Practical Quantum Information Research Articles

Decomposing large unitaries into multimode devices of arbitrary size

Decomposing complex unitary evolution into a series of constituent components is a cornerstone of practical quantum information processing. While the decomposition of an n×n unitary into a product of 2×2 subunitaries (which can for example be realized by beam splitters and phase shifters in linear optics) is well established, we show how for any m>2 this decomposition can be generalized into a product of m×m subunitaries (which can then be realized by a more complex device acting on m modes). If the cost associated with building each m×m multimode device is less than constructing with m(m−1)2 individual 2×2 devices, we show that the decomposition of large unitaries into m×m submatrices is more resource efficient and exhibits a higher tolerance to errors, than its 2×2 counterpart. This allows larger-scale unitaries to be constructed with lower errors, which is necessary for various tasks, not least boson sampling, the quantum Fourier transform, and quantum simulations. Published by the American Physical Society 2024

Steady motional entanglement between two distant levitated nanoparticles

Quantum entanglement in macroscopic systems is not only essential for practical quantum information processing, but also valuable for the study of the boundary between quantum and the classical world. However, it is very challenging to achieve the steady remote entanglement between distant macroscopic systems. We consider two distant nanoparticles, both of which are optically trapped in two cavities. Based on the coherent scattering mechanism, we find that the ultrastrong optomechanical coupling between the cavity modes and the motion of the levitated nanoparticles could be achieved. The large and steady entanglement between the filtered output cavity modes and the motion of nanoparticles can be generated if the trapping laser is under the red sideband. Then through entanglement swapping, the steady motional entanglement between the distant nanoparticles can be realized. We numerically simulate and find that the two nanoparticles with 10 km distance can be entangled for the experimentally feasible parameters, even in room temperature environments. The generated continuous variable multipartite entanglement is the key to realizing the quantum enhanced sensor network and the sensitivity beyond the standard quantum limit.

Navigating the 16-dimensional Hilbert space of a high-spin donor qudit with electric and magnetic fields

Efficient scaling and flexible control are key aspects of useful quantum computing hardware. Spins in semiconductors combine quantum information processing with electrons, holes or nuclei, control with electric or magnetic fields, and scalable coupling via exchange or dipole interaction. However, accessing large Hilbert space dimensions has remained challenging, due to the short-distance nature of the interactions. Here, we present an atom-based semiconductor platform where a 16-dimensional Hilbert space is built by the combined electron-nuclear states of a single antimony donor in silicon. We demonstrate the ability to navigate this large Hilbert space using both electric and magnetic fields, with gate fidelity exceeding 99.8% on the nuclear spin, and unveil fine details of the system Hamiltonian and its susceptibility to control and noise fields. These results establish high-spin donors as a rich platform for practical quantum information and to explore quantum foundations.

Integrated Manipulation and Addressing of Spin Defect in Diamond.

Scalable and addressable integrated manipulation of qubits is crucial for practical quantum information applications. Different waveguides have been used to transport the optical and electrical driving pulses, which are usually required for qubit manipulation. However, the separated multifields may limit the compactness and efficiency of manipulation and introduce unwanted perturbation. Here, we develop a tapered fiber-nanowire-electrode hybrid structure to realize integrated optical and microwave manipulation of solid-state spins at nanoscale. Visible light and microwave driving pulses are simultaneously transported and concentrated along an Ag nanowire. Studied with spin defects in diamond, the results show that the different driving fields are aligned with high accuracy. The spatially selective spin manipulation is realized. And the frequency-scanning optically detected magnetic resonance (ODMR) of spin qubits is measured, illustrating the potential for portable quantum sensing. Our work provides a new scheme for developing compact, miniaturized quantum sensors and quantum information processing devices.

Frequency-bin photonic quantum information

Discrete frequency modes, or bins, present a blend of opportunities and challenges for photonic quantum information processing. Frequency-bin-encoded photons are readily generated by integrated quantum light sources, naturally high-dimensional, stable in optical fiber, and massively parallelizable in a single spatial mode. Yet quantum operations on frequency-bin states require coherent and controllable multifrequency interference, making them significantly more challenging to manipulate than more traditional spatial degrees of freedom. In this mini-review, we describe recent developments that have transformed these challenges and propelled frequency bins forward. Focusing on sources, manipulation schemes, and detection approaches, we introduce the basics of frequency-bin encoding, summarize the state of the art, and speculate on the field’s next phases. Given the combined progress in integrated photonics, high-fidelity quantum gates, and proof-of-principle demonstrations, frequency-bin quantum information is poised to emerge from the lab and leave its mark on practical quantum information processing—particularly in networking where frequency bins offer unique tools for multiplexing, interconnects, and high-dimensional communications.

