Researchers Develop Qubit Gate Operations in Polariton Condensates

Researchers Luciano S Ricco, Ivan A Shelykh, and Alexey Kavokin have explored the use of bosonic condensates of exciton-polaritons in elliptical traps as optically tunable qubits. They have developed a set of universal single-qubit gates that can control the shift of the Bloch vector using an auxiliary laser beam. The team also examined interaction mechanisms between two neighboring traps that could enable the design of two-qubit operations. The study also considered error sources in the context of polariton traps. The findings could lead to the realization of various quantum algorithms in a planar microcavity with optically induced elliptical traps.

What are Qubit Gate Operations in Elliptically Trapped Polariton Condensates?

The study, conducted by Luciano S Ricco, Ivan A Shelykh, and Alexey Kavokin, explores the concept of bosonic condensates of exciton-polaritons optically confined in elliptical traps. These condensates are represented by a point on a Bloch sphere and are considered as an optically tunable qubit. The researchers describe a set of universal single-qubit gates that result in a controllable shift of the Bloch vector through an auxiliary laser beam. They also consider interaction mechanisms between two neighboring traps that enable the design of two-qubit operations such as CPHASE and CNOT gates.

The researchers analyze both single and two-qubit gates in the presence of error sources in the context of polariton traps. These error sources include pure dephasing and spontaneous relaxation mechanisms, which lead to a reduction in the fidelity of the final qubit states and quantum concurrence, as well as an increase in Von Neumann entropy. The researchers also discuss the applicability of their qubit proposal in the context of DiVincenzo’s criteria for the realization of local quantum computing processes.

The developed set of quantum operations could pave the way for the realization of a variety of quantum algorithms in a planar microcavity with a set of optically induced elliptical traps.

What is the Role of Solid-State Exciton-Polariton Systems in Quantum Computing?

The placement of solid-state exciton-polariton systems, or simply polaritons, in the quantum computing race remains questionable to date. This is despite the growing quality of patterned optical cavities and available active materials therein, and steadily advancing techniques in potential landscape engineering and optical control to minimize decoherence processes.

In inorganic II-VI semiconductor microcavities, significant cross-phase modulation, squeezing, blockade effect, and interactions at the single polariton level have already been demonstrated. This is due to the large interaction strengths between polaritons, owing to the generous size of their underlying Wannier-Mott exciton component. Quantum computing proposals using polaritons can be divided into two categories: single-particle and macroscopic field strategies. The researchers in this study are concerned with the latter, based on nonequilibrium polariton Bose-Einstein condensates.

What are Polaritons and How are They Used in Quantum Computing?

Polaritons are hybrid particles that arise in the strong coupling regime between matter (excitons) and light (confined photons). They possess extremely light effective mass, large interparticle interaction strengths, and can be reversibly adjusted through all-optical techniques. Information about the polariton state is encoded in the emitted cavity light, which can be measured through standard optical techniques.

Being bosonic quasiparticles, polaritons can be stimulated into a macroscopically coherent state, which lies at the interface between nonequilibrium Bose-Einstein condensates and polariton lasers. A condensate of polaritons can be conveniently described by a single macroscopic wavefunction.

How are Polariton Condensates Used in Quantum Computing?

Polariton condensates are driven-dissipative objects, with particles being generated from an external laser excitation and losses naturally occurring through the cavity mirrors. They can possess equilibrium points that do not coincide with the many-body system ground state in thermodynamic equilibrium. In particular, they can populate and stabilize into the excited state manifolds of their transverse trapping configuration.

The concept of using polaritons as macroscopic quantum states for continuous-variable quantum computation was recently visited by Xue et al. using the superposition of colocalized and nondegenerate ring-shaped polariton condensates of opposite circulation. These polariton qubits are similar to their superconducting counterpart, the flux qubits. However, instead of circulating basis states, polariton condensates might be more conveniently described in terms of p states corresponding to their spatial dipolar distribution along the two main axes of the transverse trap, i.e., px and py.

What is the Future of Digital Quantum Computers Based on Polariton Qubits?

For the realization of digital quantum computers based on polariton qubits, one should aim at maximizing the coherence times of each individual qubit and coupled arrays of qubits. Interestingly, the dynamics of polariton condensates are characterized by a range of time scales, and it is a nontrivial question of which of them would be responsible for the decoherence of polariton qubits. The shortest timescale is given by the single polariton lifetime that is dependent on the specific system parameters.