Welcome to the Bosonic Kingdom.

Bosonic code Also called an oscillator or a continuous-variable (CV) code. Encodes logical Hilbert space, finite- or infinite-dimensional, into a physical Hilbert space that contains at least one oscillator (a.k.a. bosonic mode or qumode). States of a single oscillator are elements of the Hilbert space of $$\ell^2$$-normalizable functions on $$\mathbb{R}$$). Ideal codewords may not be normalizable because the space is infinite-dimensional, so approximate versions have to be constructed in practice. Protection: An error set relevant to Fock-state bosonic codes is the set of loss operators associated with the amplitude damping (a.k.a. photon loss or attenuation) noise channel, a common form of physical noise in bosonic systems. For a single mode, loss operators are proportional to powers of the mode's annihilation operator $$a=(\hat{x}+i\hat{p})/\sqrt{2}$$, where $$\hat x$$ ($$\hat p$$) is the mode's position (momentum) operator, and with the power signifying the number of particles lost during the error. For multiple modes, error set elements are tensor products of elements of the single-mode error set. A definition of distance associated with this error set is the minimum weight of a loss error that implements a nontrivial logical operation in the code. Parents: Quantum error-correcting code (QECC). Cousin of: Fermionic code, Group-based quantum code.
Bosonic stabilizer code Bosonic code whose codespace is defined as the common $$+1$$ eigenspace of a group of mutually commuting displacement operators. Displacements form the stabilizers of the code, and have continuous eigenvalues, in contrast with the discrete set of eigenvalues of qubit stabilizers. As a result, exact codewords are non-normalizable, so approximate constructions have to be considered. A stabilizer code admitting a set a set of group generators such that each generator consists of either position or momentum operators is called a bosonic CSS code. Protection: Protective properties can be delineated in terms of the annihilators or displacements, and the most natural noise model for such codes is displacement noise. If an error operator does not commute with a stabilizer group element, then that error is detectable. Oscillator-into-oscillator stabilizer codes protect against erasures of a subset of modes, while GKP codes protect against sufficiently small displacements in any number of modes. Parents: Stabilizer code, Bosonic code. Parent of: Homological bosonic code, Multi-mode GKP code. Cousin of: Group GKP code.
Hybrid qudit-oscillator code Encodes a $$K$$-dimensional logical Hilbert space into $$n_1$$ qudits of dimension $$q$$ and $$n_2 \neq 0$$ oscillators, i.e., the Hilbert space of $$\ell^2$$-normalizable functions on $$\mathbb{Z}_q^{n_1} \times \mathbb{R}^{n_2}$$. Parents: Bosonic code. Parent of: Very small logical qubit (VSLQ) code. Cousins: Qudit-into-oscillator code.
Encodes Hilbert space of $$\ell^2$$-normalizable functions on $$\mathbb{R}^k$$ into that on $$\mathbb{R}^n$$. Usually denoted as $$((n,k))_{\mathbb{R}}$$. Parents: Bosonic code. Parent of: Homological bosonic code, Niset-Andersen-Cerf code.
Qudit-into-oscillator code Encodes $$K$$-dimensional Hilbert space into Hilbert space of $$\ell^2$$-normalizable functions on $$\mathbb{R}^n$$. Parents: Bosonic code. Parent of: Fock-state bosonic code, Multi-mode GKP code. Cousin of: Hybrid qudit-oscillator code.
An $$[[n,1]]_{\mathbb{R}}$$ oscillator-into-oscillator CSS stabilizer code defined using homological structres associated with an $$n-1$$ simplex. Relevant to the study of spacetime replication of quantum information [4]. Protection: Protects against certain types of erasure errors (depending on the specific dimension). Certain constructions also protect arbitrary sized errors on multiple photon states.
Generalization of the GKP code to $$n$$ bosonic modes whose stabilizer group is an infinite countable group of oscillator displacement operators. Protection: The level of protection against displacement errors is quantified by the Euclidean code distance $$\Delta=\min_{x\in {\mathcal{L}}^{\perp}\setminus {\mathcal{L}}} \|x\|_2$$ [7].
The two logical codewords are $$|\pm\rangle \propto (|0\rangle\pm|2\rangle)(|0\rangle\pm|2\rangle)|0\rangle|0\rangle$$, where the total Hilbert space is the tensor product of two qudits (whose ground states $$|0\rangle$$ and second excited states $$|2\rangle$$ are used in the codewords) and two oscillators. In the original proposal for implementation, the single logical qubit is given by the two lowest energy states of a circuit composed of two transmons coupled to two lossy resonators, but the resonators can also be thought of as qubits since only a few low-lying Fock states are used by the code. Protection: Passively protects against single photon loss. Parents: Hybrid qudit-oscillator code. Cousins: Quantum repetition code.
