|bibcode = 1995PhRvL..75..346L }}</ref> A [[quantum algorithm]] can be instantiated as a [[quantum circuit]].<ref>{{Cite journal|last1=Yazdani|first1=Maryam|last2=Zamani|first2=Morteza Saheb|last3=Sedighi|first3=Mehdi|date=2013-06-09|title=A Quantum Physical Design Flow Using ILP and Graph Drawing|journal=Quantum Information Processing Journal|volume=12|issue=10|page=3239|doi=10.1007/s11128-013-0597-6|arxiv=1306.2037|bibcode=2013QuIP...12.3239Y|s2cid=12195937}}</ref><ref>{{Cite journal|last1=Whitney|first1=Mark|last2=Isailovic|first2=Nemanja|last3=Patel|first3=Yatish|last4=Kubiatowicz|first4=John|date=2007-04-02|title=Automated Generation of Layout and Control for Quantum Circuits|url=https://archive.org/details/arxiv-0704.0268|journal=ACM Computing Frontiers|arxiv=0704.0268}}</ref>
A '''logical''' qubit specifies how a single qubit should behave in a quantum algorithm, subject to quantum logic operations which can be built out of quantum logic gates. However, issues in current technologies preclude single [[two-state quantum system]]s, which can be used as '''physical''' qubits, from reliably encoding and retaining this information for long enough to be useful. Therefore, current attempts to produce scalable quantum computers require [[quantum error correction]], and multiple (currently many) physical qubits must be used to create a single, error-tolerant logical qubit. Depending on the error-correction scheme used, and the error rates of each physical qubit, a single logical qubit could be formed of up to 1,000 physical qubits.<ref name="FowlerMariantoni2012">{{cite journal|last1=Fowler|first1=Austin G.|last2=Mariantoni|first2=Matteo|last3=Martinis|first3=John M.|last4=Cleland|first4=Andrew N.|title=Surface codes: Towards practical large-scale quantum computation|journal=Physical Review A|volume=86|issue=3|year=2012|page=032324|issn=1050-2947|doi=10.1103/PhysRevA.86.032324|arxiv=1208.0928|bibcode=2012PhRvA..86c2324F|s2cid=119277773}}</ref>
