Heat-bath algorithmic cooling with correlated qubit-environment interactions

Abstract

Cooling techniques are essential to understand fundamental thermodynamic questions of the lowenergy states of physical systems, furthermore they are at the core of practical applications of quantum information science. In quantum computing, this controlled preparation of highly pure quantum states is required from the state initialization of most quantum algorithms to a reliable supply of ancilla qubits that satisfy the fault-tolerance threshold for quantum error correction. Heat-bath algorithmic cooling has been shown to purify qubits by controlled redistribution of entropy and multiple contact with a bath, not only for ensemble implementations but also for technologies with strong but imperfect measurements. In this paper, we show that correlated relaxation processes between the system and environment during rethermalization when we reset hot ancilla qubits, can be exploited to enhance purification. We show that a long standing upper bound on the limits of algorithmic cooling Schulman et al (2005 Phys. Rev. Lett. 94, 120501) can be broken by exploiting these correlations. We introduce a new tool for cooling algorithms, which we call 'state-reset', obtained when the coupling to the environment is generalized from individual-qubits relaxation to correlated-qubit relaxation. Furthermore, we present explicit improved cooling algorithms which lead to an increase of purity beyond all the previous work, and relate our results to the Nuclear Overhauser Effect.

Influence of the nonradiative decay of qubits into a common channel on the transport properties of microwave photons

“…Specially prepared common bath can also be used for the creation of non-decaying entangled states in multi-qubit systems [13,14].…”

Correlation-Enhanced Algorithmic Cooling

“…In practice, however, the qubits generally possess correlations of both classical and quantum origin, generated thermally and through interactioninduced entanglement respectively. Here, we generalize HBAC to allow the presence of correlations -and we show that these correlations provide a resource that can be used to improve the efficiency of HBAC methods beyond previously established limits.Indeed, recent work has suggested that quantum correlations are important in work extraction and entropy flows in cooling protocols [15][16][17][18][19][20]. However, current algorithms such as PPA (Partner Pairing Algorithm [4, 9]) do not make use of correlations in the system.…”

“…Indeed, recent work has suggested that quantum correlations are important in work extraction and entropy flows in cooling protocols [15][16][17][18][19][20]. However, current algorithms such as PPA (Partner Pairing Algorithm [4,9]) do not make use of correlations in the system.…”

“…We have compared our results with two other HBAC methods: the PPA-HBAC [9] for two qubits and three qubits (let us call it PPA-2 and PPA-3 respectively) and a new cooling algorithm [32], SRΓ n -HBAC, based on the Nuclear Overhauser Effect (NOE) (which improves over PPA-HBAC).…”

“…1b is called SRΓ 2 -HBAC [32]. In this method, the coupling to the environment is not limited to just rethermalization, but could also include correlations between the qubits of the system and the bath.…”