arXiv subjects

Ryan J. Marshman

Publications and source records attributed to Ryan J. Marshman.

At least 19 recordsLinked to original sources

arXiv2026

Rigorous no-go theorems for heralded linear-optical state generation tasks

A major challenge in photonic quantum technologies is developing strategies to prepare suitable discrete-variable quantum states using simple input states, linear optics, and auxiliary photon measurements to identify successful outcomes. Fundamentally, this challenge arises from the lack of strong non-linearities on the single-photon level, meaning that photonic state preparation based on linear optics cannot benefit from the deterministic gate-based approach available to other physical platforms. Instead, the preparation of quantum states can be probabilistically implemented using single photons, linear-optical networks, and photon detection. However, determining whether an input state can be transformed into a target state using a specific measurement pattern - a problem that can be mapped to deciding the feasibility of a system of polynomial equations - is a complex problem in general. To solve it, we apply the Nullstellensatz Linear Algebra algorithm from algebraic geometry to quantum state generation; this can provide definitive no-go results by proving infeasibility when the state preparation task in question has no solution. We demonstrate this capability to validate and establish lower bounds on the physical resource requirements for the realization of several ubiquitous optical states and gates.

quant-ph↗

arXiv2025

Enhancing Long-distance Continuous-variable Quantum-key-distribution with an Error-correcting Relay

Noiseless linear amplifiers (NLAs) serve as an effective means to enable long-distance continuous-variable (CV) quantum key distribution (QKD), even under realistic conditions with non-unit reconciliation efficiency. Separately, unitary averaging has been suggested to mitigate some stochastic noise, including phase noise in continuous-variable states. In this work, we combine these two protocols to simultaneously compensate for thermal-loss effects and suppress phase noise, thereby enabling long-distance CV QKD that surpasses the repeaterless bound, the fundamental rate-distance limit, for repeaterless quantum communication systems.

quant-ph↗

arXiv2025

Noise-reduction of multimode Gaussian Boson Sampling circuits via Unitary Averaging

We improve Gaussian Boson Sampling (GBS) circuits by integrating the unitary averaging (UA) protocol, previously demonstrated to protect unknown Gaussian states from phase errors [Phys. Rev. A 110, 032622]. Our work extends the applicability of UA to mitigate arbitrary interferometric noise, including beam-splitter and phase-shifter imperfections. Through comprehensive numerical analysis, we demonstrate that UA consistently achieves higher fidelity and success probability compared to unprotected circuits, establishing its robustness in noisy conditions. Remarkably, enhancement is maintained across varying numbers of modes with respect to the noise. We further derive a power-law formula predicting performance gains in large-scale systems, including 100-mode and 216-mode configurations. A detailed step-by-step algorithm for implementing the UA protocol is also provided, offering a practical roadmap for advancing near-term quantum technologies.

quant-ph↗

arXiv2025

A Spin-Based Pathway to Testing the Quantum Nature of Gravity

A key open problem in physics is the correct way to combine gravity (described by general relativity) with everything else (described by quantum mechanics). This problem suggests that general relativity and possibly also quantum mechanics need fundamental corrections. Most physicists expect that gravity should be quantum in character, but gravity is fundamentally different to the other forces because it alone is described by spacetime geometry. Experiments are needed to test whether gravity, and hence space-time, is quantum or classical. We propose an experiment to test the quantum nature of gravity by checking whether gravity can entangle two micron-sized crystals. A pathway to this is to create macroscopic quantum superpositions of each crystal first using embedded spins and Stern-Gerlach forces. These crystals could be nanodiamonds containing nitrogen-vacancy (NV) centres. The spins can subsequently be measured to witness the gravitationally generated entanglement. This is based on extensive theoretical feasibility studies and experimental progress in quantum technology. The eventual experiment will require a medium-sized consortium with excellent suppression of decoherence including vibrations and gravitational noise. In this white paper, we review the progress and plans towards realizing this. While implementing these plans, we will further explore the most macroscopic superpositions that are possible, which will test theories that predict a limit to this.

quant-ph↗

arXiv2025

Coherently mitigating boson samplers with stochastic errors

Sampling experiments provide a viable route to show quantum advantages of quantum devices over classical computers in well-defined computational tasks. However, quantum devices such as boson samplers are susceptible to various errors, including stochastic errors due to fabrication imperfections. These cause the implemented unitary operations to deviate randomly from their intended targets, following distributions with finite variance. Whilst full-scale quantum error correction remains challenging in the near term, quantum error mitigation schemes have been devised to estimate expectation values, but it is unclear how these schemes would work for sampling experiments. In this work, we demonstrate that, given access to multiple stochastic unitaries, it is possible to mitigate the effect of these errors in sampling experiments. We adopt the unitary averaging protocol which employs multiple stochastic boson samplers to generate a distribution that approximates the ideal boson sampler distribution as the number of samplers increases. We derive a rigorous upper bound on the trace distance between the output probability distributions induced by invertible vacuum-heralded networks based on the Schur-Weyl duality. This result can be seen concretely as an error mitigation scheme in sampling experiments against stochastic errors. On a broader level, it suggests a path towards understanding error mitigation for sampling experiments and developing analysis tools for photonic circuits incorporating measurements and feed-forward. We further provide other applications of unitary averaging, including its use in implementing the linear combination of unitaries and benchmarking fabrication repeatability in linear optics.