Encodes two-mode coherent states $$\{|\alpha\rangle, |\beta\rangle\}$$ over two modes into four modes such that the values $$(\alpha,\beta)$$ are recoverable after a single-mode erasure. There are two variations of the storage procedure: a deterministic protocol that offers recovery against a single mode erasure, and a probabalistic that can protect against multiple errors with post selection. This code is effectively protecting classical information stored in $$(\alpha,\beta)$$ using quantum operations. Protection: The deterministic protocol protects against a single erasure error on a known mode. This recovers one state perfectly and the other state with fidelity $$F = \frac{1}{1 + e^{-2 r}}$$ for an initial EPR pair squeezed with variance $$e^{-2r}$$. The probabalistic protocol utilizes post-selection to protect against multiple erasures with state-dependent fidelity. Parents: Oscillator-into-oscillator code. Cousin of: Homological bosonic code.
Fock-state bosonic code Qudit-into-oscillator code whose protection against amplitude damping (i.e., photon loss) stems from the use of disjoint sets of Fock states for the construction of each code basis state. The simplest example is the dual-rail code, which has codewords consisting of single Fock states $$|10\rangle$$ and $$|01\rangle$$. This code can detect a single loss error since a loss operator in either mode maps one of the codewords to a different Fock state $$|00\rangle$$. More involved codewords consist of several well-separated Fock states such that multiple loss events can be detected and corrected. Protection: Code distance $$d$$ is the minimum distance (assuming some metric) between any two labels of Fock states corresponding to different code basis states. For a single mode, $$d$$ is the minimum absolute value of the difference between any two Fock-state labels; such codes can detect up to $$d-1$$ loss events. Multimode distances can be defined analogously; see, e.g., Chuang-Leung-Yamamoto codes. Parents: Qudit-into-oscillator code. Parent of: Bosonic rotation code, Chuang-Leung-Yamamoto code. Cousins: Linear binary code, Qubit code. Cousin of: Fusion-based quantum computing (FBQC) code.
Bosonic Fock-state code that encodes $$k$$ qubits into $$n$$ oscillators, with each oscillator restricted to having at most $$N$$ excitations. Codewords are superpositions of oscillator Fock states which have exactly $$N$$ total excitations, and are either uniform (i.e., balanced) superpositions or unbalanced superpositions. Codes can be denoted as $$[[N,n,2^k,d]]$$, which conflicts with stabilizer code notation. Protection: Protects against amplitude damping for up to $$t = d-1$$ excitation losses. Defining the spacing between two Fock states $$|u_1\cdots u_n\rangle$$ and $$|v_1\cdots v_n\rangle$$, \begin{align} \text{Spacing}(u,v) = \frac{1}{2}\sum_{i=1}^n |u_i - v_i|, \end{align} the code distance $$d$$ can be defined as the minimial spacing between Fock states making up the codewords. Cousin of: Binomial code.
Stub. Parents: Multi-mode GKP code.
Bosonic qudit-into-oscillator code whose stabilizers are oscillator displacement operators $$\hat{S}_q(2\alpha)=e^{-2i\alpha \hat{p}}$$ and $$\hat{S}_p(2\beta)=e^{2i\beta \hat{x}}$$. To ensure $$\hat{S}_q(2\alpha)$$ and $$\hat{S}_p(2\beta)$$ generate a stabilizer group that is Abelian, there is another constraint that $$\alpha\beta=2k\pi$$ where $$k$$ is an integer. Codewords can be expressed as equal weight superpositions of coherent states on an infinite lattice, such as a square lattice in phase space with spatial period $$2\sqrt{\pi}$$. The exact GKP state is non-normalizable, so approximate constructs have to be considered. Protection: For stabilizer $$\hat{S}_q(2\alpha),\hat{S}_p(2\beta)$$, code can correct displacement errors up to $$\frac{\alpha}{2}$$ in the $$q$$-direction and $$\frac{\beta}{2}$$ at $$p$$-direction. Approximately protects against photon loss errors [12], outperforming most other codes designed to explicitly protect against loss [13]. Very sensitive to dephasing errors [14]. A biased-noise GKP error correcting code can be prepared by choosing $$\alpha\neq \beta$$. Parents: Multi-mode GKP code. Cousin of: Bosonic rotation code, Rotor GKP code.
A single-mode Fock-state bosonic code whose codespace is preserved by a phase-space rotation by a multiple of $$2\pi/N$$ for some $$N$$. The rotation symmetry ensures that encoded states have support only on every $$N^{\textrm{th}}$$ Fock state. For example, single-mode Fock-state codes for $$N=2$$ encoding a qubit admit basis states that are, respectively, supported on Fock state sets $$\{|0\rangle,|4\rangle,|8\rangle,\cdots\}$$ and $$\{|2\rangle,|6\rangle,|10\rangle,\cdots\}$$. Protection: Losses or gains less than $$N$$ are detectable. Dephasing rotations $$\exp(\mathrm{i}\theta \hat{n})$$ can be detected whenever $$\theta$$ is roughly less than $$\pi/N$$. To get precise bounds on $$\theta$$, one needs to analyze the particular bosonic rotation code. Parents: Fock-state bosonic code. Parent of: Binomial code, Cat code, Number-phase code. Cousins: Gottesman-Kitaev-Preskill (GKP) code.
