High Fidelity Single-NV Qubit Quantum State Tomography by Photoelectric Readout

arXiv:2603.16502 · quant-ph · Submitted 2026-03-17 · Read on arXiv

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Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: We're looking at the paper: "High Fidelity Single-NV Qubit Quantum State Tomography by Photoelectric Readout".

Mira: Here's the gist of the paper. The gist: The fidelity 0.995 ± 0.0062 for state reconstruction using photoelectrical detection is comparable to optical measurements, demonstrating that employing a photoelectric readout does not degrade the fidelity of state reconstruction significantly Ambient solid-state qubits offer an alternative to cryogenic requirements for quantum computing.

Kai: The nitrogen-vacancy (NV) center in diamond is a primary candidate for room temperature quantum computing architectures because it has proven competitive in terms of gate fidelity, quantum error correction, and couplings For a single NV center, single qubit gates with a fidelity of 0.99995 and two-qubit gates with a fidelity of 0.992 have been demonstrated.

Mira: Optical magnetic resonance (ODMR) is currently the main detection method, but it faces limitations such as resolution restricted by the Abbe limit and low collection efficiency caused by the high refractive index of diamond.

Kai: Photoelectric Readout Mechanism The alternative solution investigated is photoelectric detection of magnetic resonance (PDMR).

Title and authors: Kai: So, looking at the title of this paper, "High Fidelity Single-NV Qubit Quantum State Tomography by Photoelectric Readout," it sounds like they’re really pushing the boundaries of how we read out these qubits.

Mira: It suggests that they’ve managed to get a high-fidelity reconstruction of the quantum state using this new electrical method, and they're comparing it directly against the standard optical measurements.

Lev: What does that comparison tell us about whether this readout method is actually reliable enough for real error correction protocols?

Kai: Well, the paper claims they achieved a fidelity of zero point nine nine five plus or minus zero point zero zero six two across twenty-one different measurements on a selection of pure states <ref:2603.16502#pg2>.

Mira: That's the key result they're highlighting, and what’s interesting is that those individual measurement fidelities are about as good as what you get with conventional optical detection, according to their comparison to work in fifteen <ref:2603.16502#pg2>.

Lev: So the point is that this readout doesn't introduce a significant error compared to the established methods we already have.

Kai: That’s right, they explicitly state that using the current readout doesn't degrade the fidelity of state reconstruction significantly. And they even found an optimized fidelity of zero point nine nine eight plus or minus zero point zero one when you consider the standard deviation in theta and phi measurements <ref:2603.16502#pg2>.

Mira: That is a very strong result for a room-temperature setup, which really supports their hope that this photocurrent readout could be useful for scalable NV spin registers envisioned as platforms for fault-tolerant quantum computing.

The paper's summary: Kai: To summarize the core of "High Fidelity Single-NV Qubit Quantum State Tomography by Photoelectric Readout," they’re demonstrating that you can reconstruct the density matrix of an NV electron spin using this photocurrent-detected Rabi phase quantum state tomography, or PC-RPQST.

Mira: The paper is essentially showing that even though we're using an electrical readout instead of the usual optical one, the resulting fidelity for determining the quantum state remains very high, specifically zero point nine nine five plus or minus zero point zero zero six two on average over twenty-one measurements <ref:2603.16502#pg2>.

Lev: But what’s the underlying assumption here? What makes them claim this fidelity holds up when you switch from photoluminescence detection to this charge carrier detection?

Kai: The underlying assumption they are testing is that the photocurrent readout itself doesn't introduce errors that destroy the quantum information being measured. They compare their attained fidelities to mathematical modeling and conventional optical detection.

Mira: They’re saying that because those fidelities are approximately equal to what conventional optical detection achieves, it suggests that using this photoelectric detection introduces no significant error into the state reconstruction process itself.

Lev: From an engineering standpoint, if you were trying to run a quantum error correction code on this system, would you trust these numbers immediately without knowing the noise characteristics of the charge collection?

Kai: The paper shows that they've adapted their pulse protocols for slow detection, averaging the current signal over multiple runs of the Rabi protocol—dividing it into envelopes one through n <ref:2603.16502#pg1>.

