Seminar

IQC Math and CS seminar featuring Natalie Parham

Takudzwa Chipo Valerie Mudzongo — Fri, 11 Jul 2025 18:24:22 +0000

IQC Math and CS seminar featuring Natalie Parham Takudzwa Chipo… Fri, 07/11/2025 - 14:24

Quantum circuit lower bounds in the magic hierarchy

Natalie Parham | Columbia University

In this talk I’ll introduce the magic hierarchy, a quantum circuit model alternating between arbitrary Clifford circuits and constant-depth circuits with two-qubit gates (QNC0). This framework unifies several existing models, including those with adaptive measurements. I’ll present new lower bounds at the first level of the hierarchy and explain how extending these bounds above a certain level would imply major breakthroughs in classical complexity theory—making the hierarchy a natural testing ground for lower bound techniques.

In particular, I’ll show that certain explicit quantum states—such as ground states of topological Hamiltonians and non-stabilizer codes—cannot be approximately prepared by a Clifford circuit followed by QNC0. These proofs go beyond standard light cone arguments and reveal an infectiousness property: approximating even one state in a high-distance code forces the entire code space to lie near a perturbed stabilizer code. Based on this paper: Quantum circuit lower bounds in the magic hierarchy

Location

QNC 1201
Online on Zoom
- Meeting ID: 948 5318 0071
  
  Passcode: 592573

IQC Seminar featuring Soo-hyon Phark

Takudzwa Chipo Valerie Mudzongo — Fri, 11 Jul 2025 07:07:12 +0000

IQC Seminar featuring Soo-hyon Phark Takudzwa Chipo… Fri, 07/11/2025 - 03:07

Quantum-coherent Science on a Solid Surface using Bottom-up Approach

Soo-hyon Phark | Center for Quantum Nanoscience (QNS)

Atom-by-atom addressability using a scanning tunnelling microscope (STM)[1] enables bottom up design of functional quantum devices. Furthermore, recent advance of a STM equipped with electron spin resonance (ESR) allows an atomic scale characterization of quantum states of individual spins on surface with an energy resolution down to nano electron volt.[2,3]

In this talk, I aim at an introduction to coherent quantum platforms crafted atom-by-atom on solid surfaces using an STM-ESR: First, utilization of single spins for sensing quantum objects with an atomic precision and a 10 neV energy resolution.[4]

Second, coherent multi-qubit systems using tailored spin nano structures on a surface, driven and read out in an all-electrical fashion.[5,6]

Third, atomic scale benchmarking of quantum electrodynamics and quantum thermodynamics using coupled spin qubits.[7] A survey on challenges in hand and outlook of this noble quantum platform will be followed.[8]

References

[1] D. M. Eigler, E. K. Schweizer, Nature 344, 524-526 (1990)

[2] S. Baumann et al. Science 350, 417-420 (2015)

[3] K. Yang et al. Science 366, 509-512 (2019)

[4] T. Esat et al. Nat. Nanotechnol. 19, 1466-1471 (2024)

[5] S. Phark et al. ACS Nano 17, 14144-1415118 (2023)

[6] Y. Wang et al. Science 382, 87-92 (2023)

[7] H. T. Bui et al. ACS Nano 18, 12187-12193 (2024)

[8] C. Wolf, A. J. Heinrich, S. Phark, ACS Nano 18, 28469-28479 (2024)

Location

RAC 1 - 3003

IQC Student seminar featuring Shlok Ashok Nahar

Takudzwa Chipo Valerie Mudzongo — Fri, 11 Jul 2025 06:53:28 +0000

IQC Student seminar featuring Shlok Ashok Nahar Takudzwa Chipo… Fri, 07/11/2025 - 02:53

Modelling imperfect detectors for adversarial tasks

Shlok Ashok Nahar

Real-world detectors are imperfect but theoretical analyses often assume ideal behavior. In this talk, I will present a framework for bridging this gap between real-world models and theoretical analyses in adversarial tasks, such as quantum key distribution. The talk will be based on recent work, Imperfect detectors for adversarial tasks with applications to quantum key distribution, but will be far more introductory.