Single site-controlled inverted pyramidal InGaAs QD–nanocavity operating at the onset of the strong coupling regime

Precise positioning of single site-controlled inverted pyramidal InGaAs quantum dots (QDs) at the antinode of a GaAs photonic crystal cavity with nanometer-scale accuracy holds unique advantages compared to self-assembled QDs and offers great promise for practical on-chip photonic quantum information processing. However, the strong coupling regime in this geometry has not yet been achieved due to the low cavity Q-factor based on the (111)B-oriented membrane structures. Here, we reveal the onset of phonon-mediated coherent exciton–photon interaction on our tailored single site-controlled InGaAs QD–photonic crystal cavity. Our results present the Rabi-like oscillation of luminescence intensity between excitonic and photonic components correlated with their energy splitting pronounced at small detuning. Such Rabi-like oscillation is well reproduced by modeling the coherent exchange of the exciton-photon population. The modeling further reveals an oscillatory two-time covariance at QD-cavity resonance, which indicates that the system operates at the onset of the strong coupling regime. Moreover, by using the cavity mode as a probe of the virtual state of the QD induced by phonon scattering, it reveals an increase in phonon scattering rates near the QD–cavity resonance and asymmetric phonon emission and absorption rate of even around 50 K.

Interferometry-Integrated Noise-Immune Quantum Memory.

A quantum memory with the performances of low noise, high efficiency, and high bandwidth is of crucial importance for developing practical quantum information technologies. However, the excess noises generated during the highly efficient processing of quantum information inevitably destroy quantum state. Here, we present a quantum memory with built-in excess-noise eraser by integrating a photon-correlated quantum interferometry in quantum memory, where the memory efficiency can be enhanced and the excess noises can be suppressed to the vacuum level via destructive interference. This quantum memory is demonstrated in a rubidium vapor cell with a 10-ns-long photonics signal. We observe ∼80% noise suppression, the write-in efficiency enhancement from 87% to 96.2% without and with interferometry, and the corresponding memory efficiency excluding the noises from 70% to 77%. The fidelity is 93.7% at the single-photon level, significantly exceeding the no-cloning limit. Such interferometry-integrated quantum memory, the first expansion of quantum interference techniques to quantum information processing, simultaneously enables low noise, high bandwidth, high efficiency, and easy operation.

Entanglement recovery by weak measurement reversal in tripartite systems

Entanglement protection from noisy environments is an essential task in practical quantum information and communication. In this paper, we investigate the entanglement recovery of an amplitude-damped tripartite GHZ state by a weak-measurement reversal procedure. In particular, we emphasize the key importance of the inequivalency of probability amplitudes of the tripartite system under the recovery technique. We explore the maximal and non-maximal tripartite entangled state scenarios under amplitude damping noise in the absence and presence of weak measurement reversal procedures. Importantly, the non-maximal entangled state turns out to be a good choice for the entanglement recovery via weak-measurement reversal procedure.

Quantifying high-dimensional spatial entanglement with a single-photon-sensitive time-stamping camera.

High-dimensional entanglement is a promising resource for quantum technologies. Being able to certify it for any quantum state is essential. However, to date, experimental entanglement certification methods are imperfect and leave some loopholes open. Using a single-photon-sensitive time-stamping camera, we quantify high-dimensional spatial entanglement by collecting all output modes and without background subtraction, two critical steps on the route toward assumptions-free entanglement certification. We show position-momentum Einstein-Podolsky-Rosen (EPR) correlations and quantify the entanglement of formation of our source to be larger than 2.8 along both transverse spatial axes, indicating a dimension higher than 14. Our work overcomes important challenges in photonic entanglement quantification and paves the way toward the development of practical quantum information processing protocols based on high-dimensional entanglement.

