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Towards practical secure delegated quantum computing with semi-classical light

Quantum 9, 1943 (2025). https://doi.org/10.22331/q-2025-12-12-1943 Secure Delegated Quantum Computation (SDQC) protocols are a vital piece of the future quantum information processing global architecture since they allow end-users to perform their valuable computations on remote quantum servers without fear that a malicious quantum service provider or an eavesdropper might acquire some information about their data or algorithm. They also allow end-users to check that their computation has been performed as they have specified it. However, existing protocols all have drawbacks that limit their usage in the real world. Most require the client to either operate a single-qubit source or perform single-qubit measurements, thus requiring them to still have some quantum technological capabilities albeit restricted, or require the server to perform operations which are hard to implement on real hardware (e.g isolate single photons from laser pulses and polarisation-preserving photon-number quantum non-demolition measurements). Others remove the need for quantum communications entirely but this comes at a cost in terms of security guarantees and memory overhead on the server's side. We present an SDQC protocol which drastically reduces the technological requirements of both the client and the server while providing information-theoretic composable security. More precisely, the client only manipulates an attenuated laser pulse, while the server only handles interacting quantum emitters with a structure capable of generating spin-photon entanglement. The quantum emitter acts as both a converter from coherent laser pulses to polarisation-encoded qubits and an entanglement generator. Such devices have recently been used to demonstrate the largest entangled photonic state to date, thus hinting at the readiness of our protocol for experimental implementations.

Quantum Journal

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Exploiting many-body localization for scalable variational quantum simulation

Quantum 9, 1942 (2025). https://doi.org/10.22331/q-2025-12-12-1942 Variational quantum algorithms (VQAs) represent a promising pathway toward achieving practical quantum advantage on near-term hardware. Despite this promise, for generic, expressive ansätze, their scalability is critically hindered by barren plateaus–regimes of exponentially vanishing gradients. We demonstrate that initializing a hardware-efficient, Floquet-structured ansatz within the many-body localized (MBL) phase mitigates barren plateaus and enhances algorithmic trainability. Through analysis of the inverse participation ratio, entanglement entropy, and a novel low-weight stabilizer Rényi entropy, we characterize a distinct MBL-thermalization transition. Below a critical kick strength, the circuit avoids forming a unitary 2-design, exhibits robust area-law entanglement, and maintains non-vanishing gradients. Leveraging this MBL regime facilitates the efficient variational preparation of ground states for several model Hamiltonians with significantly reduced computational resources. Crucially, experiments on a 127-qubit superconducting processor provide evidence for the preservation of trainable gradients in the MBL phase for a kicked Heisenberg chain, validating our approach on contemporary noisy hardware. Our findings position MBL-based initialization as a viable strategy for developing scalable VQAs and motivate broader integration of localization into quantum algorithm design.

Quantum Journal

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LUCI in the Surface Code with Dropouts

AbstractRecently, usage of detecting regions facilitated the discovery of new circuits for fault-tolerantly implementing the surface code. Building on these ideas, we present LUCI, a framework for constructing fault-tolerant circuits flexible enough to construct aperiodic and anisotropic circuits, making it a clear step towards quantum error correction beyond static codes. We show that LUCI can be used to adapt surface code circuits to lattices with imperfect qubit and coupler yield, a key challenge for fault-tolerant quantum computers using solid-state architectures. These circuits preserve spacelike distance for isolated broken couplers or isolated broken measure qubits in exchange for halving timelike distance, substantially reducing the penalty for dropout compared to the state of the art and creating opportunities in device architecture design. For qubit and coupler dropout rates of 1% and a patch diameter of 15, LUCI achieves an average spacelike distance of 13.1, compared to 9.1 for the best method in the literature. For a SI1000(0.001) circuit noise model, this translates to a 36x improvement in median logical error rate per round, a factor which increases with device performance. At these dropout and error rates, LUCI requires roughly 25% fewer physical qubits to reach algorithmically relevant one-in-a-trillion logical codeblock error rates.Featured image: LUCI Diagrams and their circuit interpretation. (left) LUCI diagram showing the two rounds of a distance-5 three-coupler surface code circuit [10]. Colors indicate the mid-cycle stabilizers of the surface code being measured, with X stabilizers in red and Z stabilizers in blue. The gray shapes inside the squares indicate how that stabilizer will be measured. (right) Circuit compilations for U-shapes on X-type (red) and Z-type (blue) squares. The colored regions depict the detecting region contracting and being measured out, and then a new detecting region expanding from a reset. Shades of gray in the shap

Quantum Journal

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Artificial Intelligence (MS)