Mira: And they’re using a picoammeter instead of something more complex, which is important because every charge cycle produces two charge carriers: an electron during NV ionization and a hole during recovery.

Lev: So, what’s the practical challenge they addressed regarding those slow detection rates and the current size of the signal?

Kai: They noted that because the current size is in the range of one to one hundred picoamperes for a single NV center, it absolutely needs substantial amplification for detection. That necessity forces them to adapt their pulse sequences and use methods like lock-in detection where possible.

The paper's improvements: Kai: The paper points out a few specific ways this approach is better than just using optical readout, and they frame these as improvements for the technique itself.

Mira: They suggest that the main improvement is showing that adopting this electrical readout doesn't reduce the fidelity of tomography at all, which is a big deal because it validates using current readout for state reconstruction.

Lev: Beyond just maintaining fidelity, what else does this method offer in terms of practical advantages for building these larger NV spin registers?

Kai: They highlight that they are using the photoelectric detection of magnetic resonance, or PDMR method as an improvement because it offers spatial resolution, detection rates, compactness, and better integration with electronics.

Mira: That compactness is huge for scaling up; it means you can fit more NV centers in a smaller footprint, which addresses one of the major limitations they discussed earlier about optical resolution being restricted by the Abbe limit.

Lev: So they are not just proving fidelity; they are proposing a viable architecture where this electrical readout could facilitate the creation of compact integrated systems for large-scale NV spin registers.

Kai: And looking at those specific metrics, the paper also shows a visible correspondence between the experimental fidelity and theta and phi measurements and the simulated curves for fidelity.

Mira: That visual confirmation between simulation and experiment is encouraging, especially when you're dealing with room temperature operations where thermal noise is a real concern.

Lev: So they’ve shown that the methodology itself is sound, not just that the final result numbers look good on paper or in simulation.

Conclusion: Kai: So to wrap up this discussion on "High Fidelity Single-NV Qubit Quantum State Tomography by Photoelectric Readout," the main implication is that you can achieve high-fidelity state reconstruction for an NV center using a photoelectric readout without suffering a significant fidelity drop compared to optical measurements.

Mira: Essentially, they’re showing that the photocurrent readout shall be a useful tool for measuring spin states with high fidelity in scalable NV spin registers, which is what they hope this research will enable for fault-tolerant quantum computing.

Lev: For real hardware deployment, the overhead here lands on adapting the pulse protocols for slow detection and managing that picoampere current size needing amplification.

Kai: And looking at what they’ve shown in "High Fidelity Single-NV Qubit Quantum State Tomography by Photoelectric Readout," we see a strong push toward using this electrical readout as a viable path forward for room temperature quantum processors.

Mira: It really boils down to proving that the fidelity doesn't suffer from the adapted readout, which is the central message they want listeners to hear about this work.

Lev: I just think if they can improve on those detection rates and PDMR contrast, it could seriously contribute to more precise measurements in the near future.

Kai: Exactly, and we’ll keep an eye out for how these compact integrated systems start coming together as we look at this paper.

Hasselt University Institute for Materials Research IUMAT · imec

quant-ph

Submitted: 2026-03-17

Updated: 2026-10-21

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 92/100

The gist: The gist: The fidelity 0.995 ± 0.0062 for state reconstruction using photoelectrical detection is comparable to optical measurements, demonstrating that employing a photoelectric readout does not

Key concepts

Nitrogen-Vacancy (NV) Center: The NV center is a specific defect within the diamond crystal structure. It acts as a single quantum bit (qubit) because its electronic spin can be manipulated to store and process quantum information. It is chosen for room-temperature quantum computing due to its stability.
Photoelectric Readout Mechanism: This method detects charge carriers excited from the NV center using light. A two-photon process excites an electron into the conduction band, which is then captured by electrodes on the diamond surface. This change in charge state allows for electrical detection of the spin state.
Quantum State Tomography (QST): QST is a technique used to fully reconstruct the quantum state of a qubit. By performing multiple measurements on different states, researchers can build up enough data to determine the exact probability distribution of the qubit's spin and phase, allowing for precise characterization.