A broadband high-brightness quantum-dot double solid immersion lens single photon source

High-brightness single photon sources (SPSs) are key components for practical quantum information processing systems. Although the performances of recently reported high-brightness SPSs are excellent, it remains challenging to match the emission wavelength of a quantum dot (QD) to the cavity since the high-Q cavity structures have narrow spectral bandwidths. Here, we propose a highly bright and broadband QD SPS that can be deterministically fabricated with a simple yet precise method. The optimized GaAs-polymer double solid immersion lens structure is capable of a brightness of 88% at 0.5 NA and has an operation band of 65 nm with a brightness of over 80% from numerical simulations. Experimentally, we achieved a brightness of 51.6% ± 2% and pure single photon emission [g(2)(0) = 0.029 ± 0.005] at saturation. We believe that our result can pave the way to a practical high-brightness QD SPS, considering its simple QD geometry together with its low cost and precise deterministic fabrication without using expensive and complicated e-beam lithography and dry etching processes.

Bell Inequalities, Entanglement, and Quantum Information

The Royal Swedish Academy of Sciences awarded the Nobel Prize in Physics 2022 jointly to Alain Aspect, John Clauser, and Anton Zeilinger for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science. The present article provides a brief overview on the significance of their contributions and practical quantum information applications of Bell inequalities and entanglement.

Deterministic N-photon state generation using lithium niobate on insulator device

The large-photon-number quantum state is a fundamental but nonresolved request for practical quantum information applications. We propose an N-photon state generation scheme that is feasible and scalable, using lithium niobate on insulator circuits. Such a scheme is based on the integration of a common building block called photon-number doubling unit (PDU) for deterministic single-photon parametric downconversion and upconversion. The PDU relies on a 107-optical-quality-factor resonator and mW-level on-chip power, which is within the current fabrication and experimental limits. N-photon state generation schemes, with cluster and Greenberger–Horne–Zeilinger state as examples, are shown for different quantum tasks.

Room Temperature Triggered Single Photon Emission from Self‐Assembled GaN/AlN Quantum Dot in Nanowire

AbstractRoom temperature (RT) operation is one of the crucial requirements for the practical applications of single photon emitters. Here, RT triggered single photon emission from self‐assembled GaN quantum dots (QDs) embedded in AlN nanowires grown by molecular beam epitaxy on Si (111) substrate is demonstrated. Photoluminescence of GaN/AlN QDs show strong emission in the range of 3.6–4.1 eV at 4.7 K, with average full width at half maximum of ≈2.7 meV, the sharpest one reaches as narrow as 1.6 meV. The QD exhibits a fast‐radiative lifetime of ≈0.98 ns at 4.7 K due to strong quantum confinement. A clear antibunching effect is observable up to RT, with a second‐order correlation function at zero time delay g(2)(0) = 0.35 ± 0.05, confirming single photon emission at RT, manifesting strong potential for practical quantum information applications.

High-Fidelity Ion State Detection Using Trap-Integrated Avalanche Photodiodes.

Integrated technologies greatly enhance the prospects for practical quantum information processing and sensing devices based on trapped ions. High-speed and high-fidelity ion state readout is critical for any such application. Integrated detectors offer significant advantages for system portability and can also greatly facilitate parallel operations if a separate detector can be incorporated at each ion-trapping location. Here, we demonstrate ion quantum state detection at room temperature utilizing single-photon avalanche diodes (SPADs) integrated directly into the substrate of silicon ion trapping chips. We detect the state of a trapped Sr^{+} ion via fluorescence collection with the SPAD, achieving 99.92(1)% average fidelity in 450 μs, opening the door to the application of integrated state detection to quantum computing and sensing utilizing arrays of trapped ions.

Practical multipartite entanglement distribution in noisy channels

Entanglement among multiple parties is vital for practical quantum information applications. As two kinds of typical multipartite entangled state, the W and Greenberger-Horne-Zeilinger (GHZ) states have attracted a lot of attention. In this paper, we propose two practical linear optical schemes for multipartite entanglement distribution in noisy channels. Using the time-bin or spatial-mode degree of freedom (DoF) encoding, the influence of the noise will be eliminated and perfect W or GHZ state can be achieved among different nodes in a deterministic way. Moreover, we construct quantum circuits, which can be implemented with current technology. These results contribute to long-distance quantum communication both in theoretical and experimental points of view.

Retrieving High-Dimensional Quantum Steering from a Noisy Environment with N Measurement Settings.