quant-ph↗

arXiv2024

Improving Continuous-variable Quantum Channels with Unitary Averaging

A significant hurdle for quantum information and processing using bosonic systems is stochastic phase errors which occur as the photons propagate through a channel. These errors will reduce the purity of states passing through the channel and so reducing the channels capacity. We present a scheme of passive linear optical unitary averaging for protecting unknown Gaussian states transmitted through an optical channel. The scheme reduces the effect of phase noise on purity, squeezing and entanglement, thereby enhancing the channel via probabilistic error correcting protocol. The scheme is robust to loss and typically succeeds with high probability. We provide both numerical simulations and analytical approximations tailored for relevant parameters with the improvement of practical and current technology. We also show the asymptotic nature of the protocol, highlighting both current and future relevance.

quant-ph↗

arXiv2023

Gravito-diamagnetic forces for mass independent large spatial superpositions

Creating a massive spatial quantum superposition, such as the Schrödinger cat state, where the mass and the superposition size within the range $10^{-19}-10^{-14}$ kg and $Δx \sim 10~{\rm nm}-100~μ{\rm m}$, is a challenging task. The methods employed so far rely either on wavepacket expansion or on a quantum ancilla, e.g. single spin dependent forces, which scale inversely with mass. In this paper, we present a novel approach that combines gravitational acceleration and diamagnetic repulsion to generate a large spatial superposition in a relatively short time. After first creating a modest initial spatial superposition of $1~μ{\rm m}$, achieved through techniques such as the Stern-Gerlach (SG) apparatus, we will show that we can achieve an $\sim 10^{2}-10^{3}$ fold improvement to the spatial superposition size ($1~{\rm μm}\rightarrow 980~μ{\rm m}$) between the wave packets in less than $0.02$~s by using the Earth's gravitational acceleration and then the diamagnetic repulsive scattering of the nanocrystal, neither of which depend on the object mass. Finally, the wave packet trajectories can be closed so that spatial interference fringes can be observed. Our findings highlight the potential of combining gravitational acceleration and diamagnetic repulsion to create and manipulate large spatial superpositions, offering new insights into creating macroscopic quantum superpositions.

quant-ph↗

arXiv2023

Unitary Averaging with Fault and Loss Tolerance

We consider the impact of the unitary averaging framework on single and two-mode linear optical gates. We demonstrate that this allows a trade-off between the probability of success and gate fidelity, with perfect fidelity gates being achievable for a finite decrease in the probability of success, at least in principle. Furthermore, we show that the encoding and decoding errors in the averaging scheme can also be suppressed up to the first order. We also look at how unitary averaging can work in conjunction with existing error correction schemes. Specifically, we consider how parity encoding might be used to counter the extra loss due to the decreased probability of success, with the aim of achieving fault tolerance. We also consider how unitary averaging might be utilised to expand the parameter space in which fault tolerance may be achievable using standard fault tolerant schemes.

quant-ph↗

arXiv2023

Entanglement of Magnetically Levitated Massive Schrödinger Cat States by Induced Dipole Interaction

Quantum entanglement provides a novel way to test short-distance quantum physics in a non-relativistic regime. We provide entanglement-based protocols to potentially test the magnetically induced dipole-dipole interaction and the Casimir-Polder potential between the two nano-crystals kept in a Schrodinger Cat state. Our scheme is based on the Stern-Gerlach (SG) apparatus, where we can witness the entanglement mediated by these interactions for the nano-crystal mass m~10^-19 kg with a spatial superposition size of order 0.1 micron in a trap relying on diamagnetic levitation. We show that it is possible to close the SG interferometer in position and momentum with a modest gradient in the magnetic field.

