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Physical Review A

Extracting an entangled state of n−t qubits from ann-qubit entangled state after errors attsites

Chui‐Ping Yang, Julio R. Gea-Banacloche

Published Aug 20, 2001• 4 citations
Publisher

Abstract

We present a method to protect the entanglement in a maximally entangled n-qubit state of the Greenberger-Horne-Zeilinger or ``Schr\"odinger cat'' form against decoherence, by pairing up each qubit with another one in such a way that each pair of qubits undergoes collective decoherence. Under these conditions, if t pairs experience an error, we show how one can still, in general, extract a maximally entangled state of the remaining $n\ensuremath{-}t$ pairs. Two versions of the method are considered: one of them applies when the locations of the decohered qubits are known, and it might be useful, for instance, in secret-sharing protocols. Another version, which is free from this restriction, provides only partial protection (the resulting state is, generally, not a maximally entangled state), but could nonetheless be useful for the important special case in which the source of error is spontaneous emission.

DOI
10.1103/physreva.64.032309

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We present a method to protect the entanglement in a maximally entangled n-qubit state of the Greenberger-Horne-Zeilinger or ``Schr"odinger cat'' form against decoherence, by pairing up each qubit with another one in such a way that each pair of qubits undergoes collective decoherence. Under these conditions, if t pairs experience an error, we show how one can still, in general, extract a maximally entangled state of the remaining n\ensuremath−tn\ensuremath{-}tn\ensuremath−t pairs. Two versions of the method are considered: one of them applies when the locations of the decohered qubits are known, and it might be useful, for instance, in secret-sharing protocols. Another version, which is free from this restriction, provides only partial protection (the resulting state is, generally, not a maximally entangled state), but could nonetheless be useful for the important special case in which the source of error is spontaneous emission.
We investigate the decay of entanglement, due to decoherence, of multiqubit systems that are initially prepared in highly (in some cases maximally) entangled states. We assume that during the decoherence processes each qubit of the system interacts with its own independent environment. We determine, for systems with a small number of qubits and for various decoherence channels, the initial states exhibiting the most robust entanglement. We also consider a restricted version of this robustness-optimization problem that only involves states equivalent, under local unitary transformations, to the Greenberger-Horne-Zeilinger ∣GHZ⟩|\text{GHZ}⟩∣GHZ⟩ state.
Secret sharing is a procedure for sharing a secret among a number of participants such that only the qualified subsets of participants have the ability to reconstruct the secret. Even in the presence of eavesdropping, secret sharing can be achieved when all the members are quantum. So what happens if not all the members are quantum? In this paper, we propose two semiquantum secret sharing protocols by using maximally entangled Greenberger-Horne-Zeilinger-type states in which quantum Alice shares a secret with two classical parties, Bob and Charlie, in a way that both parties are sufficient to obtain the secret, but one of them cannot. The presented protocols are also shown to be secure against eavesdropping.
Entanglement is the quintessential quantum phenomenon and a key enabler of quantum algorithms. The ability to faithfully entangle many distinct particles is often used as a benchmark for the quality of hardware and control in a quantum computer. Greenberger-Horne-Zeilinger (GHZ) states, also known as Schrödinger cat states, are useful for this task. They are easy to verify, but difficult to prepare due to their high sensitivity to noise. In this Letter we report on the largest GHZ state prepared to date consisting of 120 superconducting qubits. We do this via a combination of optimized compilation, low-overhead error detection and temporary uncomputation. We use an automated compiler to maximize error-detection in state preparation circuits subject to arbitrary qubit connectivity constraints and variations in error rates. We measure a GHZ fidelity of 0.56(3) with a post-selection rate of 28%. We certify the fidelity of our GHZ states using multiple methods and show that they are all equivalent, albeit with different practical considerations.
We show a way to correct collective error through a three-qubit quantum code. The encoding and decoding (i.e., error recovery) operations for such a code, which protects one qubit of quantum information perfectly (with infinite ``distance'') against collective decoherence, are presented. The code is actually a manifestation of a noiseless subsystem first reported by Knill, Laflamme, and Viola [Phys. Rev. Lett. 84, 2525 (2000)], and provides a good illustration of the relationship between noiseless subsystems and error-correcting codes already noted by these authors. We also show in detail how this code can be used to preserve an arbitrary joint state (including entangled states) of two qubits, by pairing each of them with two other qubits, even in the case in which the two resulting clusters interact in nonequivalent ways with a common environment.