Bosonic rotation codes designed to approximately protect against errors consisting of powers of raising and lowering operators up to some maximum power. Binomial codes can be thought of as spin-coherent states embedded into an oscillator [13]. The $$q$$-dimensional qudit $$(N, S)$$ binomial codeword states are $$\{|\overline{i}\rangle\mid i\in \mathbb Z_q \}$$, where \begin{align} |\overline{i}\rangle = \frac{1}{\sqrt{q^N}} \sum_{\substack{p=0\\p\equiv i \pmod{q}}}^{(q-1)(N+1)} \sqrt{\binom{N+1}{p}_q} \ket{p(S+1)}. \end{align} The set $$\ket{i} \mid i \in \mathbb{N}$$ is the set of Fock states. Also, $$\binom{N+1}{p}_q$$ are extended binomial coefficients, or polynomial coeffiients, defined recursively as \begin{align} \binom{n}{m}_1 \equiv 1,\quad \binom{n}{m}_q \equiv \sum_{k=0}^n \binom{n}{k}\binom{k}{m-k}_{q-1}. \end{align} The extended binomial coefficients $$\binom{n}{m}_q$$ are also the coefficients of $$x^m$$ in the polynomial $$(1 + x + \cdots + x^{q-1})^n$$. Protection: An $$(N, S)$$ binomial code protects against $$L$$ boson losses, $$G$$ boson gains, and dephasing up to $$\hat{n}^{D}$$, where $$S=L+G$$ and $$N = \mathrm{max}(L,G,2D)$$. Binomial codes approximately protect against continuous-time amplitude damping, boson loss and gain, and dephasing. Parents: Bosonic rotation code. Cousin of: GNU permutation-invariant code.
Rotation-symmetric bosonic Fock-state code encoding a $$q$$-dimensional qudit into one oscillator. Codewords for a qubit code ($$q=2$$) consist of a coherent state $$|\alpha\rangle$$ projected onto a subspace of Fock state number modulo $$2(S+1)$$. The logical state $$|\overline{0}\rangle$$ is in the $$\{|0\rangle , |2(S+1)\rangle , |4(S+1)\rangle \cdots \}$$ Fock-state subspace, while $$|\overline{1}\rangle$$ is in the $$\{|(S+1)\rangle, |3(S+1)\rangle , |5(S+1)\rangle , |7(S+1)\rangle \cdots \}$$ subspace. An alternative basis, valid for for general $$q$$ and $$\alpha\neq 0$$, consists of $$q$$ coherent states distributed equidistanctly around a circle in phase space of radius $$\alpha$$. Protection: Due to the spacing between sets of Fock states, the distance between two distinct logical states is $$d=S+1$$. Hence, this code is able to detect $$S$$-photon loss error. Two-legged cat codes ($$S=1$$) do not protect against loss events, but there exist modifications based on sign alternation [20] or squeezing [21] that add such protection. Parents: Bosonic rotation code. Cousins: Number-phase code, Hamiltonian-based code. Cousin of: Binomial code.
Bosonic rotation code consisting of superpositions of Pegg-Barnett phase states [22], \begin{align} |\phi\rangle \equiv \frac{1}{\sqrt{2\pi}}\sum_{n=0}^{\infty} \mathrm{e}^{\mathrm{i} n \phi} \ket{n}. \end{align} Since phase states and thus the ideal codewords are not normalizable, approximate versions need to be constructed. The codes' key feature is that, in the ideal case, phase measurement has zero uncertainty, making it a good canditate for a syndrome measurement. Protection: Number-phase codes of order $$N$$ detect up to $$N$$ photon loss or gain errors, and dephasing up to $$\theta = \pi/N$$. Parents: Bosonic rotation code. Cousins: Rotor GKP code. Cousin of: Binomial code, Cat code.
Two-mode code encoding a logical qubit in Fock states with one excitation. The logical-zero state is represented by $$|01\rangle$$, while the logical-one state is represented by $$|10\rangle$$. Protection: This is an error-detecting code against one photon loss event; it is often used in photonic quantum devices because of its ease of realization. A single loss event can be detected because, after the loss occurs, the output state $$|00\rangle$$ is orthogonal to the codespace. Recovery is not possible, so a successful run of a quantum circuit is conditioned on not losing a photon during the circuit.
Three-oscillator Fock-state code encoding a single logical qubit using codewords \begin{align} \begin{split} |\overline{0}\rangle &= \frac{1}{\sqrt{3}}(|003\rangle+|030\rangle+|300\rangle)\\ |\overline{1}\rangle &= |111\rangle \end{split}. \end{align} Protection: Protects against single photon loss in any one mode.

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