Welcome to the website of Parth Girdhar. 

Researcher in theoretical physics, computer science and quantum engineering, based at the University of Oxford. Postdoctoral research and teaching positions at Oxford, University of New South Wales. Ph.D. and B.Sc. (Advanced) (1st Class Hons) from the University of Sydney.

Research Publications

Google Scholar.

Topics:

• Foundations of quantum mechanics and gravity (with applications from open quantum systems, optomechanics and quantum information theory)

• Machine learning and quantum physics/engineering

• Cosmology and quantum field theory

Foundations of quantum mechanics and gravity

Probing Foundations of Quantum Mechanics: A Study into Nonlocality and Quantum Gravity, Parth Girdhar 

Ph.D. thesis, link. Supervisors: Prof. Andrew Doherty, A/Prof. Eric Cavalcanti

This thesis is about probing aspects of the foundations of quantum mechanics. Firstly, two notions of quantum nonlocality are explored: EPR-steering, the ability to control a remote quantum state, and Bell nonlocality, the inconsistency of a theory with local causality. A necessary and sufficient witness of Einstein-Podolsky-Rosen (EPR) steering is derived for a two qubit system employing only correlations between two arbitrary dichotomic measurements on each party. It is demonstrated that all states that are EPR-steerable with such correlations are also Bell nonlocal, a surprising equivalence between these two fundamental concepts of quantum mechanics. Next, testing modifications of the quantum mechanical canonical commutation relations is addressed. These are properties of some quantum gravity theories that involve an effective minimal length. It is shown that optomechanical probes of position noise spectrum of macroscopic oscillators can produce constraints on these theories. A comparison with current and future realistic experiments reveals the potential to beat constraints from direct experiments on elementary particles. Finally, it is studied how such modifications of quantum mechanics manifest in the theory of general continuous quantum position measurements. Several behaviours are found that deviate strongly from that of standard commutation relations.

Testing Generalised Uncertainty Principles with Quantum Noise, Parth Girdhar and Andrew C. Doherty, New J. Phys. 22 093073, arXiv:2005.08984v2 (Talk)

Motivated by several approaches to quantum gravity, there is a considerable literature on generalised uncertainty principles particularly through modification of the canonical position–momentum commutation relations. Some of these modified relations are also consistent with general principles that may be supposed of any physical theory. Such modified commutators have significant observable consequences. Here we study the noisy behaviour of an optomechanical system assuming a certain commonly studied modified commutator. From recent observations of radiation pressure noise in tabletop optomechanical experiments as well as the position noise spectrum of advanced LIGO we derive bounds on the modified commutator. We find how such experiments can be adjusted to provide significant improvements in such bounds, potentially surpassing those from sub-atomic measurements.

All two-qubit states that are steerable via Clauser-Horne-Shimony-Holt-type correlations are Bell nonlocal, Parth Girdhar and Eric G. Cavalcanti, Phys. Rev. A 94, 032317, arxiv:1601.01703

We derive a new inequality that is necessary and sufficient to show EPR-steering in a scenario employing only correlations between two arbitrary dichotomic measurements on each party. Thus the inequality is a complete steering analogy of the CHSH inequality, a generalisation of the result of Cavalcanti et al, JOSA B, 32(4), A74 (2015). We show that violation of the inequality only requires measuring over equivalence classes of mutually unbiased measurements on the trusted party and in fact assuming a general two qubit system arbitrary pairs of distinct projective measurements at the trusted party are equally useful. Via this it is found that for a given state the maximum violation of our EPR-steering inequality is equal to that for the CHSH inequality, so all states that are EPR-steerable with CHSH-type correlations are also Bell nonlocal.

Machine learning and quantum physics/engineering

End-to-End Analysis of Charge Stability Diagrams with Transformers, Rahul Marchand, Lucas Schorling, Cornelius Carlsson, Jonas Schuff, Barnaby van Straaten, Taylor L Patti, Federico Fedele, Joshua Ziegler, Parth Girdhar, Pranav Vaidhyanathan, Natalia Ares. arXiv:2508.15710  

Transformer models and end-to-end learning frameworks are rapidly revolutionizing the field of artificial intelligence. In this work, we apply object detection transformers to analyze charge stability diagrams in semiconductor quantum dot arrays, a key task for achieving scalability with spin-based quantum computing. Specifically, our model identifies triple points and their connectivity, which is crucial for virtual gate calibration, charge state initialization, drift correction, and pulse sequencing. We show that it surpasses convolutional neural networks in performance on three different spin qubit architectures, all without the need for retraining. In contrast to existing approaches, our method significantly reduces complexity and runtime, while enhancing generalizability. The results highlight the potential of transformer-based end-to-end learning frameworks as a foundation for a scalable, device- and architecture-agnostic tool for control and tuning of quantum dot devices. 

