computation and simulation. While several ground-state tune-out wavelengths have been determined, similar experimental studies for metastable excited states are challenged by inelastic collisions and Raman losses, so far prohibiting precise measurements of excited-state tune-out conditions. In this work we report on the measurement of a tune-out wavelength for the metastable 3 P 0 clock state in 174 Yb at 519.920 ( 9 ) THz. In order to circumvent collisional losses, we isolate individual 3 P 0 atoms in a two-dimensional clock-magic-wavelength lattice at 759 nm. To minimize the limitation imposed by Raman scattering, we further implement resolved sideband cooling on the clock transition, which allows us to reduce the lattice depth and surpass lifetimes of 5 s. The precision of the tune-out measurement is further enhanced by fluorescence imaging in a triple-magic configuration, where we implement molasses cooling on the 3 P 1 intercombination line and identify a magic angle of 38.5 ( 9 ) ∘ in the clock-magic lattice.