Location

QNC 1201

IQC Student seminar featuring Einar Gabbassov

Takudzwa Chipo Valerie Mudzongo — Thu, 26 Jun 2025 17:05:15 +0000

IQC Student seminar featuring Einar Gabbassov Takudzwa Chipo… Thu, 06/26/2025 - 13:05

Adiabatic Dynamics of Entanglement

Einar Gabbassov

In the world of quantum computing, entanglement is a powerful resource. Our research investigates how entanglement behaves during a special kind of quantum process called 'adiabatic evolution,' where a quantum system slowly transforms from one state to another. We've discovered that entanglement isn't just changing randomly; instead, it's precisely 'woven' and re-woven at specific points during this evolution, almost like threads in a fabric. These crucial points are called 'avoided energy level crossings,' where the system's energy levels get very close but don't quite touch.

This weaving process has significant implications for how fast we can run adiabatic quantum computations. We found that the more efficiently entanglement needs to be manipulated (which happens at very 'narrow' avoided crossings), the slower the quantum computer must operate to maintain accuracy. Intriguingly, the amount of entanglement that builds up during a quantum computation is directly related to how 'rugged' or complex a quantum problem's energy landscape is. This provides new insights into why some quantum problems are inherently harder than others and could offer fresh perspectives on where the 'quantum advantage' truly comes from. Our work offers new tools to precisely control and study entanglement in adiabatic quantum computation.

Location

QNC 1201

IQC Math and CS seminar featuring Zhaoyi Li

Takudzwa Chipo Valerie Mudzongo — Thu, 19 Jun 2025 13:16:32 +0000

IQC Math and CS seminar featuring Zhaoyi Li Takudzwa Chipo… Thu, 06/19/2025 - 09:16

Designing and Implementing Optimal Quantum Purity Amplification

Zhaoyi Li | Massachusetts Institute of Technology (MIT)

Quantum Purity Amplification (QPA) offers a new framework for mitigating noise in quantum systems. In this talk, I will present the optimal QPA protocol for general quantum states under global depolarizing noise, a problem that has remained open for two decades. We prove the optimality of the protocol and show that it achieves an exponential improvement in sample complexity compared to previous methods in the strong-noise regime. I will also describe an efficient implementation based on generalized quantum phase estimation (GQPE), and introduce SWAPNET, a shallow and sparse circuit design that makes QPA practical for near-term devices. Finally, I will share simulation results demonstrating that QPA can improve fidelity in noisy Hamiltonian evolution, suggesting its potential utility in realistic applications with limited quantum resources.

Location

QNC 1201
Online on Zoom
- Meeting ID: 942 9381 6499
  
  Passcode: 539850

IQC Student seminar featuring Adam Teixido-Bonfill

Takudzwa Chipo Valerie Mudzongo — Wed, 11 Jun 2025 13:18:43 +0000

IQC Student seminar featuring Adam Teixido-Bonfill Takudzwa Chipo… Wed, 06/11/2025 - 09:18

Towards experimental entanglement harvesting in superconducting circuits

Adam Teixido-Bonfill

Entanglement harvesting is the surprising prediction that two quantum systems can become entangled by locally interacting with a quantum field, even if the two systems are far apart and never directly interact. Moreover, this can occur even if the field is in its vacuum state. In relativistic quantum information, entanglement harvesting is typically modeled using Unruh-DeWitt (UDW) particle detectors.

In this talk, we show how to extend the standard UDW detector model to better match a proposed realization of entanglement harvesting in superconducting circuits. The experiment consists of a pair of tunable superconducting qubits that interact with one-dimensional quantum fields (transmission lines). We investigate how these experimental features impact entanglement harvesting, paving the way to implement this protocol in the lab.

Location

QNC 1201

IQC seminar featuring Marek Żukowski

Naomi Grosman — Tue, 10 Jun 2025 18:03:10 +0000

IQC seminar featuring Marek Żukowski Naomi Grosman Tue, 06/10/2025 - 14:03

Violations of Bell inequality by a single photon and weak homodyne measurements

Marek Żukowski | University of Gdansk

The 1991 precursory idea of such a "Bell experiment" is criticized. First, the used "Bell inequalities" are using an additional assumption. Second, we find an exact local realistic model for the 1991 correlations. Nevertheless, we show that one can find genuine violations of local realism, if one uses on-off homodyne measurements, as suggested by Hardy. We search for optimal version of such experiment, and its modifications, as well as quantum informational applications of the correlations which genuinely violate local realism.