Terminology

Fidelity — 4× in this paper · explained in 2 episodes

Summary

The gist: The fidelity 0.995 ± 0.0062 for state reconstruction using photoelectrical detection is comparable to optical measurements, demonstrating that employing a photoelectric readout does not degrade the fidelity of state reconstruction significantly

Introduction and Motivation

Ambient solid-state qubits offer an alternative to cryogenic requirements for quantum computing. The nitrogen-vacancy (NV) center in diamond is a primary candidate for room temperature quantum computing architectures because it has proven competitive in terms of gate fidelity, quantum error correction, and couplings For a single NV center, single qubit gates with a fidelity of 0.99995 and two-qubit gates with a fidelity of 0.992 have been demonstrated. Optical magnetic resonance (ODMR) is currently the main detection method, but it faces limitations such as resolution restricted by the Abbe limit and low collection efficiency caused by the high refractive index of diamond.

Photoelectric Readout Mechanism

The alternative solution investigated is photoelectric detection of magnetic resonance (PDMR). This method relies on the direct, on-chip electric detection of charge carriers excited from NV centers to the diamond conduction band by a two-photon process under optical pumping. The process involves a two-step sequence: excitation (first photon) and ionization (second photon), which brings the electron into the excited state and then into the conduction band. The free electron is then driven by an applied electric field and captured via electrodes fabricated on the diamond surface, causing the charge state of the NV− center to switch to a neutral NV0 complex.

Experimental Setup and Protocol

All measurements were performed on a home-built confocal setup that allows for simultaneous optical and electrical detection. The laser excitation originated from a yellow (561 nm) gem laser with a continuous-wave power tuned to 3.5 mW, which was focused onto an acousto-optic modulator (AOM). A dichroic mirror allowed fluorescence (637 nm) from the NV centers to pass through towards the detector while blocking the strong laser light. The photocurrent detection used a suction voltage of approximately 2V/µm delivered by a picoammeter/voltage source. For Rabi measurements, the pulse sequence is divided into envelopes (1, 2, n) during which a certain part of the pulse protocol is being repeated.

Tomography and Fidelity Results

The work extends the photocurrent readout to quantum state tomography by performing Rabi phase quantum state tomography (RPQST) on the electron spin of a single NV center at room temperature. The pure state is constructed as ψ⟩ = cos θ2/20⟩ + eiϕ sin θ2/21⟩. Across 21 measurements on a representative selection of pure states, the fidelity was 0.995 ± 0.0062 for PC-RPQST. The attained fidelities of individual measurements are approximately equal to those attained by conventional optical detection in [15], indicating that using current readout introduces no significant error. In 21 such room-temperature measurements of 10 different electron spin states, an average fidelity of 0.995 ± 0.0062 for PC-RPQST was found. By considering only the standard deviation of ∆θ and ∆ϕ, an optimized fidelity of 0.998±0.01 was found.

Conclusion and Outlook

This study demonstrates that adopting the electrical readout does not reduce the fidelity of the tomography methodology. The results suggest that photocurrent readout shall be a useful tool to measure spin states with high-fidelity in scalable NV spin registers, which are envisioned as a platform for fault-tolerant quantum computing. Further improvements in speed and PDMR contrast are expected to contribute to more precise measurements. The study shows a visible correspondence between experimental F and ∆θ and the simulated curves for fidelity.