Graduate Degrees in Artificial Intelligence Graduate Degrees in Artificial Intelligence Artificial Intelligence How to Apply  Request More Information The professional master’s in AI prepares engineers, applied scientists and technical professionals for career advancement in advanced technical leadership roles in the rapidly growing ﬁeld of artiﬁcial intelligence engineering. The core curriculum addresses a breadth of areas central to AI engineering expertise including machine learning, statistical learning, data mining and ethics.The MS-AI is offered online and on campus. Whether you are seeking a program that promotes a flexible learning schedule or one that offers in-person interaction with peers and faculty, you’ll find the same rigorous curriculum that will help you advance your career. Degree Types MS-AI College of Engineering and Applied ScienceDepartment of Computer ScienceComputer Science Faculty Degree Options & Application Requirements Artificial Intelligence Artificial Intelligence Artificial IntelligenceProfessional Master's in Artificial Intelligence (On Campus)30 credit hoursMust be completed within 4 yearsApplication Deadlines & RequirementsTo learn about the upcoming term application deadlines, please visit the Graduate School website. For program details, review the course requirements.Professional Master's in Artificial Intelligence (Online)The online MS-AI is a program offered through the University of Colorado Boulder and hosted online through Coursera’s learning platform. The MS-AI is a nonthesis degree that requires 30 credit hours of coursework.Enroll right away into a series of courses. Complete your pathway with a GPA of 3.0 or better to be admitted to the program, all while making direct progress toward your degree. No transcripts or applications are required.Application Deadlines & RequirementsTo learn about the upcoming term application deadlines, please visit the Graduate School website. For program detai

University of Colorado Quantum

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Polynomial-Time Classical Simulation of Hidden Shift Circuits via Confluent Rewriting of Symbolic Sums

Quantum 9, 1926 (2025). https://doi.org/10.22331/q-2025-12-02-1926 Implementations of Roetteler's shifted bent function algorithm have in recent years been used to test and benchmark both classical simulation algorithms and quantum hardware. These circuits have many favorable properties, including a tunable amount of non-Clifford resources and a deterministic output, and moreover do not belong to any class of quantum circuits that is known to be efficiently simulable. We show that this family of circuits can in fact be simulated in polynomial time via symbolic path integrals. We do so by endowing symbolic sums with a confluent rewriting system and show that this rewriting system suffices to reduce the circuit's path integral to the hidden shift in polynomial time. We hence resolve an open conjecture about the efficient simulability of this class of circuits.

Quantum Journal

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Consistent circuits for indefinite causal order

AbstractOver the past decade, a number of quantum processes have been proposed which are logically consistent, yet feature a cyclic causal structure. However, there is no general formal method to construct a process with an exotic causal structure in a way that ensures, and makes clear why, it is consistent. Here we provide such a method, given by an extended circuit formalism. This only requires directed graphs endowed with Boolean matrices, which encode basic constraints on operations. Our framework (a) defines a set of elementary rules for checking the validity of any such graph, (b) provides a way of constructing consistent processes as a circuit from valid graphs, and (c) yields an intuitive interpretation of the causal relations within a process and an explanation of why they do not lead to inconsistencies. We display how several standard examples of exotic processes, including ones that violate causal inequalities, are among the class of processes that can be generated in this way; we conjecture that this class in fact includes all unitarily extendible processes.Featured image: The routed graphs corresponding to the quantum switch and to the Lugano process.► BibTeX data@article{Vanrietvelde2025consistentcircuits, doi = {10.22331/q-2025-12-02-1923}, url = {https://doi.org/10.22331/q-2025-12-02-1923}, title = {Consistent circuits for indefinite causal order}, author = {Vanrietvelde, Augustin and Ormrod, Nick and Kristj{\'{a}}nsson, Hl{\'{e}}r and Barrett, Jonathan}, journal = {{Quantum}}, issn = {2521-327X}, publisher = {{Verein zur F{\"{o}}rderung des Open Access Publizierens in den Quantenwissenschaften}}, volume = {9}, pages = {1923}, month = dec, year = {2025} }► References [1] D. E. Deutsch, ``Quantum computational networks,'' Proceedings of the Royal Society of London: A. Mathematical and Physical Sciences 425 no. 1868, (1989) 73–90. https://doi.org/10.1098/rspa.1989.0099 [2] D. Aharonov, A. Kitaev, and N. Nisan, ``Quantum circuits with mixed states,'

Quantum Journal

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Quantum error correction for long chains of trapped ions