One of the most often implied benefits of high-dimensional (HD) quantum systems is to lead to stronger forms of correlations, featuring increased robustness to noise. Here, we experimentally demonstrate the n-setting linear HD quantum steering criterion. We verify the large violation of the steering inequalities without full-state tomography. The lower bound of the violation is 2.24±0.01 in 11 dimensions, exceeding the bound (V<2) of two-setting criteria. Hence, a higher strength of steering has been revealed. Moreover, we demonstrate the method for enhancing the noise robustness without increasing dimension, alternatively, by increasing measurement settings. Using the entanglement in 11 dimensions, we experimentally retrieve steering nonlocality with 63.4±1.4% isotropic noise fraction, surpassing the 50% limitation of two-setting criteria. Our Letter offers the potential for practical one-sided device-independent quantum information processing that tolerates the noisy environment, lossy detection, and transcends the present transmission distance limitation.

High-performance cavity-enhanced quantum memory with warm atomic cell

High-performance quantum memory for quantized states of light is a prerequisite building block of quantum information technology. Despite great progresses of optical quantum memories based on interactions of light and atoms, physical features of these memories still cannot satisfy requirements for applications in practical quantum information systems, since all of them suffer from trade-off between memory efficiency and excess noise. Here, we report a high-performance cavity-enhanced electromagnetically-induced-transparency memory with warm atomic cell in which a scheme of optimizing the spatial and temporal modes based on the time-reversal approach is applied. The memory efficiency up to 67 ± 1% is directly measured and a noise level close to quantum noise limit is simultaneously reached. It has been experimentally demonstrated that the average fidelities for a set of input coherent states with different phases and amplitudes within a Gaussian distribution have exceeded the classical benchmark fidelities. Thus the realized quantum memory platform has been capable of preserving quantized optical states, and is ready to be applied in quantum information systems, such as distributed quantum logic gates and quantum-enhanced atomic magnetometry.

Robust method for certifying genuine high-dimensional quantum steering with multimeasurement settings

Genuine high-dimensional (HD) quantum steering was recently proposed for certifying HD entanglement in the one-sided device-independent setting, as quantified by the Schmidt number [Phys. Rev. Lett. 126, 200404 (2021)PRLTAO0031-900710.1103/PhysRevLett.126.200404]. As one of the central challenges in the steering test is the tolerated noise threshold, here we develop a more robust method for certifying genuine HD steering in noisy environments. We derive genuine HD steering criteria using multiple mutually unbiased bases, surpassing the previous restriction of only two measurement settings. Although the present multisetting criteria are not tight, we theoretically show that they are more robust to noise than the tight two-setting criteria. To test the practicality of our method under realistic conditions, we report an experimental demonstration using photonic orbital angular momentum entangled states in dimensions d = 3 , 4 , 5 . Our work offers a more robust way to witness the entanglement dimension in practical one-sided device-independent quantum information processing.

Quantum teleportation of hybrid qubits and single-photon qubits using Gaussian resources

In this paper, we compare single-photon qubits and hybrid qubits as information carriers through quantum teleportation using a Gaussian continuous-variable channel. A hybrid qubit in our study is in the form of entanglement between a coherent state and a single photon. We find that hybrid qubits outperform photonic qubits when coherent amplitudes of the hybrid qubits are as low as $\alpha\lesssim 1$, while photonic qubits yield better results for larger amplitudes. We analyze effects of photon losses, and observe that the overall character of teleportation for different qubits remains the same although the teleportation fidelities are degraded by photon losses. Our work provides a comparative look at practical quantum information processing with different types of qubits.

Simultaneous resolution of photon numbers and positions with series-connected superconducting nanowires

In this Letter, we report on a device with which to resolve photon numbers and positions simultaneously, using single-channel readout from superconducting nanowire single-photon detectors (SNSPDs). The nanowires in the SNSPDs are connected in series with parallel resistors for producing response pulses with different amplitudes, whose values obey a distribution of 1:2:4:,..,2n-1. In single-photon detection, a saturated counting rate is obtained at a low dark count rate (&amp;lt;10 cps). Furthermore, we observed 16 output modes corresponding one-to-one with 16 states (15 photon responses + 1 state without photons) in multiphoton mode. This structure not only inherits the advantages of high quantum efficiency and low dark count rate of traditional SNSPD, but also realizes the resolution of photon position and number simultaneously without complicated optical system. Thus, this proposal offers a promising platform for realizing scalable and practical quantum information chips.