Model-free quantum gate design and calibration using deep reinforcement learning, O Shindi, Q Yu, Parth Girdhar, D Dong, in IEEE Transactions on Artificial Intelligence, vol. 5, no. 1, pp. 346-357, Jan. 2024, doi: 10.1109/TAI.2023.3243187, arXiv:2302.02371

High-fidelity quantum gate design is important for various quantum technologies, such as quantum computation and quantum communication. Numerous control policies for quantum gate design have been proposed given a dynamical model of the quantum system of interest. However, a quantum system is often highly sensitive to noise, and obtaining its accurate modeling can be difficult for many practical applications. Thus, the control policy based on a quantum system model may be unpractical for quantum gate design. Also, quantum measurements collapse quantum states, which makes it challenging to obtain information through measurements during the control process. In this article, we propose a novel training framework using deep reinforcement learning for model-free quantum control. The proposed framework relies only on the measurement at the end of the control process and offers the ability to find the optimal control policy without access to quantum systems during the learning process. The effectiveness of the proposed technique is numerically demonstrated for model-free quantum gate design and quantum gate calibration using off-policy reinforcement learning algorithms.

A Modified Deep Q-Learning Algorithm for Optimal and Robust Quantum Gate Design of a Single Qubit System (I), 2022 IEEE International Conference on Systems, Man, and Cybernetics (SMC), O Shindi, Q Yu, Parth Girdhar, D Dong, doi: 10.1109/SMC53654.2022.9945165

Precise and resilient quantum gate design is important for the building of quantum devices. In this paper, we consider the optimal and robust quantum gate design problem for three classes of two-level quantum systems. The aim is to construct quantum gates in a given fixed time with limited control resources. A modified dueling deep Q-learning (MDuDQL) is employed for the optimal and robust gate design problem. To improve the performance of the classical DuDQL method, we propose a unique semi-Markov DuDQL algorithm based on a modified action selection procedure, modified replay memory, and soft update procedure. The proposed algorithm outperforms ordinary DuDQL in terms of discovering optimal global or near global optimal control protocols and faster convergence to a better policy. Moreover, the modified DuDQL agent shows improved performance in finding robust control protocols which achieve high-fidelity quantum gate design for varying uncertainties in a certain range. The effectiveness of the proposed algorithm for the optimal and robust gate design problems has been illustrated by numerical results.

Cosmology and Quantum Field Theory

Cosmological Zitterbewegung, Parth Girdhar and Archil Kobakhidze, 

This essay on our research received an Honorable Mention in the 2013 Essay Competition of the Gravity Research Foundation. Published in "Special Issue—Selected Essays from the Annual Essay Competition of the Gravity Research Foundation 2013 and Invited Papers", Int. J. Mod. Phys. D, 22(12), p.1342023, arxiv:1305.3372.

We describe a new phenomenon of zitterbewegung of a free Dirac particle in cosmological spacetimes. Unlike the similar effect theorized by Schrodinger in 1930, the cosmological zitterbewegung is a real, physically attainable effect, which originates from the mixing of positive and negative frequency modes of a field operator in cosmological spacetimes. We briefly discuss the potential for observing this effect in laboratory experiments with trapped ions.

Signatures of Dark Energy Phase Transitions in Type 1a Supernovae: A Bayesian Approach, Parth Girdhar 

Honours thesis (available on request). Supervisor: Prof. Geraint Lewis

For the last 15 years cosmology has accommodated the theory that the universe is expanding at an accelerating rate due to an unknown “dark energy” fluid. The theory rests on the Friedmann equations which describe the evolution of the universe scale factor over time, together with observations of bodies such as high-redshift supernovae. Via a Markov Chain Monte Carlo (MCMC) method I demonstrate here the limits to which currently available supernovae distance modulus-redshift data can constrain parameters within models with complicated time- dependent dark energy equation of state. In particular it is shown that wide degeneracies exist within a model which incorporates a sharp dark energy phase transition. To fine tune the analysis, I compare Bayesian evidences to assess the superiority of several models, ranging from one without dark energy to one with dark energy sharply decaying to matter. Sufficient agreement between predictions of time-dependent dark energy models and supernovae data leaves open the possibility for any of the tested models to be viable candidates.