quant-ph↗

arXiv2023

Mass-independent scheme for enhancing spatial quantum superpositions

Placing a large mass in a large spatial superposition, such as a Schrödinger Cat state is a significant and important challenge. In particular, the large spatial superposition (${\cal O}(10-100)$ $μ$m) of mesoscopic masses ($m\sim {\cal O}(10^{-14} -10^{-15})$ kg) makes it possible to test the quantum nature of gravity via entanglement in the laboratory. To date, the proposed methods of achieving this spatial delocalization are to use wavepacket expansions or quantum ancilla (for example spin) dependent forces, all of whose efficacy reduces with mass. Thus increasing the spatial splitting independent of the mass is an important open challenge. In this paper, we present a method of achieving a mass-independent enhancement of superposition via diamagnetic repulsion from current-carrying wires. We analyse an example system which uses the Stern-Gerlach effect to creating a small initial splitting, and then apply the diamagnetic repulsion method to enhance the superposition size ${\cal O}(400-600)$ $μ$m from an initial modest split of the wavefunction. We provide an analytic and numeric analysis of our scheme.

quant-ph↗

arXiv2022

Quantum optical levitation of a mirror

While the levitating mirror has seen renewed interest lately, relatively little is known about its quantum behaviour. In this paper we present a quantum theory of a one dimensional levitating mirror. The mirror forms a part of a Fabry-Perot cavity where the circulating intracavity field supports the mirror through radiation pressure alone. We find a blue and red detuned steady-state of which only the blue detuned solution with damping on the mirror and cavity is stable. We find strong entanglement (15-20 ebits) between the mirror output and cavity output and squeezing in the mirror position.

quant-ph↗

arXiv2022

Catapulting towards massive and large spatial quantum superposition

Large spatial quantum superposition of size ${\cal O}(1-10)~{\rm μ\text{m}}$ for mass $m \sim 10^{-17}-10^{-14}~\text{kg}$ is required to probe the foundations of quantum mechanics and testing classical and quantum nature of gravity via entanglement in a laboratory. In this paper, we will show that it is possible to accelerate the two spin states of a macroscopic nano-crystal sourced by the inhomogeneous nonlinear magnetic field in the Stern-Gerlach type setup. We will assume that the electronic spin can be embedded at the centre of the nano-crystal, such as the nitrogen-vacancy (NV) centre of diamond. Our analysis will be generic to any dopant or any material. We will show that we can create a desired superposition size within $1-2$ seconds by catapulting the trajectories of the two spin states with a modest magnetic field gradient and then recombine the trajectories for a coherent interference. We will show the demanding nature of the precision required in the magnetic field to recover $99\%$ spin coherence confidence level at the moment of interference.

quant-ph↗

arXiv2022

Entanglement based tomography to probe new macroscopic forces

Quantum entanglement provides a novel way to test short distance physics in the non-relativistic regime. We will provide a protocol to {\it potentially} test new physics by bringing two charged massive particle interferometers adjacent to each other. Being charged, the two superpositions will be entangled via electromagnetic interactions mediated by the photons, including the Coulomb and the Casimir-Polder potential. We will bring a method of {\it entanglement based tomography} to seek time evolution of very small entanglement phases to probe new physical effects mediated by {\it hitherto unknown macroscopic force} which might be responsible for entangling the two charged superpositions modelled by the Yukawa type potential. We will be able to constrain the Yukawa couplings $α\geq 10^{-35}$ for $r\geq 10^{-6}$m for new physics occurring in the electromagnetic sector, and in the gravitational potential $α_g \geq 10^{-8}$ for $r \geq 10^{-6}$m. Furthermore, our protocol can also constrain the axion like particle mass and coupling, which is complimentary to the existing experimental bounds.

hep-ph↗

arXiv2022

Constructing Nano-Object Quantum Superpositions with a Stern-Gerlach Interferometer

Probing quantum mechanics, quantum aspects of general relativity along with the sensing and the constraining of classical gravity can all be enabled by unprecedented spatial sizes of superpositions of massive objects. In this paper, we show that there is a feasible setup sourced by realizable magnetic field gradients ${\cal O}(10-100)$ Tm$^{-1}$ to construct a large spatial superposition of ${\cal O}(10^{-4}-10^{-8})$ m for masses ${\cal O}(10^{-16}-10^{-14})$ kg over a time period of up to $0.1-10$ seconds.

quant-ph↗

arXiv2022

Improving resilience of the Quantum Gravity Induced Entanglement of Masses (QGEM) to decoherence using 3 superpositions