Quantum 9, 1920 (2025). https://doi.org/10.22331/q-2025-11-27-1920 We propose a model for quantum computing with long chains of trapped ions and we design quantum error correction schemes for this model. The main components of a quantum error correction scheme are the quantum code and a quantum circuit called the syndrome extraction circuit, which is executed to perform error correction with this code. In this work, we design syndrome extraction circuits tailored to our ion chain model, a syndrome extraction tuning protocol to optimize these circuits, and we construct new quantum codes that outperform the state-of-the-art for chains of about $50$ qubits. To establish a baseline under the ion chain model, we simulate the performance of surface codes and bivariate bicycle (BB) codes equipped with our optimized syndrome extraction circuits. Then, we propose a new variant of BB codes defined by weight-five measurements, that we refer to as BB5 codes and we identify BB5 codes that achieve a better minimum distance than any BB codes with the same number of logical qubits and data qubits, such as a $[[48, 4, 7]]$ BB5 code. For a physical error rate of $10^{-3}$, the $[[48, 4, 7]]$ BB5 code achieves a logical error rate per logical qubit of $5 \cdot 10^{-5}$, which is four times smaller than the best BB code in our baseline family. It also achieves the same logical error rate per logical qubit as the distance-7 surface code but using four times fewer physical qubits per logical qubit.

Quantum Journal

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Bounds in Sequential Unambiguous Discrimination of Multiple Pure Quantum States

AbstractSequential methods for quantum hypothesis testing offer significant advantages over fixed-length approaches, which rely on a predefined number of state copies. Despite their potential, these methods remain underexplored for unambiguous discrimination. In this work, we derive performance bounds for such methods when applied to the discrimination of a set of pure states. The performance is evaluated based on the expected number of copies required. We establish a lower bound applicable to any sequential method and an upper bound on the optimal sequential method. The upper bound is derived using a novel and simple non-adaptive method. Importantly, the gap between these bounds is minimal, scaling logarithmically with the number of distinct states.► BibTeX data@article{PerezGuijarro2025boundsinsequential, doi = {10.22331/q-2025-11-20-1919}, url = {https://doi.org/10.22331/q-2025-11-20-1919}, title = {Bounds in {S}equential {U}nambiguous {D}iscrimination of {M}ultiple {P}ure {Q}uantum {S}tates}, author = {P{\'{e}}rez-Guijarro, Jordi and Pag{\`{e}}s-Zamora, Alba and Fonollosa, Javier R.}, journal = {{Quantum}}, issn = {2521-327X}, publisher = {{Verein zur F{\"{o}}rderung des Open Access Publizierens in den Quantenwissenschaften}}, volume = {9}, pages = {1919}, month = nov, year = {2025} }► References [1] Ryuji Takagi, Suguru Endo, Shintaro Minagawa, and Mile Gu. ``Fundamental limits of quantum error mitigation''. npj Quantum Information 8, 114 (2022). https://doi.org/10.1038/s41534-022-00618-z [2] Antonio Acín, Joonwoo Bae, E Bagan, M Baig, Ll Masanes, and Ramon Muñoz-Tapia. ``Secrecy properties of quantum channels''. Physical Review A—Atomic, Molecular, and Optical Physics 73, 012327 (2006). https://doi.org/10.1103/PhysRevA.73.012327 [3] Nicolas Gisin and Rob Thew. ``Quantum communication''. Nature photonics 1, 165–171 (2007). https://doi.org/10.1038/nphoton.2007.22 [4] C. W. Helstrom. ``Quantum detection and estimation''. Academic Press. New York (1976

Quantum Journal

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Boundaries for quantum advantage with single photons and loop-based time-bin interferometers

AbstractLoop-based boson samplers interfere photons in the time degree of freedom using a sequence of delay lines. Since they require few hardware components while also allowing for long-range entanglement, they are strong candidates for demonstrating quantum advantage beyond the reach of classical emulation. We propose a method to exploit this loop-based structure to more efficiently classically sample from such systems. Our algorithm exploits a causal-cone argument to decompose the circuit into smaller effective components that can each be simulated sequentially by calling a state vector simulator as a subroutine. To quantify the complexity of our approach, we develop a new lattice path formalism that allows us to efficiently characterize the state space that must be tracked during the simulation. In addition, we develop a heuristic method that allows us to predict the expected average and worst-case memory requirements of running these simulations. We use these methods to compare the simulation complexity of different families of loop-based interferometers, allowing us to quantify the potential for quantum advantage of single-photon Boson Sampling in loop-based architectures.Featured image: Top: A time-bin boson sampler made of a sequence of optical delay lines (loops) expressed in the spatial-mode representation, where the loops form stair-case like structures. In the progressive decomposition, we push all the measurements as far to the beginning of the circuit as possible, pushing also the relevant beamsplitters. This way we identify causal cones of the output modes, and obtain a sequence of smaller circuits which are easier to simulate. Bottom left: An example of our lattice path formalism that describes the reachable state space of a loop-based system throughout simulation. Here, we show the state space after evolving the input wavefunction through one component and measuring one mode. Each output basis state that can be found with nonzero probability corresp

Quantum Journal

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A Graphical Calculus for Quantum Computing with Multiple Qudits using Generalized Clifford Algebras

