Nature | Spin orbit microcavity laser: a new dimension of quantum communication

Gilbert

Categories:

Laser

Tags:

Optics

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2022-11-28

Research background

Bits, Qubits and Qudits

In classical digital communication and calculation, information is transmitted, stored or calculated in binary bits. One bit represents one of the two states of 0 or 1; In mainstream quantum communication and computing, the smallest information unit is qubits, which can be the superposition of two states, 0 and 1. Its information capacity and security are far superior to those in classical digital communication. Different from the current encryption security in classical digital communication, this security is guaranteed by physical laws rather than complex mathematical algorithms that can still be cracked. Especially with the rapid development of computing technology, it is more and more likely to be cracked.

However, since each quantum bit is only the superposition of two states, its single bit information capacity and interference carrying capacity still have a large room for improvement. In a quantum system, if the smallest information unit is composed of more than two states, we call it high-dimensional quantum bits (Qudits). For example, a qubit consisting of three states is usually expressed as a qudit with d=3. Improving the dimension of Qudit can not only increase the bit capacity and communication bandwidth in communication, but also further enhance the anti-interference ability, security and robustness of the channel.

The generation of high-dimensional arbitrary Qudit states mainly depends on bulk optical devices, and its dimensional expansion is constrained by both device volume and architecture complexity. On chip integration and scalability are the development trend of high-performance communication and quantum communication devices in the future, but they are also the current difficulties. Recently, Professor Feng Liang of the University of Pennsylvania has developed an integrated microcavity laser, which can realize arbitrary lasing in a four-dimensional Hilbert space. The device is also scalable. This work was published in Nature.

Research Highlights

Lasing of spin orbit vortex lasers in four dimensional hilbert space

Professor Feng Liang's team continues the basic design in their previous work (Science 368760-763 (2020)). Two spin orbit ring lasers are coupled by pure imaginary gauge fields composed of four control waveguides (left and right groups) (Figure 1), and their complex coupling coefficients can be dynamically adjusted accurately. The difference is that the previous work realized the dynamic control of the photon orbital angular momentum (OAM); By introducing and controlling the coupling between the spin angular momentum Spin and the orbital angular momentum OAM between the two rings, an additional degree of control freedom is realized in the present work, which can generate lasing in any four-dimensional Hilbert space. As shown in Figure 1, each vortex laser itself has two degrees of freedom to dynamically adjust the intensity and phase relationship between two indirectly coupled spin orbit vortex states in its cavity, so as to generate an arbitrary state (equivalent to Qubit) on a two-dimensional higher-order Poincare sphere with SU (2) symmetry, realizing arbitrary state lasing in two-dimensional Hilbert space. The coupling between the two lasers organically combines these two two-dimensional higher-order Poincare spheres to form a four-dimensional super Bloch sphere with SU (4) symmetry (Figure 2), which has six degrees of freedom of regulation.

In the experiment, the authors used the thermo optical effect to precisely control the refractive index difference inside each group of control waveguides to control the longitude coordinates of the lasing state of a single spin orbit laser on the two-dimensional high-order Poincare sphere (Fig. 3a, b). At the same time, thanks to the non Hermitian characteristics (controllable gain and loss) of quantum well materials, the latitude coordinates of the lasing state in a single spin orbit laser on a two-dimensional high-order Poincare sphere are effectively adjusted by adjusting the difference in the optical pumping intensity on the same group of control waveguides (Fig. 3c, d). Each spin orbit laser itself completely covers a two-dimensional high order Poincare sphere, which fully demonstrates the advantages and scalability prospects of the basic unit. By using two non Hermitian coupled spin orbit lasers, the authors further demonstrated the manipulation of the lasing state in a four-dimensional Hilbert space with SU (4) symmetry. As shown in Figure 2, the control of the phase and intensity between the two lasers can be represented on the two-dimensional higher-order Poincare sphere III, whose two poles correspond to the lasing states on the higher-order Poincare spheres I and II respectively. Specifically, the authors control the latitudinal coordinates of the lasing state on the sphere by controlling the difference in pump intensity between the two rings; The longitude coordinate of the lasing state on the sphere can be adjusted by adjusting the difference of the resonant frequencies of the double rings. As shown in Figure 4, the authors selected and generated two representative higher-order states, one of which is of great significance to the quantum error correction algorithm, and achieved high fidelity in the experiment.

The spin orbit microcavity laser demonstrated in this study has excellent integration and scalability, and its high dimensional lasing performance is of great significance for the next generation of large capacity secure communication and computing systems; At the same time, its design idea has some enlightenment on the generation of high-dimensional quantum states and the future high-dimensional quantum communication technology.