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Parallelization of high-dimensional single-photon quantum gates

Jaroslav Kysela

Published Jun 21, 2021• 0 citations
OpenAlex

Abstract

Local quantum gates represent the basic building blocks of quantum computers. In the photonic domain, these gates act on individual photons and are usually implemented as single optical devices. In cases when the same gate is to be applied simultaneously to multiple photons, the corresponding physical implementation consists of a stack of identical devices. We present a systematic approach that allows one to replace this stack with a single device supplemented by pre- and post-processing stages. As a result, considerable savings in resources are obtained. We exemplify our scheme for the case of the orbital angular momentum (OAM) of single photons and demonstrate the scheme for high-dimensional Pauli gates and their arbitrary powers. As a byproduct, we also present a simple argument that explicitly shows how to construct an arbitrary quantum gate acting on OAM of single photons, where only conventional optical elements are utilized.

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Local quantum gates represent the basic building blocks of quantum computers. In the photonic domain, these gates act on individual photons and are usually implemented as single optical devices. In cases when the same gate is to be applied simultaneously to multiple photons, the corresponding physical implementation consists of a stack of identical devices. We present a systematic approach that allows one to replace this stack with a single device supplemented by pre- and post-processing stages. As a result, considerable savings in resources are obtained. We exemplify our scheme for the case of the orbital angular momentum (OAM) of single photons and demonstrate the scheme for high-dimensional Pauli gates and their arbitrary powers. As a byproduct, we also present a simple argument that explicitly shows how to construct an arbitrary quantum gate acting on OAM of single photons, where only conventional optical elements are utilized.
Abstract A simple argument is presented that explicitly shows how to construct an arbitrary quantum gate acting on orbital angular momentum (OAM) of single photons. The scheme can be applied to implement subspace multiplexing, where a single high-dimensional OAM qudit represents effectively a stack of multiple independent lower-dimensional qudits. A special subclass of unitaries composed of single-photon controlled gates is studied in detail and notable examples of the general approach are discussed. The generalization of the simple argument leads to the parallelization scheme, which results in the savings of resources. The presented schemes utilize only conventional optical elements and apply not only to single photons but also to classical light.
Transformations on quantum states form a basic building block of every quantum information system. From photonic polarization to two-level atoms, complete sets of quantum gates for a variety of qubit systems are well known. For multilevel quantum systems beyond qubits, the situation is more challenging. The orbital angular momentum modes of photons comprise one such high-dimensional system for which generation and measurement techniques are well studied. However, arbitrary transformations for such quantum states are not known. Here we experimentally demonstrate a four-dimensional generalization of the Pauli X gate and all of its integer powers on single photons carrying orbital angular momentum. Together with the well-known Z gate, this forms the first complete set of high-dimensional quantum gates implemented experimentally. The concept of the X gate is based on independent access to quantum states with different parities and can thus be generalized to other photonic degrees of freedom and potentially also to other quantum systems.
Optical networks implementing single-qudit quantum gates may exhibit superior properties to those for qubits as each optical element in the network can work in parallel on many optical modes simultaneously. We present a class of such networks that implements in a deterministic and efficient way the quantum Fourier transform (QFT) in an arbitrarily high dimension. These networks redistribute the initial quantum state into the orbital angular momentum (OAM) and path degrees of freedom and offer two modes of operation. Either the OAM-only QFT can be implemented, which uses the path as an internal auxiliary degree of freedom, or the path-only QFT is implemented, which uses the OAM as the auxiliary degree of freedom. The resources for both schemes scale linearly O(d)O(d)O(d) with the dimension ddd of the system, beating the best known bounds for the path-encoded QFT. While the QFT of the orbital-angular-momentum states of single photons has been applied in a multitude of experiments, these schemes require specially designed elements with nontrivial phase profiles. In contrast, we propose a different approach that utilizes only conventional optical elements.
High-dimensional degrees of freedom of photons can encode more quantum information than their two-dimensional counterparts. While the increased information capacity has advantages in quantum applications (such as quantum communication), controlling and manipulating these systems has been challenging. Here we show a method to perform deterministic arbitrary high-dimensional Pauli XXX gates for single photons carrying orbital angular momentum. The XXX gate consists of a cyclic permutation of qudit basis vectors and, together with the ZZZ gate, forms the basis for performing arbitrary transformations. The proposed experimental setups only use two basic optical elements such as mode sorters and mode shifters and thus could be implemented in any system where these experimental tools are available. Furthermore the number of involved interferometers scales logarithmically with the dimension, which is important for practical implementation.
The recently demonstrated possibility of entangling opposite values of the orbital angular momentum (OAM) of a photon with its spin enables the realization of nontrivial one-photon spin-orbit four-dimensional states for quantum information purposes. Hitherto, however, an optical device that can able to perform arbitrary unitary transformations on such spin-orbit photon states has not been proposed yet. In this work we show how to realize such a ``universal unitary gate'' device, based only on the existing optical technology, and describe its operation. Besides the quantum information field, the proposed device may find applications wherever an efficient and convenient manipulation of the combined OAM and spin of light is required.
Unitary transformations are the fundamental building blocks of gates and operations in quantum information processing, allowing the complete manipulation of quantum systems in a coherent manner. In the case of photons, optical elements that can perform unitary transformations are readily available only for some degrees of freedom, e.g., wave plates for polarization. However, for high-dimensional states encoded in the transverse spatial modes of light, performing arbitrary unitary transformations remains a challenging task for both theoretical proposals and actual implementations. Following the idea of multi-plane light conversion, we show that it is possible to perform a broad variety of unitary operations at high quality by using only a few phase modulation planes. More importantly, we experimentally implement several high-dimensional quantum gates for up to five-dimensional states encoded in the full-field mode structure of photons. In particular, we realize cyclic and quantum Fourier transformations, known as Pauli X ^ -gates and Hadamard H ^ -gates, respectively, with an average visibility of more than 90%. In addition, we demonstrate near-perfect “unitarity” by means of quantum process tomography, unveiling a process purity of 99%. Last, we demonstrate the benefit of the two independent spatial degrees of freedom, i.e., azimuthal and radial, and implement a two-qubit controlled-NOT quantum operation on a single photon. Thus, our demonstrations open up new paths to implement high-dimensional quantum operations, which can be applied to various tasks in quantum communication, computation, and sensing schemes.