Photonic quantum computation refers to quantum computation that uses photons as the physical system for doing the quantum computation. The field is largely divided between discrete-variable (DV) and continuous-variable (CV) photonic quantum computation. In the former, quantum information is represented by one or more modal properties (e.g. polarisation) that take on distinct values from a finite set. Quantum information is processed via operations on these modal properties (e.g. waveplates in the case of polarisation), and eventually measured using single-photon detectors. In CV photonic quantum computation, quantum information is represented by properties of the electromagnetic field that take on any value in an interval (e.g. position). Both CV and DV implementations have been realized experimentally; each has a unique set of challenges that need to be overcome to achieve scalable universal photonic quantum computation. It is possible to combine both DV and CV in a hybrid CV-DV fashion to overcome the limitations of either approach.
Photonic quantum computation refers to quantum computation that uses photons as the physical system for doing the quantum computation. The field is largely divided between discrete-variable (DV) and continuous-variable (CV) photonic quantum computation. In the former, quantum information is represented by one or more modal properties (e.g. polarisation) that take on distinct values from a finite set. Quantum information is processed via operations on these modal properties (e.g. waveplates in the case of polarisation), and eventually measured using single-photon detectors. In CV photonic quantum computation, quantum information is represented by properties of the electromagnetic field that take on any value in an interval (e.g. position). Both CV and DV implementations have been realized experimentally; each has a unique set of challenges that need to be overcome to achieve scalable universal photonic quantum computation. It is possible to combine both DV and CV in a hybrid CV-DV fashion to overcome the limitations of either approach.
In the race to demonstrate a scalable and fault tolerant quantum computing (QC) platform, quantum photonics stands poised to have a major impact. In particular, by encoding quantum information over fields rather than photons (continuous-variable—CV, rather than discrete-variable—DV, quantum information), record-size quantum processors were demonstrated in spectral and in temporal quantum field combs. These demonstrations used the measurement-based QC paradigm, which is based on cluster entangled states. In this talk, I will outline two of our recent results: proposals for generating hypercubic cluster states by phase modulation of the quantum optical frequency comb and for generating fault-tolerant hybrid CV-DV encodings called Gottesman-Kitaev-Preskill states. The latter leverages CV cluster states along with our experimental ability to perform photon-number-resolving measurements up to 100 photons.
Quantum teleportation, a transfer protocol of quantum states, is the essence of many sophisticated quantum information protocols. There have been two complementary approaches to optical quantum teleportation: discrete variables (DVs) and continuous variables (CVs). However, both approaches have pros and cons. Here we take a “hybrid” approach to overcome the current limitations: CV quantum teleportation of DVs. This approach enabled the first realization of deterministic quantum teleportation of photonic qubits without post-selection. We also applied the hybrid scheme to several experiments, including entanglement swapping between DVs and CVs, conditional CV teleportation of single photons, and CV teleportation of qutrits. We are now aiming at universal, scalable, and fault-tolerant quantum computing based on these hybrid technologies.
Photons are a ubiquitous carrier of quantum information: they are fast, suffer minimal decoherence, and do not require huge cryogenic facilities. Nevertheless, their intrinsically weak photon-photon interactions remain a key obstacle to scalable quantum computing. This review surveys hybrid photonic quantum computing, which exploits multiple photonic degrees of freedom to combine the complementary strengths of discrete and bosonic encodings, thereby significantly mitigating the challenge of weak photon-photon interactions. We first outline the basic principles of discrete-variable, native continuous-variable, and bosonic-encoding paradigms. We then summarize recent theoretical advances and state-of-the-art experimental demonstrations with a particular emphasis on the hybrid approach. Its unique advantages, such as efficient generation of resource states and nearly ballistic (active-feedforward-free) operations, are highlighted alongside the remaining technical challenges. To facilitate a clear comparison, we explicitly present the error thresholds and resource overheads required for fault-tolerant quantum computing. Our work offers a focused overview that clarifies how the hybrid approach enables scalable and compatible architectures for quantum computing.
In this Near Intermediate-Scale Quantum era, there are two types of near-term quantum devices available on cloud: superconducting quantum processing units (QPUs) based on the discrete variable model and linear optics (photonics) QPUs based on the continuous variable (CV) model. Quantum computation in the discrete variable model is performed in a finite dimensional quantum state space and the CV model in an infinite dimensional space. In implementing quantum algorithms, the CV model offers more quantum gates that are not available in the discrete variable model. CV-based photonic quantum computers provide additional flexibility of controlling the length of the output vectors of quantum circuits, using different methods of measurement and the notion of cutoff dimension.
Robust continuous-variable (CV) quantum information processing requires correcting realistic errors in bosonic systems, but all existing schemes rely on auxiliary Gottesman-Kitaev-Preskill (GKP) states which the preparation and operation are demanding in many platforms. In this work, we propose a novel CV quantum error correction (QEC) scheme that utilizes a broadly accessible resource: discrete-variable (DV) ancilla. Our scheme extracts information about CV displacement to the DV ancilla, measuring that allows counteracting the unwanted displacement error. We show that a simple single-qubit ancilla can already suppress CV infidelity by more than 20%. By concatenating with DV QEC codes, our scheme is robust against the physical errors in hybrid CV-DV systems, and yields a new class of oscillator-in-oscillator code that does not involve GKP states. Our work facilitates the implementation of CV QEC on realistic platforms.
Hybridizing different degrees of freedom or physical platforms potentially offers various advantages in building scalable quantum architectures. Here, we introduce a fault-tolerant hybrid quantum computation by building on the advantages of both discrete-variable (DV) and continuous-variable (CV) systems. In particular, we define a CV-DV hybrid qubit with a bosonic cat code and a single photon, which is implementable in current photonic platforms. Due to the cat code encoded in the CV part, the predominant loss errors are readily correctable without multiqubit encoding, while the logical basis is inherently orthogonal due to the DV part. We design fault-tolerant architectures by concatenating hybrid qubits and an outer DV quantum error-correction code such as a topological code, exploring their potential merit in developing scalable quantum computation. We demonstrate by numerical simulations that our scheme is at least an order of magnitude more resource efficient compared to all previous proposals in photonic platforms, allowing us to achieve a record-high loss threshold among existing CV and hybrid approaches. We discuss the realization of our approach not only in all-photonic platforms but also in other hybrid platforms including superconducting and trapped-ion systems, which allows us to find various efficient routes toward fault-tolerant quantum computing. Published by the American Physical Society 2024