This setup models the interaction of a probe with hot, dense matter composed of deconfined partons, a quark-gluon plasma (QGP). The probe may be an energetic parton which produces a high-energy jet, or a heavy quark – heavy antiquark meson – quarkonium. The exact description of the interaction between the probes (jets or quarkonium) with the medium is complex and most phenomenological calculations of the problem use semiclassical approximations.
Quantum computing devices may eventually enable a full quantum treatment of such dynamical processes – an accurate treatment that can be achieved within a finite time. The premise of the advance to pursue quantum computation in this case (as many others) is that the problems that are classically hard to compute may be effectively formulated as “quantum-easy.”
Figure 2. Measurement of P0(t), which can be interpreted as the time-dependent nuclear modification factor. Results from the IBM Q Vigo device including error mitigations (a readout correction and a so-called RIIM correction – an extrapolation to zero-noise for quantum-gate errors) compared to results from the qiskit quantum circuit simulator (an IBM Open-Source Quantum Computing Software) for Ncycle = 1 and Ncycle = 3 approximations and the calculation using a Runge-Kutta method. Higher values of Ncycle quickly converge to the result using the Runge-Kutta method. The current quantum computer simulation corresponds to Ncycle = 1.
The new method shows how to simulate the dynamics of the process when an open quantum system (the probe) interacts with the external environment. Figure 1 shows a multi-level quantum state, such as quarkonium (S) immersed within the hot QGP environment (E) and interacting with it. Figure 2 shows the result of the simulation on an actual quantum computer (IBM-Q) compared to a simulated circuit and a classical calculation. The figure shows the probability for the quarkonium probe to remain bound, as a function of time. This is directly related to the nuclear modification factor (usually labelled RAA) that experimentalist measure. The quantum circuit simulates the dynamics of the so-called Lindblad equation which is an extension of the Schrödinger equation applicable to closed systems. The implementation on the IBM-Q device used 3 qubits to simulate the system and its interaction with the thermal environment. With current NISQ (Noisy Intermediate-Scale Quantum) era devices, the depth of the circuit is limited by qubit coherence times, and involves significant noise introduced by imperfect two-qubit gate operations. After applying error mitigation techniques, it was possible to demonstrate good agreement between the quantum and classical simulation, as is shown in Fig. 2. This pioneering work lays the ground for future applications for a range of problems. It demonstrates the feasibility of simulating open quantum systems on current and near-term quantum devices, which is of broad relevance to applications in nuclear physics, quantum information, and other fields.
References
[1] “Quantum simulation of open quantum systems in heavy-ion collisions,” Wibe A. de Jong (LBL/CRD), Mekena Metcalf (LBL/CRD), James Mulligan (LBL/NSD), Mateusz Płoskoń (LBL/NSD), Felix Ringer (LBL/NSD), Xiaojun Yao (MIT), arXiv:2010.03571 [hep-ph], submitted for publication.
[2] “Simulating subatomic physics on a quantum computer”, Symmetry Magazine,
https://www.symmetrymagazine.org/article/simulating-subatomic-physics-on-a-quantum-computer