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High Fidelity Single-NV Qubit Quantum State Tomography by Photoelectric ReadoutBoo Carmans, Michael Petrov, and Milos Nesladek1Hasselt University, Institute for Materials Research (IUMAT), Q-Lab, Martelarenlaan 42, B-3500 Hasselt, Belgium2Imec, IUMAT, Wetenschapspark 1, B-3590 Diepenbeek, BelgiumQuantum computing is a rapidly developing field. However, the most commonly used qubits require cryogenic conditions to operate. Ambient solid-state qubits provide an alternative with potential for large-scale application. The nitrogen-vacancy (NV) center in diamond is one of the main candidates for solid-state computing architectures at room temperature and has proven to be competitive in terms of gate fidelity, quantum error correction, couplings, etc. Each NV center has an associated electronic spin that is conventionally read out by photoluminescence. However, regarding the creation of small, ambient NV-based quantum processors, the optical readout introduces limitations on the collection efficiency and resolution of the readout as well as the size of the final device and its integration into standard semiconductor architectures. In this work, we investigate the competitiveness of the photoelectric readout versus the traditional optical readout. In particular, we report on using photoelectrical detection to perform quantum state tomography measurements on a single NV center. We achieve the fidelity 0.995 ± 0.0062 for state reconstruction, comparable to optical measurements, demonstrating that the fidelity does not suffer from the adapted readout, highlighting the value of photoelectric detection for NV-based quantum processors.

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I. INTRODUCTIONThe introduction of small, ambient processors as an alternative to conventional quantum architectures could significantly accelerate the development of quantum applications, as cooling or sustaining a vacuum are no longer required [1]Ambient operation is possible for a certain number of solid-state defects that are found in wide bandgap materials such as diamond The diamond lattice can host large dipole, photostable color centers [2] with the nitrogen-vacancy (NV) defect as one of the main candidates for quantum computing architectures All necessary functionalities of a quantum computer have been demonstrated on the NV platform, such as quantum error correction [3], couplings, quantum teleportation [4], etc. For a single NV center, single qubit gates with a fidelity of 0.99995 and two-qubit gates with a fidelity of 0.992 have been demonstrated Nearby NV spin qubits can be entangled via their magnetic dipole-dipole interaction (strong coupling regime), which was demonstrated for both the electronic and nuclear spins of an NV pair separated by 25nm. Currently, optical magnetic resonance (ODMR) is used as the main detection method of the NV spin state. An efficient scale-up of the technique is required for reading multiple spin qubits, as the optical resolution is limited by the Abbe limit, which is smaller than the interqubit distances that are required for dipolar NV spin entanglement Secondly there is the low collection efficiency caused by the high refractive index of diamond and by the design of conventional detection optics. Both problems can be alleviated.

Improvements for AI systems

Photonic Readout Integration for Ambient Quantum Processors: The system can perform quantum state tomography on single NV centers at room temperature with a fidelity of 0.995 plus or minus 0.0062, which is comparable to optical measurements and demonstrates that the fidelity does not suffer from the adapted readout.
Enhanced Qubit Characterization via Photoelectric Detection: The improved system can reconstruct the density matrix of an NV electron spin using photocurrent-detected Rabi phase quantum state tomography (PC-RPQST), achieving an optimized fidelity of 0.998 plus or minus 0.001 by accounting for standard deviation in measurement errors.
Scalable Qubit Readout Architecture: The system can utilize the photoelectric detection of magnetic resonance (PDMR) method, which offers advantages in spatial resolution, detection rates, compactness, and integration with electronics, facilitating the creation of compact integrated systems for large-scale NV spin registers.

Abstract

The nitrogen-vacancy (NV) center in diamond is one of the advanced candidates for solid-state quantum computing at room temperature and has proven to be competitive in terms of gate fidelity and the availability of quantum error correction codes. Ambient solid-state qubits may therefore provide quantum computing architectures as an alternative to current solutions. The NV center has an associated electronic spin that is conventionally read out via photoluminescence. As an alternative, the electrical readout is not limited by the confocal resolution of quantum microscopes, which is required in the development of NV-based quantum processors to establish a scalable individual single-spin readout on the order of a few nanometers (necessary for strong dipole-dipole coupling). In this work, the performance of photoelectric readout versus traditional optical readout is investigated to perform Quantum State Tomography measurements on a single NV center. A state fidelity of 0.995 plus or minus 0.006 is achieved using photoelectrical spin-state readout for state reconstruction, comparable to simultaneous optical measurements. We report photocurrent signal delays, which significantly slow down the photocurrent tomography. We develop a model describing their origin, and propose strategies for their mitigation.