Recently a protocol called quantum gravity induced entanglement of masses (QGEM) that aims to test the quantum nature of gravity using the entanglement of 2 qubits was proposed. The entanglement can arise only if the force between the two spatially superposed masses is occurring via the exchange of a mediating virtual graviton. In this paper, we examine a possible improvement of the QGEM setup by introducing a third mass with an embedded qubit, so that there are now 3 qubits to witness the gravitationally generated entanglement. We compare the entanglement generation for different experimental setups with 2 and 3 qubits and find that a 3-qubit setup where the superpositions are parallel to each other leads to the highest rate of entanglement generation within $τ= 5 $ s. We will show that the 3-qubit setup is more resilient to the higher rate of decoherence. The entanglement can be detected experimentally for the 2-qubit setup if the decoherence rate $γ$ is $γ< 0.11 $ Hz compared to $γ< 0.16 $ Hz for the 3-qubit setup. However, the introduction of an extra qubit means that more measurements are required to characterize entanglement in an experiment. We conduct experimental simulations and estimate that the 3-qubit setup would allow detecting the entanglement in the QGEM protocol at a $99.9\%$ certainty with $O(10^4)-O(10^5)$ measurements when $γ\in [0.1,0.15] $ Hz. Furthermore, we find that the number of needed measurements can be reduced to $O(10^3)-O(10^5)$ if the measurement schedule is optimised using joint Pauli basis measurements. For $γ> 0.06 $ Hz the 3-qubit setup is favourable compared to the 2-qubit setup in terms of the minimum number of measurements needed to characterize the entanglement. Thus, the proposed setup here provides a promising new avenue for implementing the QGEM experiment.

quant-ph↗

arXiv2021

Relative Acceleration Noise Mitigation for Nanocrystal Matter-wave Interferometry: Application to Entangling Masses via Quantum Gravity

Matter wave interferometers with large momentum transfers, irrespective of specific implementations, will face a universal dephasing due to relative accelerations between the interferometric mass and the associated apparatus. Here we propose a solution that works even without actively tracking the relative accelerations: putting both the interfering mass and its associated apparatus in a freely falling capsule, so that the strongest inertial noise components vanish due to the equivalence principle. In this setting, we investigate two of the most important remaining noise sources: (a) the non-inertial jitter of the experimental setup and (b) the gravity-gradient noise. We show that the former can be reduced below desired values by appropriate pressures and temperatures, while the latter can be fully mitigated in a controlled environment. We finally apply the analysis to a recent proposal for testing the quantum nature of gravity [S. Bose et. al. Phys. Rev. Lett 119, 240401 (2017)] through the entanglement of two masses undergoing interferometry. We show that the relevant entanglement witnessing is feasible with achievable levels of relative acceleration noise.

gr-qc↗

arXiv2021

Qudits for Witnessing Quantum Gravity Induced Entanglement of Masses Under Decoherence

Recently a theoretical and an experimental protocol known as quantum gravity induced entanglement of masses (QGEM) has been proposed to test the quantum nature of gravity using two mesoscopic masses each placed in a superposition of two locations. If, after eliminating all non-gravitational interactions between them, the particles become entangled, one can conclude that the gravitational potential is induced via a quantum mediator, i.e. a virtual graviton. In this paper, we examine a range of different experimental set-ups, considering different geometries and the number of spatially superposed states taken, in order to determine which would generate entanglement faster. We conclude that without decoherence, and given a maximum distance $Δx$ between any two spatial states of a superposition, a set of two qubits placed in spatial superposition parallel to one another will outperform all other models given realistic experimental parameters. Furthermore, when a sufficiently high decoherence rate is introduced, multi-component superpositions can outperform the two-qubit set-up. This is further verified with an experimental simulation, showing that $O(10^3)$ measurements are required to reject the no entanglement hypothesis with a parallel qubits set-up without decoherence at a 99.9$\%$ confidence level. The number of measurements increases when decoherence is introduced. When the decoherence rate reaches $0.125$~Hz, 6-dimensional qudits are required as the two-qubit system entanglement cannot be witnessed anymore. However, in this case, $O(10^6)$ measurements will be required. One can group the witness operators to measure in order to reduce the number of measurements (up to ten-fold). However, this may be challenging to implement experimentally.

quant-ph↗

arXiv2020

Quantum Gravity Witness via Entanglement of Masses: Casimir Screening

A recently proposed experimental protocol for Quantum Gravity induced Entanglement of Masses (QGEM) requires in principle realizable, but still very ambitious, set of parameters in matter-wave interferometry. Motivated by easing the experimental realization, in this paper, we consider the parameter space allowed by a slightly modified experimental design, which mitigates the Casimir potential between two spherical neutral test-masses by separating the two macroscopic interferometers by a thin conducting plate. Although this set-up will reintroduce a Casimir potential between the conducting plate and the masses, there are several advantages of this design. First, the quantum gravity induced entanglement between the two superposed masses will have no Casimir background. Secondly, the matter-wave interferometry itself will be greatly facilitated by allowing both the mass $10^{-16}-10^{-15}$kg and the superposition size $Δx \sim 20 μ$m to be a one-two order of magnitude smaller than those proposed earlier, and thereby also two orders of magnitude smaller magnetic field gradient of $10^4$Tm$^{-1}$ to create that superposition through the Stern-Gerlach effect. In this context, we will further investigate the collisional decoherences and decoherence due to vibrational modes of the conducting plate.

quant-ph↗