AbstractIn this work, we develop a graphical calculus for multi-qudit computations with generalized Clifford algebras, building off the algebraic framework developed in our prior work. We build our graphical calculus out of a fixed set of graphical primitives defined by algebraic expressions constructed out of elements of a given generalized Clifford algebra, a graphical primitive corresponding to the ground state, and also graphical primitives corresponding to projections onto the ground state of each qudit. We establish many properties of the graphical calculus using purely algebraic methods, including a novel algebraic proof of a Yang-Baxter equation and a construction of a corresponding braid group representation. Our algebraic proof, which applies to arbitrary qudit dimension, also enables a resolution of an open problem of Cobanera and Ortiz on the construction of self-dual braid group representations for even qudit dimension. We also derive several new identities for the braid elements, which are key to our proofs. Furthermore, we demonstrate that in many cases, the verification of involved vector identities can be reduced to the combinatorial application of two basic vector identities. Additionally, in terms of quantum computation, we demonstrate that it is feasible to envision implementing the braid operators for quantum computation, by showing that they are 2-local operators. In fact, these braid elements are almost Clifford gates, for they normalize the generalized Pauli group up to an extra factor $\zeta$, which is an appropriate square root of a primitive root of unity.► BibTeX data@article{Lin2025graphicalcalculus, doi = {10.22331/q-2025-11-17-1913}, url = {https://doi.org/10.22331/q-2025-11-17-1913}, title = {A {G}raphical {C}alculus for {Q}uantum {C}omputing with {M}ultiple {Q}udits using {G}eneralized {C}lifford {A}lgebras}, author = {Lin, Robert}, journal = {{Quantum}}, issn = {2521-327X}, publisher = {{Verein zur F{\"{o}}rderung des Open Access Pu

Quantum Journal

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Multipartite Entanglement Distribution in Quantum Networks using Subgraph Complementations

AbstractQuantum networks are important for quantum communication, enabling tasks such as quantum teleportation, quantum key distribution, quantum sensing, and quantum error correction, often utilizing graph states, a specific class of multipartite entangled states that can be represented by graphs. We propose a novel approach for distributing graph states across a quantum network. We show that the distribution of graph states can be characterized by a system of subgraph complementations, which we also relate to the minimum rank of the underlying graph and the degree of entanglement quantified by the Schmidt-rank of the quantum state. We analyze resource usage for our algorithm and show that it improves on the number of qubits, bits for classical communication, and EPR pairs utilized, as compared to prior work. In fact, the number of local operations and resource consumption for our approach scales linearly in the number of vertices. This produces a quadratic improvement in completion time for several classes of graph states represented by dense graphs, which translates into an exponential improvement by allowing parallelization of gate operations. This leads to improved fidelities in the presence of noisy operations, as we show through simulation in the presence of noisy operations. We classify common classes of graph states, along with their optimal distribution time using subgraph complementations. We find a sequence of subgraph complementation operations to distribute an arbitrary graph state which we conjecture is close to the optimal sequence, and establish upper bounds on distribution time along with providing approximate greedy algorithms.Featured image: Subgraph complementation operation on four verticesPopular summaryHow can we efficiently share quantum entanglement across multiple parties in a quantum network? We introduce a new method for distributing graph states—the fundamental resource in quantum networks—using a process called subgraph complementation

Quantum Journal

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From nanoscale to global scale: Advancing MIT’s special initiatives in manufacturing, health, and climate

Previous image Next image “MIT.nano is essential to making progress in high-priority areas where I believe that MIT has a responsibility to lead,” opened MIT president Sally Kornbluth at the 2025 Nano Summit. “If we harness our collective efforts, we can make a serious positive impact.”It was these collective efforts that drove discussions at the daylong event hosted by MIT.nano and focused on the importance of nanoscience and nanotechnology across MIT's special initiatives — projects deemed critical to MIT’s mission to help solve the world’s greatest challenges. With each new talk, common themes were reemphasized: collaboration across fields, solutions that can scale up from lab to market, and the use of nanoscale science to enact grand-scale change.“MIT.nano has truly set itself apart, in the Institute's signature way, with an emphasis on cross-disciplinary collaboration and open access,” said Kornbluth. “Today, you're going to hear about the transformative impact of nanoscience and nanotechnology, and how working with the very small can help us do big things for the world together.”Collaborating on healthAngela Koehler, faculty director of the MIT Health and Life Sciences Collaborative (MIT HEALS) and the Charles W. and Jennifer C. Johnson Professor of Biological Engineering, opened the first session with a question: How can we build a community across campus to tackle some of the most transformative problems in human health? In response, three speakers shared their work enabling new frontiers in medicine.Ana Jaklenec, principal research scientist at the Koch Institute for Integrative Cancer Research, spoke about single-injection vaccines, and how her team looked to the techniques used in fabrication of electrical engineering components to see how multiple pieces could be packaged into a tiny device. “MIT.nano was instrumental in helping us develop this technology,” she said.

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