Location

QNC 0101

IQC Math and CS seminar featuring Junqiao Lin

Naomi Grosman — Thu, 29 May 2025 17:03:48 +0000

IQC Math and CS seminar featuring Junqiao Lin Naomi Grosman Thu, 05/29/2025 - 13:03

MIPco=coRE and generalized compression framework

Junqiao Lin | Centrum Wiskunde & Informatica (CWI)

Nonlocal games is a mathematical model used to model entanglement. Recently, Ji et al shows that estimating the optimal success rate for a nonlcoal game under the quantum (tensor) model is uncomputable in the celebrated result MIP*=RE result. A key part of this breakthrough involves a compression theorem for nonlocal games, a way to shrink the question and answer size of the game while preserving the optimal success rate to some degree.

In this talk, I will introduce the generalized compression framework, a generalization of the compression argument used in the MIP*=RE theorem, which could be applied to other computation problems unrelated to nonlocal games. I will explore how this framework can be used to prove the coRE-completeness of the complexity class MIPco, an MIP protocol where provers are allowed access to the commuting operator model of entanglement.

This talk is based on the upcoming paper MIPco=coRE.

Location

QNC 1201
Online on Zoom
- Meeting ID: 992 2420 0124
- Passcode: 983537

IQC PhD seminar featuring Jiahui Chen

Naomi Grosman — Fri, 23 May 2025 15:43:37 +0000

IQC PhD seminar featuring Jiahui Chen Naomi Grosman Fri, 05/23/2025 - 11:43

Engineering robust and precise effective Hamiltonians

Jiahui Chen

Quantum control is essential for implementing high-fidelity operations in quantum computing, simulation, and sensing. Achieving robust control in realistic systems requires techniques that take into account limitations in the control signals, imperfections in implementations, and dispersions in system Hamiltonians.

I will introduce a general framework for robust quantum control design based on Hamiltonian engineering using Average Hamiltonian Theory (AHT). This framework allows us to characterize system controllability, systematically incorporate robustness to common control errors, and design control sequences that generate desired nonzero zeroth- and higher-order terms in the Magnus expansion.

It also accounts for higher-order corrections and cross terms, enabling comprehensive error analysis. The methods are illustrated with simple examples in gate design, quantum simulation, and sensing.

Location

RAC2 1101

IQC Math and CS seminar featuring Yangjing Dong

Naomi Grosman — Tue, 20 May 2025 21:55:19 +0000

IQC Math and CS seminar featuring Yangjing Dong Naomi Grosman Tue, 05/20/2025 - 17:55

On the Computational Power of QAC0 with Barely Superlinear Ancillae

Yangjing Dong | Nanjing University

$\mathrm{QAC}^0$ is the family of constant-depth polynomial-size quantum circuits consisting of arbitrary single qubit unitaries and multi-qubit Toffoli gates. It was introduced by Moore [arXiv: 9903046] as a quantum counterpart of $\mathrm{AC}^0$, along with the conjecture that $\mathrm{QAC}^0$ circuits can not compute PARITY. In this work we make progress on this long standing conjecture: we show that any depth-$d$ $\mathrm{QAC}^0$ circuit requires $n^{1+3^{-d}}$ ancillae to compute a function with approximate degree $\Theta(n)$, which includes PARITY, MAJORITY and $\mathrm{MOD}_k$. We further establish superlinear lower bounds on quantum state synthesis and quantum channel synthesis. This is the first superlinear lower bound on the super-linear sized $\mathrm{QAC}^0$. Regarding PARITY, we show that any further improvement on the size of ancillae to $n^{1+\exp(-o(d))}$ would imply that PARITY $\not\in$ QAC0. These lower bounds are derived by giving low-degree approximations to $\mathrm{QAC}^0$ circuits. We show that a depth-$d$ $\mathrm{QAC}^0$ circuit with $a$ ancillae, when applied to low-degree operators, has a degree $(n+a)^{1-3^{-d}}$ polynomial approximation in the spectral norm. This implies that the class $\mathrm{QLC}^0$, corresponding to linear size $\mathrm{QAC}^0$ circuits, has approximate degree $o(n)$. This is a quantum generalization of the result that $\mathrm{LC}^0$ circuits have approximate degree $o(n)$ by Bun, Robin, and Thaler [SODA 2019]. Our result also implies that $\mathrm{QLC}^0\neq\mathrm{NC}^1$.

Location

Online on Zoom
- Meeting ID: 924 2457 6407
- Passcode: 318885